Lower emissions
Who: Tests of a petrol engine developed by Lotus have shown a 10% improvement in fuel consumption over stratified direct injection engines. The Omnivore variable compression ratio, flex-fuel direct injection two-stroke engine has demonstrated homogeneous charge compression ignition (HCCI) down to extremely light loads. This results in ultra low emissions and has been achieved over a wide range of engine operating conditions, even from cold start.
Technology: The research has focused on lower speed and load conditions which represent a major proportion of an engine’s operation in a real-world environment. At 2000rpm and up to approximately 2.7 bar indicated mean effective pressure, the indicated specific fuel consumption achieved is approximately 10% better than spray-guided direct injection, spark ignition engines. Emissions are 20ppm NOx at less than 2.3 bar load with four-stroke-equivalent hydrocarbons and carbon monoxide emissions.
The engine features a variable compression ratio system and uses a two-stroke operating cycle with direct fuel injection. It is suited to flex-fuel operation with a higher degree of optimisation than is possible with existing four-stroke engines.
Omnivore features a monoblock construction that blends the cylinder head and block together, eliminating the need for a cylinder head gasket, improving durability and reducing weight. In this case, the application of a monoblock is facilitated by the absence of the requirement for poppet valves. A novel charge trapping valve in the exhaust port allows asymmetric timing of exhaust flow and continuous variation of the exhaust opening timing.
The engine uses the Orbital FlexDI fuel injection system which produces fine in-cylinder fuel preparation irrespective of fuel type and, together with air pre-mixing, allows efficient two-stroke combustion and low-temperature starting, while offering an opportunity for advanced homogenous charge compression ignition control.
The variable compression ratio is achieved by the use of a puck at the top of the combustion chamber. This system moves up and down, affecting the change in geometric compression depending on the load demands on the engine.
Application: Simon Wood, technical director of Lotus Engineering, said: “Omnivore lays the foundations for a novel vision of a variable compression ratio engine concept suitable for production. A multi-cylinder version is practical for a wide variety of vehicles and offers greatest benefit to C and D-class passenger cars which can take advantage of the low-cost architecture and improved fuel economy.”
Omnivore achieves wide-range HCCI combustion and low CO2 emissions through the application of a variable compression ratio mechanism, itself facilitated by the adoption of the two-stroke operating cycle. The automotive industry has advocated engine downsizing for four-stroke engines as a result of throttling losses at part-load, where vehicles run most of the time.
Jamie Turner, chief engineer of powertrain research at Lotus, said: “The two-stroke cycle does not suffer from significant throttling losses and in many ways is a more natural fit for automotive use. With the thermodynamic disadvantages of throttling losses removed, the two-stroke engine is free to be sized according to its improved part-load fuel consumption.”
© PE Publishing, 27 January 2010
