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

对置活塞二冲程(OP2S)发动机历史上曾被应用于飞机推进以及发电和轨道牵引发动机，并取得了巨大成功。最近，Achates Power展示了OP2S发动机在汽车应用中的潜力。OP2S发动机燃烧室的表面积体积比低，再加上没有气缸盖，导致较低的热损失产生高废气能量，使其成为涡轮增压的理想候选者，并提高了刹车热效率。然而，由于在所有工作点(进气歧管压力必须高于排气歧管压力)都要求气缸上的正增量压力以确保二冲程发动机的扫气性能，曲轴箱扫气通常被使用，因为与涡轮增压驱动的增压系统不同，它保证在所有工作点都有正的增量压力梯度。然而，其他的扫气系统，如与涡轮增压器相结合的增压机，已被证明在利用否则被浪费的废气能量的同时提供有效的扫气性能。此外，采用OP2S架构的增压/涡轮增压组合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气缸停用或其他热管理策略来满足低排放标准。此外，电动涡轮增压器的使用不仅增加了这种灵活性，还提供了一种通过涡轮组合从涡轮中提取多余功的方法，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.