Influence of EGR and oxygen-enriched air on diesel engine NO–Smoke emission and combustion characteristic

Abstract

The oxygen enriched combustion of diesel engines can reduce smoke emission and increase engine thermal efficiency; however it can also lead to an increase of NO emission. In this paper, experiment was conducted on a turbocharged direct injection diesel engine, and oxygen-enriched and EGR techniques were used to produce lower NO–Smoke emission than the unmodified engine under the same fuel supply rate curve and fuel supply quantity. The specific fuel consumption and the power loss were lower than 5% compared to the unmodified engine. The effect of oxygen enrichment on the particle size distribution was tested and analyzed. The results revealed that the optimal NO–Smoke emission can be achieved at these conditions: 1600rpm of engine speed, full load, 30–40% EGR rate and 21.5–22.5% of intake oxygen density; 2200rpm of engine speed, full load, 20–45% EGR rate and 22–24% of intake oxygen density. The result of particle size distribution tests revealed that oxygen enriched combustion can effectively suppress the diameter growth of particles and lead to fewer large particles with a diameter larger than 100nm emissions; however it did lead to an increase of 15nm small particles. A reduced n-heptane kinetic model was also developed in this research which contained NO and PAHs formation mechanisms, and the model was coupled with a CFD model to simulate the oxygen-enriched combustion of a diesel engine. The calculated results demonstrated that the coupled model can accurately predict ignition time and the change of in-cylinder pressure when the combined oxygen-enriched and EGR technique was used. The computed NO change with in-cylinder oxygen density agreed well with experiment results, and the computed result of the growth experience of PAHs showed that oxygen-enriched combustion can effectively suppress HACA reaction during PAHs formation, which leads to the reduction of large molecule PAHs, and this result agreed well with the observed situation that particle size diameter decreases with the increase of intake oxygen density.