Heat loss reduction could be one of the most promising methods of thermal efficiency improvement for modern diesel engines. However, it is difficult to fully transform the available energy derived from a reduction of in-cylinder heat loss into shaft work, but it is rather more readily converted into higher exhaust heat loss. It may therefore be favorable to increase the effective expansion ratio of the engine, thereby maximizing the brake work, by transforming more of the enthalpy otherwise remaining at exhaust valve opening (EVO) into work. In general, the geometric compression ratio of a piston cylinder arrangement has to increase in order to achieve a higher expansion ratio, which is equal to a higher thermodynamic compression ratio. It is still necessary to overcome constraints on peak cylinder pressure, and other drawbacks, before applying higher expansion ratios to current high-boost, high brake mean effective pressure (BMEP), and high exhaust gas recirculation (EGR) diesel engines.The purpose of this study was to clarify the possibility of improving the thermal efficiency of a diesel engine by variable valve timing strategies. Experiments were performed on a single-cylinder heavy duty diesel engine equipped with external supercharging system, using various combinations of effective compression and expansion ratios. Results confirmed that improvement in the gross-indicated thermal efficiency was achieved by increasing the effective expansion ratio at a fixed and relatively lower effective compression ratio (compared to expansion ratio.) The gross-indicated thermal efficiency was calculated based on the cycle work by in-cylinder pressure and volume traces only during compression and expansion strokes. The brake thermal efficiency was further improved using a piston cavity having higher geometric compression ratio than the baseline value of 18.0.