It is important to determine the phasing of a measured cylinder pressure trace and crank angle with high accuracy. The reason is that erroneous determination of the position of TDC is a major error source when calculating properties such as heat release etc. A common way to determine the TDC position is to study motored cycles. Heat transfer makes the task more complicated, since it shifts the position of the maximum pressure away from TDC.

In this paper a new method for determining the TDC position is proposed that does not require any additional sensors other than a cylinder pressure sensor and an incremental encoder. The idea is to find a point that the cylinder pressure from a motored cycle is symmetric around, since the volume is close to symmetric on either side of TDC. The new method and four published methods are tested and evaluated. Cylinder pressure data used for comparison are from simulations of a SAAB Variable Compression engine. The investigation shows that the methods have an error that is in the range of 0.1 crank angles or less, but are sensitive to errors in geometry and heat transfer information. The symmetry method is less sensitive to these errors than most of the other methods, and is not affected by an offset or gain error in the cylinder pressure signal. It is also the least sensitive method with respect to noise.