Latent Heat of Vaporization and alcohol injection

[Devilsown]

Fuel is usually supplied to the engine manifold and air stream in
liquid form. It is heated there and converted to a vapor prior to
combustion. The latent heat of vaporization is the amount of heat
required to vaporize a unit quantity of a fuel, generally measured
at one atmosphere of pressure and at the boiling point of the liquid.
The latent heat of vaporization of gasoline is approximately 900
Btu/gaL The vaporization of gasoline in a stoichiometric mixture
of liquid gasoline and air (without external heating) results in an air
temperature reduction of approximately 22°C(40°F).

The latent heat of vaporization of methanol and ethanol are
3,340 and 2,378 Btu/gal., respectively; several times that of
gasoline. Therefore, vaporization of these fuel alcohols
produces a much larger temperature drop. However, with the
same heat addition per unit mass of charge, the percent of
the alcohol evaporated in the manifold will also be less.
Because of its greater evaporative cooling effect, alcohol
reduces the charge temperature and thus usually improves
the engine volumetric efficiency.

Alcohols have a much lower molecular weight than gasoline, and
therefore their vapor occupies proportionately more volume than
an equal mass of gasoline vapor. It is the high latent heat of
vaporization which is primarily responsible for the increased power
outputs with alcohol, and which leads to the popularity of methanol
as a blending constituent for automobile racing fuels.

 

[bpestilence]

More power can be made with alcohol because of its higher autoignition temperature: 867°F vs 495°F for gasoline. That means much higher pressure ratios can be used, which means more fuel can be crammed into a similar engine. (Experimentally, a mixture of gasoline with alcohol will have a higher octane rating than gasoline by itself)

The latent heat of vaporization is of less concern, and actually makes alcohol perform worse than gasoline in actual energy output per gallon of fuel. 57,000 BTU/gallon for methanol vs. 116,000 BTU/gal for conventional gasoline. That means it takes TWICE as much alcohol to produce the same power output as gasoline can produce. Ever wonder why the jets on an alcohol carburator are so HUGE?

Let's just stick with the facts. Methanol/water injection is only good because it lowers the mixture temperature, thereby avoiding autoignition in the gasoline itself. Any significant amount of the liquid will actually reduce engine output.