Investigation of Combustion Robustness in Catalyst Heating Operation on a Spray Guided DISI Engine 2009-01-1489
In catalyst heating operation for DISI (Direct Injection Spark Ignition) engines, split injection has been generally known to improve combustion stability which is critical for the trade-off between tailpipe emissions and vehicle idle NVH. This is also the case for a spray guided DISI engine employing multi-hole injectors and with both injector and spark plug centrally located in the chamber. There are some special challenges with regard to combustion robustness because of the close proximity between injector and spark plug. Investigations have been carried out through engine testing and CFD simulation to ensure combustion robustness.
For catalyst heating operation, the first injection occurs during induction, which forms a relatively well mixed but lean mixture in the cylinder before ignition, and the second injection occurs close to ignition, which produces a stratified fuel rich mixture in the central region of the combustion chamber. Combustion initialization is found to be sensitive to spark plug protrusion and orientation, injector orientation and 2nd injection timing relative to ignition. In some extreme conditions, misfire can occur. This is likely to be associated with the close proximity between the sprays and spark plug electrodes. However, the exact mechanisms are not so apparent. Various hypotheses have been postulated. Some of findings from the engine experiments and CFD simulations suggest that the sprays close to the spark plug electrodes may affect and interfere with the sparking process either through turbulence, bulk motion or liquid fuel mechanisms. Extensive engine testing has lead to optimized design and calibration that ensure robust combustion.