Alcohol As a Fuel - A Short Primer
Alcohols have sort of a magic reputation as a racing fuel but why they work seems to be largely misunderstood and they tend to be surrounded by lots of myths.
We’ll keep the discussion of alcohols to the most common. As a race fuel Methanol is king and Ethanol is most commonly added to pump fuels in an effort to reduce emissions. Alcohols are single boiling point fuels with a hydroxyl-radical (OH) attached while hydrocarbon based race fuels are made up of hundreds of hydrogen and carbon based components with a wide range of boiling. As fuels alcohol and gasoline couldn't be much more different but what’s interesting is if you compare the heat of combustion of 1 cu. Ft of chemically correct mixture at 14.7 psia @ 60F you get the following values:
Methanol (neat methyl alcohol) = 94.5
Gasoline (SG 0.739 @ 60f) = 94.8
Benzene (aromatic hydrocarbon) = 95.7
At first glance they seem awfully similar don’t they? ;)
We have to step into science mode for a second to explain a little about latent heat of vaporization, which will hopefully make the differences between alcohol and gasoline a bit more clear and explain a bit of alcohol’s magic powers.
We all know fuel has to be a vapor for combustion to occur, and the process of changing a liquid to a vapor is sort of a two-part phase change. The temperature that a fuel's components will boil at is varied, and the temps are dependent on the pressure exerted on them. Lower the pressure (like at high altitude) and it boils at a lower temp, raise the pressure (below sea level or supercharging) and the fuel will boil at a higher temp. Here’s the tricky part, even at the boiling point temperature, for the phase change from liquid to vapor (technically gas) to occur additional latent heat is required. The heat is obtained from the surrounding air and the fuel that remains in the liquid state. In the case of methanol we have a fuel that has a relatively low boiling point, but a high latent heat of vaporization, so lots of heat is removed from the surrounding air which leads to a denser mixture reaching the combustion chamber, which is good for power (higher volumetric efficiency), but the fuel that remains in the liquid state enters the combustion chamber at a much lower temperature and will ultimately need to leech additional heat from the combustion chamber to change to a vapor that can be burned, which isn’t good for power. Some good some bad, but on the whole in the case of methanol the end result is higher volumetric efficiency, more power, higher overall cylinder temperatures but a similar heat of combustion. A common rule of thumb is for every 11 degree F drop in temperature you have the potential to make an additional 1% more horsepower due to increased air density.
To make this more understandable, keep in mind you can have two liquids with the same boiling temperature but different latent heat requirements. The liquid with the higher latent heat will steal more thermal energy from the surrounding air and liquid.
Methanol has a boiling point of 149 F and a latent heat of vaporization of 1.17 MJ/kg
Ethanol has a boiling point of 172 F and a latent heat of vaporization of 0.93 MJ/kg
Typical gasoline has boiling points in 85 - 410 F range and a latent heat of vaporization of approximately 0.18 MJ/kg
When you start getting into hydrocarbon combinations with varying latent heat requirements, you have the potential to influence the cylinder filling in similar ways with the right combinations of fuel components.
The end result is the heat of combustion changes very little, while the overall combustion temperatures will rise pretty much in direct proportion to the increase in cylinder pressure that comes with a denser mixture in the combustion chamber. Filling the cylinder more completely whether it’s done with fuel or, porting wave tuning has the same overall effect. Make more power and you get heat, increase the load and the same things happens.
Alcohols have sort of a magic reputation as a racing fuel but why they work seems to be largely misunderstood and they tend to be surrounded by lots of myths.
We’ll keep the discussion of alcohols to the most common. As a race fuel Methanol is king and Ethanol is most commonly added to pump fuels in an effort to reduce emissions. Alcohols are single boiling point fuels with a hydroxyl-radical (OH) attached while hydrocarbon based race fuels are made up of hundreds of hydrogen and carbon based components with a wide range of boiling. As fuels alcohol and gasoline couldn't be much more different but what’s interesting is if you compare the heat of combustion of 1 cu. Ft of chemically correct mixture at 14.7 psia @ 60F you get the following values:
Methanol (neat methyl alcohol) = 94.5
Gasoline (SG 0.739 @ 60f) = 94.8
Benzene (aromatic hydrocarbon) = 95.7
At first glance they seem awfully similar don’t they? ;)
We have to step into science mode for a second to explain a little about latent heat of vaporization, which will hopefully make the differences between alcohol and gasoline a bit more clear and explain a bit of alcohol’s magic powers.
We all know fuel has to be a vapor for combustion to occur, and the process of changing a liquid to a vapor is sort of a two-part phase change. The temperature that a fuel's components will boil at is varied, and the temps are dependent on the pressure exerted on them. Lower the pressure (like at high altitude) and it boils at a lower temp, raise the pressure (below sea level or supercharging) and the fuel will boil at a higher temp. Here’s the tricky part, even at the boiling point temperature, for the phase change from liquid to vapor (technically gas) to occur additional latent heat is required. The heat is obtained from the surrounding air and the fuel that remains in the liquid state. In the case of methanol we have a fuel that has a relatively low boiling point, but a high latent heat of vaporization, so lots of heat is removed from the surrounding air which leads to a denser mixture reaching the combustion chamber, which is good for power (higher volumetric efficiency), but the fuel that remains in the liquid state enters the combustion chamber at a much lower temperature and will ultimately need to leech additional heat from the combustion chamber to change to a vapor that can be burned, which isn’t good for power. Some good some bad, but on the whole in the case of methanol the end result is higher volumetric efficiency, more power, higher overall cylinder temperatures but a similar heat of combustion. A common rule of thumb is for every 11 degree F drop in temperature you have the potential to make an additional 1% more horsepower due to increased air density.
To make this more understandable, keep in mind you can have two liquids with the same boiling temperature but different latent heat requirements. The liquid with the higher latent heat will steal more thermal energy from the surrounding air and liquid.
Methanol has a boiling point of 149 F and a latent heat of vaporization of 1.17 MJ/kg
Ethanol has a boiling point of 172 F and a latent heat of vaporization of 0.93 MJ/kg
Typical gasoline has boiling points in 85 - 410 F range and a latent heat of vaporization of approximately 0.18 MJ/kg
When you start getting into hydrocarbon combinations with varying latent heat requirements, you have the potential to influence the cylinder filling in similar ways with the right combinations of fuel components.
The end result is the heat of combustion changes very little, while the overall combustion temperatures will rise pretty much in direct proportion to the increase in cylinder pressure that comes with a denser mixture in the combustion chamber. Filling the cylinder more completely whether it’s done with fuel or, porting wave tuning has the same overall effect. Make more power and you get heat, increase the load and the same things happens.
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