The Dilution, Chemical, and Thermal Effects of Exhaust Gas Recirculation on Diesel Engine Emissions - Part 2: Effects of Carbon Dioxide

Abstract

<div class="htmlview paragraph">This is the second of a series of papers on how exhaust gas recirculation (EGR) affects diesel engine combustion and emissions. It concentrates on the effects of carbon dioxide (CO<sub>2</sub>) which is a principal constituent of EGR.</div> <div class="htmlview paragraph">Results are presented from a number of tests during which the nitrogen or oxygen in the engine inlet air was progressively replaced by CO<sub>2</sub> and/or inert gases, whilst the engine speed, fuelling rate, injection timing, inlet charge total mass rate and inlet charge temperature were kept constant.</div> <div class="htmlview paragraph">In one set of tests, some of the nitrogen in the inlet air was progressively replaced by a carefully controlled mixture of CO<sub>2</sub> and argon. This ensured that the added gas mixture had equal specific heat capacity to that of the nitrogen being replaced. Thus, the effects of dissociated CO<sub>2</sub> on combustion and emissions could be isolated and quantified (chemical effect).</div> <div class="htmlview paragraph">In another set of tests, some of the nitrogen in the inlet air was progressively replaced by a carefully controlled mixture of helium and nitrogen. This ensured that the mixture replacing the nitrogen in the air had a specific heat capacity equal to that of CO<sub>2</sub>. Thus the effects of the high heat absorbing capacity of CO<sub>2</sub> on combustion and emissions could be isolated and quantified (thermal effect).</div> <div class="htmlview paragraph">In a final series of tests some of the oxygen in the inlet air was progressively replaced by CO<sub>2</sub> in order to quantify the overall effects of CO<sub>2</sub> on combustion and emissions and compare with the individual chemical, thermal, and dilution effects.</div> <div class="htmlview paragraph">It was found that CO<sub>2</sub> dissociation had small but measurable effects on exhaust emissions. The high heat absorbing capacity of CO<sub>2</sub> had only a small effect on exhaust emissions (including NO<sub>x</sub>); this finding challenges conventional wisdom as to the importance of the higher heat capacity of CO<sub>2</sub> in suppressing NO<sub>x</sub> when EGR is added to the inlet air of diesel engines.</div>