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Online combustion efficiency optimization in a variable-valve-timing (VVT) gasoline engine requires the real-time knowledge of in-cylinder pressure and its various derivatives. The in-cylinder pressure measurements, however, are still inapplicable to current light duty vehicles due to the high cost of fast pressure sensors. In this paper, an effective combustion model is developed to provide online prediction of crank-angle resolved (CAR) in-cylinder pressure evolution given five representative initial states at intake valve closing (IVC). The prediction of the combustion pressure is made by incorporating mean-value mass/energy flow models with the first law thermodynamics. To achieve real-time calculation for end-use engines, this paper improves the validity region of the existing mass/energy flow models while preserving their simplicity.