Heat Capacity Changes Predict Nitrogen Oxides Reduction by Exhaust Gas Recirculation

Earlier work has demonstrated that exhaust gas recirculation (EGR) decreases peak combustion temperature and thus reduces the concentration of nitrogen oxides (NO_x) in spark ignition engine exhaust. The present authors hypothesized that NO_x formation is primarily affected by the heat capacity of the combustion gases and recycled exhaust. The hypothesis was tested in an experimental program involving the admission of inert gases such as He, Ar, H₂, and CO₂, and water in place of EGR.

In addition to confirming the validity of the original hypothesis, the test data also indicated that engine output and efficiency were significantly affected by the heat capacity of the combustion gases. The authors conclude that EGR functions by increasing the heat capacity of the working fluid, and demonstrates that the correlative changes in NO_x and engine performance can be predicted from heat capacity considerations. These studies, together with further work on combustion pressures, should be useful in evaluating various NO_x control techniques.