Effects on Combustion and Emissions of Early and Pilot Fuel Injections in Diesel Engines

Different injection strategies applied to a common rail direct-injection (D.I.) diesel engine were tested for different engine torque and speed conditions. The injection strategies differ for the use of early and pilot injections; during the tests the injection parameters were varied, in terms of duration and timing of early, pilot and main injections. The combustion behavior and the engine performances, in terms of brake specific fuel consumption, were analysed. In addition data of nitric oxides, total unburned hydrocarbons and particulate emissions were collected. The injection strategy based on both early and pilot injections has been compared with the techniques using either pilot or early injections. Results show that, particularly at lower values of engine torque and speed, the small fuel quantity injected during early injection, coupled with the pilot injection, leads to comparable levels or even to a sensible reduction of fuel consumption compared to the only pilot or only early injection cases. Furthermore, a reduction in NOx and particulate is generally observed, while the level of unburned hydrocarbons usually increases. Experimental tests have shown that, using the early injection, a very lean premixed charge is obtained, both globally and locally, inside the combustion chamber, thus avoiding diesel problems, in particular high NOx and soot production, mainly caused by the locally rich mixture. On the other hand, by using the pilot injection the ignition delay of the main injection is reduced, contributing to the NOx reduction