A portion of the exhaust gases is recirculated into the combustion chambers. This can be achieved either internally with the proper valve timing, or externally with some kind of piping, Figure 5.1 shows this schematically.

The exhaust gas acts as an inert gas in the combustion chamber, it does not participate in the combustion reaction. This leads to a reduction of the combustion temperature by different effects.

The fuel molecules need more time to find a oxygen molecule to react with, as there are inert molecules around. This slows down the combustion speed and thus reduces the peak combustion temperature, as the same amount of energy is released over a longer period of time.

The energy is also used to heat up a larger gas portion than it would without EGR. As the air is diluted with exhaust gas, the mass of a gas portion containing the needed amount of oxygen gets bigger.

Another effect is the change in heat capacity. Exhaust gas has a higher specific heat capacity than air, due to the CO2-molecule’s higher degree of freedom. So for the same amount of combustion energy a gas mass containing EGR will get a lower temperature than pure air.

The lower combustion temperature directly reduces the NOX formation, as the NOX formation rate is highly temperature dependent, Exhaust gas recirculation has been widely used since 1970s to reduce NOx 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 NOx 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 NOx 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 NOx control is obtained by retarding injection timing. A typical EGR map for a passenger car diesel engine is shown on Fig 5.2 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.