Evaluation of fuel additives for HCCI engines operated on fuel-rich methane/air mixtures: DME, DEE, and n-heptane

Piston engines can be operated with very fuel-rich fuel/air mixtures to simultaneously produce syngas, power, and heat - so-called "polygeneration". In this context, the effects of methoxymethane (dimethyl ether, DME), ethoxyethane (diethyl ether, DEE), and n-heptane as additives in compression-ignition of fuel-rich methane/air mixtures were investigated in experiment and simulation. The experiments were performed in a single-cylinder octane-number test engine at a compression ratio of 10. Engine operating stability, auto-ignition behavior, and syngas production were examined. The simulations used a single zone model with detailed chemical kinetics. The method for pressure trace analysis was adapted to the very fuel-rich conditions examined in this study. The choice of additive does not significantly influence syngas production, but a distinct influence on auto-ignition was found. The most effective additive in terms of mass fraction was DEE which produced stable operation at around 20% by weight. While the use of DME and n-heptane resulted in similar heat release traces, DEE yielded more early heat release and less of a negative temperature coefficient (NTC) behavior. The widest stable oper-ating range in terms of additive fraction was found for DME. A reaction path analysis showed that the effect on ignition is similar for all three additives: they react early in the compression stroke and lead to H-abstraction from CH4. Comparing heat release rates and calculated cylinder temperatures indicated that not only the ad-ditive's reactivity but also its heat capacity contributes to auto-ignition behavior.