Abstract
Keywords
EGR, Miller, Swirl, VVT, FIRE
Abstract
During recent years several VVT devices have been developed in order to improve either peak power and low end torque, or part load fuel consumption of SI engines. When the target of the VVT application was an improvement of part load fuel consumption these VVT devices were often combined with devices for variable charge motion such as port deactivation, valve deactivation or tumble flaps, in order to allow acceptable combustion stability at part load with high EGR rates.
This paper describes an experimental activity concerning the integration of a continuously variable cam phaser (CVCP) and a special intake and exhaust port geometry on a family of small displacement 4 cylinder 2 valve engines. The target was to achieve significantly lower fuel consumption under normal driving conditions, compared to a standard MPFI application, without the need for a separate variable charge motion device.
A hydraulic vane-type cam phaser is used to shift the single overhead camshaft to retarded positions at constant overlap. Thus high EGR rates in the combustion chamber are generated by re-aspiration of burnt gas from the exhaust port into the cylinder. This re-aspiration phase is used to generate increased swirl with the help of a unique exhaust port design to enable stable combustion under high residual gas operating conditions. The consecutive aspiration phase through the intake port creates an additional moderate swirl motion. In this way a conventional tangential intake port can be adopted, allowing at the same time good combustion stability with high EGR rates, with a good trade-off between part load fuel consumption and full load performance. Late intake valve closure (reverse Miller cycle) is combined with these measures in order to further reduce pumping losses at part load.
Using 1.2 and 1.4 dm 3 4 cylinder 8V engines equipped with these devices the concept was developed on the engine test bed with respect to functionality, durability and cost.
Two demonstration cars with these engines were prepared. ECU calibration was performed on the test bed and in the vehicles with given targets for driveability and emissions. EURO 4 exhaust emission limits were reached using a conventional catalyst aged for 100000 km. Fuel consumption was reduced by 5% in the NEDC cycle due to the thermodynamic effects achieved with VVT, high residual gas content and late intake closing. Additional measures on the valve-train in order to reduce friction resulted in a total fuel consumption reduction of more than 6% for both vehicles compared to a standard (without any conventional EGR system) 4 cylinder 8V MPFI application.