Project Omnivore: A Variable Compression Ratio ATAC 2-Stroke Engine for Ultra-Wide-Range HCCI Operation on a Variety of Fuels

The paper describes the principal features of Omnivore, a spark-ignition-based research engine designed to investigate the possibility of true wide-range HCCI operation on a variety of fossil and renewable liquid fuels. The engine project is part-funded jointly by the United Kingdom's Department for the Environment, Food and Rural Affairs (DEFRA) and the Department of the Environment of Northern Ireland (DoENI). The engineering team includes Lotus Engineering, Jaguar Cars, Orbital Corporation and Queen's University Belfast. The research engine so far constructed is of a typical automotive cylinder capacity and operates on an externally-scavenged version of the two-port Day 2-stroke cycle, utilising both a variable charge trapping mechanism to control both trapped charge and residual concentration and a wide-range variable compression ratio (VCR) mechanism in the cylinder head. This approach permits individual control of retained and compression heat as separate inputs to the ATAC combustion process (now commonly referred to as HCCI), an ideal situation which is not possible when attempting to operate a traditional fixed compression ratio, variable valve timing 4-stroke engine in HCCI combustion. The ease of application of the VCR system due to the elimination of poppet valves for gas exchange is fundamental to the concept and this feature is discussed in detail, together with the very wide range of compression ratios that the chosen solution permits (from 10: 1 to 40: 1 in this initial configuration). In addition to describing the engine and its technologies, test results from its initial operation on 98 RON gasoline are presented, including stable operation at idle load and 450 rpm in true HCCI (i.e. without spark assistance).