Mixture formation analysis in a direct-injection NG SI engine under different injection timings

As a matter of fact, the mixture formation process in a direct injection gaseous-fuel engine differs significantly from direct-injection engines fuelled by gasoline. In fact, the gas jet momentum is lower, reducing the penetration, and the mixture formation strongly relies on the charge motion generated during the intake stroke. More precisely, the work presented in this paper showed that several factors exert an influence on the fuel-air mixing process: jet shape, interaction with piston and/or with the charge motion, and time available for mixing between the end-of-injection and the spark timing, and these may combine differently depending on the specific working point. On an average, at low load and low-medium speeds, the injection should better take place during the second part of the induction stroke. On the other hand, at high speed or high load the injection timing needs to be advanced till around 250-300° CA degrees before firing TDC, in order to increase the time available for mixing as much as possible.