For a spark-ignition engine, the parasitic loss suffered as a result of conventional throttling has long been recognised as a major reason for poor part-load fuel efficiency. While lean, stratified charge, operation addresses this issue, exhaust gas aftertreatment is more challenging compared with homogeneous operation and three-way catalyst after-treatment. This paper adopts a different approach: homogeneous charge direct injection (DI) operation with variable valve actuations which reduce throttling losses. In particular, low-lift and early inlet valve closing (EIVC) strategies are investigated.
Results from a thermodynamic single cylinder engine are presented that quantify the effect of two low-lift camshafts and one standard high-lift camshaft operating EIVC strategies at four engine running conditions; both, two- and single-inlet valve operation were investigated. Tests were conducted for both port and DI fuelling, under stoichiometric conditions.
Measurements of specific fuel consumption and exhaust emissions were carried out, while the combustion was analysed using heat release analysis. These tests were carried out in a thermodynamic single cylinder engine. In parallel, tests were conducted in a second engine having the same combustion chamber geometry but with extensive optical access through a transparent cylinder liner. The results from the thermodynamic engine were correlated with in-cylinder measurements in the optical engine of the fuel spray.
The results show that there are worthwhile fuel consumption and exhaust emission benefits to be gained through de-activation of one of the two inlet valves at part-load conditions. The performance characteristics under certain load conditions were dependant on which intake valve was actuated.