Why is methanol used in race engines?

[Topher925]

The enthalpy of formation of methanol (-22.7 KJ/g) is about half that of octane (-47.8 KJ/g). But methanol allows an ICE engine to achieve much greater specific powers than a comparable gasoline engine. My RC heli runs on methanol and will outperform any gas powered heli. But why is this?

I always thought it was because methanol engines typically have a air:fuel ratio of about ~4:1 while gasoline powered engines have ratios of ~14.1:1 which allows methanol engines to introduce much more fuel (therefor, heat) into the combustion chamber for each cycle. But my chemistry text that I'm reviewing, by Zumdahl and Zumdahl, states that its because "methanol burns much more smoothly than gasoline in high-performance engines, and this advantage more than compensates for its weight disadvantage". This seems like a stupid answer to me. Is the text right, and if it is can someone give a more descriptive answer as to why?

 

[mgb_phys]

My understanding was that it resists pre-detonation better than gasoline so you can run at much higher compression, or boost pressure on a turbo. So you put more fuel, + air, into the cylinder with each cycle.

 

[ank_gl]

Methanol powered engine will have higher thermal efficiency due to its higher heat of vaporization & hence cooler inlet & hence higher specific heat capacity. Methanol also burns faster due its higher flame velocity relative to gasoline, hence higher peak pressure. Although due to its low LHV, bsfc for methanol is higher than that for gasoline.
as mgb said, its octane rating is also higher, so higher compression ratios can be used

 

[ank_gl]

I always thought it was because methanol engines typically have a air:fuel ratio of about ~4:1 while gasoline powered engines have ratios of ~14.1:1 which allows methanol engines to introduce much more fuel (therefor, heat) into the combustion chamber for each cycle.

If a similar intake system is used for methanol, it would introduce 14 units of air for every unit of fuel, when only 4 units of air were required. It would make the engine run very lean, & probably not run at all.

 

[Topher925]

But the increase in flame speed would only yield a small and probably an almost negligible increase in peak pressure. The compression ratios for methanol engines are usually only around 16:1 while for typical high end gasoline engines are around 13:1 (NA). Theoretically, this would yield less than a 3% increase (cold air method) in power due to an increase in thermal efficiency. A higher compression ratio and flame speed can not justify an estimated 20-30% increase in specific power that methanol engines can have.

 

[Topher925]

If a similar intake system is used for methanol, it would introduce 14 units of air for every unit of fuel, when only 4 units of air were required. It would make the engine run very lean, & probably not run at all.

This doesn't make any sense to me. Obviously an engine designed to run on methanol is going to have a different design than one that runs on gasoline.

1 mole of methanol oxidizes with 1.5 moles of oxygen;

2CH3OH + 3O2 -> 4H2O + 2CO2

But 1 mole of octane oxidizes with 12.5 moles of oxygen;

2C8H18 + 25O2 -> 16CO2 + 18H20

So an engine that runs on methanol will require very different fuel to air ratios than that of a gasoline engine.

Here is an interesting link that I think helps prove my point: http://www.smokemup.com/tech/fuels.php

 

[Ranger Mike]

When i used an LM1 AFR sensor, and its goes error... "too rich" and babling from 9:1 to 21:1 afr...
Further, 6.5 is about the stoichiometric ratio for air:methanol.

Methanol will burn without fouling or miss-fire to a very rich mixture. This ability is used to run rich or cooling when the boost or compression is very high.

About 5 to 5.5 is typical for supercharged very high power engines.

With methanol it is more normal to tune by plug reading and exhaust gas than by oxygen sensor.A fuel system reacting to O2 sensing to control methanol typically responds to slow and between the sensor seeing the exhaust sample and the cylinder seeing the correction, often a hole is already in the piston.



The Four Primary Characteristics of Racing Fuels

OCTANE is a measure of a fuel's resistance to pre-ignition, pinging, and detonation. There are three octane ratings for motor fuels; Research Octane Number (RON), Motor Octane Number (MON), and the average of the two (R+M/2). Of these three ratings, MON is the most useful to race engine builders because it is tested under load and higher RPM conditions. High MON ratings allow the use of higher compression ratios and more advanced spark timing. However, there are other fuel characteristics that influence the ability of a particular fuel to resist knocking.

BURNING RATE is the speed at which a fuel burns and releases its heat energy. At higher RPM's there is less time for fuel to burn, so racing fuels tend to work better if they have a rapid burning rate. If a fuel can be almost completely burned by the time the crankshaft is 20 degrees after TDC (Top Dead Center) on the engine power stroke, peak horsepower and engine efficiency are realized.

LATENT HEAT of VAPORIZATION is the ability of a fuel to cool the intake charge and the combustion chamber. It is often expressed as BTU's/gal (British Thermal Units per gallon). A fuel with a high latent heat value will do a better job of removing heat. This makes the intake charge more dense and packs more energy per volume into the engine. The cooling effect also helps control detonation and reduces the temperatures of engine and oiling system components.

ENERGY VALUE is an expression of the total heat energy contained in a given amount of a fuel, expressed as British Thermal Units per pound (BTU/lb). The total amount of heat energy that is available to make horsepower depends on the Net Energy Value of the fuel. This is found by taking the raw energy value of the fuel and then multiplying that by the amount of fuel that can be burned. The ideal air/fuel ratio for a fuel is called its stoichiometric. The lower the stoichiometric, the more fuel is burned and the more power can be produced.

 

[Bob S]

. The compression ratios for methanol engines are usually only around 16:1 while for typical high end gasoline engines are around 13:1 (NA). Theoretically, this would yield less than a 3% increase (cold air method) in power due to an increase in thermal efficiency. A higher compression ratio and flame speed can not justify an estimated 20-30% increase in specific power that methanol engines can have.

For adiabatic compression of air, T V^gamma-1= constant, where gamma = 1.4 (T = temperature in kelvin, V=volume). If air at T = 293 kelvin (20 C) and P = 1 atm is compressed a factor of 16, the air is theoretically heated to T = 890 kelvin (615 C). Using a turbo will heat the air even more. The adiabatic heating will theoretically boost the pre-ignition pressure from 16 atm to about (890/293) x 16 = 49 bar. Wow. Some of the temperature increase is lost by convection to the cylinder walls. Somebody please check my calculation.

 

[ank_gl]

This doesn't make any sense to me. Obviously an engine designed to run on methanol is going to have a different design than one that runs on gasoline.
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that is what i meant, design of the intake system is different.

 

[Ranger Mike]

a lot of racers run methanol..you got to jet way up cause it takes a lot of alky to make um HP.
also the engine will run a lot cooler and you have to bump up the timing...comp ratio too. that's about it..cam , heads valve train pistons etc..all same as gas parts..one more thing..I was standing in the hot pits at Sandusky speedway one Saturday night..the sprint cars were running..this is super dangerous because the sprint cars must be push started and they do not have a clutch, they use a dog clutch so once started,,if they stop the engine dies..so they are flying in and out of the pits..a sprint car came barreling in and the driver jumped out of the car..started rolling on the ground..luckily the gate guard was an old hand and grabbed a fire extinguisher..alcohol burns but there is not flame..really dangerous stuff and you darn well better run a fuel cell and halon internal fire suppression system..and triple nomex driver suit...