In order to compare potential of VVL and GDI concept on a small displacement engine, tests have been made on a single-cylinder engine. Compared to multi-cylinder, single-cylinder offers good flexibility and accessibility for instrumentation but its gas dynamics non-representativeness has to be taken into account, especially for turbocharged engines. Concepts are tested on same engine basis but combustion system - chamber geometry, injection configuration, internal aerodynamics, valve train - is specifically designed for each version.
Results confirm that homogeneous stoichiometric GDI is peculiarly adapted to turbo-charged conditions: GDI engine superior knock resistance allows low-end torque improvement and higher compression ratio. In addition to cooling effect, one of the advantages of direct injection is studied: ability to scavenge residual gases without directly transferring mixture to the exhaust. Some synergies between VVL and turbo-charging are also presented in the paper.
Part load tests clearly indicate potential of VVL in reducing consumption. GDI concept has to deal with several tradeoffs in order to exploit theoretical efficiency gain coming from compression ratio increase: combustion chamber (mixture preparation / flame propagation), injection timing (mixture homogeneity / fuel impact on the piston). High pressure cycle efficiency is studied for both configurations with thermodynamic split of losses analysis.