Question for engine smarty pants
 

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

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.

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

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.

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.

@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

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.

p_i_f,

Thank you - a very informative post.

We had better get out the hechtpleisters/Band Aids. ;)

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".