Operational and Environmental Evaluation of Diesel Engines Burning Oxygen-Enriched Intake Air or Oxygen-Enriched Fuels: A Review

A method to curtail emissions of smoke and other pollutants from diesel engines is to enhance the oxygen supply to their combustion chamber. This can be accomplished by enriching either the intake air stream or the fuel stream with oxygen. Experimental studies concerning the oxygen-enrichment of intake air, have revealed large decrease of ignition delay, drastic decrease of soot emissions as well as reduction of CO and HC emissions while, brake specific fuel consumption (BSFC) remained unaffected and increasing of power output is feasible. However, this technique was accompanied by considerable increase of NO_x emissions. Experimental and theoretical studies with oxygenated fuels have demonstrated large decrease of soot emissions, which correlated well with the fuel oxygen content. Reduction of CO and HC emissions with oxygenated fuels was also obtained. However, penalties in both BSFC and NO_x emissions have been observed with oxygenation of diesel fuels. In both cases one has to weigh the tradeoffs in fuel economy, in power output and in the emissions of various pollutants. Moreover, fuel cost, availability and supply infrastructure, as well as equipment and operational costs, are among concerns that apply to these techniques. This manuscript presents a comparative evaluation of the two techniques regarding engine performance characteristics, environmental repercussions and economy of operation. The primary objective is to contrast the benefits and the drawbacks of the two techniques in view of economic, operational and environmental parameters. Results have shown that the overall economy of operation of the two techniques may be comparable, if the price of oxygenated fuel blends is similar to that of diesel fuel. Their impact on pollutant emissions may also be comparable, if the oxygen enrichment of either technique is limited to a low level (<23% by mass in the cylinder mixture). However, there are possibilities of increasing the power density of engines with oxygen enrichment of the intake air.