The relationship between engine mechanics and thermodynamics is investigated in this paper. By means of numerical simulation, the inherent mismatching between the mechanics behaviors and the thermodynamic process in internal combustion engines is identified, which is believed to be one of the important limiting factors of energy efficiency for the conventional slider crank engines available in the current market. A possible novel approach is proposed and discussed for engine efficiency improvement through Optimization of Matching between Engine Mechanics and Thermodynamics (OMBMT). A parameter of Matching Gain is defined for quantifying engine efficiency improvement by comparing with a baseline engine. Several case studies have been conducted toward the actual designs in the history of engine development. The reasons for positive gains achieved as well as for negative results obtained are interpreted with the matching concept. Using this matching concept as a guideline, discussions on possible directions for engine efficiency improvement are made based on the findings identified with this method presented. Implementing the principles proposed in this paper, a novel mechanism is being proposed and a concept engine is being modeled and developed for matching among 4 individual strokes in a 4-stroke engine.