A comparison test of the effect of piston with different squish ratios and compression ratios on combustion thermal efficiency and exhaust emissions was carried out on a stoichiometric combustion natural gas engine with exhaust gas recirculation(EGR) strategy. Results showed that at the speed of 1400 to 1800r/min and with 50% or 75% of rated load

increasing the EGR rate extended the knock boundary

thus advancing the angle of 50% mass fraction burned(CA50)

and increasing the indicated thermal efficiency. At the speed of 2300r/min and with 75% or 100% of rated load

increasing the EGR rate could significantly extend the combustion duration

thus retarding the CA50 and reducing the thermal efficiency. On the other hand

increasing the compression ratio and appropriately enhancing the squish ratio was beneficial to improve the turbulent motion in the cylinder and accelerate the flame propagation speed of natural gas

so that the CA50 was closer to the top dead center

which improved the thermal power conversion efficiency coupled to the increase of the highest indicated thermal efficiency by 0.24% and hence NOx and CH4 emissions by 2.30g/(kW·h) and 0.55g/(kW·h)

respectively. Further increase in squish ratio would be limited by knock

resulting in delayed maximum brake torque timing and extended combustion duration

which would reduce the maximum indicated thermal efficiency by 0.51% and decrease NOx and CH4 emissions by 2.20g/(kW·h) and 0.44g/(kW·h)

respectively

coupled to the increase of CO by 0.36g/(kW.h). Therefore

there was a range of squish ratio that needs to be optimized.