Diesel Combustion Enhancement through an Early Injection-Based Injection Strategy

A new multiple injection strategy was tested, obtained performing an early, a pilot and a main injection. The aim of this strategy is in fact to obtain the benefits of both the premixed lean diesel combustion and the multiple injection systems already on the market. Tests were carried out for different engine torque and speed conditions, and the effect of the injection parameters, like duration and timing of early and pilot injections was investigated. The combustion quality, in terms of nitric oxides, total unburned hydrocarbons and particulate emissions, and the engine performances, in terms of fuel consumption and combustion stability, were analysed. The performance of the aforementioned injection strategy have been compared with the results obtained using either pilot or early injections. Results show that the fuel quantity injected during the 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 both NOx and particulate is always obtained, 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 a heterogeneous distribution of air-fuel ratio and consequently temperature, leading to high NOx and soot production. On the other hand, by using the pilot injection, the ignition delay of the main injection was stabilized, thus leading to a regular engine operation.