Effect of Addition of Diluents on NO Formation

Burned residual gases left from the previous cycle act as charge diluents. A part of exhaust gas is also recirculated back to the engine for diluting the intake charge to reduce NO formation. This process is known as exhaust gas recirculation (EGR). Dilution by the burnt residual gases is also called as ‘internal EGR’. The combustion temperatures decrease as a result of charge dilution and the lower combustion temperatures result in lower NO levels. The effect of different diluents on NO emissions is shown on Fig. 2.8(a) The equal volume of different diluents gives different NO reductions.  CO₂ and H₂O being tri-atomic gases have higher specific heat and give larger NO reductions than the same volume of N₂, He or Ar. The NO emission data with different diluents correlates very well with heat capacity (mass flow rate of the dilutent x specific heat) irrespective of the  chemical nature of the diluents as shown in Fig. 2.8(b) . It shows that the effect of charge dilution on NO is almost entirely due to the heat capacity of the diluting gases.  The specific heat of the exhaust gas is higher than  for air due to presence of substantial fractions of CO₂ and H₂O.  Hence, EGR results in lower combustion temperatures compared to those from dilution by nitrogen alone or by leaning of mixture.

A negative effect of charge dilution is reduction in oxygen concentration in the charge and slowing down of flame propagation speed and the rate of combustion. It causes further reduction in the burned gas temperature and beyond a limit causes misfired combustion resulting in lower fuel efficiency and higher unburned hydrocarbon emissions