Question for engine smarty pants
Thread Starter
Join Date: Aug 2005
Location: Glens o' Angus by way of LA
Age: 60
Posts: 1,975
Likes: 0
Received 0 Likes
on
0 Posts
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.
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
I'm not sure what you mean by "runs better".
This drag-racing article states that higher humidity means less power:
Race Car Book
Thread Starter
Join Date: Aug 2005
Location: Glens o' Angus by way of LA
Age: 60
Posts: 1,975
Likes: 0
Received 0 Likes
on
0 Posts
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.
- 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.
Last edited by pattern_is_full; 14th Jun 2016 at 22:18.
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,573
Received 419 Likes
on
221 Posts
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.
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. 
Thank you - a very informative post.
Jan and I42 each hold one edge of a two-edged sword
Join Date: Feb 2007
Location: Amsterdam
Posts: 4,598
Likes: 0
Received 0 Likes
on
0 Posts
- 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.
