The combination of air charging and downsizing is known to be an efficient solution to reduce CO2 emissions of modern gasoline engines. The decrease of the cubic capacity and the increase of the specific performance help to reduce the fuel consumption by limiting pumping and friction losses and even the losses of energy by heat transfer.
Investigations have been conducted on a highly downsized SI engine to confirm if a strong decrease of the displacement (50 %) was still interesting regarding the fuel consumption reduction and if other ways were possible to improve further more its efficiency.
The first aim of our work was to identify the optimal design (bore, stroke, displacement, …) that could maximize the consumption reduction potential at part load but also improve the engine's behaviour at very high load (up to 3.0 MPa IMEP from 1000 rpm). In order to do that, four engine configurations with different strokes and bores have been tested and compared. Their performance and consumption levels have then been incorporated into a larger database with other engines to compute models that can forecast the fuel consumption and emissions of an engine on a specified point.
The impact of volumetric compression ratio and valve timing on the consumption reduction potential has also been investigated with MILLER and ATKINSON cycles. The dissociation of compression and expansion ratios permits an increase of theoretical thermodynamic efficiency which is rather interesting at part load (10 % consumption reduction at 0.4 MPa IMEP - 1500 rpm). Even if the decrease of the effective compression ratio is not always beneficial to fight against the knocking phenomenon, consumption reductions are also possible at full load with a good combustion stability (10 % consumption reduction are also possible at 3.0 MPa IMEP - 2000 rpm).