The Performance Potential of Limited Cooled Diesel Engines

A mathematical model is used to predict the performance of a turbocharged diesel engine with a partially insulated combustion chamber. The model is briefly described, with emphasis on heat transfer aspects and prediction of combustion chamber surface temperatures.

First the trade-off between performance improvement (efficiency) degree of insulation, combustion chamber surface temperature and exhaust gas energy is examined. It is shown that the high surface temperatures in the cylinder, due to insulation, cause volumetric efficiency to fall. However, this reduction is offset by increased boost pressure from the turbocharger due to greater exhaust gas energy. The contribution to overall reduction in heat loss from cylinder head, piston and liner and their effect on engine efficiency are presented and discussed.

Various turbo-compound systems are examined and it is shown that most systems become unmatched when working over the wide speed and load range necessary for vehicle application, unless a variable geometry turbine or a variable ratio gearbox (this maintains compound power at part load) are used. Furthermore, substantial overall performance benefits can only be obtained with very efficient turbomachinery.

Rankine bottoming cycles are more effective than fixed geometry turbo-compounding at low load, but their normal efficiency is limited by high condenser temperature, and they would be difficult to install in a compact European truck.

It is concluded that the ultimate improvement in engine specific fuel consumption of an adiabatic turbo-compound engine is 20 per cent (excluding the savings from fan and water pump). However, drastically new design concepts are necessary, if engines are to be built that approach this degree of insulation. Meanwhile redesigned conventional engine types will offer substantial reduction in cooling system requirements with more modest thermal efficiency benefits.