Methanol combustion formula

[sbc438]

I'm currntly crueing on an IHRA Top Dragster running a Blown 525 ci Methanol engine. Just for developement sake i'm looking to derive a combustion formula by volume to make fuel mixture changes as atmosphereic conditions change. the main point I'm struggleing with is the preasens of water measured in grains. I know the composition of air, how temperature and pressure interact
(PV/mRT), Density or the fuel (constantfor small temp changes). The form i'm looking for to help tune is % change in avalible O2 per unit volume so i can compare it to previous conditions and make jetting adjustments and keep a 7:1(ish) air fuel ratio.

in this game we have to define what time we are going to run before hand so consistancy is key!!!!!!!

 

[patprimmer]

You want the presence of water as a %age by weight in the methanol.

You then need to jet to correct for the fuel displaced by the water. Note, you will also get a density change in the fuel due to water content as well as due to temperature change.

There is no free O2 in methanol or water. The O2 is fully reacted and firmly attached in both cases.

Regards

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

 

[sbc438]

Patprimmer,

Sorrt I wasn't clear. I'm looking to take into account for the water content by volume in the air.

 

[ivymike]

the volume of water "added" to the air will displace an equivalent volume of air. The mass of the water in the air is what you have (grains). You know atmospheric temperature and pressure, so you can calculate the density of gaseous water at those conditions. You can then calculate the volume fraction of your intake flow that is occupied by gaseous water, and subtract that from your "air" to get an adjusted estimate of the O2 available.

Pay attention to Pat's remark There is no free O2 in methanol or water. The O2 is fully reacted and firmly attached in both cases. He's right about the water - I thought that the hydroxy group in the methanol "broke off" and then grabbed a hydrogen later, but the net effect on the stoichiometry is the same.

 

[patprimmer]

Isaac

I think you are right.

At peak combustion chamber temperatures, I also think water dissociates and assists in the reaction between CO and O to make CO2.

I would need to dig out some old articles to refresh my memory on the details. It also has no effect on the stoichiometry, but does alter the rates of combustion.

I failed to mention it in this thread as it is has no effect on the A:F ratio or jet calculations.

My claim that it was firmly attached was to avoid full on discussions on chemistry with non chemists and to avoid discussions on the myth that alcohol makes more power because it contains oxygen.



Regards

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

 

[sbc438]

One more quick one. Gaseous water, does it adhear to the perfect gas law?

Thanks for the input!!! As you ignored the O2 in Alcohal due to the net effect I am doing the same with many of my variables. the only variables I'm going to be concerned with are change in avalible O2, Delta T & Delta P across the blower. These should give a relatively accurate O2 avalibility at the point of Alcohal interduction.

P.S.

I have about 10 hrs to develope this. First round Qualifying is 8am Sat.

 

[ivymike]

it's certainly close enough for what you're trying to calculate!

 

[patprimmer]

I am presuming this is a roots blower.

They are positive displacement.

The same mass of air is sucked in with every turn.

Manifold temperature and pressure are dependant on blower speed relative to engine speed, blower displacement relative to engine displacement, camshaft overlap, port and manifold flow capacity and ambient conditions.

Unless you change the engine components, the only variables are ambient conditions, blower drive ratio and leakage past the blower rotors.

The only things that will effect leakage past the rotors from week to week are wear and manifold pressure.

Normal wear will be gradual and progressive, and manifold pressure will only change slightly and the effect on blow by past the rotors will be negligible relative to total air flow.

Week to week variation will be almost entirely dependant on ambient conditions of air temp, air pressure and humidity.

If you track these and adjust accordingly you will retain your previous tune.

For all practical purposes, water vapour follows the ideal gas laws across the temperature range you will race under.

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]

Correct me if I am wrong, but the water content of air measured in grains is meaningless, and your units will be something like grains per cubic foot, ie mass of water per unit volume of air. You will also have an air density figure (or be able to calculate one from temperature and pressure), and this will also be a mass per unit volume measurement and thus by division you will be able to find the mass of water per unit mass of air, ie the percentage of water in the air.

I calculate the conversion factor for grains per cubic feet as 1 grain per cubic foot is 0.00228835191 kg/m3. So for example, with an air density of 1.2 kg/m3, 1 grain of water per cubic foot of air would 0.190695993 percent water by mass.

If your aim is just to release a defined amount of energy during combustion in a cylinder for your consistency reasons, I assume you would allow for wet air conditions, and adjust the AFR so that no extra fuel is added if the air is drier, eg if the air got 5 grains per cubic foot drier, (which calculating as above is about 1 percent less water by mass in the air), then I guess you would drop your AFR by 1% to keep the fuel quantity (and thus energy released) the same.
(1% water sounds quite a lot, take the figures as illustrative figures only.)

 

[crysta1c1ear]

After I posted, I asked myself whether I had the air and fuel the right way round in my 'air fuel ratio' and whether the ratio should whether it should increase or decrease as the air got drier. I was just concerned that I had posted too hastily, and I believe I did.

But more important than the question of it being the right way round, is the question of whether to actually change the air fuel ratio. If the air gets drier then more oxygen is available for a certain mass of air, and more fuel could be burnt giving more power. Thinking aloud, that's more fuel for the same mass of damp intake air, or equivalently a lower air to fuel ratio, to give you the best possible power. However the criteria in the question was consistency, which we might assume comes from having a defined amount of energy, ie burning a defined amount of fuel.

In that case, isn't the answer to set the AFR as required for the dampest air we are expecting to encounter, and not to add any more fuel if the air is drier (although by adding fuel we could run faster when the event actually takes place). That is, hold the AFR constant (for humidity changes). If the air is damp, the water cannot take part in the reaction. If the air is dry, the extra air wouldn't have the extra fuel it needs to produce power and behaves the same.

Sorry for the two contradictory postings.
Still, it's food for thought.
Talking about food, this is another rushed posting of a sort, as I have been called to eat!

 

[sbc438]

You are correct in your theories but we actually run rich to protect the engine from meltdown.... If the air gets worse we can lean out to compensate for the power loss and or add timing. It's a ballancing act that has really bad consequences if you go lean or add too much timing.

FYI Last weekend we were low qualifier in Bowlin Green, KY
with a 4.35 1/8 mile @ 162 mph 60 ft time 1.047 sec.