29-11-2012, 05:58 PM
A review of free-piston engine history and applications
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Abstract
This document reviews the history of free-piston internal combustion engines, from the air compressors and gas
generators used in the mid-20th century through to recent free-piston hydraulic engines and linear electric generators.
Unique features of the free-piston engine are presented and their effects on engine operation are discussed, along with
potential advantages and disadvantages compared to conventional engines. The paper focuses mainly on developed
engines where operational data has been reported. Finally, the potential of the free-piston engine is evaluated and
the most promising designs identified.
Introduction
Extensive use of fossil fuels as an energy source
for both sea and land based transport leads to sig-
nificant amounts of CO2 and other pollutants be-
ing produced. Much research, particularly within
the automotive industry, is being undertaken to de-
velop more environmental friendly fuel chains, and
the fuel cell vehicle stands out as a promising tech-
nology for the future. Although superior in vehi-
cle efficiency, the implementation barriers for such
radical technology change are huge, and the com-
plete fuel chain (‘well-to-wheel’) efficiencies are not
yet superior to those of conventional technology [1].
Hybrid electric vehicles powered by conventional in-
ternal combustion engines have the potential to re-
alise large emission reductions within a significantly
shorter timescale.
The original
R.P. Pescara [2] is usually credited with the inven-
tion of the free-piston engine with his patent dating
from 1928, but other vendors, among others Junkers
in Germany, were also working on free-piston ma-
chinery at this time. Since then, a high number of
patents describing free-piston machinery or related
to such machinery have been published. 1 The orig-
inal Pescara patent describes a single piston spark
ignited air compressor but the patent seeks to pro-
tect a large number of applications utilising the free-
piston principle.
Pescara started his work on free-piston engines
around 1922 and he developed prototypes with both
spark ignition (1925) and diesel combustion (1928).
The latter led to the development of the Pescara
free-piston air compressor [4]. Pescara continued his
work on free-piston machinery and also patented a
multi-stage free-piston air compressor engine in 1941
[5].
Gas generators
Free-piston gas generators (or gasifiers) are free-
piston engines feeding hot gas to a power turbine.
The only ‘load’ for the engine itself is that to su-
percharge the intake air, the output work is taken
out entirely from the power turbine. Free-piston gas
generators were used in some large-scale marine and
stationary powerplants in the mid-20th century, and
attempts were made to use this principle in auto-
motive applications. Figure 4 illustrates an opposed
piston free-piston gas generator plant.
Frequency control
For an engine with a gas filled bounce chamber,
the spring-mass nature of the system means that
the frequency and stroke length are closely related.
The system will operate at its natural frequency,
and the pressure in the gas springs (i.e. spring stiff-
ness) can only be varied over a limited range. The
stroke length is strictly limited by the need for suf-
ficient compression and scavenging, hence there will
likely be limitations in the frequency control pos-
sibilities of the engine. This also limits the power
output range of such free-piston engines, which has
been noted by a number of authors as a problem
with the free-piston design.
For engines with different types of rebound de-
vices, very high levels of controllability can be
achieved. The best example of this is the Pulse
Pause Modulation (PPM) scheme for hydraulic
free-piston engines, presented by Achten et al. [13].
The same principle has been employed by several
other authors.
The combustion process
Some reports have indicated that the combustion
in free-piston engines benefits from the high piston
speed around TDC. This leads to higher air veloc-
ity and turbulence level in the cylinder, which ben-
efit air-fuel mixing and increase the reaction rate
and flame speed. The high piston acceleration just
after TDC leads to a rapid expansion, and time-
dependent chemical reactions, such as NOx forma-
tion, are potentially reduced.
Achten et al. [13] present experimental results
showing significantly faster combustion in the In-
nas Free-Piston Engine compared to conventional
engines. Values for ignition delay are found to be
comparable to those found in conventional engines.
Tikkanen et al. [9] present similar experimental
results, and both groups indicate that combustion
takes place predominantly in the premixed phase.
Free-piston air compressors
The original free-piston configuration proposed
by Pescara was an air compressor, and these ma-
chines proved to possess some very attractive fea-
tures. Despite the large research efforts on the free-
piston gas generator during 1940-1960, the air com-
pressor is by many considered to be the only really
successful free-piston engine concept. The excellent
performance of the air compressors was a strong con-
tributor to the later significant research efforts put
into the free-piston gas generator.
These engines were of the opposed piston type,
making them vibration-free. Farmer [4] discusses
how the interaction between the bounce chamber
and the compressor cylinders controls the compres-
sion energy delivered to the combustion cylinder and
makes the engine essentially self-regulating.
Despite its apparent good performance, the free-
piston air compressor did not gain widespread com-
mercial success. No reports of serious lacks or flaws
in the concept explaining this can be found, except
that the free-piston air compressor had a narrow
output range. Most reports are, in fact, of the oppo-
site opinion, such as McMullen and Payne [24] who
state that the free-piston air compressor has proved
’reliable and efficient under all conditions of service’.
Conclusions
The basic design and unique features of the free-
piston engine have been discussed. Although several
reports have confirmed that the free-piston engine is
a viable concept, more research is required to inves-
tigate potential advantages over conventional tech-
nology. The area with most uncertainty is clearly
that of piston motion control. Most of the poten-
tial advantages of the free-piston engine depend on
an appropriate control system being realisable. Op-
timisation of the combustion process and multi-fuel
or HCCI operation require compression ratio con-
trol, and elimination of vibrations by running sev-
eral units in opposite phases requires accurate con-
trol of engine speed.