Off throttle lean condition benefits

[Birel99]

Hey F1T,

In the past 5 years I have noticed that in most premier level motorsports, that off throttle backfire has become normal.

I am assuming this is caused by extremely lean fuel mapping during closed throttle.

My question is why is this desirable and what are the negative effects of it? What are exhaust temps like during this lean closed throttle stage VS. a normal 'non super lean' condition off throttle?

My own theory is it is an extended measure to save fuel but maybe this is too neglagable to make it worth the effort.

We know that fuel is often used to cool combustion chambers during off throttle conditions, but would this also help as the AFR is so lean that no combustion can occur in the combustion chamber and is detonated in the exhaust?

Any and all information is appreciated.

[Cold Fussion]

The back fire you speak of doesn't occur in the F1 V6 power units and to my memory, it didn't happen with the V8 engines (at least in 2013) either.

[gruntguru]

The best economy measure during zero throttle conditions is no fuel at all. This also produces optimum internal cooling since there is no fire and no heat addition. This will also eliminate backfiring.

If economy is important (and it always should be) the part throttle mixtures should be leaned as far as possible without compromising driveability and durability. This will require additional spark advance.

Traditional tuners (I have banged heads with a few) will often tune much to rich - especially in the part load areas. If you see flames from the exhaust this is probably rich mixture on overrun.

Another problem I have seen several times, is slavish obedience to wide-band-lambda sensors. Conversation goes something like this:
"You can't run leaner than 0.9"
"but the engine makes more power at 0.95"
"No mate - you'll kill your engine"

What he is missing is the fact that the engine is a highly sophisticated race engine making 200 bhp/litre with a sequential injection system and is almost certainly scavenging clean air through the combustion chamber during valve overlap. The extra air is included in the AFR measured by the lambda sensor, but not in the mixture being burned by the engine. So the engine is seeing 0.92 and the lambda sensor sees 0.95.

The other factor permitting leaner mixtures is consistent fuel and air distribution between cylinders. In the situation where a carburettor choke feeds multiple cylinders, the average mixture has to be excessively rich to ensure the leanest cylinder doesn't run too lean.

[Cold Fussion]

Another problem I have seen several times, is slavish obedience to wide-band-lambda sensors. Conversation goes something like this:
"You can't run leaner than 0.9"
"but the engine makes more power at 0.95"
"No mate - you'll kill your engine"

What is the cause behind lean mixtures 'killing' engines? Is it purely from the hotter running conditions as there is less fuel absorbing the heat? If that's the case then presumably you could use a more heat resistant material for the piston?

[langwadt]

Another problem I have seen several times, is slavish obedience to wide-band-lambda sensors. Conversation goes something like this:
"You can't run leaner than 0.9"
"but the engine makes more power at 0.95"
"No mate - you'll kill your engine"

What is the cause behind lean mixtures 'killing' engines? Is it purely from the hotter running conditions as there is less fuel absorbing the heat? If that's the case then presumably you could use a more heat resistant material for the piston?

must thing will burn if hot enough and oxygen is available

[gruntguru]

1. Higher peak combustion temperature as mixture is leaned toward stoich'. Beyond stoich' the temperature falls again.
2. Slower flame veolocity = more heat rejected to chamber, less work done on piston and higher temperature at exh opening.
3. Tendancy towards detonation. (mainly due to 1. and 2.)