Oxymethylene Ethers: Quantifying the Effect of Fuel Chain Length and Water Emulsification on Emissions and Combustion in a Heavy-Duty Diesel Engine via Linear Regression Analysis

Due to the nature of diffusive combustion, diesel engines display a distinct trade-off between nitrogen oxide (NO_X) and particulate matter (PM). Since emission regulations become ever stricter, the relevance of dissolving this trade-off increases steadily as it hinders engine development from achieving ultralow emission levels. Seeking new opportunities to approach the problem, the modification of fuel properties has gained much attention. In particular, oxygenated fuels reduce particle emissions drastically, while having little adverse impact on NO_X. Similarly, water (H₂O) emulsification of diesel is commonly reported to reduce both NO_X and PM. Both methods appear very promising, yet only few investigations were conducted in an effort of combing the benefits of the two.

With this work, the authors provide a detailed study on combustion and emissions for both neat oxymethylene ethers (OME₂-OME₅) and an H₂O-emulsified OME mixture (OME_mix). By varying injection pressure and fuel properties (i.e., fuel chain length/H₂O fraction in OME_mix) separately, the sole impact of altered fuel on the investigated measurands (NO_X, carbon dioxide [CO₂], combustion speed, cylinder pressure, heat release) could be distinguished and quantified on the basis of a linear regression analysis. It was found that the impact of fuel properties does not interact with varied operating conditions (i.e., rail pressure); thus the impact of altered fuel (chain length, H₂O content) onto the measurands is nearly constant. With respect to NO_X, the effect of increased fuel chain length (−0.73 g/kWhi with an increase in chain length of one) was found to be approximately twice as pronounced when directly compared with increasing H₂O concentration (−0.41 g/kWhi with a 1% increase in H₂O share). It was found that CO₂ emissions are directly correlating to a rising fuel chain length (approx. +23 g/kWhi with an increase in chain length of one). The latter also affects the combustion process (i.e., combustion speed, max. heat release, max. cylinder pressure) in near-linear correlation. Contrary to the impact of neat OME_n chain length, an additional H₂O share in OME_mix barely affected the combustion process.