Neat Oxymethylene Ethers: Combustion Performance and Emissions of OME₂, OME₃, OME₄ and OME₅ in a Single-Cylinder Diesel Engine

Diesel engines are arguably the superior device in the ground transportation sector in terms of efficiency and reliability, but suffer from inferior emission performance due to the diffusive nature of diesel combustion. Great research efforts gradually reduced nitrogen oxide (NO_X) and particulate matter (PM) emissions, but the PM-NO_X trade-off remained to be a problem of major concern and was believed to be inevitable for a long time. In the process of engine development, the modification of fuel properties has lately gained great attention. In particular, the oxygenate fuel oxymethylene ether (OME) has proven potential to not only drastically reduce emissions, but possibly resolve the formerly inevitable trade-off completely. Although intensified investigations with OME were conducted within the past decade, little is known about the specific influence of fuel properties inherent to unimolecular, high chain-length OME on combustion characteristics, emission performance and particle size. The latter is of special concern, as studies on oxygenate fuels reported increased formation of nanoparticles, which are known to have adverse effects on human health. In this paper, the authors present a detailed analysis of emissions, combustion characteristics, and particle size of neat oxymethylene ethers. A single-cylinder diesel engine was fueled with neat OME₂, OME₃, OME₄ and OME₅ to evaluate effects that directly correlate with chain length. In the process, hydrogenated vegetable oil (HVO) was used as diesel reference fuel. It was found that a high chain length beneficially affects NO_X with little drawbacks on thermal efficiency for the operation without exhaust gas recirculation. This trade-off clearly evolves favorable with high chain lengths, as NO_X emissions are reduced in greater extent than engine efficiency. Particle size is not adversely influenced by the additional, fuel-bound oxygen. In comparison to HVO, all OME display a significant efficiency advantage in lean combustion due to enhanced burnout speed.