An experimental investigation is conducted to examine the effect of the main parameters influencing the compression stroke of a direct injection Diesel engine. The aim is to develop a methodology that can be used as a diagnostic tool to determine the compression condition of DI Diesel engines. However, conclusions derived from the present investigation can be extrapolated to other types of reciprocating internal combustion engines. The compression stroke itself is an important index concerning engine operation since engine conditions at the end of the compression stroke have a major effect on its overall performance. Such information is especially important for large scale Diesel engines used for stationary or marine applications. In these engines, it is important to develop non-catastrophic methods for estimating the cylinder compression condition without dismantling the engine cylinder. The outcome could be a serious reduction of maintenance costs, since unnecessary labour required for inspection could be avoided. When using measurement techniques, what is usually available is the cylinder pressure trace during the compression stroke. However, it is widely recognized that the compression stroke and peak compression pressure is strongly affected, beyond heat losses, by the initial pressure at the inlet valve closure, the compression ratio and the blowby rate. The last three parameters can vary significantly during engine operation, while the heat losses vary mainly due to engine operating conditions and their effect on the compression stroke can be considered. Thus, the knowledge of the peak compression pressure resulting from the cylinder compression pressure diagram is not adequate to define the cylinder compression condition. For this reason, an experimental investigation is conducted to examine the effect of the inital presssure at the inlet valve closure, the compression ratio and the blowby on the cylinder pressure trace. From analysis of the measured data, it is revealed that each parameter has a different effect on the different parts of the compression pressure trace. As revealed, it is possible to determine the compression condition of an engine cylinder based on the measured cylinder pressure trace. (c) 2005 Elsevier Ltd. All rights reserved.