151 Exhaust Gas Recirculation

Two items in Fig. 1.18, the electronic vacuum regulator at (30) and the exhaust gas recirculation (EGR) valve at (31), play an important part in this and many other engine management systems and they warrant some attention. In order to reduce emissions of NOx it is helpful if combustion chamber temperatures do not rise above approximately 1800°C because this is the temperature at which NOx can be produced. Exhaust gas recirculation helps to keep combustion temperatures below this figure by recirculating a limited amount of exhaust gas from the exhaust system back to the induction system, on the engine side of the throttle valve. Figure 1.20 shows the principle of an EGR system.

Fig. 1.20 Exhaust gas recirculation system

In order to provide a good performance, EGR does not operate when the engine is cold or when the engine is operating at full load. The inset shows the solenoid valve that controls the EGR valve and this type of valve is operated on the duty cycle principle. Under reasonable operating conditions it is estimated that EGR will reduce NOX emissions by approximately 30%.

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