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ALL ABOARD THE CLEAN ENGINE

Automotive Emissions and Onboard Diagnostics

by John West & Mark Stachew

Start ý Why Onboard Diagnostics? ý Major Components ý Catalytic Converter ý Oxygen Sensor ý EGR ý Fuel System Adaptive Learning ý Misfire Detection ý The Need for Speed ý New Regulations ý 0 to 100
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EGR

Exhaust gas recirculation (EGR) reduces NOx emissions by cooling the combustion process. Nitrogen in the air combines with oxygen during combustion to form NOx. The EGR valve directs exhaust gas to the intake when engine loading is light. The exhaust gas is inert so it doesnýt burn and absorbs energy during combustion. This, along with diluting the air/fuel mixture, lowers the combustion temperature below the critical point where nitrogen and oxygen combine, thus reducing NOx emissions.

Precise control of the EGR valve by the EMS is necessary to optimize engine performance. EGR is not active on starts, idling, nor wide open throttle. Adjusting EGR too high will severely reduce engine performance. The normal reduction in performance associated with EGR can be compensated for by more aggressive spark timing. The lower combustion temperature allows the spark event to occur at the optimal compression with less chance of engine knock.

Figure 3 shows the EGR valve with respect to the overall emissions system.

Figure 3ýHere you can see the EGR in relation to the whole emission system.

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