Challenges and feasibility of a six-stroke engine using water direct injection

Abstract

As fuel efficiency and exhaust regulations have become more stringent, many studies have been conducted to improve fuel efficiency. One interesting proposal is to develop a six-stroke engine using water injection. However, there is a lack of research on this concept, and existing studies have only taken a theoretical approach or did not involve comparisons with a four-stroke engine, obscuring the feasibility of a six-stroke engine using water injection. So, in this study, we performed experiments and simulations of a six-stroke engine using water injection and compared the results with those of a four-stroke engine. We optimized the exhaust valve lift profile through simulation for an actual exhaust cam of the six-stroke engine, and modified a four-stroke port fuel injection engine to a six-stroke engine. Then, through experiments and simulations, we evaluated the feasibility and the challenges of building a six-stroke engine using water injection. We used the CONVERGE v2.4 software application and validated the simulation models through the results of experiments. Using these models, we conducted simulations with various injection timings, masses, and water temperatures. The results indicate that the cylinder wall is cooled due to continuous water injection making evaporation unstable and that wall-temperature control is required in order to realize a six-stroke engine using water injection. In a simulation assuming the constant temperature of the wall, as additional strokes are added, the indicated mean effective pressure (IMEP) decreases by about 0.4 bar compared to that of the four-stroke engine, and only about 40% of this loss is recovered through water injection. The wall evaporation ratio is more critical to the feasibility of the six-stroke engine using water injection than other parameters. We show that it is very difficult to achieve a six-stroke engine using water injection unless a significant amount of heat energy for evaporation is brought from sources other than the mixture in the cylinder. The challenges of a six-stroke engine using water injection were cylinder temperature control and latent heat of water. If excessive cooling does not occur, such as in steam injection, IMEP can increase due to an increase in mass. However, this is very ideal case. It is difficult to obtain energy elsewhere for evaporation, and due to the latent heat of water, the mixture is inevitably cooled down making it difficult to realize this concept of six-stroke engine.