Review of the Free Piston Linear Engine (FPLE) - E-Roadster

Journal of Advanced Engineering Research
ISSN: 2393-8447
Volume 1, Issue by: ASME, 12/10/ 2016
Research Article 1 snehishivam29@gmail.com
E-ROADSTER
-FREE PISTON LINEAR ENGINE [FPLE] - A REVIEW
Sneha Mishra1
, Shivam Snehi2
1
Student of Mechanical Engineering, AKTU, U.P, Indi
2
Student of Mechanical Engineering, AKTU, U.P, India
*Email: snehishivam29@gmail.com , Mob. : +919457918223
ABSTRACT
Unlike conventional internal combustion engines, E-Roadster ,a free-piston linear engine has no a crankshaft, and thus the
pistons move freely in the cylinder. This allows a free-piston linear engine to easily adjust the compression ratio and
optimize the combustion process. Free-piston linear engines include two main parts: a free-piston engine and a linear
alternator. The linear alternator is generally categorized as flat-type or tubular-type. Free-piston linear engines can operate
with multi-fuel and HCCI combustion because of their variable compression ratios. Furthermore, they are used to generate
the electric power applied in hybrid electric vehicles. To promote understanding of the unique features of free-piston linear
engines, this paper presents a review of their different designs and operating characteristics. We also discuss the varied
experimental systems and applications of free-piston linear engines
Keywords – FPLE, Free piston, Linear engine, FPEG/FPLG
1. INTRODUCTION
The free-piston linear engine (FPLE) is a linear energy
conversion system, and the term ‘free-piston’ is widely
used to distinguish its linear characteristics from those of
a conventional reciprocating engine. Without the
limitation of the crankshaft mechanism, as known for the
conventional engines, the piston is free to oscillate
between its dead centers. The piston assembly is the only
significant moving component for the FPLEs, and its
movement is determined by the gas and load forces
acting upon it. During the operation of FPLEs,
combustion takes place in the internal combustion
chamber, and the high pressure exhaust gas pushes the
piston assembly backwards. The chemical energy from
the air fuel mixture is then converted to the mechanical
energy of the moving piston assembly. Due to this linear
characteristic, a FPLE requires a linear load to convert
this mechanical energy for the usage of the target
application. As the load is coupled directly to the piston
assembly, the technical requirements for the free-piston
engine loads are high, which are summarized as:
(1) The load must provide satisfactory energy conversion
efficiency to make the overall system efficient.
(2) The load may be subjected to high velocity
(3) The load may be subjected to high force from the
cylinder gas.
(4) The load device may be subjected to heat transfer
from the engine cylinders
(5) The size, moving mass and load force profile are
feasible to be coupled with the designed FPEs.
Reported load devices for the FPEs include air
compressors, electric generators and hydraulic pumps. In
this research, the FPE is connected with a linear electric
generator (free-piston engine generator, FPEG) and is
investigated with the objective to utilize the
configuration within a hybrid-electric automotive vehicle
power system. Since the FPEG was first proposed, it has
attracted interest from all over the world.
Different research methods and prototype designs
have been reported using the FPLE concept. However, to
date, none of these have been commercially realized in
part due to the challenges of system control. In
conventional engines, the crankshaft mechanism
provides piston motion control, defining both the outer
positions of the piston motion (the dead centers) and the
piston motion profile. Due to the high inertia of the
crankshaft system, the piston motion cannot be
influenced in the timeframe of one cycle. In the free-
piston engine, the piston motion is determined by the
instantaneous sum of the forces acting on the mover, and
the piston motion is therefore influenced by the progress
of the combustion process. Moreover, the piston motion
profile may be different for different operating

Sneha Mishra , / Journal of Advanced Engineering Research, 2016
conditions. Variations between consecutive cycles due to
cycle-to-cycle variations in the in-cylinder processes are
also possible. Overcome controlling of the FPLE engine
is a challenging task.
2. Classification of FPLEs
2.1. NUMBER OF STROKES
Similar to traditional internal combustion engines,
FPLEs are classified into four-stroke and two-stroke
engines. The strokes of a four-stroke FPLE are intake,
compression, combustion, and exhaust. In a traditional
internal combustion engine with a crankshaft
mechanism, the four strokes happen in two revolutions
of the crankshaft, and the combustion stroke is called the
power stroke. For FPLEs, the four strokes occur in the
linear motion of the piston, and the intake and exhaust
valves are controlled by an electronic system. Xu and
Chang [31] studied the motion control of a four stroke
FPLE developed for electric power generation. The
piston strokes combined with the open/close timing of
the intake and exhaust valves were electronically
controlled. Even though the four-stroke principle can be
applied to FPLEs, it presents greater technical challenges
for motion control than two-stroke engines.
The technical challenges for motion control of the
four-stroke FPLE include the complex control of the
opening/closing times of the intake and exhaust valves
vis-à-vis the linear motion of the piston. The
opening/closing times of the intake and exhaust valves
must be controlled correctly to prevent a collision
between them and the piston crown. Therefore, four-
stroke FPLEs have been investigated less than two-
stroke FPLEs, which simplify the engine structure and
improve motion control. Jia et al [32]. simulated the
piston dynamics and thermodynamics of a two- or four-
stroke FPLE. For the two-stroke cycle,
the linear generator was used only as a generator,
whereas it functioned as both a motor and a generator in
the four-stroke cycle. They found that the piston speed
during the expansion process of the four-stroke cycle was
higher than that of the two-stroke cycle. However, for the
non-power strokes of the four-stroke cycle, the piston
speed was much lower because of the brake force of the
motor. The heat release process was more aligned with a
constant volume process when the FPLE operated in
two-stroke mode, and the peak cylinder pressure of four-
stroke cycle was higher than that of the two-stroke cycle.
This can be explained by increasing of piston
displacement in the four-stroke cycle. As can be seen in
the displacement of the piston in the four-stroke cycle
was significantly longer than that in the two-stroke cycle
because in the four-stroke cycle, piston movement could
be controlled by optimizing the motor forces. To ensure
stable and smooth engine operation using a four-stroke
cycle, the authors proposed a more complex and robust
control system. Their simulation results also indicated
that the indicated power and electric power of the two-
stroke cycle were much higher than those of the four-
stroke cycle with the same throttle opening.
Because the electric power generated in the four-
stroke cycle was used to compensate for the overall
power consumption during the motoring processes. The
strokes of the most typical two-stroke FPLE are
scavenging compression and combustion–expansion.
The scavenging process occurs in different ways
depending on the engine type. Goldsborough and
Blarigan [33] presented an optimal study for the
scavenging system of a two-stroke FPLE. They
investigated a wide range of design options, including
loop, hybrid-loop, and uniflow scavenging methods. The
uniflow method uses the exhaust valve to liberate exhaust
gas during the scavenging process. Locating the exhaust
valves in the cylinder head ensures better flushing at the
top of the combustion chamber, but increases the
mechanical complexity of the engine because the valves
must be actuated. Two stroke FPLEs using the uniflow
scavenging method are also found in other studies.
2.2. PISTON CONFIGURATION

In general, FPLEs can be classified into three piston
types: single piston, double piston (dual piston and
opposed piston), and four pistons (dual piston, opposed
piston, and complex piston configuration), Of those, the
single-piston engine has a simple design with higher
controllability than the other FPLEs; however, the
dynamic balance is not good because it has only one
piston. Mikalsen and Roskilly [16] proposed a prototype
of a single-piston FPLE for electric power generation in
large scale systems. Their engine includes a combustion
cylinder, a bounce chamber cylinder, and a linear electric
machine.
In this engine, the amount of air contained in the
bounce chamber is varied by control valves to change the
force coming from the bounce chamber. Tian et al.
replaced the bounce chamber with a rebound spring. This
allowed a simpler design, compared with the design of
Mikalsen and Roskilly [17] So far, the single-piston
FPLE is the closest to a commercial system because it
offers the simplest configuration and high controllability.
Kosaka et al [40]. developed a prototype single piston
FPLE using a cooling and lubricating system along with
control system logic, which contributed significantly to
commercialization of an FPLE. Their single piston FPLE
used a cooling oil passage and a water-cooled cylinder
head. A perfectly balanced design is the main advantage
of opposed piston configurations, but those designs make
engines complicated. Pontus Ostenberg [5] presented an
early opposed-piston FPLE in 1943, Therein, a denotes a
free-piston engine with opposed pistons (piston 2 and
piston 2a), and B denotes a single-phase linear alternator.
In Pontus Ostenberg’s [5] engine.
3.2. Combustion characteristics
3.2.1. Spark ignition combustion
Similar to a traditional internal combustion engine, an
SI FPLE uses spark plugs installed in the cylinder head
to ignite the air/fuel mixture in the cylinder when
generating power. To investigate the combustion
characteristics of an SI FPLE, many studies have been
conducted, including both simulations and experiments.
Mikalsen and Roskilly [15] compared the performance
of an SI-FPLE with that of a conventional engine using
a computational fluid dynamics (CFD) simulation
model.
They showed that the FPLE obtained a slight efficiency
advantage over the conventional engine at low speeds,
but that the efficiency of the free-piston engine dropped
as the speed increased because the effects of volume
change during combustion were greater at higher
speeds.
They also found that the free-piston engine is lower
than that of the traditional hydrogen engine, this engine
had a slight benefit in NO emissions when compared
with the conventional engine, Because the shorter time
spent around TDC and the faster expansion in the free
piston engine influenced the NOx levels Yuan et al also
showed a lower level of NO emissions in a free-piston
hydrogen engine compared with a traditional hydrogen
engine. Because the mean in-cylinder gas temperature
of the free-piston hydrogen x
3.2.2. Compression ignition combustion
CI in an internal combustion engine is a process in which
the necessary high temperature is produced by
compressing the air in the cylinder before the fuel is
injected into the combustion chamber. For FPLEs, CI is
generally investigated with diesel fuel Mao et al.
presented a simulation study of a free-piston diesel
engine using a zero-dimensional numerical simulation
combined with a CFD model (AVL-FIRE) to simulate
the gas exchange and combustion processes. They used
the two-stage Wiebe function to model the combustion
process in time, one stage for premixed and one stage for
diffusive combustion. They derived the ignition delay
and combustion duration from the CFD calculation for
diesel FPLE combustion.
In another simulation study, Mikalsen and Roskilly
[11] investigated the combustion process of a free-piston
diesel engine using a CFD model (Open FOAM) and
compared the results with those from a conventional
engine. They found that the free-piston diesel engine had
a higher heat release rate from the pre-mixed combustion
phase because of an increased ignition delay, compared
with the conventional engine. In another simulation
study conducted by Mikalsen and Roskilly [15], they
compared the simulation results of a two-stroke free
piston CI engine with those from a respective
conventional CI engine. Therein, a single-zone model

was used to simulate combustion, while in-cylinder heat
transfer was modeled according to Hohenberg. They
found that the indicated efficiency of the free-piston
engine was higher than that of the conventional engine
because of reduced heat transfer losses and lower
frictional losses. Both peak gas temperature and
temperature levels during expansion were lower in the
free-piston engine, and that resulted in lower heat
transfer losses.
Yuan et al investigated the combustion characteristics
of a free-piston diesel engine coupling with dynamic and
scavenging models. Their coupled model used an
empirical heat release model of the Wiebe function to
calculate the piston motion profile based on the initial
boundary conditions. They used a scavenging CFD
model to calculate the gas exchange performances
according to the calculated piston motion. They then
imported the calculated scavenging results and piston
motion into a combustion CFD model to calculate the
combustion performances and fed those results with the
gas exchange results back to the dynamic model to
calculate the next iteration.
Afterward, they’re established the scavenging CFD
model and calculated a new using the updated results
from piston motion and combustion, repeating the
procedure until they met the iterative convergence
conditions. Their simulation results showed benefits for
reducing temperature dependent emissions (NO) because
the in-cylinder average gas temperature of the free-piston
engine was generally lower than that of the traditional
engine. This is also similar to the results obtained by
Mikalsen and Roskilly [11] However, Chenheng Yuan
found that a free-piston engine had no advantage in
particulate emissions when compared with a traditional
crank engine, Shoukry et al. presented a numerical
simulation for a parametric study of a two-stroke direct-
injection linear engine fueled with diesel. They
investigated the effects of parameters such as load
constant, reciprocating mass, injection timing, and
combustion duration on the dynamic and combustion
characteristics of an FPLE, defining injection timing as
piston position before the maximum possible stroke.
To simulate the combustion process, they used the
Wiebe function converted to time and calculated the heat
transfer based on the Woschni model. Their simulation
results showed that the increased reciprocating mass
increased the piston stroke and peak in-cylinder
combustion pressure by increasing the inertial force. The
change of injection timing also contributed to increasing
the peak in-cylinder combustion pressure. Adjusting the
injection timing closer to the maximum stroke led to
higher in-cylinder combustion pressure because of
moving the combustion event toward that of the ideal
Otto case.
3.3.1 Homogenous charge compression ignition
Homogenous charge compression ignition (HCCI)
engines compress a premixed charge until it self-ignites,
resulting in very rapid combustion but with poor control
of ignition timing. The free-piston engine is well suited
for this since the requirements for accurate ignition
timing control are lower than in conventional engines.
Potential advantages of HCCI include high efficiencies
due to close to constant volume combustion and the
possibility to burn lean mixtures to reduce gas
temperatures and thereby some types of emissions.
HCCI operation of free-piston engines has been
attempted by among others Aichlmayr and van
Blarigan[48]. A quasi-HCCI approach is mentioned by
Hibi and Ito. Diesel fuel is injected very early in the
compression process but after the intake and exhaust
ports have closed. The fuel does not ignite at injection
because the temperature
4.1 Basic operational characteristics
The engine will have characteristics similar to a dual
piston engine, as described above. However, replacing
one firing cylinder with a variable-pressure bounce
chamber adds a control variable to the engine, but with
the cost of reduced engine power density and higher
frictional losses. Starting of the engine is done using
stored electric energy and running the electric machine
in motoring mode.
Table 1 Main free-piston engine specifications and
predicted engine performance [12].
Stroke 0.150m
Bore 0.131m
Mover mass 22kg
Nominal compression ratio 15:1
Nominal speed 30Hz
Output power 44.4kW
Engine efficiency 0.42
The electric machine will drive the mover back and forth
to build up sufficient compression for fuel to be injected.
When running, the engine will resemble a spring-mass
system; the bouncing frequency and the endpoints of the
motion will depend on the moving mass and the stiffness
of the springs.
4.2.1 FPLG-Subsystems
The FPLG consists of the three subsystems ’internal
combustion engine’, ’linear generator’ and ’gas spring’.
The subsystems have been developed over the last years
and have now reached a stage of development allowing
them to be used in a complete FPLG-system. The
subsystems have been tested up to frequencies of 30 Hz.
The specifications of the subsystems are summarized in
Tables 2 & 3. Theoretical and experimental
investigations have been performed in [4] and [5]. In [6],
the development of the linear generator has been
presented.

Table 2: Specifications of the internal combustion
engine
Bore 82.5 mm
Stroke 40-95 mm
No. Inlet Valves 2
No. Outlet Valves 2
Inlet Pressure 0-3 bar
Fuel Pressure 100 bar
Valvetrain electromagnetic
Injection direct injection
Injector swirl injector
The internal combustion engine is based on a two-stroke
concept. The cylinder head is equipped with two inlet
and two outlet valves. The valves are actuated by means
of an electromagnetic valve train, which allows for
individual valve timing. In order to achieve low HC
emissions, fuel is injected directly after the exhaust
valves close. The cylinder dead volume at TDC has been
minimized in order to achieve a sufficient compression
ratio at low strokes.
Table 3: Specifications of the linear generator
Force 3620 N
Design Current 113 A
Maximum Force 6120 N
Maximum Current 235 A
Force Density 54 kN/m2
Motor Parameter 32 N/A
Time Constant (0-90) 6.6 ms
The linear generator has been designed for maximum
force density and at the same time minimum time
constant. Since the linear generator will be used for
controlling the fully-autarkic FPLG-system, a highly
dynamic behaviour of
the generator is essential. The design force of the
generator is sufficient to extract the energy brought into
the system by the combustion. In order to minimize costs
and the weight of the piston rotor, the energy is extracted
during the compression and expansion stroke of the
system.
4.2.2 Engine simulation model
An advanced full-cycle simulation model of the free-
piston engine generator was presented in [12]. The piston
dynamics are solved numerically using single-zone
submodels for engine combustion, heat transfer, gas
properties, frictional losses, etc. A scavenging model
allows the thermodynamic consequences of poor
scavenging to be taken into account, which is crucial for
investigations into engine dynamic performance such as
those presented here. The model accounts for factors
such as the effects of varying compression ratio on
ignition delay and combustion, and the effects of changes
in BDC position on the scavenging and the following
combustion process. The model therefore provides a
powerful tool for investigating engine control issues. All
investigations in this paper are based on predictions from
this engine model. For further details of the model the
reader is referred to [12].
5.1. Applications of FPLE
FPLEs are used to convert chemical energy stored in fuel
into electrical energy. They have been investigated and
developed by scientists and researchers around the
world. The high efficiency of a linear alternator
combined with the simple structures of a free-piston
engine are prompting researchers to further develop
FPLEs for hybrid electric vehicles (HEVs). A group of
authors from General Motors and West Virginia
University provided an integrated design methodology to
select a free-piston engine and linear alternator
combination for use as an HEV auxiliary power unit.
They developed integrated models of the engine and
linear alternator and simulated the electric power output
while varying system parameters. They also presented an
optimization method for selecting the design that best
met output voltage and power requirements.
Goertz and Peng[13] reviewed feasible hybrid
powertrain concepts, evaluating them based on
additional weight, power per size, fuel efficiency,
reliability, local emissions, production costs, comfort,
safety, and development risk. They found that a free-
piston engine coupled with a linear alternator and battery
was the most promising candidate for a high-efficiency
hybrid vehicle. In a simulation study, Huang developed
an opposed-piston FPLE for an HEV. The simulation
results showed that the newly designed FPLE was
feasible and could obtain a 15 kW average electric power
output with a generating efficiency of 42.5%.
Carter and Wechner[14] designed an FPLE to meet
the highest levels of fuel efficiency and exhaust
emissions performance in a compact size for use in
HEVs. Their FPLE was a combination of a free-piston
engine and an integral generator and included an integral
compressor and a passive intake valve in the head of the

piston, which eliminated common FPLE problems such
as piston ring wear and the need for an external
compressor, and allowed a significant increase in power
density. Cosic et al. compared the total efficiency of a 12-
ton truck HEV using a conventional combustion engine
and an FPLE. They found that replacing a conventional
combustion engine with an FPLE increased the total
efficiency of the system by 25%. Hansson et al
investigated the performance gain achieved by using an
FPLE in a medium-sized HEV, compared with a
conventional diesel-generator, and found a potential
decrease in fuel consumption of up to 19% when using
the equivalent consumption minimization strategy
(ECMS),
A group of researchers at Toyota Central R&D Labs
Inc. is developing a prototype 10 kW FPLE for electric
drive vehicles with a thin and compact design, high
efficiency, and high fuel flexibility. This prototype
includes a two stroke combustion chamber, a linear
alternator, and a gas spring chamber. Its main feature is
a stepped piston shape that Toyota calls a ‘‘W-shape”
that has advantages such as decreased heat loss from the
gas spring chamber, a hollow structure to ensure piston
cooling, improved generating efficiency because of a
small clearance between the magnet and the coil, and a
heated magnet to prevented degaussing.
6. CONCLUSION
In this paper, we have reviewed and summarized the
literature on FPLEs with varied designs and operating
features. For piston stroke type, two-stroke FPLEs are
most-commonly investigated and developed because of
their advantages in structure and control. Published
results show that dual-piston FPLEs have a higher
power/weight ratio than other piston arrangements.
However, the combustion process occurs alternately in
each cylinder in a dual-piston engine, which leads to
varied combustion pressure at each cylinder and engine
cycle.
Meanwhile, single-piston FPLEs have a simple
design with higher controllability than the other FPLEs;
however, the dynamic balance is not good because they
have only one piston. Unlike single-piston FPLEs, a
perfectly balanced design is the main advantage of
opposed-piston FPLEs, but those designs make engines
complicated. Besides description of various piston types,
we also described different linear alternator designs for
FPLEs. Namely, we classified linear alternators into
three main groups, including linear alternator shapes
(flat-type and tubular-type linear alternators), phase
structure (single-phase and three-phase linear
alternators), and arrangements of magnets (moving-
magnet, moving-iron, and moving-coil linear
alternators). In a simulation study, flat-type linear
alternator is considered to be better than tubular one in
efficiency, specific power, output voltage and current;
however, it needs to be further examined by both
simulation and experiment. For phase structure, much
research has shown that three-phase linear alternators are
appropriate for high-power FPLEs, whereas single-phase
linear alternators are suitable for small power FPLEs. In
addition to the designed features, we classified FPLEs by
their operating characteristics, such as piston dynamics,
combustion, and electric power generation
characteristics.
For piston dynamics, FPLEs decrease heat transfer loss
in the cylinder by increasing piston acceleration,
compared with conventional engines. The
implementation of springs in FPLEs shows benefits for
increasing piston velocity and engine performance. In
addition to benefit of piston dynamics, published results
show that the thermal efficiency of FPLEs is higher than
that of conventional engines. Furthermore, the
simulation results of FPLEs show benefits for reducing
temperature-dependent emissions (NO) because the in
cylinder gas temperature of FPLEs is generally lower
than that of conventional engines. X The variable
compression ratio in FPLEs is a great benefit for
combustion. By changing the compression ratio, FPLEs
can optimize the combustion process and operate with
various kinds of fuels and HCCI combustion

To obtain successful HCCI combustion in a free-
piston engine, simulation studies have utilized the
transition from SI to HCCI combustion. Published results
show that the engine performance in HCCI combustion
is higher than in SI combustion, while the in-cylinder
peak temperature in HCCI combustion is much lower
than that in SI combustion, which results in decreasing
NO emissions. A free-piston engine can not only be
operated as a conventional x internal combustion engine.
It can also be integrated with a linear alternator to
generate electric power. The electric power can be
optimized by adjusting parameters such as piston
assembly mass, ignition timing, equivalence ratio,
electrical resistance, and air gap. Much research has
shown that a linear alternator with a high efficiency
power source is an excellent power-unit candidate for
HEVs. With the potential offered by high-efficiency
linear alternatorsin FPLEs, we expect integrated systems
to be further developed applied in the near future.
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