Investigation into a novel pressure-balanced free-piston engine as a high efficiency chemical energy convertor

对置活塞二冲程（OP 2S）发动机在历史上已被应用于飞机推进以及发电和铁路牵引发动机，并取得了巨大成功。最近，Achates Power展示了OP 2S发动机在汽车应用中的潜力。OP 2S发动机中燃烧室的表面积与体积比低，加上没有气缸盖，导致热损失较低，产生高废气能量，使其成为涡轮增压的理想候选者，并提高了制动热效率。然而，由于需要在所有操作点处跨气缸的正压力差（进气歧管压力必须高于排气歧管压力）以确保2冲程发动机的扫气性能，因此通常使用曲轴箱扫气，因为与涡轮增压器驱动的增压系统不同，曲轴箱扫气保证了在所有操作点处的正压力差梯度。然而，其它扫气系统，例如与涡轮增压器结合的增压器，已经显示出提供有效的扫气性能，同时利用否则浪费的排气能量。此外，使用具有0 P2 S架构的组合式增压器/涡轮增压器增压系统在空燃比控制和排气温度管理方面提供了更大的灵活性，而传统的四冲程发动机预计需要使用停缸或其他热管理策略来满足低排放标准。此外，电辅助涡轮增压器的使用不仅增加了这种灵活性，而且提供了一种通过涡轮复合从涡轮机提取多余功的手段，同时简化了进气路径。

The opposed-piston 2-stroke (OP2S) engine has historically been applied to aircraft propulsion as well as engines for power generation and rail traction with great success. More recently, Achates Power have shown the potential of the OP2S engine for automotive applications. The low surface area to volume ratio of the combustion chamber in OP2S engines, combined with its lack of a cylinder head, results in lower heat losses yielding high exhaust gas energy, making it an ideal candidate for turbocharging, as well as increased brake thermal efficiency. However, due to the requirement for a positive delta pressure across the cylinder at all operating points (intake manifold pressure must be higher than exhaust manifold pressure) to ensure the scavenging performance of 2-stroke engines, crankcase scavenging is typically used instead as, unlike a turbocharger-driven charging system, it guarantees a positive delta pressure gradient at all operating points. Nevertheless, other scavenging systems, such as a supercharger in conjunction with a turbocharger, have been shown to provide effective scavenging performance whilst utilising the otherwise wasted exhaust gas energy. Moreover, the use of a combined supercharger/turbocharger charging system with an OP2S architecture provides greater flexibility in the air-fuel-ratio control and exhaust temperature management, whereas conventional 4-stroke engines are expected to require the use of cylinder deactivation or other thermal management strategies to meet the low emissions standards. Furthermore, the use of electrically assisted turbochargers not only increases this flexibility but also provides a means of extracting excess work from the turbine by turbocompounding, whilst simplifying the intake air path.