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The Effects of Carbon Dioxide in Egr on Diesel Engine Emissions
Published June 24, 1996 by Institution of Mechanical Engineers in United Kingdom
Event: International Seminar on Application of Powertrain and Fuel Technologies to Meet Emissions Standards
This paper investigates the way that exhaust gas recirculation (EGR) influences diesel engine combustion and emissions. The different effects of carbon dioxide (CO2), which is a principal constituent of EGR, were analyzed and quantified. The engine tests were carried out while the engine speed, fueling rate, injection timing, inlet charge total mass rate and inlet charge temperature were all kept constant.
The overall and individual effects of carbon dioxide on combustion and emissions were studied by replacing oxygen or nitrogen in the inlet air with carbon dioxide and/or inert gases. Additional tests were carried out which investigated the effects of oxygen replacement on ignition delay. Also tests were carried out in which the inlet charge temperature was raised gradually in order to quantify the effects of EGR temperature on emissions.
Findings from the tests included: the effect of CO2 dissociation (chemical effect) on exhaust emissions is small; the high heat absorbing capacity of CO2 (thermal effect) had only a small effect on exhaust emissions including NOx. The reduction in the inlet charge oxygen (dilution effect) is the dominant effect on emissions, resulting in very large reductions in exhaust NOx at the expense of higher particulate and unburnt hydrocarbon emissions and lower engine power output and fuel economy. The higher inlet charge temperature that is normally associated with hot EGR increased NOx; it also increases the exhaust smoke and particulate emissions, but also reduces unburnt hydrocarbon emissions.