AbstractPrime movers and boilers have traditionally been optimized independently to convert fuel to power and heat, respectively. the design of devices to optimize the simultaneous production of heat and power has been neglected. This paper considers the optimization in this mode of diesel engines and concludes that simple modifications of design could increase the heat recoverable in the exhaust by the equivalent of 10 per cent of the fuel input. In monetary terms this is equivalent to about a 4 per cent increase in brake efficiency, recent improvements in brake efficiency of this magnitude have taken ten or fifteen years of continuous research and development. the changes now proposed would increase thermal loading and some development might be necessary.