Initial Study on a Mechanical Starting Mechanism for Two-Stroke Free Piston Engine
Nguyen Huynh Thi, Nguyen Van Trang, Huynh Thanh Cong, Dao Huu Huy, Huynh Van Loc, Truong Hoa Hiep
Transactions on Machine Design, Vol. 13, No. 2, 2025.
DOI: https://doi.org/10.6025/tmd/2025/13/2/68-79
Link: https://www.dline.info/tmd/fulltext/v13n2/tmdv13n2_3.pdf
Abstract:
The Free-Piston Engine (FPE) offers a flexible alternative to conventional Internal Combustion Engines (ICE)
by allowing variable compression ratio control and eliminating the crankshaft. The advantages of FPEs lie in
their simple design with fewer moving parts, resulting in a compact engine with lower maintenance costs,
reduced friction losses, and improved fuel efficiency. However, the main technical challenges to adopting this
technology are engine startup and stable operation. This study proposes a dual two-stroke FPE model using
gasoline fuel and spark ignition. An independent starting mechanism for the FPE was developed, which does
not utilize an integrated linear motor as in previous studies. The simulation results show that with a starting
speed of 0.7 m/s, a starting compression ratio of 3.3, fuel injection of 3 mg, and a combustion duration of 2 ms
to 5 ms, the FPE can ignite with pressures ranging from 9 bar to 11 bar. The mechanical resonance starting
method was experimentally validated, achieving the desired compression ratio of 3.3, a fixed electric motor
pull force of approximately 400 N over 1.4 seconds, and a piston displacement speed of about 0.7 m/s. The
pressure during the startup process was around 4 bar, and the ignition process produced a peak pressure of 13
bar. The FPE model then transitioned to continuous operation over several cycles with peak cylinder pressures
reaching 12 bar.