The effects of long chain alcohol blends on engine performance and spray characteristics in CI engines

Licentiatavhandling, 2018

Replacing fossil fuels with alternatives derived from renewable sources is one way of meeting society’s increasing need for mobility while also significantly reducing greenhouse gas emissions from vehicles with internal combustion engines. To facilitate such a replacement, this thesis presents optical spray studies and engine experiments conducted to evaluate the potential of blends of biomass-derived alcohols and vegetable oils to serve as drop-in fuels for compression ignition engines.

A drop-in fuel is a fuel whose properties match those of fossil Diesel closely enough to allow its use in unmodified conventional Diesel engines without any adjustment of their calibration settings. Two C8-alcohols (n-octanol and its isomer 2-ethylhexanol) and two C10-alcohols (n-decanol and its isomer 2-propylheptanol) were blended with hydrotreated vegetable oil (HVO) and rapeseed methyl ester (RME) or fossil Diesel. The percentage of fossil Diesel in the blends was 10% or 20%. Blends without any Diesel contained 7% RME. The impact of blend composition on the performance and emissions of a Volvo D13 single cylinder heavy duty research engine operated with standard settings was then investigated by considering four load points adapted from the European Stationary Cycle.

To complement the engine studies, the spray characteristics of selected blends of 2-ethylhexanol and HVO with either 20% Diesel or 7% RME were investigated in a high-pressure/high-temperature chamber. Optical experiments were performed under non-combusting and combusting conditions at injection pressures of 120 and 180 MPa, respectively.

The engine experiments revealed that the performance with the studied blends was comparable to that achieved with standard Diesel in terms of the indicated thermal efficiency. Because of the blends’ lower heating value, they yielded slightly lower indicated mean effective pressures than conventional Diesel, leading to increased specific fuel consumption. The yields of HC and NOx achieved with the blends did not differ significantly from those seen with Diesel. However, the blends produced less carbon monoxide and significantly less soot. Fully renewable blends with RME and no fossil Diesel produced the lowest soot emissions because they had the highest oxygen content and contain no aromatic compounds, which are known soot precursors. Particle size distribution analyses showed that standard Diesel contained considerably more particles with diameters above 23 nm than the blends. The blends thus yielded large numbers of small soot particles (> 23nm) and a higher total number of particles than Diesel.

Several optical methods were used to characterize the combustion process. Shadow imaging under non-combusting conditions was used to measure the blends’ liquid and vapor penetration lengths. The HVO and EH blends exhibited greater liquid penetration lengths than the other tested fuels, but all fuels exhibited similar vapor penetration.

Under combusting conditions, the lift-off length, ignition delay, start of soot formation, and soot volume fraction were measured by simultaneously recording time-resolved two-dimensional laser extinction, flame luminosity and OH*chemiluminescence images. All tested fuels had similar liquid penetration lengths. Despite their different CNs, Diesel and HVO had similar ignition delays. Conversely, the EH blends had longer ignition delays than Diesel despite having the same CN. The ignition delay must thus depend on other parameters such as the latent heat of evaporation. The lift-off length was found to be highest for the blend with the highest content of EH. In accordance with the engine experiments, the blends yielded a lower soot volume fraction than conventional Diesel.

 

Overall, the obtained results indicate that blends based mainly on long-chain alcohols could be viable replacements for fossil Diesel fuel.