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

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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.