Quote:
Originally Posted by lazaris
"When watching a F1 engine on a dyno, do you see exhaust flame out the exhaust during a power pull? No. You do see it during coastdown overrun. So fuel can definitely be ignited by a hot exhaust system, but why do you only witness it on a dyno during overrun?"
I believe you only see during overrun because of the pressure wave/pulse causing fresh air to be introduced into the pipe thus igniting the unburned fuel.
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I believe this is true. To add to this, during coastdown, the injector pulse widths are generally kept at a value near what you'd see at normal idle. During coastdown and very low manifold pressures, this causes an over rich condition and allows a lot of unburned fuel to enter the exhaust.
With the fact that we can see sporadic igniting of fuel in the exhaust system during coastdown in some situations, does that gaurantee that extra fuel is always burned in the exhaust system during normal acceleration? Assuming the car has no air pump installed, pumping air directly into the exhaust manifold near the exhaust valve, the answer would be no. That is what catalytic converters are for. During a smog test, if the catalytic converter is bad, you will see a rise in hydrocarbons. The hydrocarbons is fuel that is making its way out of the exhaust system. Unburned.