Fuel Consumption on High Altitude
September 10, 2011 9:45 PM 
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I've just talked with a physician and he told me that car performances are worst on altitude because the quantity of air running into the engine, also the engines with more valves per cylinder optimizes the mix of air and fuel (using more air) which reduces fuel consumption. Based on this I asume that fuel consumption at a higher altitude should increase since the air available reduces.
I really appreciate any comments on this issue, I'm looking for theorical and concrete answers but logical opinions are also useful, so I'm glad if someone is willing to participate on this discussion.
Actually fuel consumption improves at altitude due to the drop in air resistance (which is the dominant factor at highway speeds). I'm not sure of the effect if all you do is city driving.
The drop in engine performance due to restricted incoming oxygen relates to the peak horse-power (hence why nax speed & 0-60 times are worse), but this is irrelevant to fuel economy.
>I've just talked with a physician and ...
I presume you mean a physicist rather than a physician :-)
>... nax speed ...
And here I meant 'max speed' not 'nax speed' :-)
If you are driving a modern fuel injected engine the fuel injection system should reduce the fuel charge to match the available air for combustion. Old carbureated engines had to be retuned to run at high altitude because a carb doesn't change the fuel charge it delivers a constant amount per butterfly valve position and in thin air they would sometimes literally rich out to the point where an engine's fuel air mixture got so rich that it wouldn't continue to combust. Places like Independance Pass in the Rocky Mountains were famous 25 years ago for cars left on the side of the road that just couldn't climb the mountain. This is the reason that reciprocating (piston powered, not turbine or jet) engines in airplanes have a mixture lever, so that the pilot can lean or enrichen the fuel to air mixture depending on alititude and power requirements. I can tell you absolutely that at altitude airplanes get better mileage if they are leaned properly. That is not only theoretical data, but I have years of experience flying and leaning engines and literally hundreds of hours of first hand experience with the increased range provided by proper leaning at cruising altitude.
While I agree with the poster that the air resistance drops as altitude increases I'm not sure that it will affect your mileage significantly as rolling resistance (the friction between your tires and the road surface) won't be dependent on altitude and remains more or less constant for a given weight on the same vehicle.
Finally, for improved performance you might be well served to utilize a turbo charger or belt driven blower (super charger) or nitrous oxide system to add more free oxygen to the combustion chambers which will aid combustion and horsepower.
>as rolling resistance (the friction between your tires and
>the road surface) won't be dependent on altitude and remains
>more or less constant for a given weight on the same vehicle.
This is true, but rolling resistance is a relatively minor factor at highway speeds (it is more important at lower speeds).
I will agree that higher altitudes = less wind resistance, but with less air, you burn more fuel, since you have less to combine it with during the combustion process.
If you ever go to 1/4 mile drag strips, and try one at high elevations vs one at normal elevations, they actually go slower on higher elevations. So the less resistance thing really doesn't factor in as much as the difference in how your engine burns fuel due to having less air.
I would imagine the actual MPG difference to be very low though for most vehicles, but it would be less as an average.
The source of the fuel savings at high altitude is the fact that you must open the throttle MORE to get the same amount of air into the cylinder. The fact that an otto-cycle (gasoline) engine must use a throttle to restrict air entering the cylinders, and then fight that vacuum on every stroke of the piston, means that REDUCING the atmospheric density will correspondingly DECREASE pumping losses. Notice that when you are driving at a constant speed, you do not keep the accelerator open all the way -- that is because you must restrict the amount of air entering the engine. Opening the throttle more adds more air and causes you to accelerate. Diesels are not the same, since they regulate power with the air/fuel mixture rather than with a throttle.
Air drag is also decreased, but we are talking minute changes. And any car built since the 1980s will compensate for the difference in air density at high altitude -- any car with electronic fuel injection will maintain the correct air/fuel ratio. Rolling resistance is not affected. Peak horsepower is also decreased (significantly) since the air mass available on each piston stroke is proportional to the density of the atmosphere.
What's better, you can use lower-octane gas, since power is decreased -- lower octane = cheaper. I recall on my last trip to Denver you could buy 85 octane (R+M/2) as "regular." So you get better mileage and pay less for the gas, at the cost of more than a couple horsepower.
Well, the biggest thing to take into consideration is that there is less oxygen in the air at higher altitudes.
Now, fuel is burnt in the presense of oxygen. The more oxygen, the easier the fuel burns.
So, if you have less oxygen you also have less fuel burning. Hence the better fuel economy and decrease in performance.
Now what would happen with a carb motor:
The same amount of fuel would be supplied but less oxygen. This is why it would "rich out". The fuel would not be able to burn as the Oxygen/Fuel ratio would be too low.
With modern fuel injection engines:
The Oxygen/Fuel ratio is kept at an optimum by changing the amount of fuel injected. Hence it will still burn properly but to get to the proper ratio less fuel is injected. Now with this, less fuel is burnt. This then causes the decrease in performance and better fuel economy.
I have family that lives in Bogota!!! How exciting!
The following MPG calculator is awesome, it includes the effect of air pressure, so you can experiment with different altitudes.
http://ecomodder.com/forum/tool-aero-rolling-resistance.php
Other effects to think about involve cooling efficiencies. The thinner air at high elevations can not cool the radiator as efficiently, so there will be more drag from heat transfer losses. This will be offset by less induced drag from the shape of the car through the air. I'm not sure what the net effect would be, and it probably depends on the car.
Also, while rolling resistance doesn't change with air pressure, it does change with temperature, and the tires and brakes will be hotter in the thinner air, especially at higher speeds, which will increase rolling resistance.
Just some thoughts...
I would have to imagine these subtle changes are minute at best. Probably less than .10 MPG
70mph
At sea level, 15c
MPH KM/H Aero (a) Rolling (b) Total % Aero % Rolling Watts HP HP Aero HP Rolling MPG(US) L/100km
70 113 377.36 65.14 442.51 85.28% 14.72% 13,847.24 18.57 15.84 2.73 35.45 6.63
At 3500 metres, 15c (rho 0.854)
MPH KM/H Aero (a) Rolling (b) Total % Aero % Rolling Watts HP HP Aero HP Rolling MPG(US) L/100km
70 113 264.15 65.14 329.30 80.22% 19.78% 10,304.63 13.82 11.09 2.73 47.64 4.94
The only difference in these two scenarios is the altitude. The ground level one gives 35mpg, and the one at 3500 metres is 47mpg. This is entirely due to the extra 4hp required to overcome aerodynamic resistance at sea level.
According to the math.
But for the real life factor, living in high elevations also usually means more hills. So that will throw that math out the window... LOL
Math calc = perfect world. Unfortunately there is no such thing.
>But for the real life factor, living in high elevations
>also usually means more hills. So that will throw that
>math out the window... LOL
The original poster (judacomadc) lives on a plateau. So no hills.
You have to isolate each factor and treat it separately. But then you have to combine everything back again to see the overall effect. You can't just hand-wave and say that there is only going to be a 0.1 MPG effect without any grounds to say that.
I should also point out that 3500 metres was just a simple example of the effect of altitude on MPG - it wasn't a specific example tailored for Bogotá. The only thing I changed between the sea level example and the 3500 metre example was the altitude, it did not include temperature, or any specific factors related to the OP's vehicle or engine. But all these things can be added to the calculator as needed.
If an engine is properly tuned for the higher altitude, there will be an increase in MPG. Less power is required to move a vehicle through less dense air. Simple.
Well gentlemen I´m adding more details:
Travelling in the city of Bogotá at highway speeds (70mph+) is something rare because if there's no traffic jams the speed limit inside the city is 80km/h
(about 50mph) and on highway it increases to 100 (62mph).
Anyways I'll keep reading these more comments, then I'll find a clear conclusion.
Thanks for your opinions, I'm amazed of the knowledge of Fuelly users!