Simulation Study on Thermal Efficiency of Marine Diesel Engine

The recent competition in reduction of specific fuel oil consumption (FOC) among the marine engine manufacturers has realized engines whose FOC is as low as on the order of 120g/(PS・h) {163g/(kW・h)}, and a further improvement is still under way. Under these circumstances it will be valuable to estimate the limit of FOC reduction, and in view of this, the diesel engine thermal efficiency is thermodynamically analyzed by computer simulation of the performance and thermal load. The mean effective pressure, maximum combustion pressure, scavenging air pressure,and rate of explosion (or compression ratio) are the factors influencing the theoretical thermal efficiency, and their effects on the FOC are examined. In connection with this, the effects of the turbo-charger efficiency and the use of a longer stroke on the FOC are also examined. Next, the effects of the indicator diagram related factors on the FOC are quantitatively clarified; these factors include the combustion period, exhaust-valve opening speed, asymmetical scavenging method, cooling and insulating comditions of the combustion chamber. Based on these results, then the limit of the thermal efficiency improvement on the engine alone is discussed and given as a function of the brake mean effective pressure. Also examined in order to improve the overall thermal efficiency are the use of low air-fuel ratio cycle and a compound system to recover power from the turbo-charger and/or from the exhaust waste-heat by Rankine cycle.