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Experimental Comparison of Different Strategies for Natural Gas Addition in a Common-Rail Diesel Engine
Published May 23, 2004 by Society of Automotive Engineers of Korea in South Korea
A diesel engine, equipped with a common-rail injection system, has been tested when operating in dual-fuel mode, using diesel fuel and natural gas simultaneously as fuels. The natural gas was premixed with the engine intake air and used in two operating modes: to provide the full power required by the engine, thus eliminating the diesel fuel main injection, and as an additive, in order to improve the engine combustion and to reduce pollutants emissions. During tests, engine efficiency, in terms of fuel consumption, and pollutant emissions, in terms of NOx, opacity, CO and total hydrocarbons, have been measured.
In the first operating mode, the main injection was eliminated and the liquid fuel was injected only during the pilot injection, in order to ignite the premixed charge of air and natural gas. For this operating mode, the dual-fuel combustion shows a higher ignition delay and a lower heat release rate during the diffusive combustion phase with respect to the normal combustion, obtained using only the diesel fuel as a fuel. During the tests, pilot injection timing has been varied, and its effect on engine behavior has been analyzed. Generally, HC, CO and NOx emissions levels at the exhaust are higher, when pilot advance increases. In terms of particulate emissions, this operating mode shows a better behavior with respect to the full diesel fuel operation, thanks to the lower injected quantity of liquid fuel, only limited to the pilot injection.
Using the natural gas in the engine, as an additive fuel, while performing the diesel fuel main injection, leads to maintain unchanged engine efficiency, with respect to the traditional diesel fuel operation mode. Concerning the emissions levels at the exhaust, the use of small quantities of gas (10-30% respectfully to the total fuel energy) improves the NOx-soot tradeoff; however, at the same time, HC and CO emission are characterized by higher values.