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8/12/2019 Rcci Final

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Reactivity Controlled Compression Ignition

(RCCI)

by

Abhijeet Chausalkar


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Table of Contents

List of Figures ________________________________________________ 3

Nomenclature ________________________________________________ 4

Section 1 Introduction to RCCI___________________________________ 5

Section 2 Working Principle of RCCI______________________________ 7

Section 3 Chemical Kinetics of RCCI______________________________ 8

Section 4 Flame Propagation in RCCI_____________________________ 10

Section 5 Comparison of RCCI with conventional diesel regime_________ 10

Section 6 Advantage of RCCI____________________________________ 12

Section 7 Challenges in RCCI___________________________________ 12

Section 8 Conclusion__________________________________________ 12

References__________________________________________________ 13


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List of Figures

Figure 1: LTC, PCCI, HCCI, and conventional combustion regime [1]..5

Figure 2: High efficiency clean operating regimes shown in -T space [2].5

Figure 3: Ignition delay Characteristics of different fuels calculated using SENKIN

code (47) and reduced PRF mechanism (46). Initial pressure = 70 bar, Equivalence

Ratio = 0.5.[3]6

Figure 4: Schematic of cylinder injection and fuel distribution (Curran et al. SAE

2014-01-1324) [4]7

Figure 5: Cylinder Pressure Rise & Heat release [3]..8

Figure 6: In-cylinder RCCI Combustion [3] . 8

Figure 7: First Stage of RCCI [3] 9

Figure 8: Second Stage of RCCI [3] 9

Figure 9: Variation of Species [3] 9

Figure 10: Flame Growth versus the Distance [3] .10

Figure 11: Propagation of Laminar Flame from Injector in RCCI [3] .10

Figure 12: Comparison of Cylinder Pressure [3] 10

Figure 13: Comparison of Heat release rate [6] 11

Figure 14: Comparison of experimental values of NOx, Soot & thermal efficiency [3]

.11

Figure 15: Comparison of experimental values of NOx and Soot emissions of RCCI

vs Conventional Diesel at variable load [7] 11


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Nomenclature

Abbreviations

AFR Air Fuel Ratio

ATDC After Top Dead Center

CA Crank Angle

CDC Conventional Diesel Combustion

CH2O Formaldehyde

CI Compression Ignited

CO Carbon Monoxide

DEF Diesel Exhaust Fluid

DI Direct Injection

EGR Exhaust Gas Recirculation

GRI Gas Research Institute

HCCI Homogeneous Charge Compression Ignition

HRR Heat Release Rate

IC Internal Combustion

LD Light-Duty

LTC Low Temperature Combustion

NOx Nitrogen Oxides

OH Hydroxyl Radical

PM Particulate Matter

PRR Pressure Rise Rate

RCCI Reactivity Controlled Compression Ignition

TDC Top Dead Center

UHC Unburned Hydrocarbons


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Section 1 Introduction to RCCI

In recent years, many advanced

combustion strategies have been

proposed in order to meet current and

future emissions mandates. Most ofthe current strategies can be grouped

in the category of premixed, low

temperature combustion (LTC). By

operating at low in-cylinder

temperatures and maintaining longer

ignition delay period, it is possible to

reduce NOx, soot emissions with the

added advantage of achieving high

thermal efficiency. Thermal NOx is the

dominant form of NOX emission whenengine operate at high temperatures

since activation energy required for

NO formation is high. Therefore, lower

in-cylinder temperature in LTC does

not allow to reach the threshold

activation energy resulting in low NOX

emissions. Further, longer ignition

delay allows more time for mixing

hence rich regions inside the cylinder

are reduced, inhibiting soot formationin those regions. High thermal

efficiency is achieved in LTC due to

reduced heat transfer losses. Fig. 1

shows different operating windows in

terms of equivalence ratio and local

temperature of various combustion

strategies such as LTC, HCCI, PCCI.

From fig 1, it is clear that NOx and

soot are generated when engine

operates between local temperature of2200 - 3000K and local equivalence

ratio of 2-6.Diesel engine operates in

the range of 2200 to 2600 K and

equivalence ratio range of 5 to 0.5

generating high level of soot and NOx

emissions. Other combustion concepts

shown in the figure such as HCCI,

PCCI and LTC operate in the

combustion regimes that generate very

low soot and low NOX emissions.Based on LTC, researchers have

Fig.1:LTC, PCCI, HCCI, and dieselengine [1]

Fig. 2: High efficiency clean operating

regimes shown in -T space [1]

shown experimentally and using

simulation that Homogeneous Charge

Compression Ignition (HCCI) and

Premixed Charge Compression

Ignition (PCCI) concepts are promising

techniques for simultaneous NOx and

soot reduction. HCCI is achieved by

creating a perfectly premixed charge

prior to ignition. An early injection, wellbefore TDC, is used to create a


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premixed charge. PCCI combustion is

achieved by enhancing the pre

combustion mixing time and

introducing high levels of EGR .This is

done to reduce the peak equivalence

ratio from the threshold soot formation

equivalence ratio limit and to reduce

temperature for lower NOx. From the

above discussion and figure 2, it is

inferred that when engine operates

under conditions when local

equivalence ratio is in between 0.2-1

and local temperature is in between

1500-2000K, the combustion is highly

efficient (near 100% combustion

efficiency).But premixed low

temperature combustion

strategies(HCCI & PCCI) face two

major problems. They are:

1. Control of Combustion Phase

2. Control of Heat Release Rate

Further, it was inferred from various

studies on HCCI using gasoline,

diesel, gasoline/diesel blends that

optimum efficiency depends on

reactivity of the fuel in different

operating conditions [3]. For example,

in a diesel engine when in cylinder

charge temperature is around 725 K &

combustion phasing of 8 CA , neat

diesel provides optimum fuel reactivity

to achieve optimum combustion

efficiency whereas when the

temperature is increased to 780K ,

optimum combustion efficiency is

maintained using 70-30 blend of diesel

and gasoline. This is because with the

combustion duration of 8 CA at

elevated temperature of 780K, neat

diesel could not achieve optimum

combustion efficiency due to increase

heat losses and reduced thermal

efficiency [3]. Hence, reactivity of

fuel becomes critical with the

operating conditions i.e changing

operating conditions results in

different in cylinder conditions,

demanding different fuel reactivity

conditioned to maintain optimum

combustion efficiency. Further,

controlling the combustion phase &

heat release rate is important in

HCCI engine to maintain optimum

combustion efficiency.

Fig. 3: Ignition delay Characteristics of

different fuels calculated using

SENKIN code (47) and reduced PRFmechanism (46). Initial pressure = 70

bar, Equivalence Ratio = 0.5. [3]

The figure 3 implies that different fuels

due to distinct reactivitys has different

ignition delay characteristics and has a

distinct narrow operating range where

the combustion efficiency is maximum.

Hence, there is need to use different

fuel blends (fuel reactivitys) to ensure

optimum combustion efficiency so that

combustion phase can be

appropriately controlled. Partial

ultimately NOX and soot emissions.

Therefore, a new concept evolved by

using approaches for overcoming

problems in HCCI and it is called as

RCCI, abbreviated as Reactivity

controlled charge ignition. It is also

called as dual fuel PCCI. RCCI

combustion is achieved using in-


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cylinder blending of two fuels with

different auto ignition characteristics

utilizing the reactivitys of different

fuels to control the combustion phase

and heat release rate. Section 2

explains the working principle of RCCI.

Section 2 Working Principle of RCCI

Fig. 4 Schematic of cylinder injection

and fuel distribution in RCCI [4].

In a four stroke engine working on

RCCI, low reactivity fuel say gasoline

is injected in the cylinder in the early

phase of compression stroke. A pulse

is used for injection and this pulse is

timed to mix with intake air so that it is

too lean to produceappreciable soot

or nitrogen oxides upon combustion,

but not so lean that it createssignificant amounts of unburned

hydrocarbons and carbon

monoxide.Since low reactivity fuel is

injected early, more time is available

for the formation of mixture of gasoline

and air till this mixture is compressed

and ignited. Hence in RCCI, ignition

delay is longer than diesel engine.

Long ignition delay ensures

homogenous premixed mixture withinthe cylinder at the end of compression

stroke. High reactivity fuel say diesel

using a direct injector is injected in this

mixture to start the combustion. Diesel

provides local rich air fuel mixture

required for auto ignition. Diesel is

injected to maintain blending ratio of

two fuels (gasoline/diesel) for optimum

combustion phase and heat release

rate. The timing and volume of these

pulses are optimized to control the

combustion event to maximize

efficiency. Since RCCI combustion is

controlled by the reactivitys of the

fuels hence the combustion occurs in

stages. For example, RCCI using

gasoline/diesel blend has two stage of

combustion, low temperature and high

temperature combustion. First stage of

combustion is diesel like i.e. low

temperature combustion since diesel is

injected for starting the combustion.

No flame propagation is observed but

as the combustion progresses flame

starts to form at some distance from

the tip of the injector initiating second

stage of combustion i.e high

temperature combustion. A laminar

flame propagates from the cylinder

periphery to the centre during RCCI

combustion.Therefore, the RCCI

engine integrates combustion phasing

and duration control to achieve high

efficiency and low emissions by using

fuels with differing reactivitysdelivered

through multiple injections to achieve

optimum fuel reactivity stratification.

Among the fuel choice for RCCI,

gasoline is well suited for high load

conditions whereas diesel is best for

low load conditions due to their

reactivity towards combustion.

Blending ratio of gasoline and diesel

are used to optimize the combustion

and emissions. This enhanced

combustion process improves

performance, at all load conditions.


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Section 3 Chemical Kinetics of

RCCI

RCCI is a new combustion concept

and research is still ongoing to

understand the chemical mechanismgoing during RCCI process within the

cylinder. Many aspects of RCCI

chemical mechanism are still under

study. In a study done on heavy-duty,

Fig. 5: Cylinder Pressure Rise & Heat

release [3]

single cylinder, optically accessible

research engine, it is found that

radicals CH2O (formaldehyde), OH

play a dominant role in the combustion

process of RCCI [3]. This study uses

n-heptane and isooctane as primary

fuels representing gasoline for RCCI.

Fig 5 shows the cylinder pressure

distribution and heat release rate

during RCCI in study. In fig 5, lowtemperature heat release is primarily

due to n-heptane decomposition. The

high temperature heat-release begins

near -6 ATDC and peaks near 2

ATDC. The relatively symmetric shape

of the high-temperature heat release

curve suggests that no mixing

controlled combustion is present,

consistent with the long ignition dwell.

Fig. 6: In-cylinder RCCI Combustion[3]

The figure confirms that the

combustion in RCCI occurs in two

stages, low temperature and high

temperature combustion stages. The

mechanism of reaction zone in RCCI

appear as sequential auto ignition

[3]. Here, it is important to understand

what is sequential auto ignition?From fig 6, it is clear that during RCCI

combustion, initially small auto ignition

pockets appear at -5.ATDC and then

by -3. ATDC, these small pockets

merge to form reaction zone. Image at

+1 ATDC to +5 ATDC indicate that the

reaction zone move towards the

injector located at the centre. Hence

from these images, it is inferred that

auto ignition process in RCCI issequential. Further, it is interesting to

understand the role of H2CO

(formaldehyde) and OH radicals during

the RCCI combustion. Imaging studies

has been conducted to understand the

onset, distribution and location of

these radicals during first and second

stage of combustion [3]. Consider

following chemiluminescence images:


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Fig. 7: First Stage of RCCI [3]

Fig. 8 Second Stage of RCCI [3]

Fig. 9 Variation of Species [3]

Fig 9 indicates that formaldehyde

radical (red) are formed during the first

stage of combustion and OH radical

(green) are formed during second

stage of combustion. No OH radical

built up is observed in the low

temperature combustion (first stage).

Fig 7 & 8 imply that the formaldehyde

(red) is formed near the centre of the

piston bowl rim because it contains

large concentration of n-heptane and

therefore it is inferred that low

temperature combustion is due to

decomposition of low reactivity fuel.

The second stage starts when piston is

near to TDC (-11ATDC). Initial

presence of OH radical (green) is

observed with the onset of second

stage of combustion at -3 ATDC.

Presence of OH radical can be

observed by combustion luminosity.

Decomposition of high reactivity

fuel initiates the formation of OH

radical.It is important to note here that

two stages of combustions having

different radical pool is the result of

different reactivitys of the fuels (n

heptane and isooctane). The high-

temperature reaction zone grows from

the liner towards the center of the

combustion chamber. The point at

which the ignition starts and reaction

zone starts growing is mainly

controlled by difference in the fuel

reactivity. The ignition location is

controlled by the location of the

highest concentration of high reactivity

fuel (e.g., diesel fuel) and that the

combustion process proceeds in a

staged event from regions of high to

low fuel reactivity.

Other study [5] suggest that as the in-

cylinder fuel blending ratio is varied

towards lower global reactivity blends

(up to 25/75% diesel/gasoline ratio),the ignition delay gets longer and the


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fuel reactivity stratification is improved.

That implies a premixed stage of

combustion slightly lowered and a

flame propagation enhanced,

shortening combustion duration.

Section 4 Flame Propagation in

RCCI

Flame growth is not observed close to

the injector due to sequential auto

ignition points at different location but

as the distance from the injector

Fig 10: Flame Growth versus theDistance from Injector in RCCI [3]

increases, the flame starts to form and

follow laminar pattern. Fig 10 shows

the growth of flame as function of

distance from the injector.

There is no flame close to the injector

due to lean nature of charge that is

insufficient to support the growth of

flame. Fig 11 shows that the laminarflame propagates after some distance

from the injector and the flame speed

increases as it moves away from the

centre of combustion chamber. Flame

speed is typically in the range of 20

cm/s to 75 cm/s.

Fig 11: Propagation of laminar flame

[3]

Section 5 Comparison of RCCI with

Conventional Diesel combustion

regime (CDC) & alternate

approaches (EGR etc)

In this section, comparison of diesel

and RCCI combustion regime has

been done.

Fig 12: Comparison of cylinder pressure

[6]

Figure 12 & 13 compares experimental

and simulation results of RCCI and

conventional diesel engine. At 10 CA

after TDC, the peak pressure and heat

release rate is 120MPa and 600 J/CA

for RCCI combustion whereas for

conventional diesel engine it is

100MPa and 300J/CA. It implies that inRCCI, the peak pressure is 20%


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higher and heat release rate is 100%

above the conventional diesel engine

combustion parameters.

Fig.13 Comparison of Heat release

rate [6]

Fig.14 Comparison of in cylinder

temperature of RCCI and Diesel

combustion [6]

Hence, thermal efficiency of RCCI

combustion is higher than diesel

engine since it takes advantage of

the optimisation strategy used onthe basis of reactivity of the fuel to

control the combustion phase and

heat release rate. Further, it will be

interesting to study the comparison of

emission characteristics of RCCI with

conventional diesel engine. Consider

table 1 for comparing the emission

characteristics. From the table, it is

inferred that reduction of 99.89% in

NOx and 84.2% in soot is achieved

using RCCI over diesel combustion

regime, but level of incomplete

combustion in the form unburnt

hydrocarbons is higher by order of

19.Increase in UHC is due to large ring

pack crevice volume.

Table 1: Comparison of emission

characteristics of RCCI and Diesel

engine [3]

RegimesNOx(g/kWh)

Soot(g/kWh)

IncompleteCombustion(%)

RCCI 0.011 0.012 2

Diesel 10 0.076 0.1

Several experiments have been done

to evaluate the validity andeffectiveness of RCCI combustion with

respect to US tier 2 Bin 5 emission

norms and other alternate approaches

The results are shown in fig 14 & 15

Figure 14: Comparison of experimental

values of NOx, Soot & thermal

efficiency [3]


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Figure 15: Comparison of experimental

values of NOx and Soot emissions of

RCCI vs Conventional Diesel at

variable load [7].

Figure 14 implies that use of RCCI in adiesel engine increases its thermal

efficiency by 17.5% as compared to

baseline Euro 4 diesel engine. Higher

thermal efficiency of RCCI is due to

lower heat losses in the cycle. In

addition to this, it is higher than all the

other alternate approaches .Further,

NOx emission reduction is 93.33% and

soot reduction is 78.2% as compared

to baseline Euro 4 diesel engine.Further, RCCI approach indicates

significant emission benefits as

compared to other alternate

approaches using 50% EGR levels in

Euro 4 engine. Results in figure 15

imply that RCCI generates lower NOx

and Soot emissions at low and high

load conditions as compared to

conventional diesel engine hence

RCCI strategy is consistently reducingNOx and soot emissions at all load

conditions in engine.

Section 6 Advantages of RCCI

1. RCCI shows extremely low engine

out NOx and soot emissions, well

below the US Tier 2 bin 5 emission

standards without after treatment

devices.

2. RCCI combustion show higherthermal efficiency than conventional

diesel engine due to lower heat losses.

3. Fuel savings of 20 percent as

compared to conventional diesel

engines

4. Lower engine costs since expensive

high pressure fuel injector is replaced

by relatively low pressure fuel injector.

5. Use of DOC with RCCI reduces the

PM emission by 47% compared to

30% with conventional diesel engine

and 9% with PCCI

6. Many advanced engines provide

high output and efficient fuel use, but

performance declines markedly at low

loads or while idling. The RCCI engine

overcomes this obstacle through

stratified fuel reactivity and a throttle

upstream from the intake port to

maintain the optimal fuel/air mixture.

Section 7 Major Challenges in RCCI

1. Higher hydrocarbon emissions

due to large crevice volume of

the piston.

2. Higher carbon monoxide

emissions that are inline with

the emissions due to gasoline

fuel.

3. Lower exhaust temperature

presenting difficult oxidation of

HC and CO emissions.

Maximum exhaust temperature

in RCCI is around 300 K where

in a conventional diesel engine

(CDC) maximum exhaust

temperature is 550K. Lower

exhaust temperature will cause

difficulty in the oxidation of HC

emissions since hydrocarbons

oxidation also take place in the

exhaust of the engine.

Section 8 Conclusion

From the above study, it is concluded

that RCCI is an innovative approach

that can be effectively used to increase

fuel efficiency, reduce NOX and soot

emission to meet stringent emission

norms. Difference in fuel reactivitys is

used to control the combustion phase

and heat release rate in low

temperature premixed engine. Engine

is designed to inject low reactivity fuel

early in the compression stage andhigh reactivity fuel is injected near the


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TDC for initiating the combustion. Low

reactivity fuel generates low

temperature combustion stage

whereas high reactivity fuel produces

high temperature stage in RCCI

combustion. The mechanism of

reaction zone in RCCI appear as

sequential auto ignition. Formaldehyde

and OH radical pool plays critical role

in the combustion stages of RCCI. No

flame is observed near the injector but

as the distance from the injector

increases, the flame growth is

observed. Recent studies in RCCI

include experimental and simulations

studies on different fuels such as

gasoline/diesel,ethanol/diesel,gasoline

/biodiesel. RCCI generates higher CO

and HC emissions as compared to

diesel which is one important

challenge in further optimising the

RCCI. Higher HC emission is due to

large crevice volume. Another

challenge in RCCI is its lower exhaust

temperature than diesel.

References

1. Neely, G. D., Sasaki, S., Huang,

Y., Leet, J. A., and Stewart, D.

W. New Diesel Emission

Control Strategy to Meet US

Tier 2 Emissions Regulations,

SAE Technical Paper 2005-01-

1091, 2005, doi:10.4271/2005-

01-1091

2. Park, S. W. and Reitz, R. D.,Numerical Study on the LowEmission Window of Homog-eneous Charge CompressionIgnition Diesel Combustion,Combustion Science andTechnology, 179(11):2279-2307,2007,doi: 10.1080/00102200701484142

3. Sage Lucas Kokjohn, Rolf D.Reitz,REACTIVITY

CONTROLLEDCOMPRESSION IGNITIONRCCI, PhD Thesis, 2012,UMINumber: 3503916

4. Curran, S., Gao, Z., and

Wagner, R., "ReactivityControlled Compression IgnitionDrive Cycle Emissions and FuelEconomy Estimations UsingVehicle Systems Simulationswith E30 and ULSD," SAE Int.J. Engines 7(2):2014,doi:10.4271/2014-01-1324

5. Jess Benajes, Santiago

Molinaa, Antonio Garcaa, An

investigation on RCCI

combustion in a heavy duty

diesel engine using in-cylinder

blending of diesel and gasoline

fuels, Applied Thermal

Engineering Volume 63, Issue

1,5 February 2014, Pages 66

76.

6. Kokjohn, S. L., Hanson, R. M.,Splitter, D. A., and Reitz, R. D.,Fuel Reactivity Controlled

Compression Ignition (RCCI): APathway to Controlled High-Efficiency Clean Combustion,International Journal of EngineResearch, 12(3):209-226, 2011,doi:10.1177/1468087411401548

7. WARF Brochure, The RCCIEngine, Breakthrough FuelEfficiency, Low NOx & SootEmissions,March 2014.
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