Diffusion Combustion Phenomena in GDi Engines caused by Injection Process 2013-01-0261
Particulate matter emissions are no longer only a concern in the development of Diesel engine powertrains. In addition to particulate mass requirements, the new European legislation for Euro 6 includes a proposed particulate number requirement for all vehicles with gasoline direct injection engines. Euro 6b will establish the first requirement in 2014 which will then be significantly reduced with the implementation of Euro 6c in 2017. This might coincide with the introduction of the World Light Duty Testing Procedure vehicle drive cycle test, raising the bar even higher to reach compliance to the particulate number legislative requirements. Several different investigations revealed that the particulate number emission will become very challenging while the limit for particulate mass can already be met with today's applications.
The injection system has a dominant impact on the mixture formation and homogeneity and therefore is in the center of detailed investigations targeting reduced particulate number emissions. Optical investigations inside the combustion chamber of a single cylinder engine have shown that late in the combustion cycle some diffusive combustion radiation is happening in the vicinity of the tip of a coked injector (i.e. deposits on the tip). A quantitative measurement method has been developed by evaluating the size and duration of the late diffusive radiation event. This derived value - called the injector diffusion flame - correlates quite well with the particulate number measured with a particulate counter as well as with soot mass measurements.
In order to reliably reduce the critical particulate number emissions, both engine control strategies as well as injector component characteristics need to be optimized. On the control side the increase of the fuel pressure has shown the strongest effect reducing injector diffusion flame, while during warm engine operation multiple injections very often lead to an increase of the injector diffusion flame. On the component side, a detailed optimization of the relevant injector tip parameters controlling the atomization as well as the fuel injector tip interaction is necessary to lower the particulate number emissions. With further ongoing refinement on the injector and control system, the particulate number emissions target seems to be in reach for gasoline direct injection engine powered vehicles without the application of a particulate filter.