From the recent vapor carb thread, I got the idea that the fuel economy gain from warm air intakes is possibly due to:
- Increased vaporization of injected fuel as it hits the warmed-up air, causing more of the fuel to become a gas with its own partial pressure,
- Causing an increase in the total pressure of the fuel-air mix going into each cylinder on its intake stroke (due to Dalton's Law of Partial Pressures),
- Thereby reducing the amount of work required to pull the fuel-air mix into said cylinder.
Since then, I've been toying around with this idea, first trying to figure out how to further heat up the intake air without adding too much more complexity, and then last week it hit me - what if did water injection, but with the water (50/50 ethanol/water mix, if you will) already near its boiling point?
I am thinking of using a water injection system, heating the water to near its boiling point using engine coolant and a heat exchanger, and injecting this warmed water into the intake manifold near the intake ports of the cylinders.
The idea is that the heated water will be injected into the intake, near the intake valve, as a fine mist. As it does so, the fine mist will be encouraged to vaporize into a gas because it is already heated and the intake pressure will already be low due to part-throttle conditions.
That will increase the partial pressure of the already-present water vapor, and add the partial pressure of ethanol. Again, due to Dalton's Law, that should increase the total pressure of the gas mixture being drawn into each cylinder, thereby reducing the amount of work the piston has to do WRT drawing in this gas mixture, which will then reduce the amount of fuel consumed.
And it seems that, 35 years ago, the US EPA has already published a paper that shows that this effect does in fact happen. However, they chose to focus on the the drop in volumetric efficiency caused by their methanol injection setup, so they did not evaluate whether this method would increase part-throttle fuel economy.
Methanol Vaporization: Effects on Volumetric Efficiency and on Determination of Optimum Fuel Delivery System
A benefit of this system I am thinking about doing, is that it can be shut off at will, say at wide open throttle, for when full engine power is needed.