A new approach referred to as (P-ω) method, has been developed at Wayne State University to determine the instantaneous friction and its components in internal combustion engines. The method is based on the fact that the instantaneous cylinder gas forces and the instantaneous frictional, inertia and load forces cause the instantaneous variation in the flywheel angular velocity.The instantaneous total friction forces have been computed for a single diesel engine, under idling conditions. A breakdown of the friction into its components and a formulation for each component has been made. By applying linear regression, correlations between the individual components of the friction losses and the different parameters have been developed. The components are classified into two categories: piston assembly losses and crankcase assembly losses.The correlations have been applied to a multi-cylinder and a single cylinder diesel engine to compute frictional losses under operating and motoring conditions. The computed results have been found to be in fairly good agreement with the experimental results. The correlations were also applied to the gasoline engine and compared with publised experimental results in the literature.