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DVCP, a system with continuously adjustable intake and exhaust cam positions, was investigated in terms of residual gas content, intake manifold pressure, pumping losses and fuel economy by means of engine cycle simulation on a 16-valve 4 cylinder naturally aspirated SI engine at part load conditions. Using the simulation results a phasing strategy for part load operation with the primary emphasis on improved fuel consumption has been developed. To verify simulation predictions subsequently, measurements were made at the test rig. It was found that cycle simulation was able to predict properly the behavior of the engine even far away from calibration point. The simulated and measured cam positions for maximum fuel economy matched. Test rig results showed that fuel economy improvement by DVCP is limited by residual gas content tolerance of the engine investigated.

A 2-step valve train offers a cost effective alternative to fully variable valve trains. Using the small valve lift, which is usually combined with an early intake closing strategy, reduced pumping losses are opposite to decreasing combustion efficiency due to lower charge motion. To work out the trade off between these two effects an extensive coupled CFD investigation is performed. The 1-D engine model delivering the pumping losses is complemented by an empirical combustion model, that relates combustion duration with residual trapped gas content. The model ensures right prediction of fuel economy. Additionally the influence of a small intake valve event on charge motion can also be demonstrated by 3-D in-cylinder flow simulation.