Which Chemicals Make the Best Fuels for Turbocharged Engines?

[samblohm]

I'm not sure how much background someone reading this may have so I'm going to go over it kinda thoroughly. In a turbocharged engine, when the turbo compresses air into the intake piping, it heats up and becomes less dense. If the air isn't dense, you can't put as much fuel in so you get less power. When fuel is sprayed, it evaporates and cools the intake charge so even more fuel can be put in. Another important thing is the octane rating of the fuel. (resistance to detonation when it's compressed in the cylinder. The higher the octane, the more it can be compressed and so the more efficiently it will burn) A cooler air/fuel mixture will increase the effective octane rating of the fuel. 2,2,4-Trimethylpentane is used as a the standard for octane rating with it being 100. Heptane is the other standard with an octane of 0. The less surface rating a chemical has the high its octane rating is (Toluene, Benzene, etc) Now you can make more power generally with something like ethanol because more fuel is required per unit of air, so more of a cooling effect. It also has an octane of something like 120. Methanol is even better because of the same reasons (it's octane is 129, which is really high). Sometimes water is injected to further cooler the intake charge. With that being said, what are some good choices of chemicals for fuels? (assuming price isn't an issue.) Possibly Methanediol? It may be to stable based on the stability of ethylene glycol. And I'm asking for a point of view from a chemist.

 

[256bits]

There are some misconceptions:

In a turbocharged engine, when the turbo compresses air into the intake piping, it heats up and becomes less dense. If the air isn't dense, you can't put as much fuel in so you get less power. When fuel is sprayed, it evaporates and cools the intake charge so even more fuel can be put in.

The compression heats the air, and compression means making more dense, not less.
The fuel turns into tiny droplets that burn with the oxygen. Whether it is sprayed from fuel injection or with a carburator, any cooling of the mixture does not dictate if any more fuel can be added. What is strived for is a stoichiometric mixture where there is enough fuel to burn in the air, no more, no less. Operating conditions of the engine will then dictate whether a more lean or rich mixture is best.

Any cooling of the air/fuel mixture means more of that mixture can enter the combustion chamber, of if it just the air that is cooled, then more of that can enter the combustion chamber, where fuel can be added.

The higher the octane, the more it can be compressed and so the more efficiently it will burn) A cooler air/fuel mixture will increase the effective octane rating of the fuel

The higher the octane the more resistant to pre-detonation the fuel will be in that the activation energy for self ignition is higher.
More compression means that more work can be obtained from burning of the fuel. The phrase " more efficiently it will burn" is to vague to even determine to what the efficiency may refer to.

Now you can make more power generally with something like ethanol because more fuel is required per unit of air, so more of a cooling effect

More ethanol is required because the energy content is less than that of gasoline. ( 21.2 MJ/L versus 34.8 and 26.8 MJ/kg versus 44.4 MJ/kg - see http://en.wikipedia.org/wiki/Ethanol )
To consider your statement about more of a cooling effect, one would have to compare the heat of vaporization of ethanol and gasoline.

Other choices for fuel, of that I will leave to the chemist point of view.

 

[samblohm]

Should have been more specific in some areas. I first meant that compressing the air makes it less dense than what would be ideal because air is not an ideal gas. It heats up when compressed. Obviously compressing it makes it more dense than ambient though.

The whole point of this is for power, not efficiency so a slightly rich mixture is what is needed. Especially for a turbocharged engine.

And it is important to note that fuel should never detonate. It deflagrates. So "pre-detonation" is kinda of misleading.

Yes more ethanol is required but the limiting factor in an performance engine is always going to be the amount of air in the engine. You can put as much fuel in as what can be supported by the amount of air present. Ethanol does contain less energy per unit of ethanol, but per unit of air, it is the same as gas because the more is used which makes up for it.

The heat of vaporization of ethanol is higher than gasoline, and more ethanol is required so there is an even greater cooling effect.

Any other input would be appreciated.

 

[MrSid]

As a minor point methanediol is formaldehyde in its acetal form and hasn't been used as a good fuel that I know of- it is too prone to polymerize, is toxic in the smallest of doses and wouldn't be energy dense like hydrocarbons.

On the other hand the point of incorporating part of the oxygen needed to combust completely is why there is a nitro fueled class of racing. Here the fuel is nitromethane and the nitro group adds extra energy.

 

[chemisttree]

As a minor point methanediol is formaldehyde in its acetal form and hasn't been used as a good fuel that I know of- it is too prone to polymerize, is toxic in the smallest of doses and wouldn't be energy dense like hydrocarbons.

Actually, the cyclic trimer of formaldehyde has been used for many decades as a fuel pellet by the Army. It is called TRIOX.