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Smokeless and low NOx combustion in a dual-fuel diesel engine with induced natural gas as the main fuel

Abstract In a compression ignition engine, using a rich and lean biform mixture composition that avoids both slightly lean and extremely over-rich regions would be effective in suppressing NOx formation without increasing smoke when the overall air-fuel ratio approaches the stoichiometric ratio. To realize the formation of rich and lean mixtures and the control of ignition timing, a dual-fuel diesel engine with an induced gas with resistance to self-ignition as the main fuel and with a small quantity of diesel fuel for the ignition source has potential merits. However, this method has the problem of knocking and misfiring when the percentage of inducted fuel is increased. In this research smokeless and very low NOx combustion without knocking over a wide operating range was established in a single-cylinder dual-fuel diesel engine with induced natural gas as the main fuel. Optimizations of the combustion chamber shape and operating factors, including exhaust gas recirculation (EGR) and intake air throttling, which determine conditions of the in-cylinder gas, were investigated at several i.m.e.p. (indicated mean effective pressure) conditions. The results of the experiments showed that a combination of the divided cavity, EGR and intake air throttling was effective in simultaneously eliminating knocking and reducing total hydrocarbon (THC) and NOx over a wide i.m.e.p. range. At high i.m.e.p. silent and smooth combustion without knocking was achieved, even with a large amount of induced natural gas. Moreover, the maximum i.m.e.p. increased in comparison with conventional diesel operation with a lower injection pressure system because smoke emission was not a limitation. At medium i.m.e.p. it was effective to adopt EGR and intake air throttling for very low NOx under relatively lower THC. However, at low i.m.e.p. it was difficult to avoid increases in THC and i.s.e.c. (indicated specific energy consumption) while realizing very low NOx and smokeless operation. At lower overall excess air ratio conditions, NOx reduction was shown with a biform mixture composition without slightly lean or extremely rich regions.