Thanks for the additional information.
I already understood why it happens but did not know that the endurance guys
purposely run that "comfortably rich" setup. Though once you said that, it
"clicked" as to why they'd do that.
Oh, and to reply to another's comment... yes, it is admittedly cool to watch.
-STEFAN
In a message dated Mon, 18 Jun 2001 1:13:20 PM Eastern Daylight Time,
jac73@daimlerchrysler.com writes:
<< On Mon, 18 Jun 2001 10:13:37 EDT, Stefanv@aol.com said:
<SNIP>
>As I understand it.
To add to your understanding: The flames out the exhaust were only on
"overrun", or closed-throttle conditions. In an endurance race, it's
customary to set the fuel mixture into the "comfortably rich" zone, a bit
richer than the best fuel/air ratio for peak power production, in order to
assist the longevity of the engine for 6, 10, 12, or 24 hours. This is
particularly true for turbocharged engines (like the Audis, Cadillacs,
Bentleys), but is also done on normally-aspirated engines (I've seen
overrun flames from the Corvettes and Vipers, for example).
The reason for "comfortably rich" as opposed to stoiciometric or even
slightly lean in an endurance racing engine is simple: lean means higher
combustion temperatures, and a far greater potential for Bad Things to
happen to pistons (like holes getting torched in the piston crowns).
Running a little rich will actually cool the burn slightly, and having more
fuel than oxygen means there's some leftover fuel -- very little under
load, but there's a bit more on an overrun condition where the throttle
closes and there's a wee bit of lag between that action and the injection
system reducing the fuel shot. That extra bit of fuel passes through the
cylinders, the turbocharger (if equipped) and lights off in the exhaust --
or right at the exit of the exhaust where there's more oxygen available.
You won't see the flames out the exhaust under load, except in Hollywood
movies.
Yes, it uses a wee bit more fuel than running stoic or lean, but it's also
cheap insurance that the engine will be running in a fit state at the end
of the race (because, of course, you have to be running at the end to be
classified as a finisher...).
I watched an awful lot of SpeedVision this weekend...
Jim "Yeah, I'm an engineer" Crider
autojim@att.net
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