Objective As the world moves toward a decarbonized society, the CO2 emitted by automobile engines must be drastically reduced. Various attempts have been made to reduce losses. We have studied a mechanical Atkinson cycle engine with a large expansion ratio to improve thermal efficiency. The mechanical Atkinson cycle engine has a different component structure from that of conventional engines, which makes the load direction more complicated and increases the weight of the components, resulting in a higher load on each component. This paper describes the reliability of each part and the establishment of the entire structure. Methodology The authors have built a four-cylinder mechanical Atkinson cycle engine prototype and tested its performance. The connecting bridge and sliding parts, which are subjected to particularly high loads, may be affected by deformation due to deflection, and there are concerns about seizure due to sliding resistance. The feasibility of the parts as individual components was confirmed in advance by using CAE and unit tests, and then the performance was evaluated in tests on actual equipment. Results The connecting bridge has a four-bolt fastening structure to prevent deflection, opening, and slippage. The sliding parts are coated with DLC, have adjustable clearances, and are designed to prevent misalignment in the thrust direction. By taking the above measures, a mechanical Atkinson cycle engine capable of withstanding an engine speed of 6000 rpm was realized. As a characteristic of the overall structure, it was found that the parts are easily deformed by bending loads, and the piston TDC position changes more than in conventional engines. This was addressed by adjusting the combustion chamber shape, shifting the phase of the crankshaft and sub-crankshaft, and lowering the exhaust TDC relative to the compression TDC position. Limitations of this study In this study, tests were conducted on a prototype engine. Tests have been conducted at an engine speed of 6000 rpm, but no tests have been conducted in such areas as low RPM high load and high RPM high load. Also, the manufacturing method of parts was not studied for this paper. What does the paper offer that is new in the field in comparison to other works of the author?. In this paper, an engine with a new mechanical system studied by the authors is provided for the first time. In addition, the paper provides the feasibility of a component system that is subjected to higher loads and from more multiple directions than conventional engines. Conclusion (1) The mechanical Atkinson cycle was realized by ensuring reliability in parts and structures not found in conventional engines. (2) By adjusting the structure, shape, and clearance of the connecting bridge and sliding parts, a mechanical Atkinson cycle engine capable of withstanding an engine speed of 6000 rpm could be realized. (3) The TDC position changes more significantly than in conventional engines due to component deformation, so the design must take thermal efficiency and interference into consideration.

Mr. Yasukazu Kuriyama, Staff Engineer, Honda R&D Co.,Ltd.