An experimentally validated diesel engine code is used to study the crankshaft torsional deformations originating in the difference between instantaneous engine and load torques. The analysis aims in studying the phenomena under critical conditions, namely operation when one cylinder malfunctions ('open valves' or motoring situation) as well as during transient conditions. A detailed crankshaft torsional model is formulated; this takes into account cylinder gas, inertia, friction, load and stiffness and damping torques. Details are provided concerning the underlying mechanism of the crankshaft torsional deformations, which can assume significant values depending on the specific configuration, being important for safe engine operation.