The internal combustion engine has served as the foundation of modern transportation and power generation for more than a century, with the four-stroke engine being the prevalent design in both automotive and industrial sectors. However, due to growing concerns about fuel efficiency and environmental impact, alternative engine designs have developed. One such innovation is the six-stroke engine, which expands upon the traditional four-stroke design by adding two extra strokes to the cycle. This modification seeks to improve thermal efficiency, lower emissions, and enhance overall fuel economy. In a typical six-stroke engine, the additional strokes involve injecting water or air to cool the combustion chamber and recover more power. Consequently, the working principle of the six-stroke engine features a distinct combination of increased power output and reduced operating temperatures, potentially resulting in lower emissions and improved engine longevity. This paper examines the operational principles of six-stroke engines, highlighting the mechanisms that set them apart from traditional four-stroke designs. We investigate various six-stroke engine architectures, particularly those that incorporate water vapor or air for additional cooling, and assess their performance in terms of fuel efficiency, emissions, and mechanical complexity. Furthermore, the paper compares the six-stroke engine with the four-stroke engine, emphasizing key metrics such as thermal efficiency, fuel consumption, and pollutant emissions. We discuss the potential advantages of six-stroke engines, along with the engineering challenges related to their development and mass production. Additionally, we evaluate practical considerations, including cost, scalability, and the applicability of six-stroke engines across different industries such as automotive, marine, and power generation. These results indicate that although six-stroke design may be a potentially viable technological solution for enhanced efficiency and emissions performance, there are substantial technical–economic–manufacturing barriers to implementation on a widespread basis. That said, the six-stroke engine has Ericsson on the right path at least in the search for more ecological and optimized internal combustion technologies. Finally, this paper ends with an outlook on the future of six-stroke engines and their place in the development path toward cleaner and more resource-efficient powertrains.
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