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Why a rich mixture helps to reduce temperature?

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Hi there! Hope you're doing well!

Why a Rich mixture helps to reduce temperature in the combustion chamber? What is the process that happens inside the combustion chamber that helps to reduce temperature? is it a chemical reaction which is eliminated through the exhaust step?

The process of vaporizing fuel requires energy, this comes in the form of heat, so when the fuel is atomized the temperature goes down. More fuel, more temperature change.

Hello all,

i have an additional question to that topic:

In the webinar 065 | Planning your AFR targets

Andre talked about richening the high rpm / vacuum area to cool the combustion chamber for a racecar.

Now this made my thinking: How long does it take too cool the combustion chamber?

Its hard to explain since im not a native speaker so i try to give an example:

N/A Honda K20 Racecar (Street legal tho) at Zandvoort (old layout), at turn 3 i found that it works best for me to stay in 3rd gear, which made the RPM drop quite significant to lets say 3000 rpm. On the track, this low rpm is only seen for a short moment while breaking into that specific corner. Would it make sense to enrichen the vacuum area that low (3000 rpm)? Or is the combustion chamber already cooled down enough by the rich vacuum from lets say 9000 down to 3000 rpm? What would you do when consumption is not an issue?

Hope that makes sense :)

Cheers, Mark

Normally you would not need to cool down combustion chamber at 3000 rpm as the load there is not that high and engine does not generate a lot of heat. So even at the lean afr at low load the temperature in combustion chamber is not critical and does not usually require any enrichment especially at deceleration- you can get away with lean mixture.

Hi Mark. good question.

There is a class where you can see how long it takes to increase temperature due to a lean mixture. in that example, Andre increases from lean to rich AFR to show how it affects the exhaust temperature.

I have no experience but what I think about your example because theory is that if you shiftdown to 3th gear and you drop to 3000RPM. So you have had change throttle position quickly and it means it can result in a momentary rich condition until when the fuel film (acceleration enrichment) finds equilibrium again. So your temperature in the combustion chamber should be fine because you should have a rich AFR when you are coming from 9000 RPM.

If someone else could answer this would be great!

Hi Victor, Georg,

@Georg: You are right - at 3k rpm there isnt much load. But there certainly is alot of load on the engine, when coming from a straight at the racetrack to a rapid stop. So thats what my question aims for.

@Victor: I also saw that, but he is measuring the EGT, the Gas. Andre is talking about cooling the chamber itself, not the Gas. Sooo i guess you are slightly mistaken :) But also slightly right, since the Heel-Toe downshift might be catched by the acc-enrichment.

Still, is the chamber cooled down enough by that? Or would it make sense to cool the chamber additionally by enrichen the vacuum down to 2500,3000 rpm

Greetings from Germany


the chamber cools very fast and is not needed below the speed the engine comes off the cam in time attack cars i tune for minimal richness off throttle to make it faster to recover. where as a endurance car i will use this as both a combustion chamber cooler and vale train cooler to help the engine last.

there is no right and wrong way to set it up and full your preference as to why you would or would not do it

Regards Ross

Thanks Ross, for clarifying. Helped alot.

Would you mind sharing some Lambda target values for endurance engines for off-throttle? Thanks man!