Effect of Active Piston-Movement Control on Thermal Efficiency in Different Heat Release Profiles

In order to improve thermal efficiency of spark ignition engines, the authors have studied means to improve degree of constant volume. The ideal Otto cycle realizes the maximal degree of constant volume with an instantaneous combustion at TDC. However, it is actually impossible to achieve instantaneous combustion as the combustion speed is limited. Thereby, the authors thought of an idea to increase degree of constant volume. That is to make the piston speed slow during combustion period by active piston-movement control, allowing more time for combustion. As a result, degree of constant volume was improved, but indicated thermal efficiency, estimated by integrating P-V diagram, was deteriorated. A longer expansion stroke was found to keep a longer period of high temperature and then, heat loss increased, leading to a decrease in indicated work. In this study, the authors built another test engine that has equal strokes of compression and expansion but has a slow piston speed in the first half of expansion stroke by making the length of connecting-rod extremely long and made some tests. In the previous report, because the numerical calculation predicted that the shorter the combustion period becomes, the worse the thermal efficiency does, a direct fuel injection system was employed to take a longer combustion period and a different profile of heat release rate compared to the port injection system. As a result, thermal efficiency did not depend on the length of connecting-rod very much, and the numerical simulation predicted the same tendency as experiment, depending on the profile of heat release rate.