This paper is based on extensive experimental research with lean burn, high compression ratio engines using LPG, CNG and gasoline fuels. It also builds on recent experience with highly boosted spark ignition gasoline and LPG engines and single cylinder engine research used for model calibration. The final experimental foundation is an evaluation of jet assisted ignition that generally allows a lean mixture shift of more than one unit in lambda with consequential benefits of improved thermal efficiency and close to zero NOx.
The capability of an ultra lean burn spark ignition engine is described. The concept is operation at air-fuel ratios similar to the diesel engine but with essentially homogenous charge, although some stratification may be desirable. To achieve high thermal efficiency this engine has optimized compression ratio but with variable valve timing which enables reduction in the effective compression ratio when desirable. High specific power output is achieved by supercharging the engine and no NOx reduction is proposed with only an oxidation catalyst needed to meet Euro 6 standards.
The 2.6L four-cylinder engine is optimized using the group's PSO (particle swarm optimizer) and a first order engine simulation model that at a particular torque-speed, can rapidly find the local optimum engine configuration. It has been shown that the model accurately predicts the performance for all air-fuel ratios to the lean limit of both normal spark ignition and jet assisted ignition with LPG and other fuels.
Mechanical constraints of the four valve pent roof combustion chamber limit the compression ratio to 15 or to lower values because of knock or excessive friction losses. Variable cam phasing is essential and the performance of variable inlet valve duration and fixed inlet valve duration cams are compared. In addition to power, thermal efficiency, and NOx emissions, operational characteristics of the engine are reported: optimum ignition timing, lambda, cam-phasing, boost/throttle and more.
The engine's predicted maximum performance is a torque of 283 Nm and power of 150 kW with a fuel consumption reduction of around 40% over conventional SI. This prediction is based on hot-start steady state data so real engine performance will likely be less.