The recent oil crisis has once again emphasized the need to develop both fuel efficient engines and alternately fueled engines, particularly for automotive applications. Engines which burn coal or coal pyrolysis products are attractive, but ignition delay and metal erosion problems continue to limit high speed operation of such engines. Further, the throttled spark ignition engine often used with methanol and natural gas does not prove an efficient or tolerant device for the combustion of a wide range of fuel. Therefore, an novel approach must be taken in order to achieve the efficient and flexible operation of such an engine.A novel design of a fuel tolerant engine suitable for burning coal fuels separates the combustion from the piston in order to have more careful flame control and to exclude the particulate matter from the engine's piston rings. Such a design consists of a piston compressor, a combustion zone, and a piston expander so that the products of combustion are used as the working gas. The flame in the combustion zone would be constant and thus the problems with intermittent combustion would be avoided. Some benefits of this engine would include wider fuel tolerance, the lowering of some emissions, and a higher efficiency than more conventional engines.Initial work on this design includes an ideal model of the thermodynamic cycle in order to compare the thermal efficiency of the novel engine with that of more conventional engines. The actual thermal efficiency of the novel engine will be determined by the displacement ratio (the ratio of the compressor displacement to the expander displacement) of the engine. This idealized model of the novel engine has shown that its thermal efficiency would lie between that of the Diesel and Brayton cycles.