Combustion phasing and emission control by using multiple injections on a Heavy-Duty Diesel engine.
Paper i proceeding, 2005
Very early injection strategies are used to get more premixed combustion the level of EGR must be increased substantially to suppress NOx formation and ignition delay at higher loads. This normally leads to higher equivalence ratio and poor energy conversion efficiency thus higher fuel consumption. When less EGR are used, to be able to keep the fuel consumption on an acceptable level, the mixture ignites too early and NOx levels become very high when all the fuel is injected early. This leads to a combination of early injections and more conventional injection timing.
A diesel combustion where all fuel is premixed before it is burned suffers from combustion phasing control problems at higher loads; the heat release is too early. One way to delay ignition and improve phasing is to substitute fresh air with EGR. This is possible to some extent for low loads and light duty. But no publication has shown high EGR usable for high load in a heavy-duty application. Combustion mode shift can be a possible way to reach higher loads, a change to more conventional diesel combustion at high loads. But the inertia of the gas leads to difficulties going from one mode to another, when EGR levels needs to be changed rapidly. By using multiple injections strategies the combustion rate can be phased and the ignition controlled. Combined with high EGR levels and lean mixtures low emission output can be achieved. Multiple injections allow control over how much fuel that is burned at premixed conditions and how much is burned at more diffusion like conditions.