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 influenceof cetane number (CN) and fuel oxygen on overall engineperformance 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 degreeof mixing prior to auto-ignition. With respect to fuel oxygen,it is known that this has a favorable impact on soot emissionsby means of carbon sequestration. This makes the use of lowCN oxygen fuels an interesting route to improve theapplicability of PCCI combustion in diesel engines. In earlierstudies, performed on a heavy-duty engine, cyclic oxygenateswere found to consistently outperform their straight andbranched counterparts with respect to curbing soot. This wasattributed to a considerably lower CN.The oxygenate in question, cyclohexanone (C6H10O), has theadvantage of being producible in a renewable way fromlignin, a second generation biomass waste stream (e.g. paperpulp industry). To investigate the impact of cyclohexanoneon diesel combustion and pollutant emissions in greaterdetail, a parametric test program was carried out in a jointproject between Istituto Motori (Naples) and the Universityof Technology Eindhoven.To decouple the influence of a low cetane number and fueloxygen content on the engine performance, diesel(commercial high quality diesel fuel), gasoline (commercialhigh quality gasoline) and cyclohexanone were blended intofive mixtures, with varying cetane number or oxygen content.These blends were tested and compared on a modern singlecylinderlight-duty (LD) direct injection (DI) research dieselengine. The results suggest that it is not possible to attributefavorable performance to either CN or fuel oxygen, but ratherto the combination of both properties. In nearly allinvestigated work points, a decrease in CN led to a decreasein nitric oxides (NOx) and particulate matter (PM), whilstslightly increasing carbon monoxide (CO) and unburnedhydrocarbons (UHC). At an equal CN, the results suggest thatfuel oxygen reduces soot emissions and also plays a role insuppressing UHC and CO emissions.