Double Compression-Expansion Engine Concepts : Experimental and simulation study of a split-cycle concept for improved brake efficiency

Abstract: The four-stroke engine was invented by Nikolaus Otto during the second half of the 19th century. Although a lot of technological improvements have been made over the years, the operating principle with four-strokes remains the same. The need and desire of engines with even higher fuel-efficiency is a motivation to study alternative engine concepts. This thesis presents studies of the Double Compression-Expansion Engine (DCEE) concept. It belongs to the split-cycle engine family because the engine cycle is performed in two or more cylinders. The potential advantages of these concepts are reduced heat transfer loss, while it can be over-expanded which reduces the exhaust loss. These loss reductions provide a potential for higher fuel-efficiency compared to the conventional four-stroke cycle. However, there are also potential drawbacks such as increased gas exchange loss due to multiple gas exchange events to complete the cycle. Another potential drawback is increased engine displacement requirement to obtain the same power as a conventional engine, which means the power density is reduced. The results of experimental and simulations studies indicates that a brake efficiency of 51.0 % can be achieved with the proposed DCEE concept. With subsystem optimizations and insulation of certain engine components the efficiency can reach 53.7 %. Morevoer, the obtained results indicate a trend for further improved efficiency at even higher loads. But to maintain the same fuel/bulk gas- ratio (dilution rate), a higher inlet pressure is required which also cause higher peak cylinder pressures. The performed engine experiments were limited to 210 bar to preserve the structural integrity and it is expected that the efficiency can improve further with mechanical upgrades. This thesis also presents simulation studies of other DCEE concepts. They proved to reach even higher efficiencies, but were also operated at much higher peak cylinder pressures compared to the engine experiments. It was also assumed that a low temperature combustion mode could be used, which has the advantage of decreased heat transfer loss, but also pose challenges in controling the combustion process.