This paper details a simulation-based investigation into a novel forced-induction boosting system with a centrifugal-type supercharger driven from the engine crankshaft via a continuously variable transmission (CVT). This system acts as a pre-boost to a traditional fixed-geometry turbocharger and has been identified as a possible solution to improving the low-speed engine torque and transient response of future downsized and existing turbocharged engines. The concept was modelled around an existing baseline high-speed directinjection diesel engine model featuring a variable-geometry turbocharger.Conclusions are drawn comparing the potential of the proposed system with the baseline engine in terms of the brake specific fuel consumption, and both the steady state and the transient performance. A design-of-experiments approach is applied to investigate the effects of the supercharger compressor size, the turbocharger compressor and turbine size, the CVT ratio, and the engine compression ratio on the system performance. Optimization techniques are then applied to identify the best settings for these parameters in the proposed system. Transient simulation was undertaken in a MATLAB/Simulink Ricardo WAVE co-simulation environment to develop the required control strategies for the CVT supercharger.The proposed system demonstrates a significant improvement in the low-speed engine torque and transient response of the boosting system during tip-in pedal events, which it is proposed will result in a significant improvement in the vehicle performance and driveability.

Downsizing is an established trend in the development of passenger car engines. However, the benefits of an improved fuel economy are often obtained at the expense of the engine's dynamic response (owing to increasing demands on the boosting system) and, consequently, the vehicle driveability. The use of a continuously variable transmission in the supercharger driveline offers increased control flexibility over the air path, which could allow more suitable calibrations to be developed. This paper gives details of a co-simulation-based investigation into the trade-off between the steady-state part-load fuel efficiency and the resulting tip-in transient response for a highly boosted downsized gasoline engine. The engine was a 2.0 l in-line four-cylinder unit, designed to replace a 5.0 l, naturally aspirated V8, equipped with a positivedisplacement supercharger in a sequential series arrangement with a fixed-geometry turbocharger with an external wastegate. The supercharger can be de-clutched and bypassed, and therefore three separate supercharger engagement regimes were investigated for part-load operation, defined as follows: with the supercharger disengaged and bypassed; with the supercharger engaged with a fixed drive ratio; with the supercharger engaged using a variable ratio (i.e. through a continuously variable transmission). For each of these supercharger engagement regimes, design-of-experiments and optimisation techniques were used to find the best settings for the key engine control parameters such as the intake and exhaust valve timings and the exhaust gas recirculation rate. Using these calibrations as a starting point, the transient performance was then assessed in fixed-speed tip-in simulations. The trade-off situation was found to be highly complex; identifying the best overall balance of the steady-state efficiency and the dynamic performance requires a subjective assessment. However, the continuously variable transmission does provide the best potential for dynamic response combined with a satisfactory fuel economy. It is suggested that the most suitable solution would be to have multiple userselectable calibrations, such as the 'economy' and 'sport' modes used on many modern vehicles.