Energy conversion path improvement by optimizing motion profile on a two-stroke rod-less opposed pistons engine

•A cam profile model was established to study its effect on energy conversion path.•Modifying cam profile can redistribute and optimize the energy conversion fraction.•Constant volume combustion degree can be increased by piston decelerating near TDC.•Rapid compression and expansion reduce energy loss by shortening heat transfer time.•Constant volume stage is extended to meet longer combustion at higher engine speed. The two-stroke rod-less opposed pistons engine provides a new possibility to improve the energy conversion path by optimizing cam profile. Constrained by the optimization algorithms and profile control equations, previous research could not optimize the profile with a high-degree freedom under practical engine operating conditions. A thermodynamic model with complete working process, a cam profile control equation providing rich optimization variables, and a Simplex optimization model without solving gradient were established to optimize the cam profile. It is found that the thermal cycle is modified by optimizing cam profile, realizing the redistribution of work, heat transfer and exhaust energy, making the indicated thermal efficiency maximumly increase by 2.38% based on an existing prototype engine. By decelerating the piston motion velocity near the top dead center, constant volume combustion could be realized to enhance the thermal-power transition ability, what’s more, by accelerating the compression and expansion process, heat transfer energy loss could be reduced by shortening the duration of high-temperature states. Only with the optimum cam profile, the best cooperation could be achieved among the volume change rate, heat transfer rate and heat release rate. To cope with the increasing combustion duration with the increase of engine speed, the constant volume combustion stage is appropriately extended and the velocity of the expansion process is remained to reduce the exhaust energy loss at the cost of increasing the heat transfer energy loss, making the heating energy used for working to the maximum extent. Overall, the optimum piston motion profile and the optimization strategy of energy conversion path proposed in the research could be used to guide the improvement of most reciprocating heat engines in thermal-power transition.