Effect of the stroke-to-bore ratio on the performance of a dual-piston free piston engine generator

The free piston engine generator (FPEG) is considered as one of the next generation efficient energy conversion device because of its compact structure, high geometric power ratio and low pollution. This paper investigated the effect of stroke-to-bore (S/B) ratio on the system operation characteristics and engine performance, constructed a detailed numerical model in MATLAB/Simulink and verified the experimental data whose difference value could be controlled within 5%. The effect of five S/B ratios (0.84, 0.91, 0.99, 1.07 and 1.14) and three compression ratios (8, 9 and 10) was analysed at a constant bore diameter. The simulation results indicated that the operation frequency increased from 28.2 Hz to 48.3 Hz when the S/B ratio decreased from 1.14 to 0.84. The highest indicated power is 4.1 kW when the S/B ratio is 0.84 and the compression ratio (CR) is 10. While for high thermal efficiency and fuel economy design, larger S/B ratio and higher operating compression ratio should be selected while keeping the periodic energy input unchanged. The heat transfer loss decreased from 29.0% to 20.4% when the S/B ratio increased from 0.84 to 1.14. And in the long stroke, ignition position needs to lean back (from 6.8 mm to 24.8 when S/B increased from 0.84 to 1.14) so as to keep the compression ratio unchanged under different S/B ratios.