Consideration on Theoretical Thermal Efficiency of Diesel Engine Under Polytropic Change

If the factors which defferentiate a theoretical cycle frome an actual cycle and the factors which constitute a theoretical thermal efficiency of the theoretical cycle are analyzed, the thermal efficiency of the actual cycle can be evaluated with these factors. As the change of the thermal efficiency is directly compared to the change of the factors, so it will be possible to get the measure how to and much improve the thermal efficiency.In the present paper, a new idea named the polytropic cycle is introduced. The polytropic cycle is related to the actual cycle with two diagram factors, and the theoretical thermal efficiency is expressed with the cycle factors and the cycle indexes which constitute the polytropic cycle. The heat balance is also expressed based on the polytropic cycle. To estimate the theory, some systematical experiments have been conducted changing the injection timing, compression ratio and boost pressure using a single cylinder fore-cycle test engine and a test equipment.As a result of applying the theory to the experiment, the factors which strongly concern the thermal efficiency have been analyzed, and some characteristic tendencis have been found in the change of the factors. In addition, the calculated break thermal efficiency and exhaust gas temperature agree well with the measured data, so this theory can be applied to the practical analysis and evaluation of the engine performance.