Using exhaust gas recirculation in internal combustion engines: a review

The aim of this work is to review the potential of exhaust gas recirculation (EGR) to reduce the exhaust emissions, particularly NOX emissions, and to delimit the application range of this technique. A detailed analysis of previous and current results of EGR effects on the emissions and performance of Diesel engines, spark ignition engines and duel fuel engines is introduced. From the deep analysis, it was found that adding EGR to the air flow rate to the Diesel engine, rather than displacing some of the inlet air, appears to be a more beneficial way of utilizing EGR in Diesel engines. This way may allow exhaust NOX emissions to be reduced substantially. In spark ignition engines, substantial reductions in NO concentrations are achieved with 10% to 25% EGR. However, EGR also reduces the combustion rate, which makes stable combustion more difficult to achieve. At constant burn duration and brake mean effective pressure, the brake specific fuel consumption decreases with increasing EGR. The improvement in fuel consumption with increasing EGR is due to three factors: firstly, reduced pumping work; secondly, reduced heat loss to the cylinder walls; and thirdly, a reduction in the degree of dissociation in the high temperature burned gases. In dual fuel engines, with hot EGR, the thermal efficiency is improved due to increased intake charge temperatures and reburning of the unburned fuel in the recirculated gas. Simultaneously, NOX is reduced and smoke is reduced to almost zero at high natural gas fractions. Cooled EGR gives lower thermal efficiency than hot EGR but makes possible lower NOX emissions. The use of EGR is, therefore, believed to be most effective in improving exhaust emissions.