Alcohol-based fuels are a viable alternative to fossil fuels for powering
vehicles. As a drop-in fuel, an oxygenated fuel blend containing the
C8 alcohol 2-ethylhexanol (isomer of octanol), hydrotreated
vegetable oil (HVO) and rapeseed methyl ester (RME) can reduce soot and NOx
emissions whilst maintaining engine performance. However, fuel injection
strategy significantly affects combustion and hence has been investigated with a
view to reducing emissions whilst maintaining engine efficiency. In a single
cylinder light-duty compression ignition research engine, the effect of
different injection strategies (main, main/post, double pre/main, double
pre/main/post injection) and EGR levels (0%, 19%) on specifically NOx, soot
emissions and particle size distribution was investigated for three different
fuels: fossil diesel fuel, HVO and the oxygenated blend. The blend was designed
to have diesel-like combustion properties (cetane number of 52) and had an
oxygen content of 5.4% by mass. The crank angle used when measuring MFB50, fuel
consumption and IMEP was kept constant.
The engine efficiencies were similar for all tested fuels and injection
strategies. Heat release analysis revealed a strong influence of the cetane
number on main and main/post injection strategy. However, when using double
pre-injection, the start of combustion was similar for all fuels. Combustion
characteristics, particle mass and number were more affected when using double
pre-injection rather than post-injection. With 19% EGR and double pre-injection,
soot mass increased as agglomerated particle mode increased in the PSD. Further,
the in-cylinder temperature and pressure were lower compared to combustion
without EGR, leading to a reduction of NOx emissions by a factor of 2.5 while
soot emissions increased by a factor of 10. There were just minor differences in
NOx emissions with variations in injection strategy. The PSD moved towards
smaller particle diameters without EGR. In conclusion, the soot reduction
potential of all fuels tested was coupled to the use of double pre-injection and
EGR rather than post-injection.