air density

[nemesis]

I was driving somewhere in Tennessee on a 6 grade road of 8 miles or so, and the elevation change got me thinking. What effects does elevation/baro has on fe. To expend it little more lets add air temperature to that. I've been playing with increasing my air temp temps and watching fe, it's been increasing up to about 120F ish, anything over that and the results stayed about the same, but I started experiencing more of a spark knock. I haven't had any time to experiment with different baro/elevation. In theory I could see your fe increasing from a lower baro pressure due to having less O2, but how would increased air temp would effect the engine?

Ideally, I am probably looking at 3 variables that I'd like to know how they both work together and what effects it does to fe and if there is a perfect combo. The 3 variables are engine static compression, air baro pressure, and air temperature. The reason I added static compression into the equation is because baro pressure effects dynamic compression. Basically at higher elevation you can run higher static compression on pump gas than at sea level. Excessive static compression, excessive air temps, higher baro pressure will make the engine to experience spark knock more likely.

[Occasionally6]

How about this too: with low ambient atmospheric pressure engine will need to be throttled less for a given power output. In general that should help FE.

Lower air pressure should help with drag too.

[Xist]

I remember posting about this and finding an air density calculator. The higher altitude seemed to have sufficiently less dense air to overcome the colder air being denser.

[redpoint5]

This has been discussed before in other threads. As pointed out, lower density air will increase engine efficiency by reducing throttling losses, and it will improve aerodynamic losses. Warm air has a similar effect as reducing density.

Another factor that will increase FE is humidity, with higher humidity further reducing density and aerodynamic drag.

[user removed]

Originally Posted by redpoint5

This has been discussed before in other threads. As pointed out, lower density air will increase engine efficiency by reducing throttling losses, and it will improve aerodynamic losses. Warm air has a similar effect as reducing density.

Another factor that will increase FE is humidity, with higher humidity further reducing density and aerodynamic drag.

How does higher humidity reduce drag?

100% humidity is less drag than 10%?

Atmospheric pressure is 50% at 18k feet of what it is at sea level under otherwise identical conditions.

regards
mech

[spacemanspif]

The amount of water vapor in the air also effects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.

Taken from here: WHY IS MOIST AIR LESS DENSE THAN DRY AIR AT SAME TEMPERATURE

I knew the law but never knew why...but now we all know

[RedDevil]

Moist air has more water molecules replacing the other gases, so it will have less oxygen molecules, further reducing the vacuum on low loads to get the 'right' amount of oxygen - less pumping loss.

The hotter the air, the more water it can contain... The race to build a wet WAI is on!