Low heat rejection engines

A special class of experimental prototype internal combustion piston engines has been developed over several decades with the goal of improving efficiency by reducing heat loss.[98] These engines are variously called adiabatic engines; due to better approximation of adiabatic expansion; low heat rejection engines, or high temperature engines.[99] They are generally piston engines with combustion chamber parts lined with ceramic thermal barrier coatings.[100] Some make used of pistons and other parts made of titanium which has a low thermal conductivity[101] and density. Some designs are able to eliminate the use of a cooling system and associated parasitic losses altogether.[102] Developing lubricants able to withstand the higher temperatures involved has been a major barrier to commercialization.