Methanol as an alternative to Petrol
Discussion
This may be a really stupid question and easily answered but I am wondering why if methanol has a higher octane rating than petrol but it has an enthalpy change (delta H) of -1277 kJ mol-1 and petrol has an enthalpy change (delta H) of -10240 kJ mol-1 and the energy release from methanol is 20000 kJ kg-1 and petrol is 45000 kJ kg-1 then why does methanol have this much higher octane rating than petrol?
Surely, the higher energy release from petrol equates to a higher octane rating?
Octane ratings can't work in reverse of the energy release, can they?
If methanol releases less energy per kilogram of fuel then it is must be less efficient and less economical?
Is this anomaly something to do with the relative atomic mass of methanol, perhaps if it is smaller than that of petrol there is more room in the cylinder for ignition with oxygen making it more combustible and hence the higher octane rating?
In which case the opposite is probably correct, the lower energy release is less significant than RAM perhaps?
As you can see, I'm getting quite confused over which is correct!
Some insight would be much appreciated.
Cheers
P
Surely, the higher energy release from petrol equates to a higher octane rating?
Octane ratings can't work in reverse of the energy release, can they?
If methanol releases less energy per kilogram of fuel then it is must be less efficient and less economical?
Is this anomaly something to do with the relative atomic mass of methanol, perhaps if it is smaller than that of petrol there is more room in the cylinder for ignition with oxygen making it more combustible and hence the higher octane rating?
In which case the opposite is probably correct, the lower energy release is less significant than RAM perhaps?
As you can see, I'm getting quite confused over which is correct!
Some insight would be much appreciated.
Cheers
P
Edited by P9 on Monday 24th May 00:17
Put simply, petrol vapour is horribly explosive.
Methanol burns in a more controlled fashion so is considered better for many racing applications, even though if it catches fire it's hard to see - it burns without a visible flame.
Consider also diesel fuel. A gallon of diesel contains more energy than a gallon of petrol (hence better mpg) but it behaves very differently.
Methanol burns in a more controlled fashion so is considered better for many racing applications, even though if it catches fire it's hard to see - it burns without a visible flame.
Consider also diesel fuel. A gallon of diesel contains more energy than a gallon of petrol (hence better mpg) but it behaves very differently.
Isn't octane rating different to the total energy? People think 99 ron has more "energy" than 95 ron, but it is not so. The 99 does nto burn faster either I dont think, it burns slower. As someone else posted 99 will have higher resistance to detonation, ie buring without a spark, so can run more ignition advance and in turbos, more boost, withou the motor going to Kingdom come.
Methanol was used on many old race cars, where there was crude (if any at all) engine management and yet a desire to get oodles of power. So they bolted mahoosive turbos and superchargers on things, would up the boost, then ran methanol or a methanol mix to stop it detonating to s
t. Even with lower total energy per litre, they could get huge power just by wopping the boost and ignition timing up. Some 1950's motors of 1600cc or less were making 300hp this way with not a knock sensor in sight!
OH and if it is all about "energy per litre" diesel has more than both petrol and methanol!
Methanol was used on many old race cars, where there was crude (if any at all) engine management and yet a desire to get oodles of power. So they bolted mahoosive turbos and superchargers on things, would up the boost, then ran methanol or a methanol mix to stop it detonating to s
t. Even with lower total energy per litre, they could get huge power just by wopping the boost and ignition timing up. Some 1950's motors of 1600cc or less were making 300hp this way with not a knock sensor in sight!OH and if it is all about "energy per litre" diesel has more than both petrol and methanol!
Edited by poo at Paul's on Sunday 23 May 21:35
mdm123 said:
i'll give a hint, have a look at what the octane rating is.
You are saying that energy release is the same as octane?That was my question and clearly you don't know either but thanks anyway.
jagracer said:
Do you really need to ask such a stupid question?
oh dear 
Edited by P9 on Sunday 23 May 23:15
P9 said:
mdm123 said:
i'll give a hint, have a look at what the octane rating is.
You are saying that energy release is the same as octane?That was my question and clearly you don't know either but thanks anyway.
jagracer said:
Do you really need to ask such a stupid question?
oh dear Edited by P9 on Sunday 23 May 23:15
what you think it is, is way off what it means. for a start a fuels octane rating has got nothing to do with it's energy release.
P9 said:
This may be a really stupid question and easily answered but I am wondering why if methanol has a higher octane rating than petrol but it has an enthalpy change (delta H) of -1277 kJ mol-1 and petrol has an enthalpy change (delta H) of -10240 kJ mol-1 and the energy release from methane is 20000 kJ kg-1 and petrol is 45000 kJ kg-1 then why does methanol have this much higher octane rating than petrol?
Surely, the higher energy release from petrol equates to a higher octane rating?
Octane ratings can't work in reverse of the energy release, can they?
Octane rating is a measure of activation energy. Higher octane=harder to ignite. Surely, the higher energy release from petrol equates to a higher octane rating?
Octane ratings can't work in reverse of the energy release, can they?
Heating value/energy content of the fuel is different. This is the energy released per unit mass once the fuel is ignited.
Because Methanol needs far less oxygen than petrol to burn - which means you can feed a give cylinder of air roughly 2.3-2.5 times as much methanol as petrol..
The methanol also has a high latent heat of vaporisation, so injecting twice as much meth as petrol means you have a huge cooling effect on the incoming air charge - if you inject it over a throttle plate you can quite easily have a buildup of ice jam the throttle open/shut.
The methanol also has a high latent heat of vaporisation, so injecting twice as much meth as petrol means you have a huge cooling effect on the incoming air charge - if you inject it over a throttle plate you can quite easily have a buildup of ice jam the throttle open/shut.
Edited by PhillipM on Monday 24th May 00:29
P9 said:
OK thanks, I get the octane ratings now but still not too clear on why methanol is a better and more efficient fuel than petrol when the energy release from methanol is 20000 kJ kg-1 and petrol is 45000 kJ kg-1.
A higher energy release makes a less efficient fuel?
Higher Octane rating means you can use a higher compression ratio and more ignition advance, and thus the combustion process is more efficient.A higher energy release makes a less efficient fuel?
Methanol is also good because it's easy and not particularly energy intensive to create from scratch from CO2 in the air. Basically you could quite feasibly have a closed loop process for producing methanol, powered by renewable energy, to keep all of the 'old' petrol engines running while keeping the environmentalists happy.
The only downside is that you need to double the size of the fuel tank in the car, or travel half as far.
The Wookie said:
P9 said:
OK thanks, I get the octane ratings now but still not too clear on why methanol is a better and more efficient fuel than petrol when the energy release from methanol is 20000 kJ kg-1 and petrol is 45000 kJ kg-1.
A higher energy release makes a less efficient fuel?
Higher Octane rating means you can use a higher compression ratio and more ignition advance, and thus the combustion process is more efficient.A higher energy release makes a less efficient fuel?
Methanol is also good because it's easy and not particularly energy intensive to create from scratch from CO2 in the air. Basically you could quite feasibly have a closed loop process for producing methanol, powered by renewable energy, to keep all of the 'old' petrol engines running while keeping the environmentalists happy.
The only downside is that you need to double the size of the fuel tank in the car, or travel half as far.
PhillipM said:
Because Methanol needs far less oxygen than petrol to burn - which means you can feed a give cylinder of air roughly 2.3-2.5 times as much methanol as petrol..
The methanol also has a high latent heat of vaporisation, so injecting twice as much meth as petrol means you have a huge cooling effect on the incoming air charge - if you inject it over a throttle plate you can quite easily have a buildup of ice jam the throttle open/shut.
Thanks guysThe methanol also has a high latent heat of vaporisation, so injecting twice as much meth as petrol means you have a huge cooling effect on the incoming air charge - if you inject it over a throttle plate you can quite easily have a buildup of ice jam the throttle open/shut.
Edited by PhillipM on Monday 24th May 00:29
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