EXHAUST GAS RECIRCULATION

Exhaust gas recirculation has been widely used since 1970s to reduce NO_x emissions in the SI engines. During 1990s, when emission standards for diesel passenger cars in Europe were tightened, EGR found application for the first time in small high speed engines and later in the heavy duty diesel engines as well.
With the use of EGR, reduction in NO_x is accompanied with an increase of smoke, particulate   and HC emissions. Fuel consumption also increases with the use of EGR.
 As the EGR is applied, excess air decreases. With 25% EGR in a turbocharged engine at full load operation, the excess air ratio decreased from around 1.7 to 1.3. Simultaneously with 25% EGR, the NO_x reduced by 85%, smoke increased manifold from around 0.5 Bosch smoke units to 3.5 Bosch units and BSFC increased by 8%.  Smoke and BSFC increased sharply beyond about 12% EGR rate.  
At part loads when air-fuel ratios are high, EGR rates even up to 50% can be applied. In practice, on the production engines, EGR is applied at part loads and at high loads NO_x control is obtained by retarding injection timing.  A typical EGR map for a passenger car diesel engine is shown on Fig 6.9. EGR rates are varied with engine load and speed and an electronic control of EGR is usually employed. In addition to the input data on engine speed and load, EGR is controlled based on air mass flow rate. Air mass flow rate is measured by a flow meter. From air flow signal the engine control unit determines EGR rate from a look-up table to control EGR rates. Measurement of airflow rate compensates for any changes in volumetric efficiency that may result due to deposit build-up in combustion chamber, valve timing adjustment etc.