The influence of Michael acceptors on the structural reactivity of renewable fuels

Abstract

Expanding the use of sustainable fuels in hard to decarbonise transport vehicles utilising heavy-duty engines is urgently required to reduce greenhouse gas emissions from sectors reliant on these engines. As biofuel production turns to alternative sources of biomass with a differing chemical composition to fossil fuel, it is increasingly important to understand how chemical functional groups which may be present in biomass influence the process of combustion. Biofuels are more homogeneous in chemical composition than fossil diesel or gasoline, which are a variety of compounds with a common range of boiling points. As the transport industry progresses in replacing fossil volumes with renewable liquid fuels it also moves towards fuels which are more homogeneous in chemical composition and therefore reactivity. Esters and carbon–carbon double bonds are two common functional groups found in biodiesel and many other classes of bioderived molecules. When adjacent to each other in a specific conformation, they are classed as a Michael acceptor functional group which has a unique reactivity with free radicals separate to either the ester or alkene alone and may play a key role in the low temperature reactions preceding auto-ignition in the combustion process. In this study, the combustion and emissions characteristics of a series of saturated and unsaturated fatty acid esters were tested as single component test fuels, to observe how the inclusion of a Michael acceptor group in esters influences reactivity in a heavy-duty compression ignition engine. Under constant injection duration and timing conditions, it was found that the inclusion of the Michael acceptor in methyl non-2-enoate reduced the duration of ignition delay and increased the IMEP relative to methyl nonanoate and methyl non-3-enoate. Conversely, the inclusion of the Michael acceptor in C₈ and C₁₀ ethyl esters resulted in a longer duration of ignition delay and similar observed IMEPs.