Timmer

Stoichiometric Air/fuel Ratio

14 posts in this topic

In my post "Air/fuel ratio" I discussed the basics of air/fuel ratio as it pertains to gasoline. I talked about how to measure it and also gave a basic scale of rich and lean for gasoline.

 

But what about other fuels?

 

Just the other day Cobie asked me some questions about the use of some of the newer fuels like E15 and E85 and the affects it has on an engine. The answers to his questions lied in some additional data about air/fuel ratio, so I figured I would post on it.

 

In my earlier post I defined stoichimetric air/fuel ratio. Below is quick refresher...

Stoichiometric air/fuel ratio(abbreviated "stoich"): A ratio of air and fuel such that exactly enough air is provided to completely burn all of the fuel.

 

Many people know that the stoichiometric air/fuel ratio of gasoline is 14.7:1. This means that 14.7 parts of air mixed with 1 part of gasoline is the proper proportion or "recipe" to get a complete burn of all the fuel. Since other fuels have different chemical compositions than gasoline they also have different stoichiometric air/fuel ratios.

 

Stoichiometric Air/Fuel Ratio of Common Fuels

Gasoline(Petrol) = 14.7:1

Methanol = 6.4:1

Compressed Natural Gas(CNG) = 17.45:1

Diesel = 14.5:1

Nitromethane = 1.7:1 (Stuff used in top fuel dragsters)

E10(10% Ethanol, 90% Gasoline) = 14.13:1

E15(15% Ethanol, 85% Gasoline) = 13.8:1

E85(85% Ethanol, 15% Gasoline) = 9.7:1

Ethanol(E100) = 9:1 (Alcohol)

 

So looking at the above list you can see the stoichiometric air/fuel ratios are very different between different fuels. For example, nitromethane requires almost 1/3 of the mixture to be fuel in order to get a complete burn...that is a lot when compared to gasoline!

 

I'll make some more posts about the affects of running some of the new alternative fuels like E15 and E85, but I just wanted to post up this info about air/fuel ratio as a reference.

 

If you guys have any questions about this stuff, please fire away!

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Hey Tim -

 

Good post / good info!

 

When tuning an engine, do you target (or does Autotune target) different A/F ratios based on rpm, throttle position, & load? If so, what do some of those numbers look like, and how do they vary by fuel type?

 

Thanks!

Dan

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I was looking at the other post on octane, maybe we shoudl combine these Timmer? Or are they going to go in distinctly different directions?

 

CF

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Hey Dan,

 

Excellent questions!

 

To answer your first question....Yes, different A/F ratios are typically used for different operating conditions of the bike(throttle position, RPM, etc.). In general, most engine tuners/manufacturers will use leaner air/fuel ratios(13.8:1-14.7:1) in the low throttle position, low RPM areas to get the best gas mileage. At high throttle position, high RPM areas they will run richer air/fuel ratios(13:1-12.8:1) to get the best performance. The Autotune for Power Commanders allows you to tune for different A/F ratios in different areas of the map.

 

I'm glad you asked "how do they vary with fuel type?" because it brings up another interesting technical point. Those silly engineers like me have another term to describe air/fuel ratio called LAMBDA. It is useful when describing "richness" or "leanness" when dealing with different fuels.

 

Example:

Let's say we were running ethanol(E100) and gasoline in the same bike and wanted to tune it properly for both fuels. Since the stoichiometric air/fuel ratios are very different between the two fuels, we would be dealing with two sets of air/fuel ratios to tune for.

 

We put gasoline in the bike first, put it on a dyno and start tuning. We decide that we want to run it on the rich side, and tune the bike perfectly to have an air/fuel ratio of 13:1. Now we empty all the gas out of the bike, put the ethanol(E100) in the gas tank and get ready to tune again.

 

So...since the stoichiometric air/fuel ratio of ethanol(E100) is different, what air/fuel ratio should we shoot for to get the same richness we want? Well there's an easy way to figure that out. We do a simple division of the AFR we chose divided by its stoichiometric AFR. This would be 13:1 / 14.7:1 = .884.

 

.884 is our LAMBDA! The 13:1 we chose is 88.4% of the 14:7:1 stoichiometric air/fuel ratio of gasoline.

So...LAMBDA is a percentage of stoichiometric air/fuel ratio.

 

Now we take that percentage and multiply it by the stoichiometric air/fuel ratio of ethanol(E100).

.884 x 9:1 = 7.96:1

 

BOOM...Now we know what air/fuel ratio number to tune for with the ethanol to get the same amount of richness!

 

Make sense?

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Cobie,

 

I decided to separate the two posts because they are pretty different, and I didn't want questions of each to get mixed together in the same post.

 

Hope that's ok. I can always combine them later if we want to.

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Ok lets go deeper with a question.

How is the AFR been affected by the weather conditions?Is the AFR getting richer or leaner in hot,cold conditions?Is there a rule?

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Another good question DrIoannis.

 

The properties of the air coming into the motor(pressure and temperature mainly) are important factors to take into account when trying to figure out how much fuel needs to be delivered to the motor. Fortunately for all of us, modern sportbikes automatically do this for us. All the modern fuel injected bikes have sensors which tell the on-board computer(ECU) what the pressure and air temperature are.

 

Air Pressure

We all know that air at higher altitudes is less dense or "thinner". There is less oxygen in a given volume of air. This is why you have to breathe a lot more to catch your breath when you're on top of a mountain. This means that there is also less oxygen getting to your motor when it breathes. So, if your bike is delivering the same amount of fuel on top of a mountain as it did at the bottom of the mountain, you would get a RICHER air/fuel ratio at the top of mountain. There is less air in the air/fuel mixture at the top of the mountain.

 

Modern fuel injected bikes have pressure sensors that will detect this change in altitude. Once the on-board computer learns that you have gone up in altitude it will actually SUBTRACT fuel to maintain the same air/fuel ratio it had at the bottom of the mountain.

 

Also, when bad weather hits(like a thunderstorm) you will hear meteorologists say that it is a "low pressure system". They are referring to the air pressure. Air pressure typically goes down when the weather is bad, and goes up when it is clear and beautiful outside.

 

Air Temperature

We also know that hotter air is less dense or thinner than cold air. This is why a hot air balloon works. The hotter air inside the balloon is less dense than the atmospheric air, so the balloon floats higher to where the air outside is the same density as in the balloon.

 

So if your bike was delivering the same amount of fuel for all air temperatures, you would see a RICHER air/fuel ratio if the air coming into the motor got hotter. There is less air in the air/fuel mixture with the hotter air.

 

Modern fuel injected bikes also have a sensor to detect the temperature of the incoming air. Once the on-board computer computer learns that the air coming into the motor has gotten hotter it will SUBTRACT fuel to maintain the air/fuel ratio that it had with the colder air.

 

Hope that answers your question. Let me know if you have any others! :)

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Excellent information Tim - thanks!!!

 

I love this stuff and since I plan to pick up the Autotune this year, I want to learn as much up front about it as I can.

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Thanks Dan!

 

I remember you talking to me in Jersey about getting an Autotune once you got everything else set up how you wanted.

 

Please post up any other questions you have on air/fuel ratio. If you have any specific questions on how to get the Autotune working, feel free to shoot my a private message. :)

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Thanks Dan!

 

I remember you talking to me in Jersey about getting an Autotune once you got everything else set up how you wanted.

 

Please post up any other questions you have on air/fuel ratio. If you have any specific questions on how to get the Autotune working, feel free to shoot my a private message. :)

 

Thanks Tim, will do! Will you be in Jersey again this year? I've got Level IV booked for 5/20, 5/21 and again on 7/29, 7/30.

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Hey Dan,

Not totally sure if I will be in New Jersey yet. I've told Cobie I'm available for those dates, but not sure if he has scheduled me for those days. Hopefully I will see you there! :)

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C'mon Cobie - schedule Tim for Thunderbolt; he needs to continue his challenge of helping me correct my horrible body position :).

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