IN DEGREE PROJECT MATERIALS SCIENCE AND ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2021

Experimental Study of Internal Injector Deposits In Internal Combustion Engines Using Renewable Fuels

PATRICK KIPROTICH KORIR

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

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Abstract

The strive to minimize emissions in the automotive industry keeps gaining

momentum. Continuous improvement of engine designs and development of more

efficient fuel systems in diesel vehicles is a solution to be applauded. More

importantly is the growing shift to use of renewable fuels in internal combustion

engines. With countries implementing tighter regulations on emissions, and

markets have witnessed a rise in the use of biofuels. Subsequently, the fuel

quality varies from market to market. Blending of different fuels changes the

properties of fuel as solubility of some compounds reduce. Consequently, soft

particles which are precipitated in the process have been linked to deposit

formation of internal diesel injector deposits (IDIDs).

This project aims at investigating IDIDs and possible conditions that enhance

their formation in the injector. An injector test rig operating at actual engine

pressures (>2000 bars) has been constructed for this purpose. Test fuel for use in

the rig is prepared at Scania by introducing soft particles into B10 fuel. Start of

the test rig was performed by checking component functionality and pressure test.

Due to leakage problem, a redesign of fuel collection cup was done. Evaluation of

test fuel was carried to determine the suitability for deposit formation in the

injector. Two screening tests were carried to investigate sticky deposit formation

using the test fuel. Autoclave test was carried out at temperature of 150 0C over

a period of up to four days. Frying pan test was performed to evaluate formation

of deposits with increase in temperature between 90 0C to 230 0C. Analysis was

carried out using SEM-EDX, GC-MS and FTIR instruments.

The test fuel prepared at Scania for replication of deposits in the injector yielded

positive results. Sticky deposits formed during the frying pan test evidenced by

stretchy and sticky residue on the pan. FTIR analysis showed that the presence

of metal carboxylate which is as a result of the metal ion soft particles. Autoclave

tests showed formation of brown deposits on the vessel. SEM-EDX analysis of the

brown deposits gave great insights on the morphology of the deposit contrasted to

the structure of soft particles initially present in the test fuel. Soft particles are

small and smeary with a regular shape while the deposits are large, irregular,

agglomerated and rough in texture. This is important in understanding the

transformation mechanism of soft particles to deposits. A combination of calcium

and sodium soft particles in the test fuel showed better ability to form deposits

during the autoclave test. GC-MS analysis showed huge decrease in the

concentration of soft particles in test fuel after autoclave tests compared to initial

test fuel.

In conclusion, the test fuel prepared works as expected and thus can be scaled up

for running the injector test rig. Additionally, test fuel containing calcium and

sodium soft particles have a higher probability to form deposits. Deposits were

indeed proven to be metal carboxylates as expected.

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Sammanfattning

Strävan efter att minimera utsläppen inom fordonsindustrin fortsätter att ta fart.

Kontinuerlig förbättring av motorkonstruktioner och utveckling av effektivare

bränslesystem i dieselfordon är en lösning som bör applåderas. Ännu viktigare är

den ökande övergången till användning av förnybara bränslen i

förbränningsmotorer. Med länder som inför strängare utsläppsregler har

marknaderna sett en ökad användning av biobränslen. Därefter varierar

bränslekvaliteten från marknad till marknad. Blandning av olika bränslen

förändrar bränslets egenskaper när lösligheten hos vissa föreningar minskar.

Följaktligen har mjuka partiklar som fälls ut i processen kopplats till

avlagringsbildning av interna dieselinjektoravlagringar (IDID).

Detta projekt syftar till att undersöka IDID:s och möjliga förhållanden som

förbättrar deras bildande i injektorn. En injektortestrigg som arbetar vid faktiska

motortryck (>2000-bar) har konstruerats för detta ändamål. Testbränsle för

användning i riggen bereds på Scania genom att mjuka partiklar förs in i B10-

bränsle. Testriggens start utfördes genom kontroll av komponentens

funktionalitet och trycktest. På grund av läckageproblem gjordes en omdesign av

bränsleuppsamlingskoppen. En värdering av testbränslet genomfördes för att

fastställa lämpligheten för deponeringsbildning i injektorn. Två screeningtester

utfördes för att undersöka klibbig avlagringsbildning med hjälp av testbränslet.

Autoklavtest utfördes vid en temperatur av 150 0C under en period av upp till

fyra dagar. Autoklavtest utfördes för att utvärdera bildandet av avlagringar med

temperaturökning mellan 90 0C till 230 0C. Analysen utfördes med hjälp av SEM-

EDX, GC-MS och FTIR instrument.

Testbränslet som förbereddes i Scania för replikering av avlagringar i injektorn

gav positiva resultat. Klibbiga avlagringar som bildas under stekpannans test

framgår av stretchiga och klibbiga rester på pannan. FTIR-analys visade att

förekomsten av metallkarboxylat som är ett resultat av metalljonens mjuka

partiklar. Autoklavtester visade bildandet av bruna avlagringar på fartyget.

SEM-EDX-analysen av de bruna avlagringarna gav stora insikter om

depositionens morfologi i motsats till strukturen hos mjuka partiklar som

ursprungligen fanns i testbränslet. Mjuka partiklar är små och utsmetade med

en regelbunden form medan avlagringarna är stora, oregelbundna,

agglomererade och grova i konsistensen. Detta är viktigt för att förstå

omvandlingsmekanismen för mjuka partiklar till avlagringar. En kombination av

kalcium- och natriummjuka partiklar i testbränslet visade bättre förmåga att

bilda avlagringar under autoklavtestet. GC-MS-analysen visade en enorm

minskning av koncentrationen av mjuka partiklar i testbränsle efter

autoklavtester jämfört med det ursprungliga testbränslet.

Sammanfattningsvis fungerar testbränslet som förväntat och kan därför skalas

upp för att driva injektortestriggen. Dessutom har testbränsle som innehåller

mjuka kalcium- och natrium partiklar större sannolikhet att bilda avlagringar.

Avlagringarna visade sig faktiskt vara metallkarboxylater som förväntat.

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Table of Contents

1 Introduction .................................................................................................... 1

1.1 Objectives ..................................................................................................... 2

1.2 Significance .................................................................................................. 2

1.3 Delimitations ................................................................................................ 2

1.4 Social and Ethical aspects of the project ..................................................... 3

2 Background ..................................................................................................... 4

2.1 High pressure common rail systems ........................................................... 5

2.2 Internal Diesel Injector Deposits (IDIDs) ................................................... 6

2.2.1 Causes of Internal diesel injector deposits ........................................... 6

2.2.2 Types of internal diesel injector deposits ............................................. 7

2.2.3 Problems of internal diesel injector deposits........................................ 8

2.3 Replication of internal injector deposits using test rigs ............................. 9

2.4 Influencing parameters in internal deposit replication rig ...................... 10

2.4.1 Temperature ........................................................................................ 10

2.4.2 Injection pressure ................................................................................ 11

2.4.3 Used Fuel ............................................................................................. 11

2.5 Deposits indicators during test ................................................................. 12

2.6 Autoclaves .................................................................................................. 14

3 Methodology .................................................................................................. 15

3.1 Test rig description .................................................................................... 15

3.1.1 Temperature control ............................................................................ 16

3.1.2 Pressure testing ................................................................................... 17

3.1.3 Leakage prevention ............................................................................. 17

3.1.4 Calibration ........................................................................................... 19

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3.1.5 Selection of parameters to be controlled during replication of injector

deposits ............................................................................................................. 19

3.2 Experiments for evaluation of test fuel ..................................................... 20

3.2.1 Autoclave test ...................................................................................... 21

3.2.2 Frying pan screen test ......................................................................... 24

3.3 Used fuel ..................................................................................................... 24

3.4 Instruments used for deposit study and analysis ..................................... 25

3.4.1 Scanning Electron microscopy - Energy dispersive X-ray ................. 25

3.4.2 Fourier Transform Infrared Spectroscopy (FTIR) ............................. 25

3.4.3 Gas Chromatography- Mass Spectroscopy (GC/MS) .......................... 26

3.4.4 Visual inspection ................................................................................. 26

4 Results & Discussion ................................................................................... 27

4.1 Test protocol ............................................................................................... 27

4.2 Test rig start up ......................................................................................... 28

4.3 Frying pan test tests .................................................................................. 29

4.4 Autoclave test results ................................................................................ 31

5 Conclusion ..................................................................................................... 36

6 Future work ................................................................................................... 37

7 Acknowledgement ........................................................................................ 38

8 References ...................................................................................................... 39

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List of Acronyms

EOI – End of Injection

EVs – Electric vehicles

FAME - Fatty Acids Methyl Esters

FTIR – Fourier Transform Infra-red spectroscopy

GC/MS – Gas chromatography- mass spectroscopy

HVO – Hydrogenated Vegetable Oil

IDIDs – Internal diesel injector deposits

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1 Introduction

Earth is warming and necessity to mitigate global warming is more real than

ever. Carbon dioxide (CO2) emission is one of the major greenhouse gases

associated with climate change. Combustion of fossil fuels to power industrial

processes, generate electricity and propel vehicles is one source of CO2 [1] as

shown in Figure 1-1. Due to stringent measures governing carbon emission by

various countries, there has been a paradigm shift towards renewable sources of

energy. Transport sector is a large consumer of fossil fuel at 65% of total world oil

products [2]. It is responsible for 24% of global CO2 emissions courtesy of

combustion of fuels. Monumental solutions have been witnessed in the

automotive industry in push for reduction of emissions. Use of electric vehicles

(EVs) has risen as countries have developed policies to reduce emissions [3]. Like

any technology, it is still developing and probably will take some time to replace

the internal combustion engine (ICE). ICEs still occupy an important role in the

transport sector because of several associated merits over the EV counterparts,

at least as at present. Intense debates around the ICEs continue to mount [4]. As

one of the oldest technologies of mobility, its development and improvement has

led to milestone achievements. Shift to renewable fuels in ICEs has been

accredited to significant reduction in CO2 emissions. Biofuel is evidently a

sustainable alternative to finite fossil fuel resource for use in ICEs and can lead

to a reduction of CO2 emissions by 80% [5]. One great benefit of biofuels is its

application in nearly all ICEs without requirement of any fundamental technical

modification. In addition to fuel change, several improvements have been done

to increase the efficiency for example improved injection and exquisite monitoring

of engine performance. Blending of fossil fuel with biodiesel is a move that most

fuel markets have undertaken. Consequentially, fuel quality keeps fluctuating

especially with the advent of reduced sulphur content which significantly affects

the fuel ability to hold products from semi-soluble reactions. Moreover, it prompts

use of lubrication additives and corrosion inhibitors. FAME has increased

contributing to more less stable compounds into fuel [6]. Prominently, deposits

and precipitates readily observed.

Figure 1-1: Energy consumption by transport sector [5]

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1.1 Objectives

This project is aimed at running a previously constructed injector test rig under

controlled variables to enable the formation of deposits on the internal parts of

the injector. The zone of interest is the lower part of contact of injector needle and

inner wall. The test involves non-combustion process in which a heater is installed

to generate the threshold temperature for formation of deposits. Specific

objectives are as follows:

a) To evaluate and understand other existing injector test rigs.

b) To start and run the injector rig previously constructed at Scania.

c) To perform tests of replication internal diesel injector deposits on a test rig

d) To establish a test protocol for running the injector rig for repeatable

results.

e) To evaluate the influence of temperature and soft particle type on deposit

formation using frying pan and autoclave tests.

f) To investigate the suitability of the proposed test fuel for formation of

deposits

1.2 Significance

Sustainability in the automotive sector has continuously taken shape to minimize

emissions. Through engine developments and fuel improvements, positive

progress has been observed. Renewable fuels are on a rise in heavy transportation

diesel engine trucks. This is foreseen to have an upward trend in the future where

fossil fuel is blended with HVO, biodiesel and other upcoming forms of biofuels.

Consequently, solubility challenge in the different components of the fuel which

fall out from the solution. As such, soft particles will become more prevalent

posing expected challenges in the filters and injectors. The outcome of this will

accompanied prominent failure of injector due to internal deposits. It is therefore

very cautious enough to foresee such problems as Scania explores new markets

which use fuels of different qualities. This project thus seeks to ensure readiness

to handle these anticipated issues related with the fuel by having available test

methods that create more understanding through replication of this possible

problem in the field.

1.3 Delimitations

1. The project focuses on the physical running of the experimental set-up with

little deliberation on the chemistry.

2. Evaluation and analysis of the formed deposits are not done but only

replicating deposits in the injector.

3. Time frame is limited to 20 weeks.

4. No focus on the mechanisms how the deposits are attached in the surface

of the injector.

5. Only deposits from calcium and sodium soaps in aged biodiesel B7/B10

previous work filter clogging.

6. Only injectors from Scania system to be included in the rig.

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1.4 Social and Ethical aspects of the project

Climate change has had a lot of ramifications on the environment and society in

principle. Use of fossil fuels has been identified as major source of carbon dioxide

that highly results in global warming. It is therefore important to mention that

this project ultimately drives a shift towards the use of renewable fuels in the

transport sector. The research herein works towards ensuring robust engine

system which increases the use of renewable fuels. This project greatly seeks to

fulfil sustainable development goal (SDG) 13 on climate action. However, it has

far reaching positive impact across the society through reduction of pollution in

cities from transportation sector. Aspects such as predictable weather patterns

will be witnessed again with successful increase in the use of renewable fuels and

decrease in fossil fuel consumption. Continuous use of biofuels such as biodiesel

and hydrogenated vegetable oil plays a role in cleaner exhausts from truck

engines. This thesis work enhances the adoption of such fuels by creating better

understanding of their possible problems and solutions during use in engines. A

decrease in the use of fossil fuels is a gain for environments across the land, sea

and air. Results obtained in this work thus go a long way in achievement of

Scania’s quest for sustainable solutions in transport industry. In-depth

understanding of internal diesel injector deposits is crucial in handling its

occurrence in the future as the markets increase the use of renewable fuels.

Ethical aspects are not considered herein as the experimental work performed did

involve situations that necessitate this consideration. However, as the fuel used

was flammable, safety procedures were followed strictly according to internal

standards to prevent any risk fires or accidents to the people around the

laboratories as well as those undertaking the experiment. This work mainly

involved instruments or machines for analysis and obtaining results.

2 Background

The need to shift to purely renewable fuels in the automotive industry is presently

at the peak. In a bid to solve the climate change menace, the solution of using

biodiesel has come with numerous challenges witnessed in the performance of

the engine alongside the life span of the fuel system components. Internal diesel injector deposits (IDIDs) are key noticeable nuisance associated with

the adoption of biodiesel. Different markets have reported the issue which

has necessitated a research in this area. It can be confidently mentioned

that the major cause of the injector deposits is the presence of soft particles

in the fuel. These soft particles are present in the form of metal ion

carboxylates. The conditions in the truck engine greatly affect the rate of

formation of these deposits. Deposits form because of different

mechanisms on different inner sections of an injector. They have

varying morphologies and composition depending on the causes. Factors that

influence the development of these deposits are diverse [7].

Numerous ways have been used to create an understanding of this phenomenon.

A successful test entails replication of the engine conditions at least to a

significant percentage. The ability to have a wide range of controllable variables

is desired in running of experiments aimed at reproducing injector deposits. There

are several variables and conditions during the operation of an injector. The

dynamic nature of these conditions requires exquisite selection of major variables

to be controlled in a lab at Scania with no combustion and still yield near-engine

conditions in a truck engine. Profound parameters for instance are temperature

and test fuel.

There are complex heat transfer characteristics from the combustion chamber in

the cylinder, the outer part of the nozzle, the needle and the fuel. Thermocouples

provide a means of obtaining temperature of points of concern. However, it is

difficult to have a sensor inside the injector to have real time temperature.

Temperature is of interest as it has been closely linked to the driving force for the

formation of deposits on the needle of the injector. Based on previous calculations,

the needle temperature is given as 1500C. The fuel contains the particles from

fuel contaminants which cause the deposits when they precipitate. The formation

of IDIDs on needle impedes the efficiency of the injector. Effects such as late

opening and closing as well as incomplete opening and closing contribute to

incoherent performance of the truck engine.

The high-pressure rail systems have increased the need for reduced clearances in

the injector. IDIDs are bound to inhibit the flow additionally by reducing further

these clearances. Their presence coupled with the small clearances contribute to

the ultimate deterioration of the functioning of the injector [8].

Test rigs provide a platform to integrate the different parameters presumed to be

the drivers of formation of IDIDs without involving combustion. Since it takes a

long time for the injectors to start clogging or exhibiting sticky characteristics,

the test rig also aims at significantly reducing this time by speeding up the

formation of deposits.

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2.1 High pressure common rail systems

Current injection systems in truck engines utilize high pressures as shown in

Figure 2-1. Designs of the injector have undergone several improvements for this

reason. There are numerous benefits of high injection pressures which has

progressively been higher and higher. It enables achievement of better mixing

characteristics between air and fuel during combustion in the chamber. The

current design of common rail comprises of fuel being in constant high pressure

in the common rail where several injectors are connected to it. This provides

multiple injection capability and fuel economy. High fuel pressure has made it

possible to have pressure-controlled injector systems (piezoelectric).

Figure 2-1: High pressure common rail system [9]

The high-pressure pump is powered by electric motor which raises fuel pressure

to 2000 bars in the common rail. The fuel held in the common rail (accumulator)

is at constant high pressure. All the injectors are connected to the rail from which

the pressurized fuel flows.

There are two types of injector needles commonly used in the market: flat surface

and orifice type. The orifice type comprises holes to allow fuel during injection

while the flat surface type has a circular contact with flat section for fuel flow

during injection. Figure 2-2 shows a disassembled injector and the components.

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Figure 2-2: Injector components (injector body, tip with nozzle holes and

needle)

2.2 Internal Diesel Injector Deposits (IDIDs)

Internal deposits can be attributed to various sources of the elements (Na, Ca)

present in the soft particles. The needle and inner surface of the injector have a

sliding motion owing to the small clearance between them. Convergence of several

conditions in this zone is believed to be the reason for formation of stick deposits

as shown in Figure 2-3. The narrow clearance causes the effects of the deposits to

manifest in observable ways such as fuel consumption and engine power.

Figure 2-3: Common section where IDIDs (sticky deposits) form courtesy

Cummins

2.2.1 Causes of Internal diesel injector deposits

In the current fuel injection systems, lubrication occurs as the fuel flows. There

is significant acid level in some diesel oils in use presently. The common cause of

internal injector deposits is poor fuel stability. This hugely depends on the

percentage composition of Fatty acids methyl esters (FAME) commonly used as

biodiesel. Chemical structure of FAME consists of high content of oxygen and

chains of unsaturated hydrocarbons in the fuel. Reduced fuel stability has been

linked to this property [10]. Contamination is another major cause of IDIDs.

Contaminants vary from market and the sources. Elements found in fuel filters

from the market can be linked to biodiesel impurities, engine oils, or from fuel

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degradation [11]. Calcium has been traced and confirmed to present in deposits

found in failed Scania injectors. Deposits associated with metal ions are waxy or

soapy with sticky characteristics. Different researches have also confirmed

sodium contamination [12]. IDIDs related to these ions have similar properties.

Studies on calcium soft particles have been performed in Scania previously that

confirm their role in deposit formation. Additionally, the presence of oxidation

promoting catalysts in the fuel storage units or longer storage periods in humid

and hot conditions in oxygen abundance leads to poor fuel properties [13]. Galante

et al. states that solubility of carboxylate salts of various compounds is the

overriding mechanism for formation of injector deposits [14]. Factors that give

rise to low solubility of these compounds are temperature, high internal injector

pressure, chemical structure, and concentration of compound and base fuel

properties. Eventually, combination of low solubility together with tight

clearances in the injectors enhances the formation of internal injector deposits

[14].

2.2.2 Types of internal diesel injector deposits

There are several types of internal deposits which on the combination of factors

at play inside the injector. The mechanism of formation and growth of these

deposits vary prominently due to composition of fuel (e.g. additives), operations of

the engine, temperatures attained in the fuel injection and injection settings [7].

Based on the analysis of deposits from the market, the identification of the

causative agent is established. Figure 2-4 shows some of the deposit types which

occur in the injector. Soft, sticky deposit occurring between the injector needle

and injector wall is associated with reducing the performance of the injector.

Figure 2-4: Illustration of the different types of internal injector deposits

[6]

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2.2.3 Problems of internal diesel injector deposits

Engine designs have continuously undergone development and modification to

meet demands of high performance and efficiency. To attain this, high injection

pressures are presently used up to 2000 bars. This in turn calls for narrow

clearances of about 1 µm and precise timing controls during injection [6].

Presence of deposits on the sliding internal components can impede the proper

and efficient functionality of the injector. Amidst other observable problems,

deposits affects the fuelling timing and volume which subsequently leads to issues

with cold starts, power loss, delays in acceleration, and failure to start [15]. The

deposits observed in the market show sticky or waxy characteristics. Figure 2-5

shows the variation of injected volume upon occurrence of IDIDs.

Figure 2-5: Consequence of presence of deposits on the injector operation [16]

The phenomenon of the internal injector deposits has gained more and more focus

with the continuous rise in the use of biofuels in the transport industry. Analysis

of failed injectors from the field show the presence of these deposits.

Characterization of these deposits using techniques such as GC/MS, scanning

electron microscopy and FTIR has enabled the conclusion of their composition.

These results help understand the probable causes of the deposits and the

ultimate sources. With a clear knowledge of the formation of the injector deposits,

test rigs have been designed to simulate the process as in a truck. Fuel quality

play a major role in the formation of injector deposits. Traces of metallic elements

such as calcium in the deposits has been linked to the lubrication fluid, additives

and other sources depending on the fuel market. Additionally, the plugged fuel

filters have been due to calcium soaps [17]. The influence of injector temperature

on the formation of internal injector deposits is as result of causing precipitation

of the soft particles present in the fuel. Coupling these two critical factors

contributing to the occurrence of IDIDS, test rigs have been built in different

designs to replicate this phenomenon. Some rigs have employed the use of a

complete combustion in an engine and evaluate the injector for IDIDS. To speed

up the experimental studies and have more control on the rig, non-combustion

9

test rigs are used. To provide the desired temperature on the injector, controlled

source of heat is installed at the injector. Replication of the internal injector

deposits involves coalescing many variables such as test fuel, temperature of the

injector and injection characteristics (pressure, time, frequency). The robustness

of a rig greatly relies on the ability to execute all desired variables and yield

repeatable results.

2.3 Replication of internal injector deposits using test rigs

The design of a test rig determines the simplicity of operation and obtaining the

results. Process of reproducing internal diesel injector deposits (IDIDs) begins

with the identification of the deposit composition and introducing them in the test

fuel [6]. Use of artificially created soft particles gives the ability to control their

concentration in the test fuel. The type of soft particles (metal carboxylate salts

or soaps) is informed by analysing the injector deposits found in injectors from the

field. There are different types of IDIDs which can possibly form which makes

this step critical. The tests can be run over a short time and faster results

obtained. It is therefore important to have a stable test fuel with traceable soft

particles in correct concentration.

Physical rig construction forms a major component in the replication of IDIDs. It

provides a provision for delivery of the test fuel through the injector. Some tests

have used full-scale engine tests while some on the hand have implemented non-

engine tests [18]. The tests are driven by electric motors to create injection

pressure. Advantages of this design is the reduction of required fuel volume and

complexity in comparison with combustion engine tests. In each of the cases, the

parameters measured are different. Full-scale tests usually require specifically

designed stands and presents difficulty in elimination of possibility of residual

surface active contaminants [19].

High pressure pumps run by electric motors can provide the required pressures

in the common rail. Different configurations of injector placements have been

used in different experiments. Heating mechanisms to generate the necessary

temperature at the injector tip in non-combustion tests. The choice of method

greatly depends on the efficiency and ability of the heating mechanism to cause

the required temperatures. Modes that have been used in heating injector to

facilitate the formation of deposits include use of heated blocks. One of the

challenges in this set up is placement so that it creates the required effect of

temperature on the injector. Additionally, it is paramount to apply heat in a

manner that simulates the scenario inside the combustion chamber. The heat

flows from the tip towards the upper part of the injector is essential in trying to

replicate the formation of deposits as in the actual engine. The temperature

profile helps in understanding the relationship between deposit location and

temperature.

In a non-full-scale test, the fuel is not burnt but collected after being sprayed by

the injector. The technique employed at this point varies from what happens to

the test fuel after the injection. Some rigs recycle the fuel while others direct it to

separate tank. Each has different goal in the route fuel takes after the injector.

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Successful replication of deposits largely depends on firm understanding on the

local conditions of the zone where deposit formation occurs. As such, full scale

tests involving engines are highly dynamic exhibiting fluctuations between

injections in temperature and flow rate. The pressure remains constant owing to

the common rail design. According to the tests carried out, the fuel temperature

suddenly increases upon pressure release up to 900C. In real case scenario, the

formation of deposits is influenced by driving style (load and speed conditions)

and the composition of the fuel [20]. The aspect of the driving style is ensured by

arbitrary selection of the injection parameters by the ECU in the test rig.

2.4 Influencing parameters in internal deposit replication rig

For deposit formation in the injector, simultaneous influence of various conditions

should be considered. It is difficult to single out one or two variables and

independently analyse their effect on the rate of deposit formation. However, the

major controllable factors that can be synchronously manipulated may be

narrowed down into to reduce the complexity. Moreover, tests have been

undertaken to determine conditions under which soft particles fall off from the

solution and stick to the metal surface. Possible parameters of control in

replication of deposits include injection pressure, frequency of injection,

temperature of fuel (from pump inlet and the return from injector or common rail),

injection time and the concentration of soft particles [6]. The experiment also

justified that soft particles pass through 25µm filter but could not pass through

1µm filter. It is therefore important to consider the filter type if filtration is

deemed necessary in the test rig set up. High amounts of IDIDs were traced in

points of high temperature and high-pressure release.

2.4.1 Temperature

A critical temperature of 150 0C was noted below which deposits were not

produced. Above this temperature, the thickness of the deposit increases to levels

that is detectable and by extension could influence the clearances inside the

injector [6]. This can be seen from Figure 2-6 where there is a steep rise in the

thickness of deposits. Many publications involving the internal injector deposits

select different temperatures for experimental analysis of deposit formation. The

source of heat is commonly heaters or heated blocks to raise the temperature to

that of the engine during operation. In the engine, the injector nozzle is in direct

contact with the combustion chamber where the heat released. Through

conduction, the heat flows to other parts of the injector [21].

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Figure 2-6: Influence of temperature on deposit formation [6]

2.4.2 Injection pressure

High pressure promotes better atomization during spraying of fuel. Injection

pressure causes different effects with respect deposit formation. Higher pressure

leads to significant stressing of fuel. The outcome is aging of fuel and variation in

flow characteristics. Pressure energy is also converted to thermal energy during

fuel release [21].

2.4.3 Used Fuel

Since there is certainty on the relevant cause of the injector deposits, soft

particles, a test fuel with controlled concentration is used to speed up the process.

This makes the test fuel a major parameter that determines the rate of formation

and amount of deposits formed in the needle of the injector. A separate procedure

for preparation of test fuel has been followed in Scania with respect to the type of

deposits identified in the injectors from the market. Calcium has been identified

in injectors from analysis. The test fuel has therefore been doped with calcium to

form calcium soaps [22].

Replication of injector deposits in non-combustion test rig involves artificial re-

creation of real-world conditions necessary for formation of deposits. The test fuel

is the point of introducing the contaminants that have been established from

analysis of injector deposits from the field. The concentration of contaminants

(metal carboxylates or soft particles) influences the rate of formation of the

deposits and thus the time required for a successful replication of deposits.

Stability of the test fuel is essential in the experiment for consistent results and

repeatability. Homogeneity contributes to the rate of formation of deposits since

the soft particles will be distributed evenly in the fuel. The ability to run with a

homogeneous test fuel enables higher possibilities of repeatable results.

12

2.5 Deposits indicators during test

Various techniques are used as an indication of deposit formation. They are

mostly offline, where the system is stopped, and the injector inspected. Hoang and

Le, 2019 noted that there is a change in flow rate and pressure upon formation of

deposits [23]. As the test rig will be coupled to an inbuilt system, the end of

injection (EOI) should be a good prediction for the occurrence of deposits. A

reduction in EOI shows formation of deposits. A set up with no combustion was

done by ENIAK team [8] to develop a non-engine fuel injector deposit. The

procedure consisted of stressing the fuel to age instead of adding the soft particles

into the fuel. The rig utilized 1500 bars while injector heated to 370 0C at the

needle shaft. This temperature is stated to be at full engine load, with the aim of

replicating IDID by using external heating to simulating engine-like conditions.

The injection parameters such as injector temperature, timing and injection

pressure which are independent of operation of the engine were varied [8]. Typical

settings employed in the rig are as follows:

• Pressure – 1500 bars

• Temperature – 2600C needle shaft

• Energizing time – 500 µs

• Injection frequency – 10 Hz

• Fuel used was B10 and B0.

The experiments done took between 70-77 h and upon opening the injectors, there

was IDID present [8]. It was noted that there was no change in flow during the

experiment in relation to the deposits. At high pressure, the flow showed a

decrease in 5%. A conclusion was arrived at that injection pressure has minimal

influence on the IDID formation while its greatly influenced by temperature. An

online diagnosis while undertaking the test is desired in this investigation. The

thickness of the deposit was also measured.

Lacey et al. carried out a test apparatus to study the soft particles (carboxylates)

which was greatly reported as a problem and studied it on a test rig. Liquid

nitrogen was used to prevent ignition within the spray chamber. Heating was

done on the block to replicate the heat flow to the injector during combustion

process. Key parameters controlled are temperature (220 0C), rail pressure and

injection period. The fuel is notably used once and directed to used fuel tank thus

no recirculation of used fuel [24]. Figure 2-7 entails the consortium of various

possible factors contributing to the deposit build up. Specifically, the amount of

biodiesel in the fuel plays a major role as illustrated in Figure 2-8.

The temperatures measured near the needle indicated 220 0C measured by a

mounted thermocouple. The results represented show the effect of temperature

on the deposit thickness and % FAME.

13

Figure 2-7: Effect of temperature on the deposit thickness with different

types of fuel [24]

Figure 2-8: Effect of % FAME on deposit thickness [24]

The study above did not try to replicate specific deposits but only identify the

factors facilitating the formation of deposits. A group at Toyota constructed a test

rig to replicate injector deposits and to study the influence of different factors such

as temperature on their formation. The main point of consideration is the role of

total acid number in the formation of injector deposits. The injector mounting was

done in an aluminium bracket with 500 W power supply. An ECU was used in the

control of rail pressure and injector actuation [16]. It is noted that the deposits

caused the sticking of injectors in small amounts. The deposits caused a drop in

the flow rate [16]. Another great effect of the sticky deposit is the increase in the

friction on sliding surfaces. The sticking force measured was done using an

14

extraction device with strain gauges. This procedure can be very accurate in

determining the gravity of effect of deposits formed.

2.6 Autoclaves

An autoclave is a pressure vessel designed to hold fluids at raised temperature

for analysis. It has pressure relief valves for safety purposes. The device is heated

in an oven space in order to elevate the temperature of the content. One of the

close similarities between the autoclave set up and the injector is the minimal

levels of fuel oxidation. Autoclave is completely sealed off from the atmosphere

where only the oxygen present during filling up is present during the experiment.

There is minimal oxygen in the injector as well while the engine is in operation.

Absence of the other essential factors such as high pressure which play a role in

deposit formation is assumed to be compensated by the longer time of test fuel in

the oven at raised temperature.

15

3 Methodology

Internal injector deposits have been linked to the presence of soft particles in the

fuel. Deposits have been linked to the calcium soaps from analysis of plugged

filters. An injector test rig for the present work had been built by a previous

master thesis student and represented in Figure 3-1 [25]. The robustness of the

construction was qualified using the engineering design considerations. The

actual rig as it stands in Scania is shown by Figure 3-2.

3.1 Test rig description

Figure 3-1: Test rig layout

The components are as described as follows:

1 – Tank for unused test fuel

2 – Low pressure pump for suction of fuel and transferring it to high pressure

pump

3 – Flow meter to monitor the flow of fuel

4 – High pressure pump to generate up to 2000 bars of fuel pressure

5 – Heat exchanger for cooling the return fuel from the cylinder head

6 – Circulation pump to ensure homogeneity of test fuel during operation

7 – Common rail

8 – Fuel injector

9 – Ceramic heater

10 – Temperature controller

11 – Fuel collection cup

12 – Heat exchanger

13 – Tank to hold used fuel

16

Figure 3-2: Injector test rig at Scania

The test rig is designed and constructed in a way that it enables generation of

near real conditions. There are a series of critical components that should work

together to realize the required output. Evaluation of the rig comprises checking

the presence and functionality of the necessary instrumentation for the input and

output parameters. As the rig is to be a stand-alone unit, it is necessary to

consider all the auxiliaries to the rig. Additional instruments to improve the

monitoring of the parameters in the rig enhances the ability to realize any

changes that occurs during the experiment. By means of a physical inspection,

the completeness of all the connections in the rig has been carried out. The

sequence of the components up to injector should be as similar to the truck

injection system as possible. A provision for separate high-pressure pump with its

motor is required to give 2000 bars of fuel after the low-pressure pump. The

cylinder head with a single injector forms the critical part of the rig. A resistance

ceramic heater with a thermocouple attached should be able to create required

temperature at the injector region.

3.1.1 Temperature control

Heater circuit comprises power supply regulator, controller and the ceramic

heater which uses a feedback loop to regulate the temperature. The functionality

of the temperature control loop is tested by adjusting the voltage input to the

heater and noting the digital output in temperature. For a greater accuracy of

temperature measurements, the wiring of the controller should be compatible

with the thermocouple used on the heater. Verification of the wiring in both

thermocouple and the wire were performed. Upon powering the circuit, variation

of voltage input causes the desired output (a change in temperature).

17

The type of thermocouple used is very important for the rig because of

requirement of precise temperature changes. Temperature controller already

existing had a wiring of type K while the heating element has type J wiring. A

synchrony was done by making all wiring be type J to avoid any errors.

3.1.2 Pressure testing

The fuel line in the rig is subjected to a high pressure. It is required that there is

barely any leakage in the line to avoid loss of fuel and pressure drops. A pressure

test was done on the rig using low pressure fuel to check for non-critical leakages

as a safety measure. It was not advisable to use working pressure of 2000 bars in

the first test since it is quite high, and any leakages would be harmful. Leakage

checks was performed visually.

3.1.3 Leakage prevention

Pressure test exposed leakage at the fuel collection cup and hence a requirement

to find solution of good sealing. Integration of a heater around the injector nozzle

and within the cylinder block presented a need to drill the cup for passage of

electric and thermocouple cables as shown in Figure 3-3.

Figure 3-3: Fuel collection cup after injector (left) and heater (right)

Injection of fuel at the cup occurs outwards as the holes spray at angles from the

centre. Therefore, an idea to constrict the flow by concentrating the spraying using

a bush was chosen clearly shown by Figure 3-4. Due to the high pressure and

safety concerns, there was need for perfect contact between the bush and cup. This

was not possible because of machining tolerances and hence there was a gap.

18

Figure 3-4: Incorporation of a bush to concentrate the nozzle hole sprays

into the cup.

Two options were considered for sealing mechanism. The factors influencing the

selection were:

i. Stability at high (test) temperatures.

ii. Compatibility with fuel.

iii. Ease of removal when need to take it away arises.

Fuel compatibility tests were performed for silicon and fireplace sealant given in

Figure 3-5. The flexibility and ease of application contributed to the selection.

Figure 3-5: Silicon and fireplace sealants .

A test to evaluate the compatibility with fuel and bonding to steel surfaces. This

was important as the sealant would be constantly be bathed by hot fuel exposing

it to erosion during spraying. Two metal pieces were joined using silicon and

fireplace sealant. After curing, they were then placed in hot fuel at 1500C for 4-5

hours. Both showed good results, but the fireplace sealant created a permanent

joint and it was easy to tear off silicon. Silicon was chosen and used to seal the

gap between the bushing and cup. Curing of the joint with silicon was done in an

oven at 320C for four days. Compressed air was used in checking for perfect

sealing. Figure 3-6 illustrates the preparation of the test specimen using sealants

and testing them for compatibility with fuel.

19

Figure 3-6: Experimental set-up for compatibility test

3.1.4 Calibration

This is a vital aspect of the aim which determines the accuracy and precision of

the results. Repeatability is also affected by the measurements from the

instruments. Components which should be calibrated are:

i. Resistance heater

ii. Power supply unit

3.1.5 Selection of parameters to be controlled during replication of

injector deposits

The ultimate target in selection of parameters to control in the rig is informed by

the practical conditions that prevail in the truck engine. Figure 3-7 demonstrates

a simplified narrowing down of the potential parameters for the replicating IDIDs

in near field scenario. The rate of internal injector deposits formation is generally

influenced by the driving characteristics and the fuel properties. Moreover, the

nature of the roads plays a significant role in the engine operations. Refinement

of the parameters to control in the rig is derived from the markets where the

Scania trucks commonly showing IDIDs operate. Some of the markets are

characterized by mountainous terrains, high level of humidity and different

grades of fuel. To constructively replicate IDIDs in the test rig and yield results

in sync with that established from the field thus involves incorporation of these

perspectives the best level possible. Three categories of factors are used in the

selection of the parameters to be controlled through the test fuel, heater and ECU.

These are environmental, engine operation and fuel properties. Environmental

conditions such as humidity and topographical characteristics, engine operations

mainly arising from driver activities such as acceleration, idling which in

principle influence the fuel injection properties while the fuel properties

component essentially encompasses the percentage of biodiesel in fuel. Analysis

of the deposits found on the injectors narrows down the scope of parameters by

establishing the main contaminant. By artificially adding soft particles into the

fuel, other factors can be studied effectively.

20

Figure 3-7: Parameter identification chart

The limitation of implementing all the possible variables on the test rig is the

length of time taken in acquisition of the results and it will lead to a huge matrix

of inputs. A simplification of the matrix of inputs has been based on the literature

and the need to make the tests take a shorter time. The following are the selected

parameters for performance of tests in the injector rig:

a) Concentration of soft particles from metal ions

b) Fuel injection quantity

c) Temperature around the injector nozzle

d) Fuel injection time

e) Amount of biodiesel in fuel

3.2 Experiments for evaluation of test fuel

Replication of IDIDs is performed in the injector test rig as shown in Figure 3-2.

Test fuel for use in the rig is prepared in Scania lab. Pre-tests of the fuel are

necessary to evaluate its ability to cause deposits at near injector conditions.

Thus, screening tests enable checking the viability of test fuel before using it in

the rig. This is made possible establishing the effect of temperature on the fuel

especially the value to be used in the rig set up.

The tests are designed to evaluate the viability of test fuel to lead to formation of

deposits in the presence of controlled heat. Apparatus for the tests are frying pan,

infra-red thermometer, and heating plate stand. Three main screen tests set ups

were performed. First involved continuous and gradual increase in temperature

from 90 0C to 230 0C, second entailed constant temperature of 180 0C and third

involved combination of constant temperature (180 0C) then ramping to 230 0C.

Observations were then made based on the appearances visually at designated

times.

Parameter

identification

Environmental

Market topography

Humidity

ContaminantsEngine

operation

Fuel injection

properties

Fuel properties

Percentage of biodiesel

21

3.2.1 Autoclave test

The autoclave test is used for tests involving fuel with flash point less than 100 0C, at elevated temperatures. It is confidently estimated that deposits form in the

injector at temperatures between 90 0C and 180 0C. This experiment is designed

to verify the possibility of formation of deposits at high temperature under

controlled environment with limited oxygen. From screen test 1, the products

included components from degradation due to excess oxygen. Two kinds of

experiments were developed. Components of the autoclave are shown in Figure

3-8.

Figure 3-8: Autoclave components

First is the use of only fuel with soft particles where the inner surface of the

autoclave is to be evaluated for deposits. Second involves the incorporation of an

injector needle positioned appropriately inside the fuel. Maximum temperature

possible for the experiment is 150 0C solely determined by the safety limit of the

oven for solvents. A total of four tests were carried out (two in each of the above-

mentioned kinds) while varying the time. Temperatures rose from room

temperature to 150 0C and dropped in the same manner. Two types of soft

particles were used in the experiment: calcium (Ca) and combination of Ca +

Sodium (Na). The following table shows the tests and time taken. Table 1 gives a

summary of the variables employed in running the experiments.

22

Table 1: Experimental time span for autoclave tests

Experiment Test fuel with soft

particles present

Time

1 Ca+Na 1 day

2 Ca+Na 4 days

3 Ca 1 day

4 Ca 2 days

The test fuel was prepared according to the developed protocol. It starts by

diluting the concentrate in a smaller volume (800 ml beaker) which makes it easy

to mix using dispersive tool. This dilution is then added in intervals of 200 ml into

a bigger volume container. For each type of soft particles, 5 litre-volumes were

prepared for autoclave tests. The autoclave has a volume capacity of 900 ml.

Steps followed during the test are:

1. Two sets of autoclaves were cleaned as possible to enable easy observation

of inner wall changes and prevent contamination from other sources.

2. Test fuel is measured into the pressure vessels and closing procedure

followed according to Scania manual ending with the set up in Figure 3-9.

For subsequent tests, an injector needle is effective positioned in the

pressure vessel to be upright using improvised wires as shown in Figure

3-10.

3. The whole assembly of parts shown in Figure 3-8 and the test fuel inside is

put in the oven.

4. Safety connections are then made, and the oven temperature settings done.

5. The tests were let to run for specific length of time as shown in Table 1.

23

Figure 3-9: Pressure vessel filled with fuel (left) and fully assembled

(right) autoclave in the oven.

Figure 3-10: Set up for test using injector needle

At the end of the tests, the autoclave assembly is gently removed from the oven

and safely transferred to a fume chamber for disassembly. Test fuel is emptied

into a separate container to allow for the evaluation of the inner surfaces. Results

are mainly by observation and physical inspection of the inner surfaces of the

pressure vessel. A straight-forward technique is use of a white piece of pape which

acts a good background to know the colour of the deposits. The texture of the

deposits is ascertained by feeling the walls using a hand. A little volume of fuel

from the bottom and top is shaken and filtered for further analysis as in Figure

3-11.

Figure 3-11: Filtration of the test fuel from the autoclave (left) and

obtained deposits on filter material (right).

24

3.2.2 Frying pan screen test

1. Add 75 ml of test fuel concentrate into the frying pan

2. Set the temperature of the heating plate to required value

3. Place the saucepan with the test fuel on the hot plate as in Figure 3-12

4. Using the infra-red thermometer, monitor the temperature of the heating

plate, outer surface of saucepan and the centre of inner surface with the test

fuel.

5. Record the changes as they occur at respective durations

6. Extract a sample of remaining content on the saucepan for analysis

7. Repeat procedure for the diluted test fuel.

Figure 3-12: Experimental set up using a pan

3.3 Used fuel

Fuel to be used in the investigation of IDIDs is prepared locally in Scania labs. A

specific protocol has been developed detailing the recipe of the test fuel. As earlier

indicated in this report, the test fuel is one of the parameters that plays important

role in the achievement of deposits in the injector. Therefore, local preparation of

test fuel gives the ability to shorten the ageing time and introduction of suspected

contaminants (deduced from studies of field injectors) which aims at simulating

real life conditions as close as possible.

Test fuel preparation starts by making a concentration of soft particles consisting

of calcium and sodium metal ions. The source of calcium is calcium oxide and

sodium hydroxide for sodium. the salts are separately mixed with water at ratios

according to Scania recipe. Ageing of fuel occurs by heating it at 1100C for 60

hours. Solution of salts is then added to aged fuel to form test fuel concentrate.

For use in the deposit formation experiments, 5 litre-volume of test fuel was

prepared both with only calcium and both calcium and sodium. The following

procedure was used in the scaling up of the concentrate.

25

1. Add 5 ml of the concentrate into 800 ml volume beaker

2. Add B10 into the beaker and mix using high speed mixer

3. Put 200 ml of the solution above into a 10-litre container

4. Add 2.4 l of B10 into the container and shake rigorously

5. Add 200 ml of remaining volume in 2 above into the container.

6. Repeat the additions of B10 at 800 ml volume for remaining portion while

maintaining 200 ml volume for diluted solution of soft particles in 2.

7. Shake the solution after completion of additions for proper homogeneity.

3.4 Instruments used for deposit study and analysis

For studying and carrying out deposit analysis, different instruments were used.

The selection was based on suitability and the availability. Visual technique of

analysis is also handy in this analysis. The following are the instruments utilized

in the deposit analysis.

3.4.1 Scanning Electron microscopy - Energy dispersive X-ray

SEM is used to provide insights on the nature of deposits, size and has been

previously used to analyse the injectors from the field. EDX analysis gives

information on the different elements found in the deposits alongside their

elemental composition and makes it easy for comparison. A general elemental

composition as well as point spectrum can be done [26]. The morphology of the

deposits gives an impression of the structure and relationship with injector

clearances. The procedure for this analysis is as follows:

1. Filter the deposits from the fuel as in Figure 3-11.

2. Wash the filter paper gently with cyclohexane

3. Let the filter dry under room conditions (3 days in this case)

4. Prepare the sample by cutting small square pieces fitting on sample holder.

5. Coat the sample with platinum (10 nm) in a sputturing machine

6. Place the sample in the SEM - EDX for analysis

3.4.2 Fourier Transform Infrared Spectroscopy (FTIR)

Fourier Transform Infrared Spectroscopy (FTIR)-ATR (attenuated total

reflection) provided the confirmation of soft particles (soaps) in the injector

deposits from the field. FTIR operates on the principle that molecules can absorb

light within infrared zone of the wavelength spectrum. Absorption of radiation

occurs when infrared radiation and molecule vibration frequencies are equal. This

leads to vibration hence excitement of the bonds between the molecules. The

spectrometer detects the absorbed radiation and depicts infrared spectrum

sensitive to various chemical bond types. The values of wavenumbers range from

650 – 4000 cm-1. With respect to soft particles, the peak corresponding to

carboxylic group is of interest. This is an indication of the presence of soft particles

(calcium soap) between 1550 – 1600 cm-1 [22].

Deposits from the pan test were in solid form and directly placed on the FTIR

sample stage. Using a spatula, deposits are gently scraped off at time intervals.

For the autoclave, the deposit is mixed with fuel hence an extraction is required.

26

A filter is used to obtain the deposits for analysis. The following is the procedure

followed to analyse deposits from the autoclave.

1. Filter 15ml of sample fuel with deposits.

2. Allow the filter to dry for a few minutes.

3. Cut the filter paper into portions focusing on the sections containing the

deposits.

4. Perform FTIR analysis of a clean filter as the reference.

5. Carry out FTIR analysis of the sample on at least three points to obtain a

good average.

3.4.3 Gas Chromatography- Mass Spectroscopy (GC/MS)

This technique makes it possible to identify the compounds present in the sample.

The compounds are obtained from the sample by a suitable solvent based on the

solubilities [27]. Washing of the sample is a critical step to take away the fuel

leaving behind the deposits before injecting into GC-MS.

The procedure for this analysis was done according to Botond et al. [11] which

was developed within Scania. Samples were obtained from initial test fuel and

that from autoclave. A vacuum pump was used to perform filtration and

subsequently followed by washing of the filter using cyclohexane. Washing

protocol involves vigorous shaking of the mixture for 45 seconds and gentle

shaking for 30 minutes to allow the reaction to take place. A centrifuge is then

used to separate the phases formed. The steps of adding methanol and copper

soap follows before letting the reaction process take place for 24 hours. The

samples are then injected to the GC-MS for analysis.

3.4.4 Visual inspection

Deposits that have been reported to cause problems in the market have distinct

characteristics in terms of colour and stickiness. The description from the analysis

of the injectors is through visual observation and qualified by other superior

analysis techniques. The deposits have a brown coloration and sticky properties.

This explains why the injector needle is impeded in up and down motions. In both

screen tests, visual evaluation, and physical feeling (or indirect evidence) of

stickiness were carried out on the deposits.

27

4 Results & Discussion

Deposit formation in the injector is brought about by various factors in the engine

truck. A challenge in replicating the deposits on injector needle is the time it takes

and considering the influence of all the possible parameters.

Injector temperature is one of the major factors associated with the formation of

deposits. Understanding of the temperature gradient from the combustion

chamber to the parts of the injector in contact with the cooled parts of the engine

is vital. Engine block temperature is maintained at a maximum of 130 0C by use

of oil and water cooling. The oil temperature ranging between 90-100 0C while

that of water is 90-110 0C. Temperatures in the combustion chamber can go up to

385 0C. Deposits have been known to form at temperatures between 150-180 0C.

This is a possible value considering the temperatures of the part of the nozzle

exposed to the combustion chamber. Since the deposits form on the needle, which

is inside the injector, exact temperature at the actual point where deposits give a

problem can only be ascertained through a simulation or use of special injectors

with thermocouple sensors close to this region.

On top of considerations to make the test rig stand-alone system with its own

utilities, the logistics of acquisition of other components took a longer time

resulted in not being able to run the rig. The scope of the project was narrowed

down to exclude the running of the injector test rig. The major work has been

solidifying the understanding of the key parameters influencing deposit formation

and refurbishment of the rig.

Two tests were designed to evaluate parameters leading to formation of deposits

excluding the effect of pressure and continuous flow which is the case in the

injector. The test fuel plays a key role in the replication of deposits in the injector

test rig. Soft particles associated with Ca and Na were developed in the lab at

concentrations of 1.4 ppm.

4.1 Test protocol

Injector rig at Scania has been designed and the aim was to run during the project.

As a result of the leakage challenge, need for reconfiguration of the test rig

assembly and acquisition of the critical components (ECU computer, high pressure

pump, stand), it has not been possible to run the rig. The following is the

methodology from a study and interaction with the test rig. It is applicable upon

completion of installation of all components of the rig.

1. System cleaning- This is done by running clean fuel through the entire

system to clean out any unwanted materials which may cause

contamination or blockage in the injector. The tanks should be flushed with

fuel and drained before holding any fuel.

2. Installation of instrumented injectors – the injectors have sensors located

at the nozzle and enable to measurement of temperature close to the nozzle

tip. The injector should only be used for the calibration purposes but not

performing the deposit replication tests.

3. Calibration of the heating unit – the heating unit consists of a power supply,

controller and ceramic heater having a thermocouple. The electrical unit is

separate from the fuel injection unit. The controllable variable is voltage

from the power supply and the controller display a digital output of the

temperature of the heater itself and not the injector. The effect of gradual

increase in voltage is noted on the temperature output at the controller.

This is done without any fuel.

4. Validation of temperature measurement – once a relationship has been

established between the voltage and temperature at the injector nozzle, the

same is repeated at a chosen constant injection parameter of clean fuel. This

data will act as a reference upon the introduction of test fuel with soft

particles. The test runs with clean fuel should be done such that consistent

and repeatable values are obtained.

5. Installation of test injectors – injectors reported to have suffered the

negative effect of deposits are to be installed for test with test fuel dosed

with soft particles.

6. Recirculation of test in the tank – this is done prior to injection of the fuel.

It is important to ensure a homogeneous test fuel to avoid high

concentration of the soft particles at short instance. This will possibly cause

the deposits but may not portray real scenario.

7. Saturation of the line – this involves running the rig with test fuel for some

time without heating.

8. Parameter selection – rail pressure from the high-pressure pump,

temperature of the heater and the injection quantity set in the ECU are

selected for operation of the rig.

9. Rig operation – the rig is to be efficiently controlled through computer

software for monitoring.

10. Data is recorded for analysis and observations of any changes. In the initial

phases of running the rig, physical analysis of the injector needle is used to

confirm the presence of injector deposits.

4.2 Test rig start up

Replication of injector deposits was to be performed on the test rig shown in

Figure 3-2 where a ceramic heater (Figure 3-3) is to provide the temperature of

at least 150 0C. The system operates under high pressure of 2000 bars and has a

cylinder head for fuel injection. A leakage test was first carried out under low

pressure and done manually. Fuel collection cup leaked a lot of fuel through the

holes drilled for the heater cables. Two remedies were tested out to seal whereby

silicone sealant was a good choice. It can withstand expected temperature in the

rig with high possibility of removal during disassembly of parts. Additionally,

silicon sealant is flexible with the ability to take compressive force upon expansion

of components. Fireplace sealant gives a permanent joint that is hard (ceramic)

and showed aspects of cracking. Hence leakage could potentially occur during

continuous operation when the parts expand, and clearances become tighter.

A perfect sealing occurred with no leakage of compressed air. Figure 4-1 shows

the sealing done using silicon and the new cup design.

28

29

Figure 4-1: Fuel collection cup and new design

The use of silicon to seal the gap between the bush and cup piece provided a good

solution. Compressed air was used to check for perfect sealing. An option for

redesigning the fuel cup collection was however adopted as a long-term remedy.

New cup design has a larger volume of fuel collection. This helps in pressure

dissipation at injection and minimizes the chances of pressure build up and

contact of fuel with the heater.

4.3 Frying pan test tests

Temperature was increased from 90 0C – 230 0C while taking samples at intervals

for FTIR analysis. Distinct peaks at 1566 cm-1 are observed at longer time and

higher temperature in Figure 4-3. This is deduced as presence of carboxylic salt

(metal soap). Deposits become more concentrated as the fuel degraded in the pan

with an increase in viscosity. Oxidation products also increase at higher

temperature which is evident at shoulder peaks, 1720 cm-1 (C=O). It is also

observed that peaks increase at 3500 associated with O-H due to oxidation

products.

Final residue from the pan was scrapped off gently and observed. The deposit is

stretchy and very sticky as shown in Figure 4-2. More exposure to higher

temperature for longer time caused the darker colour due to more oxidation

products.

Figure 4-2: Brown-sticky deposit

30

Figure 4-3: FTIR of deposits from improvised pan test

Figure 4-4: FTIR of aged fuel and deposits from diluted fuel (in frying

pan test)

The peak at 1540 cm-1 is the evidence of presence of metal soap which can be seen

in reference to only aged fuel sample from Figure 4-4. The upper curve is from

aged fuel without the soft particles while the lower peak has been doped with soft

particles. The dilution of the concentrate is done to 100 times in the process of

preparing a test fuel close to concentration from the field. The two curves enable

a comparison to be deduced where soft particles cause a peak at 1540 cm-1 and

absence in the upper curve.

31

4.4 Autoclave test results

The results from the autoclave test was obtained through visual observation of

the pressure vessel.

Figure 4-5: Brown deposits

Figure 4-5 shows the bottom of the vessel was coated with brown deposits. Walls

of the autoclave showed small amount of deposit formation that was visually

observable on wiping with white paper. This is in comparison to large amount of

deposits formed at the bottom of the vessel. Little amount of fuel left at the lower

section had traceable solid particles.

Figure 4-6: Injector needle observed for deposits after autoclave tests

Autoclave experiment is believed to be a ‘’Calm’’ process whereby the fuel is ‘still’

presumably with less convection currents during the progress of the experiment.

Most of the deposits settled at the bottom with no significant amount on the walls.

This is not the case in the injector where the pressure causes stressing of fuel

hence facilitating other mechanisms of deposit formation. It is however clear that

temperature promotes the deposit formation.

On the other hand, the injector needle shown in Figure 4-6 remained clean at

least from physical observation with naked eye. This could be that the needle

surface and fuel was at equilibrium hence the effect of temperature was minimal.

Additionally, most of the soft particles would have gradually dropped to the

bottom of the autoclave. Nonetheless, the experiment shows positive results on

32

the role of temperature on the evolution and development of deposits in the

presence of contaminants from various sources. Significant amount of deposits

was observed in the test fuel containing both calcium and sodium in comparison

to one with only calcium. Since the mixing was manually done by hand,

homogeneity may not have been as required.

Figure 4-7: FTIR of specimen from autoclave test

Deposits found in the autoclave showed the presence of the metal soaps at

1593cm-1 in Figure 4-7. The peak at 3400 cm-1 associated with O-H group become

prominent at exposure of test fuel to higher temperature. On the contrary, the

carbonyl group (C=O) peak diminishes significantly after the autoclave

experiment (1720 cm-1). This can be in connection to the reason that the

carboxylic group combines with the metal ions present in soft particles to form

carboxylate group (COO) at 1593 cm-1. Thus, temperature favours or facilitates

the reaction between metal ions and carboxylic acid to form soaps. Comparing

Figure 4-3 and Figure 4-7 there is start difference at peaks around 1750 cm-1

(esters) and 3400 cm-1 (hydroxyl) [11]. As the soaps fall out of the fuel, the ester

group diminishes as it is used up in the formation soap compounds. The result

obtained is used in conjunction with the subsequent analysis of the deposits in

the SEM which aids in understanding and give insight on their morphological

transformation.

GC-MS machine was used in analysing the soft concentrations of the test fuel

before and after the autoclave tests. This is to enable a comparison of the

concentrations of soft particles which is important in establishing their role in

deposit formation during the experiment. Based on sample preparation as

elaborated in 3.4.3, the peak determines at 10.612 shows the presence of

nonanedioic acid , a product of acid esterification.

33

Figure 4-8 shows the presence of nonanedioic acid at retention time 10.612 which

is associated with the presence of dimethyl esters which is produced during

esterification of acids (aged fuel) in sample preparation process.

Comparing the soft particle concentration in fuel tested in the autoclave and the

initial fresh fuel shows huge reduction illustrated in Figure 4-9. A possible reason

for this is that soft particles were used up in the deposit formation in the autoclave

and fell out of the solution as deposits which were observed as sediments. The

huge difference is a demonstration that the temperature has a great role in the

reaction of carboxylic acids with the metal ions, hence forming soaps. Fresh fuel

has a high concentration of soft particles as there was no exposure to condition

favouring any utilization of soft particles. It is also worth noting that the soft

particles may have sedimented in the autoclave thus, immense reduction in

concentration.

Figure 4-8: GC/MS results for deposits obtained from the autoclave.

Figure 4-9: Concentration values (ppm) of tested fuel and fresh test fuel.

0

0.5

1

1.5

2

2.5

3

Fresh test fuel Screened test fuel

Co

nce

ntr

atio

n

GC/MS Results

34

Imaging was done under variable pressure secondary electrons (VPSE) at 20kV.

Images shown by Figure 4-10 and Figure 4-11 portray the behaviour of the deposit

particles. The particles are stick together and form bigger particles. Large

deposits forming in the injector would have an impact due to narrow clearances

of 20 µm. Their effect may vary depending on the orientation in the injector.

Figure 4-12 and Figure 4-13 show the nature of soft particles in the fuel before

being exposed to conditions facilitating the formation of deposits. The particles

have a more regular shape with smeary surface. Upon heating the fuel in the

autoclave at 150 degrees, deposit particles were formed which are agglomerated

and bigger in size with irregular shape. Smeary soft particles are transformed

into sticky deposit particles that are agglomerated under the effect of heat. This

gives a hint of what could be happening in the injector as the sticky deposits form.

The size of soft particles is in the range of 10 microns while the deposits formed

have a size in the range of 20 microns.

Figure 4-10: SEM image of the deposits filtered out after 4 days in the

autoclave

Figure 4-11: SEM image showing a particle of the deposits (seems

agglomerated)

35

Figure 4-12: SEM image of soft particles from ‘fresh’ unused test fuel

Figure 4-13: SEM image showing particles are smaller with more

regular shapes.

Figure 4-14: EDX image and composition of a deposit particle

The soft particles introduced in the test fuel was done in a ratio of 4:1, Na+: Ca2+

respective. EDX results in Figure 4-14 on the analysis of one of the particles

exhibit similar sodium to calcium ratio as in the soft particles originally

introduced in the test fuel. This probably can be associated with the Ca and Na

from test fuel and not from external contaminants or cross contamination. The

rest of the elements are mainly from the filter material.

36

5 Conclusion

In conclusion, the injector test rig built in Scania was evaluated where the heater

was checked for functionality and worked as expected. The wiring of the

thermocouple in the digital controller was corrected to be same as heater

thermocouple wiring (type J). Leakage problem which occurred was solved by a

redesign of the fuel collection cup. The challenge of the exit of heater cables from

the cylinder head will be avoided by drilling holes in the cylinder head block. It is

also worth mentioning that silicon is a good sealing option in conditions involving

fuels and heat up to 150 degrees. This is more so if expansion and contraction is

expected to occur around the joint. Test fuel prepared at Scania for use in

replication of injector deposits yielded positive results and thus can be scaled up

for use in the rig. The concentration of 1 ppm which is around the value of

concentration of soft particles investigated in the field and seems to be sufficient

to cause injector deposits at conditions of 150 degrees. By performing FTIR and

GC-MS, the deposits formed were confirmed to be soap type as expected. These

instruments proved to be very reliable and provided useful results on soap

formation and deposit analysis. The autoclave experiment gave positive results

which helped in understanding how the soft particles are transformed into

deposits. The SEM images showed the morphological changes between the soft

particles and deposits. The soft particles are more regular, smeary and small in

size while the deposits are agglomerated, bigger in size and rough texture.

Comparing the test fuel with Ca soft particles with that containing Ca and Na

soft particles, the latter shows a higher probability to form deposits.

37

6 Future work

Due to the reconstruction and adjustment of the test rig, it was not possible to

run any tests. The main task for the future is to implement the protocol by

running and operating the rig under the conditions suggested herein. From the

analysis involving temperature and test fuel, the starting conditions have been

roughly determined. Volume flow rate drop after the injector is an important

aspect of online indication of deposits already forming. A simulation of the

temperature gradients in the injector during operation is necessary to ensure

precise temperature during running the rig. Percentage of biodiesel in test fuel

can be incorporated as a variable to determine the deposit formation in the

injector (for instance B10 and B7). It is recommendable to scale-up test fuel

containing Ca and Na soft particles as this showed higher probability to form

deposits during the screening tests. This can be true for the injector test rig too.

The SEM results on the comparisons between soft particles and deposits creates

an interest in understanding the mechanisms behind deposit formation. This

investigation can be carried further to gain more knowledge.

38

7 Acknowledgement

I would like to express sincere gratitude to my supervisors at Scania-Södertälje,

Henrik Hittig and Mayte Pach for being extremely resourceful in the

accomplishment of this project. Their support has been immense through

constant consultation and refining the approach of the thesis objectives. Many

thanks to my supervisor at KTH, Weihong Yang from the department of Materials

Science and Engineering for guidance, great insights and helping me in report

development. I also appreciate the entire team at Scania and Cummins for

discussions that played a significant role in shaping the outcome of this thesis

work.

The time spent at Scania was worthwhile alongside my fellow master thesis

colleagues. Much recognition to Romain Couval, Saurabh Shinkhede and Linus

Nordin for great moments and support during this thesis.

A special acknowledgement to my dear mother, who encouraged me all through

and had absolute faith in me. Her words have been a necessary fuel towards this

accomplishment.

And to all my friends who cheered me on, I appreciate your support. I cannot

mention you individually.

Finally, this project was made possible during my studies at KTH Royal Institute

of Technology funded by the Swedish Institute Scholarship.

Patrick Kiprotich Korir,

Stockholm, 2021

39

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