Question for engine smarty pants

[piperboy84]

Why does the Lycoming seem to run better on high moisture days, I understand it running better on cold days but on damp ones?

[Jan Olieslagers]

There are many instances of injecting water into engines - both piston and turbine - for better performance.

Then again, if it only SEEMS to run better the difference may be in the observer, too.

[India Four Two]

pb84,

I'm not sure what you mean by "runs better".

This drag-racing article states that higher humidity means less power:

Race Car Book (http://www.racecarbook.com/articles/humidity.shtml)

[piperboy84]

Sitting behind the same Lycoming for 600 hours it just seems to run better I.e. a few more MPH for the same RPM when it's damp out.

[pattern_is_full]

Jan and I42 each hold one edge of a two-edged sword

- water vapor in the fuel/air mix displaces some oxygen and fuel, which reduces burnable fuel and thus power. However

- water vapor in the mix acts as a coolant, both to the charge itself, and to the cylinders. That has the same effect as a cooler day (denser mixture charge). It also evaporates to steam, increasing the cylinder pressure on the power stroke, or adding "reaction mass" to a jet's exhaust.

(The actual "mix" injected is usually about 50/50 water and alcohol, with a trace of oil. The alcohol burns, and acts as an antifreeze; the oil limits water corrosion (rust) inside the engine.)

Which effect predominates depends on the mode of operation: idle, low power, cruise, TOGA, "full military power." And how dense the water load is (ambient humidity being far less than the amounts injected intentionally).

In a civvy GA engine, I venture the cooling and lower power would make the engine "purr" more sweetly, even though technically it is producing less power. Definitely, if the engine is a bit mis-tuned, and closer to detonation conditions (spark timing, overall temp, or running on totally unleaded auto gas (STC)), water vapor/humidity will act as an "anti-knock" agent.

EDIT - in re post #4: Ahhh! Humid air is less dense than dry air (Molar weight of water lower than molar weight of "dry" O2/N2/trace elements mixture). All other things being equal (temp, altitude). Lower density = lower drag.

[Jan Olieslagers]

@PiF: thanks, you brought it all back to me. The magic word is MW50 (metanol/water 50/50 mix): https://en.wikipedia.org/wiki/MW_50

[ShyTorque]

Unless your aircraft has a CS prop, at the same rpm it will produce the same power.

However, I do agree that petrol engines do run a little smoother on damp days, probably for reasons already stated. It was very noticeable on an old but highly tuned BSA 250 motorbike I used to ride to college.

[India Four Two]

p_i_f,

Thank you - a very informative post.

Jan and I42 each hold one edge of a two-edged sword
We had better get out the hechtpleisters/Band Aids. ;)

[BackPacker]

- water vapor in the mix acts as a coolant, both to the charge itself, and to the cylinders. That has the same effect as a cooler day (denser mixture charge). It also evaporates to steam, increasing the cylinder pressure on the power stroke, or adding "reaction mass" to a jet's exhaust.

That may well be true in racing, where *liquid* water is injected into the engine, but has no bearing at all to "humid days". The water in the air is already in *gas form* and cannot evaporate any further. Unless you confuse "humid" with "mist" or "fog".