Sensitivity of power to change of a:f ratio 2

[patprimmer]

These question just came up in another forum I frequent.

"Is there an optimum a/f ratio for horsepower?

If so, is it variable as a function of rpm, compression ratio, ignition lead, combustion chamber design, flame front properties, etc?

If there is a range for optimal horsepower, how much wider is this range for, say, 98% of optimal horsepower? In other words, if the range for optimal horsepower is 12.3 to 12.5, how much wider would the range be for 98% of optimal horsepower?"

The poster is an all round great guy. He is preparing an engine for the Engine Masters Competition which is an American Hot Rodding based competition based on dyno performance of engines.

He works on this with Gene Adams who is a professional drag race engine builder and they have some serious sponsors, so I am classifying them as engineering professionals.

This is work related for Gene Adams as it promotes his business.

I already answered "Yes, Yes and who knows in such detail" or words to that effect.

Does anyone here have data on what %age change you typically loose in power for say 0.1 or 0.2 change from optimum in A:F ratio.

I know the magnitude will be different in different engines and conditions due to all the reasons listed in the second question, but I am looking for typical or ballpark. A clue to any factors that cause more than normal or unexpected changes would be great.

The main aim I expect is to reduce dyno time for development of less productive factors so as to leave more time for more gainful activities.

The engine in question is a early Hemi Crysler, about 350 CI , 10.5:1 compression, single 4 barrel carby, flat tappet cam, all commercially available parts with fairly extensive modifications allowed. Petrol is provided and is a specific pump gas (98 octane I think).

The test is best average power and torque from 2500 to 6500 with an inertia dyno.


Regards

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[dcasto]

go look at the big Cat engines. They market the 3400 series that is sold as rich and lean burn. The lean burn is 425 HP and a rich burn is 400 HP.

 

[Bribyk]

Dcasto, natural gas engines don't respond the same as gasoline engines to changes in fuel mixture. As you decrease AFR (richer) on a gasoline engine you increase the amount of air entering the combustion chamber (to a point) because the evaporating fuel cools the air and increases its density. As you go richer on a natural gas engine you are displacing air with the gas, which reduces power.

Patprimmer, I've got a "typical" graph of torque vs. equivalence ratio in front of me right now for a gasoline engine. In the area of max torque, the graph is reasonably flat (little change in torque for a change in fuel mixture). Max torque occurs at about 1.25 (AFR = 11.75) but approximately 98% of torque is shown from about 1.15 to 1.32 (12.75 to 11.15 AFR). This agrees with my personal experience tuning small high-out engines (130-140 hp/L).

It would appear that once you are close to about a 12:1 air fuel mixture, your efforts are better spent elsewhere. Of course, you'll have to come back and retune (or rejet) after tuning other parameters.

 

[GregLocock]

Also check the NACA LARC archive, I'm sure they'll have good data for carburetted engines.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Ummm yes.

Thanks Greg.

I very often recommend people to that site, but forgot to this time.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[crysta1c1ear]

I've have read a technical article which indicated that flame speed peaked at 1.1 times stoichiometric fuel to air ratio. I figured the faster rise in pressure would give more power. There is so much going on in a reaction that I don't think the author clearly said why the flame speed increased, but my impression was that a long chain molecule reacts more times before it fizzles out and was thus more effective at carrying the reaction to a new location.

 

[SomptingGuy]

Themochemically there is an AFR where the products of (ideal) combustion start to produce less heat and instead start pumping loads of H2 and CO out of the exhaust. Purely a chemiacal analysis, nothing to do with combustion chamber geometry but worht considering.

 

[wgknestrick]

12-12.5 A/F

 

[drwebb]

Maximum power is experimentally found to be (almost) always rich of stoichiometric in gasoline engines. This can be explained by the fact that the enthalpy of reaction of HCs to CO is much higher than CO to CO2. (see

http://www.eng-tips.com/viewthread.cfm?qid=101915)

Thermochemical considerations predict engine output would be highest if you could selectively combust all HCs in the volume to CO and exhaust it for a fresh charge, but alas kinetic factors make that impractical.

 

[al1]

There is a finite equilibrium between air fuel ratio, ignition timing and combustion speed or mass factional burn/energy heat release. You can burn a piston in one engine with say 1.1 air fuel ratio but not burn it in another engine, the initial combustion timing, and the combustion speed being the biggest difference. If you release the energy to early before TDC the combustion chamber is smaller with less surface area to exposed the heat to. If the timing is correct the combination of energy released in proportion to the increase of dynamic displacement, which both reduce pressure and exposes additional surface area to keep the temperature at a safe level, being the difference even though both examples use the same air fuel ratio.

One other area, when I was an automotive instructor, we taught that heat over time was how to think about destructive combustion temperatures. We used to take a torch and quickly pass it under are hands to show how a 2500 degree torch would not burn our skin because there is not enough time to heat sink it or what ever you trying to heat --- it takes time. Also there is only 180 degrees in the power cycle, but there are 270 degrees of relative cooling time to cool the piston that has to be apart of the consideration when evaluation the destructive combustion temperatures. Bottom line, every so-called normal combustion temperature is high enough to melt any aluminum piston, it’s the combination of heat in and heat extracted over time that makes it all work and when the balance is broken --- problems set in.
al1

 

[Tmoose]

"single 4 barrel carby"
"all commercially available parts"

The early hemi manifolds I found on line are Weiand 2X4 and PAW universal base with exchangeable tops

http://www.holley.com/types/Chrysler Hemi - Early.aspn

I have to wonder if getting all 8 cylinders A/F ratio within .2 of each other might be a feat unto itself.

Is the goal to make the biggest peak number?

I think the engines showing the greatest variation or sensitivity for varying power with A/F are the ones with least "effective" combustion chambers regarding stuff like mixing (or maybe charge stratification) and turbulence. It makes sense to me that Best power requires giving ALL the oxygens something to burn, so if the lunch lady is serving small portions, some folks have to go back twice.