This paper presents a comparison of a hybrid and a conventional powertrain using physical based simulation models on the system engineering level. The system engineering model comprises mechanistic sub-models of the internal combustion engine including exhaust aftertreatment devices, electric components, mechanical drivetrain, thermoregulation system and the corresponding controllers. Essential sub-models are discussed in detail and their interaction on the system level is pointed out. Special attention is paid to compile a real-time capable model by combining mean value air path and drivetrain models with a crank-angle resolved cylinder description and quasi-steady state considerations applied in electrical and cooling networks. A turbocharged gasoline direct injection engine is modeled and calibrated based on steady-state measurements. The conversion performance of a three way catalyst is compared to light-off measurements. A hybrid electric vehicle model combines the turbocharged engine with the electric and driveline components of the Toyota Prius hybrid vehicle. Results of the hybrid electric vehicle are compared with the results of the conventional vehicle featuring the same chassis geometric characteristics and tires that are powered by the same engine. Both powertrains are used to simulate a legislation cycle with respect to fuel consumption, engine efficiency and engine and tailpipe emissions.