Exhaust Gas Recirculation
The role of EGR is to act as inert diluents and heat sink that reduces the oxygen concentration during combustion and lowers the combustion temperatures. The flame temperatures are reduced as a result of EGR. The NOx formation being an exponential function of temperature, even a small reduction in flame temperature has a large effect on NOx formation. Increase in heat capacity of charge caused by EGR has generally been thought to result in reduction of NOx emissions from SI engines. However, in the diesel engines, EGR can affect NO x reduction in three possible ways. These effects are:
- Dilution effect: It is the reduction in inlet charge oxygen concentration
- Thermal effect: It is the increase in inlet charge heat capacity, and
- Chemical effect: Modification in combustion process as a result of dissociation of CO2 and water vapour
The dilution effect (reduction in inlet oxygen) is the dominant effect in case of diesel engines. The chemical and thermal effects are relatively small.
Figure 3.9 |
Typical effect of effect of EGR on NOx, HC and fuel economy for a turbocharged, intercooled passenger car DI diesel engine |
Typical effect of EGR on NOx , HC and CO emissions for a turbocharged passenger car DI diesel engine is shown on Fig 3.9 . At around 10% EGR, 50% reduction in NOx is obtained with little change in CO and HC. As the EGR rate is increased beyond 15 %, NOx decreases further, but CO, smoke and HC are increased. The excess air declines with increase in EGR causing sharp increase in smoke and loss in fuel economy.
The effect of EGR at the same rate (% of intake air) on diesel NOx is lower compared to SI engine as the exhaust gas in diesel engine contains smaller amounts of tri-atomic gas CO2 .. Exhaust gas recirculation has been used on diesel passenger cars since mid-1990s to reduce NO x emissions. This is being applied now on more and more diesel engines as the emission standards are being tightened. |
