Characterizing combustion in diesel engines is not only necessary when researching the instantaneous combustion phenomena but also when investigating the change of the combustion process under variable engine operating conditions. An effective way to achieve this goal is to parametrize the combustion process using a finite combustion stage cylinder process model in which the parameters can be modeled to give a global description of diesel engine combustion. The main objective of this approach is to gain knowledge on how to calculate (simulate) the parameters defining the finite stage cylinder process model using both theoretical and experimental methods. The latter is essential but also complicated. This paper presents an experimental approach to transform the measured in-cylinder pressure signals to Seiliger process, during which the Seiliger parameters can be obtained. The reliability of this method is verified by using measurements from a test diesel engine MAN 4L 20/27.