Revealing Fuel Effects on Catalyst Heating Operation Performance in an Optical Diesel Engine

Abstract Diesel oxidation catalysts and selective catalytic reduction systems are two of the most commonly used exhaust after treatment methods, which aid on- and off-road diesel engines to comply with stringent emissions regulations. Effective operation of these devices requires them to reach their light-off temperatures as quickly as possible after engine startup. Late-cycle heat release achieved using different postinjection strategies has been identified as an effective approach to increase exhaust temperatures, which contributes to effective catalyst heating operation. Previous research has identified a tradeoff between exhaust temperatures and hydrocarbon (HC) emissions that limits postinjection retardability, and therefore the maximum achievable exhaust enthalpy. This work explores the effect of fuel properties, including cetane number, distillation characteristics, and oxygen content, on the tradeoffs between exhaust enthalpy and combustion efficiency in catalyst heating operation. Experimental investigations are performed in a single cylinder optical diesel engine for fuels with different cetane numbers and distillation characteristics, and for blends of diesel fuel with 1-octanol and di-butyl ether, with an injection calibration comprising of one pilot, one main and one postinjection. Increasing the cetane number seven units (from 43 to 50) reduces HC emissions nearly by 60% with no significant changes in exhaust heat flux, whereas the HC emissions and exhaust heat flux for fuels with different distillation characteristics show lower sensitivity to postinjection timings. Diesel blends with 20% by vol. of oxygenates (di-butyl ether, polyoxymethylene dimethyl ether, and 1-octanol) show excellent potential to reduce HC emissions even at late postinjection timings, which helps achieve better tradeoffs between exhaust temperatures and engine-out emissions. Simultaneous high-speed visible and infrared imaging techniques are applied to analyze the effects of oxygenated blends on in-cylinder HC formation. Images show that the amount of unburned hydrocarbons formed by the postinjections decreases as the oxygen content and the cetane number of the fuel increases likely because the mixing rate required to burn the fuel efficiently decreases with the oxygen content of the fuel, improving postinjection retardability. The addition of di-butyl ether improves the reactivity of the mixture, which promotes the faster penetration of main combustion in the squish region, which is also beneficial for HC reduction.