Experimental Studies on Fuel Effects in Dual-Fuel Combustion

The present dissertation belongs to the research field of experimental physics and emissions reduction from a heavy-duty engine. Various alternative fuels such as diesel-like liquid fuels, methane, ethane, hydrogen, and methanol are employed in a single-cylinder research engine to experimentally investigate their combustion characteristics at varying engine conditions. Dual-fuel (DF) combustion is the focal point in this dissertation in which port-injected low reactivity fuel is ignited by a diesel-pilot directly injected close to the top-dead center. The research includes optical study and engine experiments, which aim at extending the fundamental understanding of DF ignition and subsequent combustion progression, improving engine efficiency, performance, and reducing engine-out emissions. DF combustion is a promising engine combustion technology for adopting various fuels, however, there are still operational limitations that need further improvements to meet future clean-energy goals in transportation. The present dissertation consists of 5 journal publications. Four of them investigate primarily the use of methane and one is focusing on methanol DF combustion. Publication 1 investigates diesel-methane DF ignition and progression of subsequent combustion in an optical engine using high-speed imaging of natural luminosity. Publication 2 investigates the effects of pilot fuel properties on DF ignition, engine performance, and engine-out emissions. Publications 3 and 4 study ethane/hydrogen enriched methane DF combustion to improve combustion efficiency, stability, and engine performance at lean conditions. Publication 5 investigates methanol DF combustion with negative valve overlap (NVO) to attain high engine efficiency together with ultra-low pollutant emissions at wide range of engine operating loads. The main findings of this dissertation can be summarized as follows: 1) In diesel-methane DF combustion, premixed flames grow and propagate towards the center of the chamber and at high equivalence ratios, flame propagation is more prominent. The ignition delay time is found to be a function of methane equivalence ratio, charge-air temperature, and pilot-diesel amount. In addition, DF combustion progresses as three overlapping stages with premixed combustion as a dominant mechanism. 2) For small pilot quantities, in general, a higher cetane number fuel with higher aromatic content improves DF combustion. Additionally, high viscosity, density, and distillation temperatures avoid possible leaning out of a pilot spray within methane-air mixture. 3) Ethane or hydrogen enrichment improves the reactivity of pure methane and helps to extend the range of lean operability and attain high thermal efficiency (>50%). 4) It is found that methanol DF combustion under NVO mode can produce high efficiency (>50%) with ultra-low emissions at wide range of engine operating loads.