Experimental analysis on the effects of multiple injection strategies on pollutant emissions, combustion noise, and fuel consumption in a premixed charge compression ignition engine

Early single-injection premixed charge compression ignition (PCCI) strategies in compression ignition engines have been widely studied as a promising solution to meet the ever-increasing stringent emissions regulations. Although their application to diesel engines may provide several upsides (such as a massive and simultaneous reduction of NOx and soot engine-out emissions), especially at low to medium loads, several drawbacks, including an excessive amount of engine-out carbon monoxide (CO) and unburned hydrocarbons (HC) as well as intense combustion noise (CN), usually reveal to be major constraints. As a matter of fact, PCCI combustion systems are not yet consolidated enough for practical applications, although intensive research has been carried out to overcome its common limitations. Indeed, further research is still required. In this work, an experimental analysis has been carried out to highlight the potential benefits derived from the introduction of multiple (i.e., double and triple) fuel injection PCCI strategies on a 3.0-liter diesel engine, purposely designed to be operated with PCCI combustion concepts at low to medium engine loads. The experimental tests include the application of several fuel injection strategies: double- and triple-pulse PCCI schemes, featuring various fuel injection timing sweeps, and different fuel quantity distributions among each fuel shot were compared with a baseline single-pulse PCCI and a triple-injection conventional diesel combustion (CDC) pattern. The results are presented in terms of exhaust pollutant emissions, CN, and fuel consumption at two different engine operating points within a low to medium speed and load area of the engine map. Splitting the fuel injection into a double-stage pattern turned out ensuring appreciable drops of both engine-out HC and CO emissions, up to 50% lower than the single-injection PCCI levels, but still significantly worse than CDC outcomes, with penalties above +120%. Engine-out soot and nitrogen oxides (NOx) emissions retained considerably smaller than typical CDC values (with abatement ranging between −50% and −99%), while double-pulse PCCI calibrations featuring delayed second injection timings allowed to effectively dampen excessive CN intensity (of up to 7 dBA below the CN of the reference single-stage PCCI schedule), while slightly improving fuel economy. Finally, the introduction of a triple-stage pattern in PCCI revealed to have the potential to further reduce the emissions of incomplete combustion species and fuel consumption, when compared with single- and double-injection patterns (especially at low load), as well as to further deadening CN. However, besides being still ineffective in reaching the CDC performance even at low load (bsfc slightly over +4%, HC no better than +100%), when increasing the engine load, these benefits become milder to the point that the soaring calibration complexity required by a triple-stage PCCI pattern might not render it worthwhile