Blue Sky tuning

[af2]

so you have way more O2.

Well not way more but more.

[Erland Cox]

The colder it is the less water vapor can be kept in the air.
So when it is foggy it is because it got colder and the air can no longer hold the vapor,
So the air is more dense.
Clear blue sky's are OK as long as it is not hot, PV=nRT.



Erland

[David Redszus]

Building upon Erland graph we find that air density changes as follows:

At 0 RH and 40F baseline
at 60F =-3.85%
at 80F = -7.41%
at 100F = -10.72%
at 120F = -13.80%

At 80F and 20RH baseline
at 40RH = -0.64%
at 60RH = -1.31%
at 80RH = -1.96%
at 100RH = -2.61%

As can be clearly seen, inlet air density is much more sensitive to changes in inlet air temperature than to changes in humidity.

[roadrunner]

So would I be right if I said that on a clear cool day, the air pressure is higher (high pressure system goes with clear skies), and the air is denser, so the effective compression ratio of an engine will be higher than on a warm moist day through better cylinder filling?

[Snuffman]

According to Mariner's lore:

"Red sky at night, sailor's delight.

Red sky in morning, sailor's warning! "


Now, whether (weather) that's true...

[David Redszus]

So would I be right if I said that on a clear cool day, the air pressure is higher (high pressure system goes with clear skies), and the air is denser, so the effective compression ratio of an engine will be higher than on a warm moist day through better cylinder filling?

Yes you would be right. Just remember that what really counts is the air density (volume x density = mass) ingested into the cylinder which then determines how much fuel can be actually burned.

[roadrunner]

How would the expansion of the in-cylinder air be affected by starting with a lower compressed temperature, I'm thinking that the cooler air would absorb more heat from the burning of the fuel, and less heat would theoretically be lost to the chamber/walls/piston. Maybe the cooler intake air would absorb more heat from the chamber/walls/piston right up to the point of ignition, increasing cylinder pressure slightly! A cooler intake charge obviously gives the engine a higher power output, but a warmer charge should give better fuel economy at cruise conditions (part throttle), through less pumping losses and better evaporation of the fuel droplets, please correct me if I am wrong on this David (or anyone else)

[David Redszus]

Certainly the initial conditions will affect the chamber temperature and pressure at ignition and will result in some change in power due to the quantity of fuel that can be burned.

Assuming an engine with a nominal compression ratio of 10-1, a trapped compression ratio of 8-1 (inlet valve close 118BTC).
Shown below are the compression pressure, compression temperature, air density and power changes for various conditions.
Baro...Temp...Psi....Temp....density...power
14.7....80F....234....722F....07358
14.2....80F....226....722F....07112...-4.0%
13.7....80F....218....722F....06862...-8.0%

14.7....70F....234....700F....07497...+0.94%
14.7....60F....234....678F....07641...+1.91%
14.7....50F....234....657F....07791...+2.90%

Regardless of the initial conditions, the dominant factor is the amount of fuel that can be completly burned. While the changes in temperature seem substantial, remember that combustion temperatures are on the order of 4700F. Therefore the changes in power will closely follow the actual inlet air density.

[dezza]

I had 260psi static comp and it went its best on a foggy morning

[David Redszus]

I had 260psi static comp and it went its best on a foggy morning

Let us assume that to be true.
Then we must ask, "Why would that be true?"
Is it possible that the engine was on the ragged edge of destruction and the atmospheric conditions mitigated against those circumstances?

[roadrunner]

Pre-ignition does seem to fit with that scenario, but maybe the engine management (if fitted) could be advancing the ignition timing somewhat, or could be fattening up the mixture (or both)to compensate for the moisture laden air passing through a hot wire setup! ....just thinking out loud here, any other ideas?