Controlled Auto Ignition (CAI) as a promising future combustion process is a concept to strongly reduce fuel consumption as well as NOx emissions. The acceptance and the potential of this combustion process depends on the possible CAI operation range in the engine map and the fuel consumption benefit, as well as the complexity of the variable valve train which is necessary to realize the CAI combustion process.
The thermodynamic investigations presented in this paper were done on an engine equipped with an electromechanical valve train (EMVT), featuring Port Fuel Injection (PFI) and direct Injection. They show that the electromechanical valve train is an excellent platform for developing the CAI process. Controlled Auto Ignition has been realized with port fuel injection in a speed range between 1000 and 4500 rpm and in a load range between approximately 1 and 6 bar BMEP (about 5 bar BMEP for pressure gradients lower than 3 bar/°CA) depending on engine speed. In a wide range of the engine map NOx emission reduction of 90-99 % was realized, fuel consumption in the New European Driving Cycle (hot test) can be reduced by approximately 15 % (w/o cylinder deactivation) compared to a standard production engine.
Tests performed with direct injection with the injector in the central position have shown a significant extension of the CAI operation range in the engine map while the fuel consumption is also reduced. Further potential can be seen using split injection and multi stroke operation. Especially 2-stroke and 6/8-stroke operation with dual combustion show high potential for stable CAI operation and an extension of the Controlled Auto Ignition map area. In the 6/8-stroke with dual combustion mode a combination of stratified combustion process and homogenous combustion process with Controlled Auto Ignition is realized.
Based on the thermodynamic investigations the necessary variability of a mechanical variable valve train is identified, giving distinct targets for a mechanical variable valve train enabling CAI. Possible mechanical valve train concepts to realize the CAI combustion process are presented in the paper. The influence of these systems due to the changed variability of the valve train on the operation range and the fuel consumption benefit as well as on the transient behavior and system costs is discussed.