Experimental and Numerical Investigation on Performance and Combustion Characteristics of a Free-Piston Engine Generator Fueled with Methane/Methanol

Methanol is a very promising clean alternative fuel with low carbon content and high octane number, and this makes it well suited to the free-piston engine generator (FPEG) with variable compression ratio characteristics. To the authors’ knowledge there are no relevant studies on the application of methanol for FPEG in recent literatures. In this paper, the effects of methanol substitution ratios (MSR) and load ratios on the performances and combustion characteristics of a methane/methanol dual-fuel FPEG have been investigated experimentally and numerically. The results show that the under the gas-liquid two-phase combustion startup strategy, the methane/methanol dual-fuel FPEG can be successfully started and achieve steady operation. Due to higher laminar flame speed (LFS) and the oxygen content of methanol resulting in a faster burning rate, the peak pressure increases by 43.9% from 1.344 MPa for pure methane to 1.934 MPa for 15% of MSR and the corresponding cycle-to-cycle variation decreases from 3.36 to 1.62. The FPEG operating frequency and indicated power gradually increase with the increase of the MSR. Both of them reach maximum values of 34.6 Hz and 193 W at 15% of MSR, which are increased by 19.3% and 49.6% in comparison with the pure methane. However, under specific MSR, both of them decrease with the load ratio increasing because of the large electromagnetic resistance force from the linear generator. CO and CH emissions decrease with the increase of MSR because methanol addition promotes complete combustion of mixture. NO_x emissions gradually decrease with MSR increasing owing to the low combustion temperature resulting from the high latent heat of methanol evaporation. The numerical results show that with the increase of MSR, the methane/methanol mixture presents faster flame propagation speed and higher combustion efficiency; while the ignition delay as well as CA10, CA50 and CA90 is significantly shortened due to the increase of active radicals such as H, OH, O and H₂O₂.