Without a crankshaft mechanism, the piston in such an engine moves freely in the cylinder liner; piston dynamics are determined by the interaction of all external forces applied to the piston. Free piston engines are simple in structure and offer low friction losses and high operational flexibility.

Major design parameters for free-piston engines include the optimization of moving mass; the operating frequency (mean piston velocity); port design; and mechanical load tolerance due to high compression ratio.

Free-piston engines support multi-fuel combustion and the use of advanced combustion technologies such as HCCI. One study cited by the Korea researchers found that a free-piston engine with a high compression ratio up to 30:1, auto-ignited fuels with high octane number, such as propane or natural gas, reaching an indicated thermal efficiency of 56%.

Despite the … advantages of the free piston engines, they require complex control due to lack of energy storage device and the variations in stroke. Several control schemes, such as the prediction of TDC and BDC with respect to engine control parameters and combustion mode transition between spark ignition (SI) and HCCI, were proposed and modeled. However, the number of related studies or analysis with experimental results were few due to the difficulties in realization. Though the lack of control was not too critical just to run the engine, any misfire or abnormal combustion may cause the engine to stop operating immediately. The engine may also run at an inefficient operation zone if no correction control logic is implemented. A simple control of the dual-piston type configuration was proposed and investigated by correcting the compression ratio of the engine with fuel mass flow. However, further improved control of the piston dynamics are needed to be fulfill various load conditions and transient operation in dual-piston type engines.

The objective of the study is to reveal the factors which are considered to be crucial to the engine performance and efficiency.

—Kim et al.

Diagram of the engine under study. There are three basic types of free-piston engine: single-piston, opposed-piston, and dual-piston type. For the study, the researchers used a dual-piston configuration, with three pairs of piston assemblies connected to a common rod. The piston and the cylinder liner were made of ceramic to minimize friction and heat losses. The chambers in the middle are the combustion chambers. Four separate fresh charge compressors on both sides of the combustion chambers inhale the fresh charge through the reed valve into the compressor chambers and pump the fresh charge into the opposite combustion chamber for the successive cycle. Springs installed in each compressor chamber prevented the piston reaching the TDC and colliding with the cylinder head. Kim et al. Click to enlarge.

For their experiments, the researchers used liquefied petroleum gas (LPG) consisting of 98% propane. (They chose a gas fuel for better mixture preparation with air due to limited operation range of the engine.)

They then studied the effects of air flow rate and the alternator load on the operation characteristics of the free piston engine. The experiments were conducted under fixed spark timing of 4 mm before top dead center (BTDC) and an equivalence ratio of 1.14. Among their main findings:

• The indicated mean effective pressure (IMEP) and the electric output decreased as the air flow rate was decreased. Optimum values existed for IMEP and the electric output with respect to the air flow rate and the conductance. The piston dynamics were dependent on the conductance due to the changes in the induced current and voltage.

• Piston dynamics (instantaneous piston velocity and the stroke) was the major factor influencing the shift of optimum values in IMEP when varying the conductance. The change in the piston dynamics caused the deviation of the minimum ignition advance for best torque (MBT) timing. In other words, higher compression ratio (longer stroke) does not mean higher engine efficiency without optimizing the spark timing condition.

• The piston stroke and frequency decreased as the air flow rate decreased due to the reduced amount of delivered fuel. They also decreased monotonically as the conductance increased. The increased resistive force due to the increased current restrained the piston velocity and the stroke. The changes of piston stroke and the frequency were coupled together in the study. The maximum piston velocity increased along with the operation frequency, so the stroke also needed to be increased to provide a longer distance to decelerate or accelerate the piston.

• The optimum conductance ranges for the highest overall engine efficiency and the engine-to-electric conversion efficiency of the linear alternator did not coincide. The work conversion efficiency can be improved so that it matches with the conductance region for the highest engine-to-electric conversion efficiency. The researchers expect this to be achievable by implementing a transient control. The monitoring of the piston velocity is considered to be the key for the best engine performance.

The optimum conductance ranges for the highest overall engine References efficiency and the engine-to-electric conversion efficiency of the linear alternator did not coincide. It can be achieved by implementing a transient control. The monitoring of the piston velocity in real-time is considered to be the key for the best engine performance. It is expected that the overall operation characteristics versus the engine control parameters will still remain in similar manner with the current experimental results even after some improvements in the engine. The operation characteristics studied in this paper can be provided as a reference for the future optimization work.

— Kim et al.

Resources

• Jaeheun Kim, Choongsik Bae, Gangchul Kim (2016) “The operation characteristics of a liquefied petroleum gas (LPG) spark-ignition free piston engine,” Fuel, Volume 183, Pages 304-313 doi: 10.1016/j.fuel.2016.06.060