1.
“FREE PISTON
SUBMITTED IN PARTIAL FULFILMENT OF THE REQ
BACHELOR OF TECHNOLOGY
MECHANICAL ENGINEERING
RAJASTHAN TECHNICAL UNIVERSITY, KOTA
PACIFIC COLLEGE OF ENGINEERING, UDAIPUR,
A
Seminar Report
on
FREE PISTON LINEAR ENGINE”
SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIRMENT FOR AWARD OF
THE DEGREE OF
BACHELOR OF TECHNOLOGY
IN
MECHANICAL ENGINEERING
SUBMITTED TO
RAJASTHAN TECHNICAL UNIVERSITY, KOTA
SUBMITTED BY
(SHIVAM KUSHWAH)
13EPAME064
FEBRUARY, 2017
PACIFIC COLLEGE OF ENGINEERING, UDAIPUR,
RAJASTHAN
UIRMENT FOR AWARD OF
RAJASTHAN TECHNICAL UNIVERSITY, KOTA
PACIFIC COLLEGE OF ENGINEERING, UDAIPUR,

2.
i
DECLARATION
I, Shivam Kushwahhere by declare that this report is the record of authentic work carried by me
during the academic year 2017
Signature of the student
(H.O.D. Of Mechanical Engg.)

3.
ii
ACKNOWLEDGEMENT
We are greatly thankful to all those who have given me the opportunity of doing
Seminar in Pacific College of Engineering.
Also thanks for support by Mr. Akshat Singh Jhala for his unwavering assistance
in making Report on Stirling Engine.
We are very grateful for the encouragement, guidance and assistance that he
accorded us from the beginning of the Presentation to its successful completion.
Date..................... (Shivam Kushwah)

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iii
ABSTRACT
Free-piston engines are under investigation by a number of research groups
due to potential fuel efficiency and exhaust emissions advantages over
conventional technology. The main challenge with such engines is the control of
the piston motion, and this has not yet been fully resolved for all types of free-
piston engines.
This Report discusses the basic features of a single piston free-piston engine
generator under development at Newcastle University and investigates engine
control issues using a full-cycle simulation model. Control variables and
disturbances are identified, and a control strategy is proposed. It is found that the
control of the free-piston engine is a challenge, but that the proposed control
strategy is feasible. Engine speed control does, however, represent a challenge in
the current design.

5.
iv
INDEX
Declaration i
ACKNOWLEDGEMENTS ii
ABSTRACT iii
TABLE OF CONTENTS iv
1. Introduction 1
2. Principal 2
3. Features 2-3
4. Piston Configurations 3
4.1 Single Piston 4
4.2 Dual Piston 4
4.3 Opposed Piston 5-6
5. Applications 6
5.1 Free Piston Air Compressor 6-10
5.2 Hydraulic free Piston-Engine 10-11
5.3 Free Piston Engine Generator 11-12
Conclusion 13
Reference 14

6.
1
1. Introduction
The development of industry and technology had led to a massive energy
crisis, environmental pollution and consequently, high levels of fuel prices present a
great challenge for internal combustion engine designers. More research efforts are
put into engine technology to explore and study more efficient unconventional
engines, aiming at reducing engine emissions and improving efficiency. The free
piston engine is a kind of unconventional engine with the characteristics of
simplicity and operational flexibility, which draw a great amount of attention from
engine researchers. The advanced microprocessor-based control systems and
modern engine technologies significantly promote the development of this research
as a result of improved operational control of the free piston engine, along with
enhanced optimization possibilities for various operating conditions.
Literature Review
The free-piston engine, proposed by Pescara, during 1930‟s was a successful
alternative to conventional engines and gas turbines as it was suitable to be used as
air compressors or gas generators. However, as conventional engine and gas turbine
technology matured, the free-piston engine concept was abandoned in the early
1960s. After being abandoned, free-piston engines are being investigated by a
number of research groups worldwide as an alternative to conventional engine-
generator sets or for generating hydraulic power in off- highway vehicles. The
experimental analysis on hydraulic free piston engine. and free piston engine
generator a wide scope for such an unconventional engine in the future.
2. Principle
The piston is „free‟ because its motion is not restricted by the motion of a
rotating crankshaft, but that the piston is free to move between its endpoints, only
influenced by the gas and load forces acting upon it. By coupling with a suitable

7.
2
load device, it can be used as air compressor, gas generator, hydraulic pump or
generator.
This can be written as:
∑ F = mp (d2
x/dt2
)
Where FC is the combustion chamber force, FR bounce chamber /rebound force, FL
load force, x mover position and mp piston mass [4].
Free piston engines usually works on two-stroke compression ignition or spark
ignition operating principle, as a power stroke is required on every cycle.
3. Features
The free-piston engine has a number of unique features, some give it
potential advantages and some represent challenges that must be overcome for the
free-piston engine to be a realistic alternative to conventional technology.
As the piston motion between the endpoints is not mechanically restricted by
a crank mechanism, the free-piston engine has the valuable feature of variable
compression ratio, which may provide extensive operation optimization, higher part
Fig.3 Feature of Free Piston Engine

8.
3
load efficiency and possible multi-fuel operation. These are enhanced by variable
fuel injection timing and valve timing through proper control methods.
Variable stroke length is achieved by a proper frequency control scheme such as
PPM (Pulse Pause Modulation) control [1], in which piston motion is paused at
BDC using a controllable hydraulic cylinder as rebound device. The frequency can
therefore be controlled by applying a pause between the time the piston reaches
BDC and the release of compression energy for the next stroke.
Since there is few number of moving parts frictional losses and manufacturing cost
reduces. The simple and compact design thus requires less maintenance and this
increases lifetime.
The purely linear motion leads to very low side loads on the piston, hence lesser
lubrication requirements for the piston.
The combustion process of free piston engine is well suited for Homogeneous
Charge Compression Ignition (HCCI) mode, in which the premixed charge is
compressed and self-ignited, resulting in very rapid combustion, along with lower
requirements for accurate ignition timing control. Also, high efficiencies are
obtained due to nearly constant volume combustion and the possibility to burn lean
mixtures to reduce gas temperatures and thereby some types of emissions [4].
By running multiple engines in parallel, vibrations due to balancing issues may be
reduced, but this requires accurate control of engine speed. Another possibility is to
apply counterweights, which results in more complex design, increased engine size
and weight and additional friction losses.
With the absence of an energy storage device, like flywheel in conventional
engines, it will not be capable of driving the engine for several revolutions. Hence,
if the engine fails to build up sufficient compression or if other factors influence the
injection/ignition and combustion, the engine may stop. This result in misfiring and
the need for accurate speed control.
4. Piston Configurations
Free-piston engines are usually divided into three categories based on the
piston / cylinder configuration.

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4
4.1Single Piston
It consists of a combustion cylinder, a load device and a rebound device,
which accurately controls the amount of energy put into the compression process
and thereby regulates the compression ratio and stroke length. A simple design with
high controllability is the main strength of this design compared to others.
Fig. 4.1 Single piston hydraulic free-piston engine
4.2 Dual Piston
In this design, the need for a rebound device is eliminated since the working
piston itself acts as rebound device. This allows a simple and compact device with
higher power to weight ratio. The control of piston motion has proved difficult since
small variations in the combustion in either of the two cylinders will have high
influence on the next compression.
Fig. 4.2 Hydraulic dual piston free-piston engine

10.
4.3 Opposed Piston
This design consists of two single piston units with a common combustion
chamber. Each piston requires a re
coupled to one or both of the
pistons ensure symmetric piston motion. The main advantages are
Fig.4.3.1_Free_opposed_pistion_Hydraulic_Pump
5
This design consists of two single piston units with a common combustion
chamber. Each piston requires a re-bound device, and a load device may be
coupled to one or both of the pistons. Mechanical linkages connecting the two
pistons ensure symmetric piston motion. The main advantages are
Fig.4.3.1 Free opposed pistion Hydraulic Pump
This design consists of two single piston units with a common combustion
bound device, and a load device may be
pistons. Mechanical linkages connecting the two
pistons ensure symmetric piston motion. The main advantages are perfectly

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Fig.4.3.2 Free Opposed Piston Generator
balanced and vibration-free design and reduced heat transfer losses due to the
elimination of the cylinder head. But, it is seen that the need for a piston
synchronization mechanism together with the dual set of the main components
makes the engine complicated and bulky.
5. Applications
Since the free-piston engine was first developed around 1930, a number of
different designs have been proposed using the free-piston concept. The majority of
these were, however, not commercially successful. This section gives an overview
of known free-piston engine developments, with an emphasis on engines where
experimental results or operational performance data have been reported.
5.1 Free Piston Air Compressors
The original free-piston configuration proposed by Pescara was an air
compressor. These engines were of the opposed piston type, making them vibration-
free. In these engines, air compressor cylinders were coupled to the moving pistons,
often in a multi-stage configuration. Some of these engines utilized the air
remaining in the clearance of compressor cylinders to return the piston, thereby
eliminating the need for a rebound device.

12.
In the figure shown, during the expansion stroke, the air in the compressor
cylinder is compressed along with the sc
combustion chamber by the scavenging pump. In the compression stroke, which
results from the bounce chamber, the charge is compressed and ignited
One of the earliest successful free
described by Toutant, developed by German company Junkers and was used by
the German Navy during World War II to provide compressed
torpedoes. It had the advantages of high efficiency, compactness and low noise and
vibration. Superior fuel economy is possible due to the on/off control regulating the
delivery of compressed air, where conventional engine
The lack of success of the free
possible factors, including
Fig.5.1.1_Free_Piston_Air_Compressor
7
In the figure shown, during the expansion stroke, the air in the compressor
cylinder is compressed along with the scavenging of influx charge to the
combustion chamber by the scavenging pump. In the compression stroke, which
results from the bounce chamber, the charge is compressed and ignited
One of the earliest successful free-piston engine air compressors was
escribed by Toutant, developed by German company Junkers and was used by
the German Navy during World War II to provide compressed air for launching
. It had the advantages of high efficiency, compactness and low noise and
fuel economy is possible due to the on/off control regulating the
delivery of compressed air, where conventional engine-compressor sets run idle.
The lack of success of the free-piston air compressor may be due to some
possible factors, including
Fig.5.1.1 Free Piston Air Compressor
In the figure shown, during the expansion stroke, the air in the compressor
avenging of influx charge to the
combustion chamber by the scavenging pump. In the compression stroke, which
results from the bounce chamber, the charge is compressed and ignited [5].
piston engine air compressors was
escribed by Toutant, developed by German company Junkers and was used by
air for launching
. It had the advantages of high efficiency, compactness and low noise and
fuel economy is possible due to the on/off control regulating the
compressor sets run idle.
piston air compressor may be due to some

13.
8
(a) Stationary installations tended to use cheaper electric motors to drive
compressors.
(b) Demand for variable power output disfavored the narrow-output free-piston air
compressor for portable applications.
(c) Limited market for such applications discouraged the development of such
unconventional design.
Fig. 5.1.2 Pescara Free Piston Gas Turbine
In the late 1940‟s the free-piston gas generator attracted interest for use in
large-scale power plants, marine installations and later also for vehicle propulsion.
These engines were typically diesel powered, opposed piston engines with

14.
9
mechanical synchronization of the two pistons. The most important
advantages were: low fuel quality requirements, vibration-free design, good dynamic
response and low turbine material requirements due to lower inlet temperatures.
These engines were highly supercharged and operated on higher mean effective
pressures than conventional diesel engines.
The working of a free piston gas turbine starts with the influx air being
compressed by the piston that rebounds from the cushion cylinder. The pressurized
air then flows though the delivery valves to the combustion chamber, where
combustion occurs, producing the power stroke. The exhaust gases will be
scavenged by the further flow of fresh air and is passed to the turbine.
The model GS-34 free-piston gas generator manufactured by Soci´et´e
Industrielle G´en´erale de M´ecanique Appliqu´ee (SIGMA) in France was one of
the most successful free-piston engine gas generators ever made.
Still, free-piston gas generator never became a real competitor to either the
diesel engine or the gas turbine. Some of the reasons for its limited success are:
a) The mismatching of a pulsating-flow compressor with a continuous-flow turbine,
giving low part-load efficiency and limiting the gas generator to constant power
applications.
b) High failure rates and low lifetime were reported, related to high pressure and
temperature operation.
c) No large advantages in weight or fuel economy compared to conventional
engines and could not compete with the power to weight ratio of the gas turbine.

15.
5.2 Hydraulic Free-Piston
The hydraulic free piston engine integrates the
and the hydraulic pump. The energy released from combustion process is converted
directly into hydraulic energy. They may apply a hydraulically rebound device,
using part of the produced hydraulic energy to return the piston, or
chamber.
Experimental tests on the prototypes show generally good fuel economy and
high part load efficiency by using the PPM (Pulse Pause Modulation) control [1].
Many of the modern approaches in free
engines. Most of these units are aimed at vehicles such as fork
system. The piston assembly consists of the power piston, rebound piston and pump
piston which are rigidly connected by the piston rod. The HEUI (Hydra
Electronic Unit Injector)
reversible energy storage device to supply energy for the compression stroke.
Frequency control valve controls the supplied energy from the accumulator, which
determines the operational frequency of the engine.
Fig.5.2_The_hydraulic_free_piston_engine
10
Piston Engines
The hydraulic free piston engine integrates the internal combustion engine
and the hydraulic pump. The energy released from combustion process is converted
directly into hydraulic energy. They may apply a hydraulically rebound device,
using part of the produced hydraulic energy to return the piston, or
Experimental tests on the prototypes show generally good fuel economy and
high part load efficiency by using the PPM (Pulse Pause Modulation) control [1].
Many of the modern approaches in free-piston engine technology are hydraulic
engines. Most of these units are aimed at vehicles such as fork- lift trucks and earth
system. The piston assembly consists of the power piston, rebound piston and pump
piston which are rigidly connected by the piston rod. The HEUI (Hydra
nit Injector) is equipped for fuel injection. The accumulator is used as a
reversible energy storage device to supply energy for the compression stroke.
Frequency control valve controls the supplied energy from the accumulator, which
onal frequency of the engine.
Fig.5.2 The hydraulic free piston engine
internal combustion engine
and the hydraulic pump. The energy released from combustion process is converted
directly into hydraulic energy. They may apply a hydraulically rebound device,
using part of the produced hydraulic energy to return the piston, or a bounce
Experimental tests on the prototypes show generally good fuel economy and
high part load efficiency by using the PPM (Pulse Pause Modulation) control [1].
piston engine technology are hydraulic
lift trucks and earth-
system. The piston assembly consists of the power piston, rebound piston and pump
piston which are rigidly connected by the piston rod. The HEUI (Hydraulic
is equipped for fuel injection. The accumulator is used as a
reversible energy storage device to supply energy for the compression stroke.
Frequency control valve controls the supplied energy from the accumulator, which

16.
The working process can be described as follows: Hydraulic energy from
accumulator pushes the piston thereby compressing the gases in the combustion
cylinder along with the sucking of low pressure oil through the suction valve.
During the expansion stroke, high
valves by the pump piston and compression piston pushes the compression oil to
accumulator again.
Since the piston motion is determined by the instantaneous balance of the
cylinder gas pressure, the hydraulic forces and the friction forces, the exhaust valves
need to be driven accurately and quickly according to piston position. Since this
cannot be done by the c
valves system is developed and equipped in the cylinder head.
Thus, a number of prototypes have been developed in recent years and
experimental results from these are currently being reported. The D
Innas BV [1] is among the research leaders within free
They have developed a single piston, diesel powered, hydraulic free
5.3 Free-Piston Engine
The free piston engine generator essentially
coupled to a linear electric machine. The high efficiencies of electrical machinery,
along with flexibility and controllability, make this an interesting concept.
A driving force behind the interest and researches in free
generators is the increasing interest in hybrid
electric ship concept.
Fig.5.3.1_free_piston_linear_enginegenerator
11
The working process can be described as follows: Hydraulic energy from
accumulator pushes the piston thereby compressing the gases in the combustion
with the sucking of low pressure oil through the suction valve.
During the expansion stroke, high-pressure oil is outputted through the pressure
valves by the pump piston and compression piston pushes the compression oil to
iston motion is determined by the instantaneous balance of the
cylinder gas pressure, the hydraulic forces and the friction forces, the exhaust valves
need to be driven accurately and quickly according to piston position. Since this
cannot be done by the conventional camshaft timing system, the hydraulic exhaust
valves system is developed and equipped in the cylinder head.
Thus, a number of prototypes have been developed in recent years and
experimental results from these are currently being reported. The D
Innas BV [1] is among the research leaders within free-piston technology today.
They have developed a single piston, diesel powered, hydraulic free-piston engine.
Piston Engine Generators
The free piston engine generator essentially consists of a free
coupled to a linear electric machine. The high efficiencies of electrical machinery,
along with flexibility and controllability, make this an interesting concept.
A driving force behind the interest and researches in free
generators is the increasing interest in hybrid-electric vehicle technology and all
Fig.5.3.1 free piston linear engine generator
The working process can be described as follows: Hydraulic energy from
accumulator pushes the piston thereby compressing the gases in the combustion
with the sucking of low pressure oil through the suction valve.
pressure oil is outputted through the pressure
valves by the pump piston and compression piston pushes the compression oil to
iston motion is determined by the instantaneous balance of the
cylinder gas pressure, the hydraulic forces and the friction forces, the exhaust valves
need to be driven accurately and quickly according to piston position. Since this
onventional camshaft timing system, the hydraulic exhaust
Thus, a number of prototypes have been developed in recent years and
experimental results from these are currently being reported. The Dutch company
piston technology today.
piston engine.
consists of a free-piston engine
coupled to a linear electric machine. The high efficiencies of electrical machinery,
along with flexibility and controllability, make this an interesting concept.
A driving force behind the interest and researches in free-piston engine
electric vehicle technology and all-

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In this machine, the piston will move freely between its two endpoints, its
motion being determined by the instantaneous balance of cylinder gas forces,
electric machine force and frictional forces. The bounce chamber will be a closed
cylinder with pressure control valves to regulate the amount of gas trapped in the
bounce chamber, and thereby the gas pressure force on the piston. Thus the piston
motion is controlled.
The engine will operate on a turbocharged two-stroke diesel cycle with
direct injection / electronically controlled fuel injection like CRDI. Scavenging is
provided through scavenging ports in the cylinder liner and electro-pneumatic
exhaust poppet valves in the cylinder head.
The linear electrical machine is taken as a permanent magnet machine. By
employing appropriate power electronics, this setup may allow the use of the
electric machine in motoring mode to aid engine control and for starting.
Fig.5.3.2 Free Piston Power Pack with the specifications.
The design of the electric machine will have a high influence on the
performance since the alternator translator forms a part of the moving mass.
Increasing the moving mass reduces the bouncing frequency of the system, whereas
a low moving mass is expected to give high engine speeds. Also, due to low speed,
higher mass have more time available for scavenging, allowing lower scavenging
ports height and nearly constant volume combustion. This increases the efficiency.
But the resulting high temperatures and pressures causes appreciable heat transfer
losses. Hence an appropriate weight is required in order to achieve an acceptable
power to weight ratio and optimum efficiency.

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Conclusion
Free piston engines have been proved as a promising technology with several
developments showing favorable performance compared to conventional technology
by fully utilizing the powers of modern, microprocessor-based, control techniques.
Although several reports have confirmed that the free-piston engine is a viable
concept, with further work focusing on the parameters that influences engine
performance, such as optimal control of piston motion, combustion process,
emissions measurement etc., free piston engines can be developed as an alternative
for conventional engines.

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References
1. R. Mikalsen, A.P. Roskilly, “A computational study of free-piston diesel engine
combustion”, Applied Energy, 2009
2. https://www.google.co.in
3. https://www.bing.com