The EGR valve richens the mixture _____________________________________________
"MasterBlaster" wrote:
Thank God you don't work on cars for a living.
Perhaps you could explain how Recirculating a portion of the now-inert Exhaust Gases could possibly richen the mixture? _____________________________________________
The only reason for the existence of the EGR valve is to richen the mixture.
Modern engines run as lean as possible to reduce hydrocarbon emissions and to improve fuel mileage. During operation if the fuel/air mixture becomes too lean, combustion temperatures rise, threatening preignition, burned valves and unwanted NOx generation. The best way to reduce combustion temperature is to richen the mixture. The engine control system actuates the EGR valve to richen the mixture, thereby reducing combustion temperature.
So how does the introduction of hot exhaust gas richen the mixture?
Glad you asked. Before the EGR opens there is a mechanical (not chemical) mixture of 20 per cent reactive oxygen and 80 per cent inert nitrogen in the intake manifold, flowing toward intake valves for further mixing with metered fuel being injected into combustion chambers. If additional inert gas is added (from the EGR valve) it mixes with the nitrogen and with the oxygen and the resultant mixture has a higher percentage of inert gas than before (example: now 18 per cent reactive oxygen and 82 per cent inert gases).
If fuel is being supplied to maintain a lean air/fuel ratio of 15.0 with a 20/80 mix of oxygen/inerts, then changing the oxygen/inerts mixture to 18/82 yields a new air/fuel ratio of 15.0 x (18/20) = 13.5, a richer mixture. No fuel has been added. The available oxygen has been reduced. Result: Richer mixture.
Intake manifold gases do warm up slightly when the hotter EGR gas is mixed in, but combustion temperature can drop hundreds of degrees with a richer mixture.
The only reason for the existence of the EGR valve is to richen the mixture.
Rodan.
Note: EGR valve enrichment is for combustion temperature management, not for power demand. For more power, apply more throttle.