Effect of Injection Strategy and EGR on Particle Emissions from a CI Engine Fueled with an Oxygenated Fuel Blend and HVO

Abstract

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.