Abstract
Intake valve lifts of present SI-MPFI engines are defined by the demands of nominal performance. Within the lower range of load and speed however, an intake valve lift defined in such a way is no longer the optimum. The flow velocity in the intake valve gap area greatly decreases at such operating points. Inevitably, this results in a deterioration of the mixture formation with corresponding effects on the subsequent combustion. The flow velocity in the intake valve gap can be adapted to the decreasing mixture mass flow by reducing the intake valve lift.
The effect of reduced intake valve lifts (0.5 ... 4 mm versus 9 mm) on fuel consumption and exhaust emissions has been systematically investigated on a single cylinder research engine with intake manifold injection and a stoichiometric fuel/air mixture. The experimental investigations were completed by PDA (phase doppler anemometry) measurements of diameter and velocity distributions of fuel droplets in the area of the intake valve on a flow test bench working instationary operation. In order to gain insight into process at the valve gap which is not accessible for measurement, neither on the research engine nor on the flow test bench, the mixture formation was investigated by using videoscope technique and simulated by CFD (computational fluid dynamics) software for the area between the injection valve and the upper part of the combustion chamber.
Narrow intake valve gaps lead to an improved homogenisation of the air/fuel mixture within the cylinder. The results from the flow test bench and CFD calculations indicated that sonic velocity of flow is to expected within the investigated engine speed range from an intake valve from 2 mm and lower. Under these conditions, the intake valve gap works like an atomising sprayer. The aerodynamic forces within the gap let the droplets directly entering the intake valve gap or being torn from the wall film in the gap burst. An immense reduction in droplet diameters and a decreased droplet number can be determined near the intake valves in the combustion chamber for small intake valve lifts. The chamber fills with a fine fuel mist and more fuel vapour. Additionally, a microscopic turbulent charge motion is induced supporting the mixture formation.
These effects of narrow intake valve gaps are the reason for the possibility reducing HC emissions by up to 60 % (imep |1.5 bar) compared with normal intake valve lifts. Furthermore, the engine tests indicated that the combustion becomes faster with significantly decreased delay and duration. The reduction of cycle variations and ¡V connected to that ¡V an improvement in engine smoothness by the factor 15 in lower load range and idling were observed. Finally, the greatest motivation to implement this new technique in SI-MPFI engines is a clear improvement in fuel economy by up to 15 % at an indicated mean effective pressure of imep |1.5 bar.