"Rodan" wrote:
The EGR valve richens the mixture
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"MasterBlaster" wrote:
Thank God you don't work on cars for a living.
Perhaps you could explain how Recirculating a ****tion of the
now-inert Exhaust Gases could possibly richen the mixture?
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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.
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