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 various types of losses, but for a significant reduction in CO2 emissions, there must be an enhancement in the theoretical efficiency itself. Increasing the expansion ratio enhances theoretical efficiency but is equivalent to increasing the compression ratio in the Otto cycle. However, increasing the compression ratio causes knocking, which in turn reduces efficiency. Therefore, the effort was made to enhance thermal efficiency by using a vehicle-mountable engine with a much higher expansion ratio. Methodology: Conventionally, the Miller Cycle, which controls the intake valve closing timing, has been used to increase the expansion ratio without increasing the compression ratio. However, this has the disadvantage of requiring a larger engine size to maintain maximum torque and output. Therefore, in this study, an effort was made to realize a mechanical Atkinson cycle engine that can overcome the above shortcomings. There are countless mechanisms to accomplish this, each with different piston motions, resulting in differences in the indicated efficiency, secondary vibrations, and exhaust emissions. In this study, the mechanism with the most balanced performance was sought, and its performance was demonstrated by building a four-cylinder turbocharged prototype engine. Results: The results of the thermodynamic cycle study showed that the compression ratio is limited by knocking, and the expansion ratio is limited by over-expansion. As a result, it was found that a compression ratio of 10 and an expansion ratio of 16 can significantly reduce CO2 emissions in this case while maintaining the same engine height and maximum torque as before. A new mechanism with two crankshafts and a sliding structure that embodies the thermodynamic cycle was also proposed. Tests of the prototype engine have shown that it can reduce secondary vibration to one-tenth and CO2 emissions by approximately 10.5% while maintaining the same engine height and maximum torque as conventional engines. However, the enhanced thermal efficiency caused the heating performance of the catalyst to deteriorate, and the high-frequency vibration was also observed to worsen. Limitations of this study: In the study, tests were conducted at low speed with medium load, which is important from the perspective of CO2 reduction, and at high speed, which is important from the perspective of reliability, but exhaustive tests in all operating ranges were not conducted. In addition, this study was conducted on turbocharged engines, and the same results may not be obtained for naturally aspirated engines. What does the paper offer that is new in the field in comparison to other works of the author? This paper presents, for the first time, a method for reducing CO2 emissions with a highly efficient, small-sized engine using a new mechanism with two crankshafts and sliding components. Conclusion: (1) A new mechanism to realize a mechanical Atkinson cycle was created. (2) The prototype engine reduced CO2 emissions by 10.5 percent compared to conventional turbocharged engines. (3) The future issues to be solved are the improvement of the catalyst's heating performance and countermeasures against engine vibrations.
Mr. Keitaro Nakanishi, Chief Engineer, Honda R&D CO., Ltd