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Increasingly stringent emissions and fuel economy standards have long remained a source of challenges for research in automobile engine technology development towards the more thermally efficient and less polluting engine. Spark ignition (SI) engines have lower part-load efficiency when compared with the diesel engines. The greatest opportunity for improving SI engine efficiency is by way of higher compression ratio, variable valve timing, low friction, reducing throttling losses, boosting, and down-sizing. Variable compression ratio (VCR) technology has long been recognized as a method for improving the fuel economy of SI engines. In order to vary the compression ratio, some method of varying the geometric compression ratio through changing the clearance volume is required. There are several ways of doing this; various patents have been filed and designs presented, including modification of the compression ratio by moving the cylinder head, variation of combustion chamber volume using a secondary piston or valve, variation of piston deck height, modification of connecting rod geometry, moving the crankpin within the crankshaft, and moving the crankshaft axis. The potential of these technologies needs to be evaluated by a trade-off between cost and consumption benefit. This paper reviews the geometric approaches and solutions used to achieve VCR, considers the results of prior research, and forecasts what benefits, if any, a VCR would bring to present engine design.

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