Comparison of an opposed-piston free-piston engine using single and dual channel uniflow scavenging

The free-piston engine is an unconventional engine characterized by a free-moving piston without a crankshaft system. The scavenging process recharges the engine and is crucial to mixture formation and stable combustion. In this paper, a single-channel structure for a two-stroke opposed-piston free-piston engine was proposed. A comparative study of spray, combustion, emissions and energy balance was presented using computational fluid dynamics simulations. Grid independence and step sensitivity were discussed in order to improve the simulation accuracy. The results show that the combustion efficiency of the engine with single-channel structure increased to 98.1%, and the combustion duration decreased by 59.8%. Due to the successful combustion organization, the single-channel engine achieves low emissions even if the CO2 content is slightly higher. The thermodynamic analysis of the engine reveals that the energy loss from scavenging process contributes the largest part to the total energy loss. In addition, the study initially reveals the performance stability of the single-channel engine under different combustion conditions.