Engine throttling by means of turbomachinery

Spark ignition engines typically operate with a fixed air.fuel ratio. This implies that the required air intake is more or less proportional to engine load. Accordingly, at low loads, the engine operates less throttled than it would at full capacity. Throttling is a costly business, however, as the engine has to work hard to breathe in air through the narrow slits of a throttle or intake valve at part load. Throttle losses can amount up to nearly a quarter of total power available at the crank. In this chapter, various turbomachinery-based approaches to either reduce or recover these losses will be reviewed. From this literature study it will become clear that no single technology can completely avoid the use of some form of engine throttling. This is predominantly the case for the lowest and highest loads points. Nevertheless, throttle losses can be reduced significantly and/or be partially converted to useful electrical or mechanical energy, resulting in gains in fuel economy ranging from 1 to 5%, with the best results typically corresponding to relatively large engines and low loads. It is disappointing, however, that all reviewed technologies appear to be limited to laboratory or prototype stage of development use, given that no report of demo or commercial application could be found in the literature. This leads to asking the question what major obstacles still need to be addressed to commercialize the technology.