The development of today's powertrains focuses on the reduction of CO₂ emissions. Therefore several new technologies for internal combustion engines have been established. A further tendency is the successive electrification of powertrains in hybrid vehicles. However, these trends lead to increasing system costs which are a very important aspect at the market segment of compact cars.At the Institute of Internal Combustion Engines of the Technical University of Munich a drivetrain concept for urban and commuter traffic is under development. It is based on a lean-burn air-cooled two-cylinder natural gas engine which is combined with a hydraulic hybrid system.The paper contains detailed information about the engine as well as the hybrid vehicle powertrain in parallel structure. Particular characteristics and innovations of the hydraulic hybrid system compared to systems known so far are shown. The size of the hydraulic pump is chosen in consideration of low pump losses and suitable torque in combination with the engine map. Furthermore, the paper puts emphasis on the selection of the hydraulic storage type regarding system efficiency, complexity and package. Subsequent to the description of the hybrid system, the results of an entire vehicle simulation of the parallel hydraulic hybrid powertrain are presented and discussed.In summary, the paper describes concept, design and simulation results of a two-cylinder natural gas engine combined with an innovative hydraulic hybrid system in parallel structure.