Observations on and potential trends for mechanically supercharging a downsized passenger car engine: a review

Engine downsizing is a proven approach for achieving a superior fuel efficiency. It is conventionally achieved by reducingthe swept volume of the engine and by employing some means of increasing the specific output to achieve the desiredinstalled engine power, usually in the form of an exhaust-driven turbocharger. However, because of the perceptible timeneeded for the turbocharger system to generate the required boost pressure, a characteristic of turbocharged enginesis their degraded driveability in comparison with those of their naturally aspirated counterparts. Mechanical superchargingrefers to the technology that compresses the intake air using the energy taken directly from the engine crankshaft.It is anticipated that engine downsizing which is realised either solely by a supercharger or by a combination of asupercharger and a turbocharger will enhance a vehicle’s driveability without significantly compromising the fuel consumptionat an engine level compared with the downsizing by turbocharging. The capability of the supercharger systemto eliminate the high exhaust back pressure, to reduce the pulsation interference and to mitigate the surge issue of aturbocharged engine in a compound-charging system offsets some of the fuel consumption penalty incurred in drivingthe supercharger. This, combined with an optimised down-speeding strategy, can further improve the fuel efficiency performanceof a downsized engine while still enhancing its driveability and performance at a vehicle level. This paper firstreviews the fundamentals and the types of supercharger that are currently used, or have been used, in passenger carengines. Next, the relationships between the downsizing, the driveability and the down-speeding are introduced toidentify the improved synergies between the engine and the boosting machine. Then, mass production and prototypedownsized supercharged passenger car engines are briefly described, followed by a detailed review of the current stateof-the-art supercharging technologies that are in production as opposed to the approaches that are currently only beinginvestigated at a research level. Finally, the trends for mechanically supercharging a passenger car engine are discussed,with the aim of identifying potential development directions for the future. Enhancement of the low-end torque,improvement in the transient driveability and reduction in low-load parasitic losses are the three main developmentdirections for a supercharger system, among which the adoption of a continuously variable transmission to decouple thesupercharger speed from the engine speed, improvement of the compressor isentropic and volumetric efficiency andinnovation of the supercharger mechanism seem to be the potential trend for mechanically supercharging a passengercar engine.This paper will first review the fundamentals and types of supercharger that are currently or have been used in passenger car engines. Next, the relationships between downsizing, driveability and down-speeding are introduced to identify the improved synergies between the engine and the boosting machine. Then, mass production and prototype downsized supercharged passenger car engines are briefly described, followed by a detailed review of the current state-of-the-art supercharging technologies that are in production as opposed to the approaches that are currently only being investigated at a research level. Finally, the trends for mechanically supercharging a passenger car engine are discussed, with the aim of identifying potential development directions for the future. Low-end torque enhancement, transient driveability improvement and low load parasitic loss reduction are the three main development directions for a supercharger system, among which the adoption of a CVT to decouple the supercharger speed from the engine speed, compressor isentropic and volumetric efficiency improvement and supercharger mechanism innovation seem to be the potential trends for mechanically supercharging a passenger car engine.