linear free piston engine : study and design

7/26/2019 Linear Free Piston Engine : Study And Design

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2014/2015

FINAL YEAR PROJECT

Submitted in fulfillment of the requirements for the

ENGINEERING DEGREE FROM THE LEBANESE UNIVERSITY

FACULTY OF ENGINEERING BRANCH II

Maj!" M#$%a&'$a( E&)'#!'&)

Prepared By:

Maj* MA+HLOUF

F'!a, HADDAD

__________________________________________________________________

LINEAR FREE PISTON ENGINE "

STUDY AND DESIGN

Supervised by:

D! Ma!-a& A..I

Defended on the 23rdof July 201 in front of the !ury:

D! Fa* HANNA P!#,'*#&

D! Ma!( CORDAHI M#3#!

D! G%a' TATAH M#3#!

D! Ra& RI.+ M#3#!


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Dedi+ated to our families

for their +ontinuous support,

and to the founders of -i.ipedia,

in+ludin/ Jimmy -ales

!"#$%&'()$"DD"D ***


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A$5&-(#*)3#&0

(irst of all, e ould li.e to than. our advisor Dr !aran "33* for all

his efforts throu/hout the duration of this proe+t

*n addition, e ould li.e to than. Dr (ady $"44", Dr any *3#,

!r "tef $"6$7! and !r 8anios 97!"7% for all their support throu/hout

this proe+t

!oreover, e ould li.e to than. the head of Phoeni; !a+hinery


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A4,0!a$0

Bein/ a fully linear +ran.)less en/ine, a free piston en/ine requires in

itself a thorou/h detailed study and a +areful desi/n approa+h to ta.e into

a++ount its un+onventional aspe+ts "fter a thorou/h investi/ation of many of

the free piston en/ines that have been manufa+tured throu/hout history, it as

found that the most effi+ient and most pra+ti+al one as the Pes+ara opposed

piston en/ine, hi+h uses a pneumati+ rebound system 8he aforementioned

en/ine as the one used as a basis for the desi/n of this en/ine " detailed

numeri+al simulation has been +ondu+ted based on the Pes+ara opposed pistonen/ine, hi+h shoed the hi/her thermal effi+ien+y of these en/ines, and

provided many of the parameters that ere used later in the desi/n !oreover,

and due to the fa+t that this en/ine is a linear one, and that it operates at a

relatively hi/h frequen+y, many +hallen/es have been over+ome throu/hout the

desi/n of this en/ine, most of hi+h are related to the linear /uides required in

su+h an en/ine, and to the un+onventional re+ipro+atin/ startin/ system 8he

un+onventional aspe+ts of this en/ine also require the +on+eption of an i/nition

system that is adapted to this type of en/ines, hi+h led to the desi/n of an

ele+troni+ en/ine +ontrol unit in order to over+ome the aforementioned

requirements, althou/h su+h a feat is beyond the s+ope of me+hani+al

en/ineerin/ "ll the aforementioned details ill be thorou/hly dis+ussed

throu/hout this do+ument

+#/-!*, " free piston en/ine, linear, +ran.)less, Pes+ara, opposed piston,

rebound system, numeri+al simulation, startin/ system, ele+troni+ en/ine

+ontrol unit

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C&0#&0,

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(i/ure **> : Jun+.ers< Syn+hroni=ation !e+hanisms5

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(i/ure **15 : *44"S 6hiron $ydrauli+ 6ir+uit1A

(i/ure **1? : *44"S 6hiron 7;ternal %ayout1A

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(i/ure ***1C : 6ol/ate "lternator %ayout52

(i/ure *1 : Boun+e 6hamber olume 6ontrol System5@

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(i/ure *2 : 7arly Draft &f 8he Syn+hroni=ation !e+hanism55

(i/ure *> : (ree Piston 7n/ine (inal Desi/n5A

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I Introduction

A linear free piston engine is an unconventional type of internal combustion

engines that is relieved out of many of the common features that can be found in

other common types of engines.

In fact, conventional elements such as flywheels, crankshafts, camshafts, orany other rotating element, cannot be found in theseengines, which leads to many

advantages that include but are not limited to weight reduction, construction

simplification, efficiency improvement, emission reduction...

Unlike other conventional engines, and due to the fact that this type of engines

lacks any rotating element, its output is not a rotating motion. Therefore, other types

of outputs are extracted from that engine. These include fluid compression, gas

generation, and linear electric generation. The latter constitutes one of the most

researched topics nowadays in the automotive industry. In fact, a linear free piston

alternator can be used as the main range extender of a battery powered car, thus

preventing power shortage in case the stored electric power in the batteries runs

down.

However, the design of a free piston engine is met with a multitude of

challenges, which are mainly due to the fact that many of the conventional engine

components that are essential in order to maintain a proper engine operation are

missing, and that no full rotational motion occurs during the operation of this engine,

which reuires exceptional energy storage systems and exceptional design


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characteristics.

Therefore, the design of such an unconventional piece of machinery reuires a

lot of research and a detailed numerical study, which in itself constitutes a research

pro)ect. And in addition to the aforementioned academic work, a careful attention to

details is needed to overcome the practical challenges of such an engine, which are

many due to the unconventional features present in this pro)ect.

!oreover, much of the work necessary throughout the studyof this pro)ect

involves features that fall beyond mechanical engineering's scope,which includes

features that are usually classified as electrical and electronics engineering sub)ects. It

is however necessary for these features to be included in this work, for they are

essential since they replace the conventional featuresthat such an engine lacks.

Throughout this document will be shown in detail all the work accomplished

throughout the study and the design of a compact free piston engine. A literature

survey that details all the features that free piston engines include will be presented in

this work, followed by the detailed numerical studies that have been conducted to

verify the operational parameters of this engine, and the whole design methodology

and the resulting design will be finally presented in detail.


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II- Literature Review

II-1 Historic Overview

%ree piston engines date back to the invention of the internal combustion

engine. In fact, $tto, who is credited with the invention of the spark&ignition

combustion engine, built his first engine as a linear engine which used a rack andpinion mechanism to transfer the motion of its piston into a rotational motion. It was

known as the $ttoangen %ree +iston ngine, and it was built in (-/0(10*1.

%igure II.( 2 $tto And #angen3s %ree +iston ngine 0(1


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However, the main person that has been credited with the invention and the

development of the free piston engine was 5aul +etaras +escara 2 having invented one

of the first helicopters back in (6**, +escara noticed that his helicopter was heavily

struggling while trying to take off041. It was due to its heavy weight, which was

mostly due to the weight of its engineand its heavyflywheel, a partthat formsthe

mainenergy accumulationsystem that internal combustion engines use. Therefore, he

decided to build a lighter engine for his helicopter, and thus, the free piston engine

was conceived. He built a single piston free piston engine at first, which was found to

be unbalanced071. He later proceeded into building an opposed piston engine with

uniflow scavenging, which was found to be perfectly balanced and extremely

efficient081. 9eing the first to build that type of engines, he was able to secure a

patent for the synchroni:ation mechanism of his two piston engine01.

%igure II.* 2 +escara ;ynchroni:ation !echanism041


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In the late (6*/s, interest increased widely in free piston engines. 1. However, and due to the fact that +escara held the patent for the

synchroni:ation mechanism,


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After orld ar II, +escara resumed manufacturing his free piston engines in

partnership with a french company known as ;I=!A =;&37, a diesel engine which is

the most efficient free piston engine of all time, with a thermal efficiency of 8/B and

a mechanical efficiency of -/B061. They were gasifiers which generated hot exhaust

gases that where expanded throughturbines coupled to alternators.

%igure II.7 2 Certical ;ection $f The ;I=!A =;&4706]

Later on, many free piston engines were based on the +escara layout. In fact,

;tirling A. Dolgate patented a free piston linear alternator that used the same layout

that the +escara engine used0(/1. However, this time the power was directly extracted

from the motion of the pistons, which were euipped with permanent magnets that

induced power into a multi&turn coil that surrounded them on the outside0(/1, which

was based on the %araday Torch, a flashlight that is charged by a shaking motion that

causes a free permanent magnet to oscillate in and out of a coil.


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That engine had the advantage of providing the piston with a magnetic

coupling synchroni:ation which was due to the #aplace force that the magnets were

sub)ected to and which was eual on both pistons since this force depends on the

current flowing through the coils, and both coils were mounted in series. Thus, the

need for a mechanical synchroni:ation was eliminated, which simplified the engine

further more0(/1. However, it is unclear whether this engine has been actually built or

not.

%igure II.8 2 ;tirling Dolgate3s %ree +iston #inear Alternator0(/1

%igure II. 2 %araday Torch0((1


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!any notable other free piston engines have been built throughout history.

These include but are not limited to2

=eneral !otors =!5 7&7 3Hyprex3 2 the first and one of the only free piston

engines that have been used to power a car. It is basically a ;I=!A =;&47

replica. The motion of the exhaust turbine is however directly transmitted

through a transmission shaft to the rear wheels of the car. It wasn3t proven to be

very successful, apart from the fact that it was extemely uiet and vibration

free0-10(>1.

I??A; Dhiron 2 it is a diesel hydraulic single piston free piston pump which

uses some of the energy that it provides to the oil circuit it compresses for the

storage of the energy necessary for the bounce back operation. It is one of the

few free piston engines that are fully controlled and it can achieve a variable

stroke length operation, which is a feature that highly increases efficiency and

allows the use of different kinds of fuel. It has been used to power a forklift0-1.

;A?'IA #abs %ree +iston ngine2 it is a free piston linear alternator that is

based on the +escara layout0(41.

T$E$TA %+= 2 built and tested in */(7, this gasoline engine has the same

layout as the first +escara free piston engine, which also happens to be a two&

stroke gasoline engine with uniflow scavenging. It has been euipped with

some of the advanced features of common engines Felectronic fuel in)ection,

electronically controlled exhaust valves, ceramic cylinder sleeves...G. It is also

a linear alternator0(71. However, it suffers from a design flaw since single free

piston engines are unbalanced, which Toyota must have reali:ed by now.


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%igure II.> 2 Toyota3s %ree +iston #inear Alternator0(71

9eetron %+2 a free piston engine that is based on the +escara engine. 9eing

used as a linear alternator, it is thought to be based on the Dolgate free piston

alternator. It is a very promising engine, especially that its inventor 'aniel

Hagen did a very thorough research on every free piston engine ever made,

which he shared a great part of on his website041. Moreover, he claims that

hecombinedall the successful free piston engines and alternators ever made in

his own0(81. However, it is a privately funded pro)ect, which limits its chances

of commercial success.

%igure II.- 2 9eetron %ree +iston ngine0(81


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#ibertine %+ 2 built in */(8 by a 9ritish company named #ibertine, this

engine is the most recent free piston engine as of this date. It was revealed in

April */(8. The company has been granted nearly a million pounds to carry on

with its development of free piston engines. It is also based on the +escara

engine0(1.

In #ebanon, ;I=!A =;&47 free piston engines have been used back in (6> as

the main power sources of the @ouk +ower ;tation Falso known as the Damille

Dhamoun +ower ;tationG, and have been used in a power plant in Dhekka061. In

addition, +hoenix !achinery, one of the leading industrial companies in#ebanon,

have been experimenting with free piston linear engines for a while, which lead to the

execution of many free piston engines in the past. !ost of them are, however, of the

dual piston type.

II-2 Free Piston Engines Layouts

Throughout the history of free piston engines, three main layouts have

governed the designs of these engines 2 the single piston , dual piston, and opposed

piston configurations0-1. Although they may be different, all these layouts share some

features in common, which include the fact that all of them are linear engines, and

that all of them are two&stroke engines.

;ingle piston free piston engines are, as their name suggests, monocylindrical


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free piston engines that include only one piston. That piston is driven forwards by the

combustion process of the gases of the combustion chamber, and later bounced back

by its respective rebound system, which is essentially an energy accumulation system

that replaces the conventional flywheel that is usually found on other types of

engines, and that is the main component that stores the reuired energy that keeps the

engine running Fthe conventional rebound systems that are usually found in free

piston engines will be discussed in detail in the next sectionG. These engines have the

advantage of simplicity over other free piston engines, since the piston of such an

engine is the exclusive part that is in motion. They are also easier to control, as it can

be seen on the I??A; Dhiron 0(*1 and the Toyota %+= that were discussed early

on. However, these engines have a critical flaw, since it is impossible for them to be

balanced, which has proven to be agreat inconvenience because it would make its

supporting structure vibrate critically at very high freuencies, and thus they would

eventually be sub)ected to fatigue failure, which is what lead +escara to abandon his

initial single piston design041. This layout can be mainly found nowadays in

hydraulic pumps, such as the Dhiron0(*1.

%igure II.6 2 ;ingle +iston #ayout0-1


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%igure II.(/ 2 +escara ;ingle +iston %ree +iston ngine071

Another configuration of free piston engines is the dual piston free piston

engine, which is found on engines that include two opposed combustion cylinders.

Theirpistons arethus rigidly connected by a non&rotating connecting rod. These

engines do not reuire a rebound system for their operation, because the combustion

that drives one of the pistons forwards serves as a rebound process for the other. They


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are mainly used in linear alternators0-1. These engines can be however very

challenging to control, especially that any small variation in its ignition timing can

cause such an engine to malfunction, even if its of the order of (// microseconds.

Another main disadvantage is the fact that these engines are sub)ected to high degrees

of perforation. However, and due to the fact that its execution can be easier than

other layouts since it can be built by two conventional scooter cylinder kits, the

pistons of which would be connected by a solid rod Fwhich would be the only

custom&manufactured partG, it has been the main layout that research groups have

been using in their linear alternator pro)ects. In fact, +hoenix !achinery have built in

*// a fully functioning linear alternator that was based on this engine layout.

Although it wasn3t clear if they reached a high enough efficiency, they were able to

overcome some of the control challenges of these engines. However, the

aforementioned mechanical limitations didn3t allow them to run their engine for a

long enough duration.

%igure II.(( 2 'ual +iston #ayout0-1


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%igure II.(* 2 +hoenix !achinery3s %ree +iston Alternator

In additionto the other two layouts, a third layout, the opposed piston free

piston engine,has been proven to be the most successful. In fact, it was the layout that

was applied to +escara3s most successful free piston engines. It consists of two sliding

pistons that are moved apart by one central combustion. $n each end, a pneumatic

bounce chamber is formed by the rebound pistons and the cylinder heads. These two

bounce chambers, which are connected through an euali:ing tube, serve as the main

rebound system of these engines. These engines feature an opposed piston uniflow

scavenging that overcomes the emission problem that engines with crossflow

scavenging suffer from. It is also an important feature since it allows this engine to

feature uniflow scavenging without the inconvenience and complexion of having an

exhaust valve installed. Another main advantage that this layout features is the fact

that it is both statically and dynamically balanced, which allows it to be totally

vibration&free. In fact, =eneral !otors engineers used to demonstrate this feature by

balancing a nickel on a running =!5 7&7 3Hyprex3 engine, which adopts this same


31/202

configuration0(>1. Another advantage of this type of engine is that it features a

reduced heat transfer since it lacks a cylinder head on the top of the combustion

chamber0-1. However, these engines feature a main challenge since they reuire a

synchroni:ation linkage for a proper operation, since they belong to the opposed

piston type. This configuration has been mainly used as the main layout of air

compressors, gasifiers, and some linear alternators, which includes the Dolgate

engine that claims to achieve piston synchroni:ation through electromagnetic

coupling0(/1.

%igure II.(4 2 $pposed +iston ngine #ayout0-1

%igure II.(7 2 ;I=!A =;&47 ;lider Assembly061


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II-3 Rebound Systes

$ne of the main requirements on all of the aformentionedfree piston engine

layouts Fexcept for the dual piston configurationG is the absolute need foran energy

storage system that replaces the conventional flywheel Fwhich serves as a kinetic

energy storage systemG that is found on all other engines, and that enables both the

piston bounce&back operation, and the continuous operation of the engine. Although

most of the free piston engines3 rebound systems consist of either hydraulic and

pneumatic energy accumulators, all energy accumulation systems that can be

practically applied to these engines will be discussed in this section.

$ne of the first rebound systems that comes to mind is a mechanical spring,

which stores elastic energy that is later used for the bounce&back operation. This

system has been featured on some ;tirling free piston engines. However, it has been

found that due to the high freuencies of free piston engines Fwhich can reach / hzG

0-1, and that due to the high loads that the pistons are sub)ected to during the

combustion process, it is a sub)ect to a very early fatigue failure, which makes its use

impractical0(-1.

Another rebound system can be used exclusively on linear alternators, which

consists of using some of the energy stored in the batteries during the expansion

stroke to compress the gases during the compression stroke, since a linear alternator

is a reversible machine that can be used as a motor. This imposes a need for a highly

accurate control and the use of switches with very high switching freuency.

However, batteries, like mechanical springs, have a limited life cycle, which would

reduce their life to a very short time due to the high freuency that these engines

reach. $ne of the solutions to such a problem is the use of a separate rebound circuit


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that is independent of the charging circuit of the batteries and that includes super

capacitors that accumulate the necessary energy for the bounce&back operation. These

have a life cycle that is long enough for such a system. However, they can be a uite

expensivesolution.

Hydraulic and pneumatic energy accumulation are the two forms of energy

storage systems that are usually used in free piston engines. ach has its own

characteristics. In fact, a hydraulic energy system usually consists of a series of

accumulators which are divided into two categories based on their stored pressures 2

high pressure accumulators and low pressure accumulators0-1. High pressure

accumulators are generally connected through a check valve at the bottom dead

center of the slider that is fixed to the piston while low pressure accumulators are

connected in avery similarway at thetop dead center0-1. $n the compression stroke,

the pressure difference between the two accumulators drives the piston back to the

top dead center, while the working liuid is compressed into the high pressure

reservoir during the expansion stroke. ;uch a system is generally integrated into a

wider hydraulic circuit, where the engine is used as the central pump0-1. It has the

advantage of allowing the engine to be highly controllable which helps increase its

efficiency. ;uch a system can be found on the I??A; Dhiron0(*1, which is a

hydraulic pump that uses a hydraulic circuit as its rebound system. However,

hydraulic systems are known to have a slow response time and to take a big space

because of the fact that liuids are incompressible. They can also be pollutant in case

infiltration occurs into the engine. They can however withstand high loads.


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%igure II.(8 2 I??A; Dhiron Hydraulic Dircuit0-1

%igure II.( 2 I??A; Dhiron xternal #ayout0(*1


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$n the other hand, pneumatic systems, and though they might be less efficient

due to the heat transfer that occurs when they are compressed, and though they

cannot withstand high loads as much as hydraulic systems can, have several practical

advantages over hydraulic systems. In fact, pneumatic systems are cleaner. Therefore,

and in case infiltration occurs, the engine3s emission would be unaffected. In addition,

and due to their compressibility, the pneumatic accumulators that are used for the

bounce&back operation have a significantly smaller si:e than those used in hydraulic

systems, which enables them to be included on the ends of the engine thus reducing

the si:e of the system Fthey actually consist of the piston in itself, the cylinder head,

and the cylinder wallsG, and moreover, these accumulators, which are commonly

known as bounce chambers or air cushions, are all of the high pressure type, since air

is compressed to a pressure that is high enough to drive the piston back without the

need of a second low pressure reservoir, thus reducing the number of bounce

chambers to one in single piston engines, and to two in opposed piston engines, and

thereby eliminating the need for a complicated pneumatic circuit in contrast to that of

hydraulic rebound systems, which was a factor that limited the operational

application of their respective engines to hydraulic pumps. The pneumatic system is

therefore reduced to a simple euali:ing tube that ensures an eual pressure in both

bounce chambers, in addition to an air recovery system in case a significant blow&by

occurs. %ree piston engines with a pneumatic rebound system have a wider field of

applications, ranging from air compressors to gasifiers and alternators0-1. However,

the fact that their stroke length cannot be as controllable as that of hydraulic engines

denies this type of engines to operate as efficiently at all freuencies. Therefore, they

tend to operate at a determined regime0-1.


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%igure II.(> 2 +escara %ree +iston Dompressor081


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II-! "dvantages O# Free Piston Engines

Throughout this section will be recapitulated some of the most notable

advantages that free piston engines have over other types of engines. ;ome of these

advantages have been briefly mentioned in the previous sections, one of which being

the lighter weight that free piston engines have, which is more than an advantage for

free piston engines 2 it is the main cause that lead to the invention of this type of

engines, as it was discussed earlier. This advantage has enabled a multitude of

applications for free piston engines in the maritime and aerospace fields back in the

(64/s, especially that conventional rotational turbo&compressors and )et engines were

still in a very early development stage when free piston engines were becoming more

and more common back then0-1.

Another advantage that free piston engines have is the simplicity of their

design compared with the designs of other conventional engines, which was due to

the fact that these engines lack any rotating systems. That advantage is especially

found in single and dual piston engines, the designs of which can be very simple. It is

less significant in opposed piston engines, especially that they reuire a

synchroni:ing mechanism that can complicate their design uite a bit0-1.

That last advantage implies another, which is the fact that frictional losses are

way smaller than those of other types of engines, which is due to the fact that fewer

parts are in motion in such an engine. This fact also implies that the mechanical

efficiency of these engines is increased.

In addition to the increase in mechanical efficiency of these engines, the fact


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that the stroke length of such an engine is variable can be used as a factor to increase

the thermal efficiency of these engines, especially that of spark ignition engines. In

fact, and as it is observed on conventional gasoline internal combustion engines, the

efficiency is higher on a certain rotational speed than it is on other Fusually between

4/// and 7/// rpmG. That is due to the fact that on different rotational speeds,

different compression ratios are needed for the combustion to be optimal. However,

and due to the crank&slider mechanism that most engines have, the stroke length is

constant on all the speed regimes of the engine, which implies a constant

compression ratio. That led many engine manufacturers to develop variable

compression ratio engines, which reuire complicated linkages and mechanisms, thus

implying a further inconvenience. %ree piston engines already have variable strokes

lengths that are not limited to a constant value by any linkage Feven the

synchroni:ing linkages featured on opposed piston engines do not limit the stroke

lengths of their respective engines to a certain constant valueG, which reduces the task

of having a variable compression ratio to the proper control of the ignition of the

gases, and at most to the control of the fluid motion of the rebound systems, and thus,

higher thermal efficiencies can be observed in these engines0-1.

A variable stroke length also implies a multi&fuel operation, which was

impossible on conventional engines due to the fact that each fuel has its own reuired

compression ratio range. !oreover, homogeneous charge compression ignition,

which is a form of compression ignition Fthat doesn3t reuire a sparkG in which a

gasoline&diesel fuel mixture is in)ected to the combustion chamber during the

compression stroke Fthus combining the $tto and 'iesel cyclesG, would be possible.

$ther advantages include less fuel consumption, a better fuel&air mixture

which is due to the high speed that is featured around the top dead center, and a


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reduced heat transfer loss due to the high speed expansion featured in these engines,

which reduces the time available for heat loss, and also limits the formation of

temperature&dependent pollutants such as ?$x0-1.

II-$ Starting Systes

$ne of the main challenges that free piston engines have is the fact that they

cannot be cranked over several revolutions0-1 as it is the case with other internal

combustion engines, which implies the use of unconventional starting systems.

+neumatic free piston engines were started by the impulsive introduction of air

into the bounce chambers, which drove the engine towards the top dead center0-1.

These engines had to achieve a steady&state operation right on the first stroke since

that mode of starting was only valid for the first stroke only. 5emoving the

introduced air volume was the main challenge of this feature0-1. Although a one

stroke starting process was featured on these engines, it wasn3t reported that it was a

serious inconvenience0-1. These engines featured a staring reservoir that contained

enough compressed air for this operation. These can be seen on the following

figure061.


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%igure II.(- 2 ;I=!A =;&47 +ower +lant061

!ultiple stroke starting processes are featuredon hydraulic engines and linear

alternators, which are reversible. In fact, an external pump can be controlled to

provide the circuit with the fluid motion necessary for the operation of hydraulic free

piston engines especially that these are generally integrated into a closed hydraulic

circuit0-1. #inear alternators are also reversible electric machines0*/10*(1. Therefore,

the same alternator can be used as an electric motor to generate a multiple stroke

starting process by simply reversing the current passing through the coils0-1.

?o mechanical reciprocating system has been noted to have been used for thestarting of pneumatic free piston engines, which is uite remarkable since many

factors, including low temperatures and high altitudes, can impose a multiple stroke

starting process.


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III- %inetic "nd &'erodynaic Siu(ation

After having reviewed most of the notable free piston engines that were built

throughout history, it was found that opposed piston pneumatic free piston engines

were the most successful and that they were the only engines to have been able to

compete commercially, mostly through the +escara ;I=!A =;&47 engines.

Therefore, it was decided that it was the best choice as a basis for the design of the

free piston engine that is described throughout the rest of this document.

In fact, the engine in uestion is an opposed piston spark&ignition free piston

engine with pneumatic bounce chambers that is based on the ;I=!A design. It

features two standard pistons on each side that are connected through a rigid

connecting rod. The two sub&assemblies that are formed each by the two

aforementioned pistons, which are of different si:es, and the connecting rod, are the

main sliding elements of the engine. As it can be seen on the following picture, the

small piston is in direct contact with the combustion chamber gases. A larger piston is

selected for the bounce chamber side to decrease its pressure . A third space is formed

by the large piston and the combustion cylinder transverse walls. It is known as the

compression chamber, and it is used a a scavenge pump, which allows the workingfluid to enter the combustion chamber at a higher pressure, which is essential for the

scavenge operation of two&stroke engines, and also serves as a supercharger. Two

5eed check valves are located on both ends of this space. 'uring the expansion

stroke, pressure decreases in this space, thus allowing an atmospheric pressure air&

fuel mixture to enter this space. It is later compressed to the point where it exceeds

the pressure of the scavenge reservoir surrounding the combustion cylinder.


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At that point air is allowed into the scavenge reservoir through the 5eed valve,

thus maintaining a pressure that is super&atmospheric Fit is usually a (.- bar

pressureG0**1. ?ote that the bounce chambers are considered to be closed spaces

where air is sub)ected to a nearly isentropic compression until the slider comes to

rest. The force due to the compressed gases in the bounce chamber later pushes the

sliders towards the top dead center of the engine, where ignition occurs, and thus the

cycle is repeated.

%igure III.( 2 'iagrammatic ;ketch $f A %ree +iston =as =enerator061


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%igure III.* 2 5eed Calve0*41

ach sliding element is sub)ected to a multitude of forces that are identical on

both sides due to the fact that this engine is perfectly symmetrical, and that the sliders

are connected through a synchroni:ing mechanism. The resulting sum of these forces

generates the kinematic characteristics of the sliders. Throughout this section will be

detailed the kinetic and thermodynamic studies and simulations that have been

performed to determine the operating parameters of the engine in uestion.


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III-1 %inetic )'aracteristics O# &'e S(iders

ach of the two sliders, as discussed earlier, is sub)ected to a multitude of

forces that are shown on the following free body diagram. ach of these forces will

be discussed in the following table. The sum of these forces is obviously eual to the

acceleration of the slider multiplied by its mass according to the second law of

?ewton.

J% K !sL d*FxGMdt F. III.1G

This euation is the main differential euation that generates the motion of

these sliders. The solution of this euation is a function of time representing the

position of the slider with respect to time. Therefore, it was modeled on ;I!U#I?"

and a !AT#A9 program has been written to automatically assign the corresponding

value of each parameter. This numerical aspect of the study will be detailed later on.


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%igure III.4 2 ;lider %ree 9ody 'iagram0*71

;ymbol 'escription

%c %orce due to the pressure of the gases present in the combustion chamber.

%b %orce due to the pressure of the air inside the bounce chamber.

%comp

5eaction force of the scavenge pump.%fric %riction force on the slider.

%alt #aplace force on the slider in case the engine is also a linear alternator.

Table ( 2 %orces Acting $n The ;liders

?ote that the wide arrow represents the motion of the slider.

!any of these forces3 expressions will be determined later on while discussing

the thermodynamic cycles that are respective each of the engine chambers. The

#aplace force that is induced on the slider by the coils will be discussed later on in a

detailed study that was performed on a linear alternator free piston engine. Therefore

the only force that can be determined in this section is the friction force.


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The friction force occurs mainly on the side of the engine, between the piston

rings and the cylinder walls0**1. It is mainly due to the shear stress that occurs in the

lubricant that is located betweenthe two ofthem. Therefore0**12

%fricK NsL Ar F. III.2G

with 2 & Ns 2 shear stress occurring in the lubricant between the rings and the

cylinder walls.

& Ar2 side area of the piston rings

?oting that2

Ar K O L 9 L h with 2 & 9 2 +iston 9ore

& h 2 +iston 5ing Height

and0**12 Ns P Q L v M c with 2 & Q 2 dynamic viscosity of the lubricant

& v 2 instantaneous velocity of the slider

& c 2 piston ring&cylinder wall clearance

The expression of %fricbecomes2

%fricK O L 9 L h L Q L v M c FIII.3G 0**1

The previous expression has been entered into the numerical simulation model

that will be detailed later on.


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III-2 &'erodynaic )'aracteristics O# &'e Free Piston Engine

As it was shown earlier, and as shownon figure III.1, each of the three main

chambers of the engine has its own thermodynamic cycle. Throughout this section

will be shown and explained the +&C diagrams of each cycle, which will help in

determining the expressions of the forces that are due to each of the different

chambers3 pressures.

In fact, the two&stroke $tto cycle can be applied to the fluids of the combustion

chamber. The different processes of this thermodynamic cycle will be briefly detailed

based on its +&C diagram that is shown on the following figure.

%igure III.7 2 ;tandard ;park&ignition +ressure&Colume 'iagram0**1


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+rocess (&* 2 isentropic expansion. At any point of this process, the combustion

chamber pressure is 0*812

+cK +(L FC(MCG k with k K (.7 F.III-4G

'ue to the obvious fact that the piston surface is constant2

+cK +(L Fx(MxGk with x the position of the slider. F.III-5G

?ote that the previous expression will be the one included for this process in

the numeric simulation of the engine.

+rocess *&4 2 the exhaust blowdown. 'uring this process, the exhaust ports are

uncovered by the piston. The pressure difference between the combustion chamber

gases and the atmospheric air induces the motion of the gases that exit the engine

according to 9ernoulli3s uation. That fact has been taken into account in the

numeric ;I!U#I?" model of the exhaust process, thus allowing a simulation of the

pressure variation with respect to time, which is affected by the aforementioned fact

in addition to the expansion that occurs simultaneously with the blowdown. ?ote that

the expression of the 9ernoulli euation is0*12

Fvf*G M R S g L : S + M R K Donstant F. III-6G

+rocess 4&7&82 the admission process. 'uring this process, inlet ports are

uncovered by the piston, and a fresh air&fuel mixture enters the combustion chamber

and pushes the remaining combustion products out through a process known as

scavenging. 'uring this process, +cis considered to be constant and eual to the


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pressure of the scavenge reservoir, which is usually between (7/ and (-/ "+a0**1.

+rocess 8&2 final scavenging process. After the piston recovers the inlet ports

of the engine, and before it covers the exhaust ports, some of the remaining

combustion products are driven out during this process. +c remains constant

throughout this process as well.

+rocess &>2 isentropic compression. At any point of this process, the

combustion chamber pressure is 0*812

+cK +L FCMCG k with k K (.7 F. III-7G

And therefore2

+cK +L FxMxGk with x the position of the slider. F. III-8G

+rocess >&-2 constant volume combustion process. The combustion process is

the main process that provides the cycle with the energy that is extracted from the

engine. 'ue to the fact that it is a spontaneous process0*81, the states of the working

fluids are generally determined at the beginning and at the end of this process, and

the state of the fluid during that process is disregarded and considered unable to be

determined. Thus, the state at the end of this process has been determined based on

the standard optimal end of combustion states of standard spark ignition internal

combustion engines.


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?ote that due to the different expressions that +c takes in all of the

aforementioned processes, one ;I!U#I?" model cannot be sufficient to simulate

the engine cycle, which led to the creation of several ;I!U#I?" models for each of

the aforementioned processes, and to the creation of a !atlab program that assigns

the initial values of the parameters of each of the processes at the beginning of its

respective simulation.

In addition to the combustion chamber, the compression chamber has its own

thermodynamic cycle. The following diagram illustrates the cycle in uestion.

%igure III.8 2 ;chematic 'iagram $f The Dompression Dhamber Dycle

+rocess (&*2 isentropic expansion. 'uring this process, both the compression

chamber inlet and scavenge reservoir outlet reed valvesare closed. At any point of

this process, the combustion chamber pressure is 0*812

+compK +(L FC(MCG k with k K (.7 F.III-9G


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And therefore2

+compK +(L Fx(MxG k F.III-9G

+rocess *&42 compression chamber admission. 'uring this process the inlet

reed valve opens, and the +comp is atmospheric at any time.

+rocess 4&72 isentropic compression. 'uring this process, both the compression

chamber inlet and scavenge reservoir outlet reed valve are closed. At any point of

this process, the combustion chamber pressure is 2

+compK +4L FC4MCGk with k K (.7 F.III-10G

And therefore2

+compK +4L Fx4MxGk F.III-11G

+rocess 7&(2 scavenge reservoir admission. 'uring this process the outlet reed

valves are opened, and +comp is considered to be eual to the scavenge reservoir

pressure at any time.

In contrast with the two previous chamber, air in the bounce chamber is either

isentropically compressed or expanded depending on the direction of motion of the

slider. And since the bounce chamber is nearly a closed volume, the expression at any

time during the cycle is0*812


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+bK +(L FC(MCGk with k K (.7 F.III-12G

And therefore2

+bK +(L Fx(MxGk F.III-13G

%igure III. 2 ;chematic 'iagram $f The 9ounce Dhamber Dycle


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?ote that the forces corresponding to each of the aforementioned pressures is

eual to the product of the pressure and the area of the piston that is in contact with

the chamber, such that2

%cK +cL A( with A(2 Area of the small piston F.III-14G

%bK +bL A* with A*2 Area of the large piston F.III-15G

%compK +compL FA* & A(G F.III-16G

III-3 Siu(ation *ode(s

All of the aforementioned euations have been modeled using ;I!U#I?". As

it was mentioned earlier, each model represents the differential euation that

generates the motion of the slider. This differential euation is essentially ?ewton3s

second law, where each of the aforementioned forces are summed according to

euation III&(. 'ue to the fact that the expression of each of these forces is different

for each of the thermodynamic processes of the cycle, the simulation reuires a

multitude of ;I!U#I?" models. These models are simulated according to their

respective orders in the whole cycle, a task that is performed by a !atlab program

that makes sure this order is respected, and assigns the initial values for each of the

models, thus making sure that the simulation is continuous throughout its phases. The

!atlab code of this program can be found in Appendix A. $n the following pages

will be presented each of the ;I!U#I?" models that represent the thermodynamic

cycle of the free piston engine.


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%igure III.> 2 %irst xpansion +hase ;I!U#I?" !odel

This model represents the expansion of the combustion products that takes

place between the end of the combustion process and the opening of the inlet reed

valve.


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%igure III.- 2 ;econd xpansion +hase ;I!U#I?" !odel

This model represents the expansion process right after the inlet reed valve

opens, which occurs when +comp reaches the atmospheric pressure. 'uring this

process, +comp is constant and eual to the atmospheric pressure.


56/202%igure III.6 2 xhaust ;I!U#I?" !odel


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?ote that the previous model is that of the exhaust process, which occurs right

after the exhaust ports are uncovered. ?ote that in addition to the expansion of the

combustion products, the working fluid motion that occurs out of the exhaust is taken

into account in this model, which generates the instantaneous pressure drop in the

combustion chamber, which is an effect that is governed by the 9ernouilli euation

that was discussed earlier F.III-6G.

%igure III.(/ 2 %irst Admission +hase ;I!U#I?" !odel


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The previous model is that of the admission process that occurs after the

admission ports are uncovered. ?ote that the admission process has been divided into

two 2 the first is that of the admission that occurs before the bottom dead center is

reached. It is represented by the previous model. The second is that of the admission

that takes place after the bottom dead center is reached, and through which starts the

compression of the compression chamber mixture. This process is modeled on the

following diagram.

%igure III.(( 2 ;econd Admission +hase ;I!U#I?" !odel


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%igure III.(* 2 %irst Dompression +hase ;I!U#I?" !odel


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%igure III.(4 2 ;econd Dompression +hase ;I!U#I?" !odel

The two previous models correspond to the compression process of the cycle.

?ote that throughout the first phase, the outlet reed valve is still closed and the

compression of the compression chamber is still ongoing. After +comp reaches the

value of the pressure of the scavenge reservoir, the reed valve in uestion would

open, thus letting the mixture into the reservoir, a process that is model by the second

model.


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?ote that three additional models have been created since the reed valves are

not bound to open in the exact processes that were shown earlier. The !atlab source

code has been implemented with various tests to predict the exact process during

which each reed valve opens, and thus allowing it to select the proper model at the

proper time. These drawings have not been shown among the previous diagrams for

clarity reasons and since the opening schedules according to the final simulation

performed are those of the diagrams that were previously shown. The additional

diagrams can be found in appendix A.

Also note that in all the previous diagrams, all the compression and expansion

processes have been considered as polytropic processes with a polytropic coefficient

eual to (.4, thus taking into account the heat transfer occurring during each of these

processes0*81.

III-! Siu(ation Resu(ts

Throughout this section will be shown and discussed the results of the

previously described simulation. ?ote that after having modeled the engine, and

programmed the corresponding !atlab program, a great number of simulation runs

have been performed. The following results are those of the final run which is

considered to have provided the most optimal results. ;ome of the notable inputs that

were entered were the minimum pressure of the bounce chamber, which was

optimi:ed after a series of runs, and found to be (.8 bars, the scavenge reservoir

pressure, which was selected to be (.- barsbasedonthe scavenge pressure range of

commonly availabe two&stroke engines 0**1. In addition, the bores of each of


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the pistons have been entered. These have been initially selected to be 78 mmand 6/

mm. 9ased on that initial choice, two standard pistons that are available on the market

have been selected with close dimensions to the ones reuired. The small piston that

has been selected has a diameter of 7/ mm, and the large one is a 6* mmdiameter

piston. The si:es of these pistons were the ones included in the final run of the engine

simulation. $ther inputs can be found in appendix A.

%igure III.(7 2 +iston +osition ith 5espect To Time

?ote that according to the previous diagram, many parameters can be

extracted, including the stroke length of the engine, which was found to be 86 mm,

and the duration of the cycle, which was found to be around 7/ ms. ?ote that at

around (> ms, the slider is brought to rest by the pressure force of the bounce

chamber, which in turn drives it to its initial position at the top dead center, which is

located at >/ mm from the origin that was specified in the simulation parameters.


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%igure III.(8 2 +iston Celocity ith 5espect To Time

According to this graph, the maximum velocity of this slider is found to be

almost eual to 8 mMs, which has proved to be an inconvenience that was considered

in the design of the synchroni:ation mechanism that will be discussed later. It can

also be inferred from the previous graph that the velocity of the slider can be

interpolated into a sinusoidal function, which in turn shows that the motion of the

sliders is nearly a periodic sinusoidal one. ?ote that the instantaneous velocity of the

slider is / at the same moment where the slider comes to rest and the bounce back

operation is started Fat (> msG. Another important feature of the free piston engine

that was mentioned earlier can be seen on this graph, which is the fact that the piston

velocity at the top dead center is not null, which is not the case in conventional

engines where the piston is brought to rest as it belongs to a crank&slider mechanism,


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and which is an advantage of free piston engines that has been discussed earlier.

%igure III.( 2 Dombustion Dhamber +ressure ith 5espect To x

This graph represents the thermodynamic cycle that occurs in the combustion

chamber of the engine. It has been found that this cycle complies with the two&stroke

$tto cycle that has been shown earlier. At the top dead center of the cycle can be seen

a ma)or discontinuity, which is the combustion process of the engine that is assumed

to be a spontaneous constant volume heat addition process and that is determined by

its initial and final states0**10*810(61. The positions of the exhaust and inlet ports can

also be seen on this graph, which are located respectively at *> mm and (>.8 mm

from the selected origin.


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%igure III.(> 2 9ounce Dhamber +ressure ith 5espect To x

It can be seen on the previous graph that the maximum pressure that takes

place inside the bounce chamber is eual to 7.>8 bars, which occurs at the bottom

dead center of the engine. In addition, the fact that the expression of the pressure of

the bounce chamber is the same throughout the cycle can be verified by the

continuous hyperbolic form obtained through the simulation.


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%igure III.(- 2 Dompression Dhamber +ressure ith 5espect To x

$n the following table will be shown some additional results that were

calculated by this simulation.

+arameter Calue

5ated %reuency *8Hz

Dompression 5atio >.(7

Thermal fficiency 77.8B

!ean Dycle +ressure (4.(>8 bars

ork +roduced +er Dycle (7*.7J

5ated +ower $utput 4.2K

Ignition Advance 7.5ms

Table * 2 Additional 5esults Dalculated 9y The ;imulation


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In addition to this simulation, a simulation of a linear alternator that is based on

the ;tirling Dolgate free piston engine has been performed, since that engine was

considered to be one of the bases of the design of the free piston engine in uestion

due to the fact that its inventor claims that piston synchroni:ation is achieved through

electromagnetic coupling0(/1, which enormously simplifies the design especially that

one of the main challenges that it incorporates is the design of the synchroni:ation

mechanism as it will be discussed in the following section.

In fact, and as it can be seen on the following figure, this permanent magnet

linear alternator, which incorporates two magnets that are fixed on the rebound

pistons, has two coils surrounding the cylinders. hen the magnets move through the

coils, a variable magnetic field is created, thus inducing an electric current in the

coils. The coils are in series with a capacitive circuit F4/G which is built such that the

circuit becomes a resonant 5#D series circuit having a freuency eual to the

freuency of the engine0(/1. Therefore, and due to the fact that the same current

passes through both coils, the induced #aplace force F%alt in this caseG, which is

dependent on this current as it can be seen in the following euation0*(1, will be

exactly the same on both sliders, thus creating the synchroni:ation mechanism

necessary for such an engine.

%altK i( + with 2 & i 2 induced current F.III-17G

& ( 2 euivalent length of each of the coils

& +2 magnetic field of the magnets


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%igure III.(6 2 Dolgate Alternator #ayout

A simulation that included a finite element simulation on D$!;$#

!ultiphysics that determined the instantaneous magnetic field density that was

generated by the motion of the permanent ?eodymium magnets through the

coils0*>1, and a !atlab simulation that included several ;I!U#I?" diagrams that

modeled both the kinetic and electric aspects of such an engine, in addition to a

!atlab program similar to the one already described, has been performed.

This simulation will not be shown in detail in this document since a free piston

linear alternator was abandoned by +hoenix !achinery, and thus it will not be used in

the design of the engine. However, and because it was already performed, it can be

found along with its results in Appendix 9.


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I,- *ain esign

9ased of the simulation results that were shown earlier, and based on the

+escara free piston engine, a spark&ignition free piston engine was designed. The

output of this engine is intended to be extracted throughout a turbine that expands the

exhaust gases generated by the engine. This turbine would be thereby coupled to an

alternator that in turn generates the power reuired. The basis of the design was the

standard available pistons that have been selected. Two 7/ mm pistons and two 6*

mm pistons have been used in this pro)ect. These pistons have been divided into two

sets of pistons, each set including one 78 mm piston and one 6* mm piston. %or each

set of pistons, a central connecting rod has been conceived. This connecting rod was

connected on each end to one of the pistons, and thus the sliders have been formed.

The combustion cylinder has been designed such that its inlets and exhaust

ports would comply with the aforementioned simulation results. 9eing made out of

aluminum (A#U!D>6) due to its high thermal conductivity, it features a series of

fins that were intended to increase the heat transfer rate of the cylinder outer walls,

which is essential especially that this engine is an air cooled engine. It also features a

(/ mm spark&plug located in the middle of the combustion chamber.

The big cylinders are also made of aluminum. $n their ends are fixed the

bounce chamber covers that ensure that the bounce chamber is closed. ach of these

features a (mm thread that is intended for the connection of the communication tube

that euali:es the pressure in both these chambers. A negligible blow&by occurs

through the piston rings. Therefore, a bounce chamber volume conservation system


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has been conceived for the bounce chamber. This system makes sure that the air

volume dissipated through the rings is compensated. It includes a pneumatic

accumulator, a pressure regulating valve and a check valve that is connected to the

pressure euali:ing tube of the engine. An electric compressor provides the

accumulator with its reuired pressure. This compressor is controlled such that it

operates whenever pressure drops under a certain level, which in this case is 8 bars.

Dompression is stopped once the pressure reaches > bars. Note that a pressure

drop of /.( bars each 4/ mins occurs in the bounce chamber, which implies a limited

operation time for the electric compressor. ?ote that the check valve ensures that air

would not be transferred to the bounce chamber unless a pressure drop occurs, and

denies the return of the air.

%igure IC.( 2 9ounce Dhamber Colume Dontrol ;ystem

5eed check valves have been selected for the scavenge pump control

operation. A housing has been designed for each of these four reed valves. Two of

these valves are connected to the compression chamber, while the other two are

connected to the scavenge reservoir. The former two are connected on their other

ends to a central carburettor where a fuel&oil mixture is pulveri:ed into the inlet air.


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9oth the big and small cylinders have been designed such that they would be

properly centered during the assembly operation. ?ote that cast iron sleeves are

intended to be inserted into both the big and small cylinders to ensure less friction in

the operation of the pistons.

However, the main challenge in the design of this engine remains that of the

synchroni:ing linkage. In fact, this linkage has to ensure that the linear motion of the

sliders is maintained, and that the two slidersare properly synchronized.

Two synchroni:ing mechanisms have been designed for this purpose. A

representative drawing of the first one can be seen in the following figure. It has

been abandoned due to some of the complex aspects that it incorporates, especially in

the sliders that the central link includes.

%igure IC.* 2 arly 'raft $f The ;ynchroni:ation !echanism


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The second one is based on the standard crank&slider mechanism. However, in

this case, it consists of two connecting rods on each end that are each connected to a

sliding bar that is fixed to the main slider of the engine on one end, and to a central

common link on the other. The central link is fixed to an axis that rotates inside an

external bearing arrangement. However, the motion of this link is not a full rotation,

but rather an oscillating one. The degree of freedom of this mechanism according to

the "ut:bach&=rbler euation0*-1 is eual to one, which is the degree of freedom

reuired that makes it fully constrained once the engine is in operation. ?ote that this

mechanism is based on the mechanism used by +escara in his early opposed piston

compressor that was shown previously01. It has the advantage of providing an

external access to the synchroni:ation mechanism, which will prove to be helpful for

the starting operation.

To maintain the sliding bars in a linear motion throughout the whole process, a

linear guide has to be included in the engine assembly, which is one of the topics that

were investigated the most, especially that the linear speed of the sliders reaches

8 mMs, which is a critical speed on almost all of the commercially available linear

guides. In fact, many high speed linear bearings cannot even reach a limit of 4 mMs,

which is an enormously high linear speed in industrial applications04(104*1.

Therefore, and after a thorough search on every linear guide ever made, it was found

that the best solution was to locally design and manufacture linear plain bearings out

of sintered bron:e F;A -7(G, to which hasbeen added powdered graphite, which

according to the 9osch Automotive Handbook can withstands high speeds that can

reach */ mMs 0*61. The design of these bushings has been based on many of the

parameters of commercially available bushings. Two bushings on each side are

needed for a proper support of the bars. However, and due to the limited space on

such an engine, only one bushing has been used on each side. However, these


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bushings have been designed with a length that exceeds the reuired minimal length

of operation Fwhich was found to be (/ mmG by *.8 times, thus overcoming the

problem stated earlier, and allowing the bushing to have a higher +v limit04/1.

The connections between the linkages have been designed to include needle

bearings, which can also be found in )et&ski engines. The bearings selected have been

found to have a life that exceeds (///// hours according to the ;"% $nline 9earing

Dalculator. However, these bearings are usually in a steady&state operation where a

constant rotational speed is maintained once it is reached, which is not the case in this

application especially that cyclic accelerationsand decelerations occur continuously

during the operation of the engine. However, this oscillating motion is taken into

account by multiplying the life of the bearings by /.-, which reduces the life of these

bearings to -//// hours, which is more than enough since a two stroke engine3s life

usually doesn3t exceed (/// hours Feven if the results were exaggerated by the tool

provided by ;"%, they would still exceed the reuired conditions by a far large

numberG. ?ote that the report of the bearing life calculation can be found in appendix

D.

The final three&dimensional design can be found on the next page. ?ote that the

shop drawings of each of the 4* parts that form this engine can be found in appendix

' and that the stress analysis simulations of each of the parts can be found in

appendix . In addition to the aforementioned elements, some of the elements that

belong to the starting system of the engine and others that belong to the ignition

system can be seen on the following figure. These elements will be discussed on the

next two sections of this document.


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%igure IC.4 2 %ree +iston ngine %inal 'esign


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,- Starting Syste

As it was discussed earlier, all of the pneumatic free piston engines throughout

history were supposed to achieve a steady&state operation on the very first stroke after

they are started. However, such a feat proves to be difficult even on the most

advanced conventional internal combustion engines, especially when it is a cold start

operation, which usually reuires several strokes before a continuous series of

ignitions is achieved. ;ince the designed engine doesn3t belong to neither the linear

alternator nor the hydraulic pump type of free piston engines, which, as discussed

earlier, are the only free piston engines where a multi&stroke starting operation is

possible, it reuires a mechanical starting system where a reciprocating motion is

possible.

As it was discussed earlier, the synchroni:ation mechanism that has been

designed for this engine features the possibility of external coupling through its

central link, which allows an external starting mechanism to be coupled to the engine.

However, and due to the fact that the motion of the central link is an oscillating one, a

conventional electric starter can3t be used on this particular engine. Therefore, a

mechanical linkage that converts a rotational motion into a reciprocating one is

needed for such an engine, and such a mechanism is reuired to be disengaged once

the starting operation is complete.

Two designs have been considered for the starting system of this engine. The

first one consists of a crank&rocker four bar linkage with a clutch. The second consists

of a scotch&yoke mechanism, that converted the rotational motion of the starting


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motor into a linear reciprocating motion of a rack, which in turn transferred its

oscillating motion to a pinion that was fixed to the central link.

%igure C.I 2 ;cotch&yoke !echanism0441

A crank&rocker linkage has first been designed for the system such that the

rocking motion of the central link would be eual to 4 degrees, which was conform

to the motion of the pistons of the engine, which was limited to a maximum stroke of

>/ mm. However, a pulley reduction system was still needed to provide the central

link with its reuired range of motion, which added an inconvenience to the design of

this system. Another main disadvantage of that system was that it reuires a clutch for

the disengagement operation. $ther disadvantages include a high risk of fatigue

failure thus reuiring thicker linkages which increases the costs of manufacturing, in

addition to a reuirement for a spacious frame on which the mechanism would have

been mounted, which would make the engine less compact. An early representative

three&dimensional draft for the design of such a mechanism can be found on the

following figure.


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%igure C.* 2 arly 'raft $f The Drank&rocker ;tarting ;ystem

The other starting mechanism, which was the one adopted in the final design,

consists of a rack and pinion system that is driven by a scotch&yoke mechanism. A

module two rack is fixed to the sliding element of the scotch&yoke mechanism, and a

7 tooth pinion is fixed to the rocking link of the synchroni:ation mechanism. Thesliding part of this system is allowed to rotate around the eccentric drive of the

system thus allowing a disengagement operation once the starting operation of the

engine is complete, and thus the inclusion of a clutch into the design would not be

needed anymore. The eccentric drive of the system features an eccentricity of > mm,

which is eual to half of the stroke length traveled back and forth by the rack. This

eccentric shaft is fixed by a cross&locating bearing arrangement, which enables the


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use of a multitude of driving systems to be fixed on its end. The design of this system

can be found on the next figure.

%igure C.4 2 5ack And +inion ;tarting ;ystem


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,I- Engine E(ectronic )ontro( .nit

$ne of the main features that differentiates the free piston engine that has been

designed from the +escara free piston engine is that the former reuires an ignition

control unit especially that it is a spark&ignition engine, in contrast with +escara3s

compression ignition free piston engine. This unit however has to be conceived

especially for this type of engine, for free piston engines cannot be mounted with

neither mechanical ignition units FdistributorsG, nor conventional engine electronic

control units FDUsG that reuire the use of crank sensors due to the fact that they

reuire a full rotational motion to operate properly, which is a features that free piston

engines lack. Therefore, a linear ignition control unit has been designed for this

engine, which consists of proximity sensors, a microcontroller that is implemented

with the algorithm used to control the ignition, an ignition transistor that breaks the

current of the coil whenever a spark is to occur, an electronic circuit where all the

components are connected together and are provided with their rated voltages and

currents, in addition to the ignition coil, the spark plug and the battery.

This unit is based on an ignition control unit developed by +hoenix !achinery

in *//. However, and due to the fact that the latter belongs to a dual piston engine

which is entirely different from the currently designed engine, the ignition control

unit developed for this engine is different from the one designed in *//.

Throughout this section will be detailed all the steps that were taken in the

design of this unit, in addition to the parameters that are crucial to the ignition

process and that were included in the algorithm implemented on the microcontroller.


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,I-1 Ignition Sensors

As it was stated earlier, conventional pick&up sensors cannot be used on this

engine due to the lack of rotating elements on such an engine. Therefore, a different

type of sensors was considered for the ignition operation.

It was found that due to the availability of an uncovered sliding bar in the

synchroni:ation mechanism of the engine, proximity sensors could be used for the

sensing operation. 'ue the high freuencies that are reached during the operation of

such an engine, proximity sensors having a high switching freuency were reuired

for a proper operation of the engine, which implied the use of inductive proximity

sensors, which proved to be useful especially that the components that were supposed

to be detected were iron rod ends that were designed to feature a sensing rectangular

surface that was separated from the surface of the bars by (/ mm, which is more than

enough according to most of the sensor catalogs that were considered to prevent the

sensors from continuously detecting the bar.

It was found after a thorough search that the $!5$? *A&;/-"?/7 !-

sensor was one of the most suitable for the application in uestion. ;ome of the most

important characteristics of this sensor can be found in the following table. ?ote that

although it might be obvious that only one sensor would be used since this is a

mono&cylindric engine, two sensors of this type are needed for a proper operation,

which will be explained later on. Also note that one of the reasons that lead to the

selection of this particular type of sensors is that it is locally available, and that it has

been used by +hoenix !achinery previously which confirms that they are suitable for

this application.


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)y(inder ty/e sensing 'ead si0e !-

&y/e Unshielded

Sensing et'od Inductive type

Sensing distance 7 mm &(/B to S(/B

Setting distance / to 4.* mm

i##erentia( distance (/B !ax. of sensing distance

Sensing obect%errous metal F;ensitivity lowers with non&ferrous

metals.G

Standard sensing obect Iron (*L(*L(mm

Res/onse #reuency 'D2 ( kH: FaverageG

Power su//(y vo(tage (* to *7 C'D rippleFp&pG 2(/B !ax.

O/erating vo(tage range (/ to 4* C'D

)urrent consu/tion (/ mA !ax.

)ontro( out/ut Out/ut ty/e4 +?+ open collector output

)ontro( out/u Switc'ing ca/acity4 / to *// mA

Indicator $peration indicatorFyellowG

O/eration ode ?$

Table 4 2 Dharacteristics $f The $!5$? *A&;/-"?/70471

The position of each of these sensors is critical for a proper ignition operation,

which is a factor that will be explained later on.

The sensors are supplied with a (* C 'D voltage. Their output is connected to

the input ports of the microcontroller through opto&couplers that ensure that the input

voltage of the controller doesn3t exceed its rated value of 8C. These opto&couplers will

be discussed later on.


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,I-2 *icrocontro((er Se(ection "nd Ignition "(gorit'

The central component of the DU is the microcontroller, which is the main

component that controls the ignition operation. This microcontroller is implemented

with acontrol algorithm that is specific to the engine.

After having reviewed some of the microcontrollers used in popular ignition

kits that are currently being developed worldwide, it was found that an Arduino !ega

*8/ platform, which the basis of a popular open source DU pro)ect named

;peeduino0481, is the most suitable for this pro)ect especially that it features a (&bit

timer and that it is simple to program due to the fact that Arduino platforms are very

well documented041.

The following flowchart represents the algorithm which the aforementioned

microcontroller was programmed toexecute. ?ote that the Arduino source code

implemented on the Arduino !ega board can be found in Appendix %.

The ignition operation represented by this algorithm is similar to that of

Dapacitive 'ischarge Ignition FD'IG units that are found on two&stroke motorcycle

engines, and that feature a constant ignition timing advance, which is featured on

most of the smaller two&stroke engines Fengines with less than *// cc capacityG and

that is also featured on this free piston engine since it is a (8/ cc engine0**1. The

average timing advance has been determined by the simulation that was detailed in

section III. The calculation methodology of this parameter will be detailed at the end

of this section.


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$ne of the sensors is fixed right on the position on which the ignition signal is

sent by the microcontroller to the ignition transistor. This position is calculated by the

numerical simulation of the engine based on the aforementioned timing advance. As

soon as the aforementioned signal is received, the transistor breaks the current of

the primary winding, thus inducing a 48/// C voltage on the terminals of the spark

plug that is powered by the secondary winding, which induces the reuired spark0*61.

The other sensor is located such that it is reached before the first one during the

compression stroke. Its operation is included in a subsystem that makes sure that even

though the first sensor is covered twice during the cycle Fone time during the

compression stroke and another in the expansion strokeG, ignition only occurs one

time, which prevents misfire. Its exact position is not as necessary as that of the first

sensor.

?ote that a variable ignition timing reuires in itself several months of research

to allow the creation of a proper ignition map that optimi:es the combustion process

of the engine to the greatest extents possible. However, the effect of such an ignition

on small engines is negligible which is why most of the commonly available

commercial engines feature a constant ignition timing advance0**1. Also note that a

delay of *.5ms takes place between the beginning of the ignition and its end to take

into account the dwell time of the coil Fthe time it takes the 5# circuit of the coil to

fully chargeMdischargeG0*61.


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%igure CI.( 2 Ignition Algorithm Implemented $n The !icrocontroller


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The following methodology was used in the numeric simulation to calculate

the timing advance reuired.

The combustion process consists of three phases2 flame formation, flame

propagation, and flame termination. The first two phases take some time to occur,

which is the time that is compensated by the ignition advance featured on all internal

combustion engines0**1.

Therefore, the duration of these two phases along with the dwell time of the

coil is the timing advance reuired for the engine. This timing advance is usually

expressed in terms of angular degrees on most engines. It will be determined in terms

of time for this free piston engine because it is a linear engine. This timing advance

will be determined at a freuency of *8 h:, which is the simulation freuency of this

linear free piston engine : study and design

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