Thermodynamic Potentials of a Fully Variable Valve Actuation System for Passenger-Car Diesel Engines

Future developments of passenger car diesel engines will not only face worldwide more stringent pollution emission laws, but will also have to abide the worldwide CO2 emission commitments of engine manufacturers to reduce fleet consumption. Today very advanced exhaust gas after-treatment strategies set on the basis of diesel particulate filtration systems, de-NOx systems with catalytic storage and urea based SCR systems have been developed. Thus future emphasis will have to shift towards further decreasing raw-emissions and fuel consumption jointly. In this research paper investigations on a fully variable valve actuations systems VVA is presented with the goal to optimize load points relevant for international driving cycles. These investigations are performed on a 1-cylinder research engine with a hydraulically actuated four-valve system mounted on a serially available cylinder head of a passenger car diesel engine. First a predevelopment analysis was carried out that demonstrated the potential of realizing internal EGR for decreasing fuel consumption and still maintaining the soot-NOx trade-off balance as for standard operation. These findings were then applied to the combustion process of an advanced production diesel engine with singly external EGR. The main goal was to find a VVA strategy that could match the standard combustion process with respect to emissions and fuel consumption with the potential to replace all components relating to external EGR i.e. EGR-cooler, intake-air throttle, and EGR valve. Further VVA strategies included the temporary switch from four-stroke to two-stroke operation as well as an innovative first approach to apply internal EGR cooling. For the investigated load points all strategies showed the potential to meet raw emission levels, however further fuel consumption reductions with internal EGR could not supersede external EGR operation. Improvements regarding to combustion stability and smoothness operation (i.e. degree of deviation in mean indicated pressure) with internal EGR could also be clearly identified.