A Study on Spray-Guided Stratified Charge Systems for Gasoline DI Engines
Innovative technologies are required to meet ongoing challenges to reduce the CO2 emissions from passenger cars and their fuel consumption. Such technologies are usually complex and require detailed analyses to deliver efficient, reliable systems. One proposed approach to meet these challenges is to use Direct Injection Stratified Charge (DISC) combustion systems, which consequently have received great interest, especially in Europe and Japan.
A key component for realizing stable, lean operation in a DISC engine is the fuel injector. In this type of engine, the time available for fuel atomization and mixture preparation is significantly shorter than in conventional port fuel injected engines. Furthermore, the position of the spray and structure of the fuel cloud generated by the injector must remain stable, regardless of the speed and load, and must not be sensitive to in-cylinder cycle-to-cycle variations.
The objectives of the work presented in this thesis were to investigate the spray formation, fuel distribution and combustion of sprays generated by the most common types of fuel injectors used for spray-guided DISC engines and thus gain a better understanding of the physical properties and parameters governing the mixture formation and combustion processes. The presented results were obtained from both spray chamber and single cylinder engine tests, using advanced laser-based measuring methods and high speed video photography in conjunction with measurements of emissions and standard engine operating parameters, such as pressure, temperature and heat release. In the investigations the main focus was on the stratified charge operating mode, due to its great potential to improve fuel consumption and CO2 emissions in parts of the speed-load operating range in which the efficiency is normally very low for conventional gasoline engines.
The results show that two types of injectors (multi-hole and outward-opening A-nozzle injectors) can be used for spray-guided systems. The flexibility in terms of spray shape offered by multi-hole nozzles can be very advantageous, but the sprays they deliver have tendencies to over-penetrate and to create steep fuel concentration gradients near the spark plug. Furthermore, careful optimization of the multi-hole injector geometry is necessary to ensure misfire-free combustion in stratified charge operation. A-nozzles provide additional means to control the mixture formation since their needles are directly actuated by a piezo crystal, enabling multiple injections and variation of the fuel flow. The results show that this can help reduce the spray penetration and provide means to extend the operational window of misfire-free, stable stratified charge combustion in terms of both ignition and injection timing parameters.
Furthermore, tests performed in an optical single-cylinder engine have shown that a DISC combustion system fitted with a piezo-actuated A-nozzle has the potential to lower the fuel consumption and CO2 emissions by over 40% at low load and speed when operated in stratified charge mode compared to homogenous charge operation.
A further investigation showed that a dual-fluid air-assisted injector which uses compressed air to internally atomize the fuel displays favorable atomization properties, but limitations in possible injection timings due to high cylinder back-pressures and low fuel injection pressure severely limit the scope for using it when the engine is operated in stratified charge mode.