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 single-cylinder 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.