The ideal output characteristic required from an internal combustion engine for vehicle propulsion is constant power output over the working speed range, and the present conventional engine falls very far short of this. The paper describes the development of a diesel engine which is supercharged by a compressor driven by a differential gear in such a way that the boost, and therefore the output torque, increases with decrease in engine speed and which goes some way towards giving the desired output characteristic. The further step is described of including a hydraulic torque converter in the system so that automatic operation by a single pedal control can be achieved over the practical operating range of normal road vehicles. It is shown that the full operating range required by all normal vehicles can be obtained with a gearbox with two speeds only. The results of road tests on a truck with varying payloads are given and the fuel consumptions achieved are shown in comparison with the same engine in both its naturally aspirated and turbocharged forms. Notes are given on the component development and mechanical development of the engine necessary to achieve these results in a reliable way. Details are shown of possible production arrangements of differential gears and drive trains, one of which incorporates a hydraulic retarder which can be done cheaply and effectively. The paper concludes with a note on future developments and a summary of possible advantages.