A method and apparatus for accurately determining TDC in a cylinder of an internal combustion engine. Gas pressure is measured in the cylinder relative to crank angle, giving rise to a curve of gas pressure with crank angle. An offset is found by first computing the angular position of an inflexion point of the compression pressure curve measured as a function of the crank angle, and then computing the offset as the difference between the angular position of a piston in the cylinder and a theoretical value for said angular position of the piston dependent on a known or measured flywheel angle. The value of TDC corrected by the offset is then used to calculate a value of work output such as the Indicated Mean Effective Pressure (IMEP) which may be used for diagnostic and/or control purposes.