Continuous increased interest in hybrid drive systems enabling the reduction of fuel consumption and emissions of harmful substances into the atmosphere, as well as their use partly as zero-emission drive in vehicles provides the basis for the development of these types of drive systems. Such trend is indicated by sales analysis of hybrid drives, and this means that actions taken towards of reduction of fuel consumption are still justified. The article reviews combustion engines for hybrid drive systems as far as determination of their operating conditions, indicating mean effective pressure and power/displacement ratio are considered. The concept of hybridization was defined as a measure of the degree of utilization of vehicle electric drive systems (or its drive support). The analysis was conducted on the basis of vehicles equipped with different hybrid drive systems with combustion engines with spark ignition and compression ignition. The hybridization indexes for different groups of hybrid drive systems were determined and significant discrepancies in the way of their determination were pointed out.