Based on the tested performance data of three turbocharged gasoline direct injection engines, a quantitative analysis on the key influencing parameters of engine indicated thermal efficiency was focused in this study. Through excavating the inner relationships between the operational and performance parameters, a strong similarity between the indicated thermal efficiency and the effective expansion ratio was revealed. On this basis, the control equations for in-cylinder indicated thermal efficiency were corrected on the basis of ideal cycle thermal efficiency, which is changed from solely related to the compression ratio to simultaneously related to multi-parameters, including the expansion ratio, heat release rate, adiabatic efficiency, excess air coefficient, etc. In this way, a direct or indirect quantitative relation between the indicated thermal efficiency and the key influencing parameters of engine was established. The comparison results between the predicted and measured data demonstrate that the corrected control equation for engine indicated thermal efficiency can predict the influences of various parameters more accurately, and the consistency of predicted results between different engines is also satisfactory. Finally, the influence factors of engine heat-work conversion efficiency were disintegrated and the quantitative analysis was made. By this means, the influence mechanisms of key parameters on the engine heat-work conversion efficiency were revealed under various operating conditions. (C) 2016 Elsevier Ltd. All rights reserved.