Abstract: An extended expansion diesel cycle engine which achieves a fuel efficiency materially higher than that of an engine operating on the conventional diesel cycle. A venting valve means is operated to provide a selectable ratio of predetermined pre-compression swept volume (C) versus work producing expansion swept volume (E) in a range of substantially between 1:1.2 and 1:3. Suitable selection of this C/E ratio ensures expansion of the combustion gases to a significantly greater volume than the pre-compression volume of the original air charge, effecting a predictable drop in temperature and pressure of the wasted exhaust gases. The extended expansion feature produces the feasibility of having a fuel delivery means timed to ensure exclusively after top dead center (ATDC) fuel ignition and combustion, thereby reducing heat dissipation, thus limiting cooling loss and significantly improving torque potential.