Variable Compression Ratio (VCR) is technology to adjust the compression ratio of an internal combustion engine while the engine is in operation. This is done to increase fuel efficiency while under varying loads. Higher loads require lower ratios to be more efficient and vice versa. Variable compression engines allow for the volume above the piston at top dead centre to be changed.

VCR engine concept is gaining popularity in both scientific research and within car industry.  There has been multiple attempts to build engines having continuous adjustment of the compression ratio, some of the earliest patents dating back 1925. Since the early era of internal combustion engines both Otto and Diesel cycles have gained dominant position in car engines.  There are other technological applications such as two stroke engines, Wankel- and some other concept that have a niche market share. VCR Technology is suitable to both Otto and Diesel cycles. In the later gains are less in form of fuel economy but it may prove effective in NOx emissions reduction that are becoming more on the scope as a major health hazard and in control of peak pressure at full load.