Assessment of the effect of low cetane number fuels on a light duty CI engine: preliminary experimental characterization in PCCI operating condition

The goal of this paper is to acquire insight into the influence of cetane number (CN) and fuel oxygen on overall engine performance in the Premixed Charge Compression Ignition (PCCI) combustion mode. From literature, it is known that low reactive (i.e. low CN) fuels increase the ignition delay (ID) and therefore the degree of mixing prior to auto-ignition. With respect to fuel oxygen, it is known that this has a favorable impact on soot emissions by means of carbon sequestration. This makes the use of low CN oxygen fuels an interesting route to improve the applicability of PCCI combustion in diesel engines. In earlier studies, performed on a heavy-duty engine, cyclic oxygenates were found to consistently outperform their straight and branched counterparts with respect to curbing soot. This was attributed to a considerably lower CN. The oxygenate in question, cyclohexanone (C6H10O), has the advantage of being producible in a renewable way from lignin, a second generation biomass waste stream (e.g. paper pulp industry). To investigate the impact of cyclohexanone on diesel combustion and pollutant emissions in greater detail, a parametric test program was carried out in a joint project between Istituto Motori (Naples) and the University of Technology Eindhoven. To decouple the influence of a low cetane number and fuel oxygen content on the engine performance, diesel (commercial high quality diesel fuel), gasoline (commercial high quality gasoline) and cyclohexanone were blended into five mixtures, with varying cetane number or oxygen content. These blends were tested and compared on a modern singlecylinder light-duty (LD) direct injection (DI) research diesel engine. The results suggest that it is not possible to attribute favorable performance to either CN or fuel oxygen, but rather to the combination of both properties. In nearly all investigated work points, a decrease in CN led to a decrease in nitric oxides (NOx) and particulate matter (PM), whilst slightly increasing carbon monoxide (CO) and unburned hydrocarbons (UHC). At an equal CN, the results suggest that fuel oxygen reduces soot emissions and also plays a role in suppressing UHC and CO emissions.