Model based TDC offset estimation from motored cylinder pressure data

Finding the correct top dead center (TDC) offset for an internal combustion engine is harder than it seems. This paper introduces a novel method to find the TDC offset based on a simple assumption that the heat loss power through the combustion chamber walls is constant for motored cycles in a narrow crank angle interval around TDC. The proposed method uses nonlinear least squares optimization to find the combination of specific heat ratio and TDC offset that makes the heat loss power as constant as possible. The standard deviation of the TDC estimate is approximately 0.05° crank angle (CA) when using a crank resolution of 0.2°CA for the measurements. The bias of the TDC estimate is in the 0-0.02°CA range both when comparing to measurements with a TDC sensor and with simulated motored cycles. The method can be used both for calibration and on-board diagnostics purposes e.g. during cranking, fuel cut-off or engine switch-off.