Altitude and fuel consumption

[Caddy101]

Guys what is the relationship between altitude and fuel consumption?

From the top of my head (I'm no engineer or scientist) I always imagined that a car would be more economical at the coast because there is more air and every drop of fuel burns more efficiently. More bang per drop of fuel.

Is this so?

 

[Sankekur]

Nah the mixture in modern engines is regulated pretty well, I would think it would run just as efficiently at altitude as it would at the coast. Thing is you would be get better output so you don't need to push as hard to get to the same speed so that could make for better fuel consumption.

 

[P1000]

Not completely - the amount of fuel that enters the engine is directly related to the amount of air (by mass) that enters the engine - given that all other factors are the same ie throttle, RPM, temp, etc. So at higher altitude you get less air mass - which means less fuel.

However, at higher altitude, the effective pressure in the combustion chamber is lower, which will somewhat effect efficiency of the engine (lower it, but the amount is dependent on a LOT of factors, and there is no reference value for this). Of course this also means that for the same kW you will be using bigger throttle openings - which means less throttling losses (if the car has a throttle) which increases efficiency under those conditions.

 

[Philip Foglar]

Something often forgotten is the difference in wind resistance due to the different air density. So while a naturally aspired engine produces more power at the coast, the car experiences much more wind resistance compared to driving at altitude. So in actual fact, driving at the coast the car needs more power from the engine to move through the more dense air - and of course the higher the speed the more this becomes the case.