Abstract
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 120 g/ (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 thermal efficiency of two-stroke and four-stroke diesel engines is thermodynamically analyzed by computer simulation of the performance and thermal load.
The mean effective pressure, maximum combustion pressure, scavenging air pressure, and degree of explosion (or compression ratio) are the factors influencing the theoretical thermal efficiency, and their effects on the FOC are 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, asymmetrical scavenging method, cooling and insulating conditions 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.
