The past five years have seen considerable interest in the possibility of constructing diesel engines in which all or part of the combustion chamber is insulated from the environment. either by ceramic materials or by high-temperature alloys backed with an air gap. It is hoped that such a design will permit engine mass to be reduced by the elimination of water cooling, and engine efficiency to be increased by the use of a secondary cycle to extract heat from the exhaust gases. In assessing the practicality of an uncooled engine, it is essential to establish the degree of insulation achievable by a proposed design. This requires identification of the principal heat loss mechanisms. This paper describes the construction of a three-dimensional time-varying combustion-chamber model, and shows that the cyclic motion of the piston leads to an important heat loss not calculable by static models. Insulation is shown to increase the importance of this loss mechanism. Estimates of the loss are given. By adapting methods established for the analysis of thermal regenerators, an approximate closed-form expression for the heat loss is obtained.