Fuel Mileage at Altitude

[jaeger22]

It is often reported here that riders see much better fuel mileage from their S10 at higher altitudes. Other types of bikes also see this improvement at altitude. I have experienced this myself and the difference can be rather dramatic. Like low to mid 40's at near sea level to mid to upper 50's in the Western mountains. 20 to 25%! ??? Wow!
So I was curious as to why this is. A little research on the net quickly showed that 99% of the many explanations out there are BS. ::005:: Most involve less O2 at higher altitude so less gas used. : This is just wrong. First the % of O2 is the same at 21%. But there is less pressure so less air enters for a given throttle opening. But we riders do not use a fixed throttle opening, we just open it until we get the power/speed we want so we end up with the same amount of air and O2. And the EFI injects the correct amount of gas. (different problem for bikes with carbs) Of course this does limit the maximum power available but few of us run at max power for very much of the time so not a big factor for fuel mileage.
So what is the answer? :question: . . . two things, mostly drag. It may seem counter intuitive at first but it turns out that 80 to 95% (depending on speed) of the bikes power is use to overcome air resistance. And at altitude the air is less dense and therefore has less drag. A lot less. ???
At sea level and standard temperature the air density is 1.225 Kg/M3. At 5000 feet it drops 12%. At 7000 ft if drops 19%, and at 10,000 22%. I won't bore you with the math but that results in a rider that normally averages 45 MPG at sea level seeing 53.5 MPG at an average altitude of 7000 ft. I believe this is the right answer because the math all work. But there may also be a secondary effect because many including myself have seen even slightly better improvements than what the math predicts. I attribute that to the fact that most mountain roads are twisty and we are forced to ride at somewhat lower speeds than the flat land roads. Yes they are also up and down and that takes more fuel. We gain back some but not all on the down hill so overall it takes a bit more fuel for the hills. But I believe that is small compared to the well understood effect of speed on fuel consumption.
Just my theory. OK shoot holes in it! ;D

 

[jbrown]

I think jaeger's explanation is the only one I've heard that makes sense. Engineering courses I took long ago tell me that the same engine at high altitude would make less efficient use of fuel (lower volumetric efficiency due to a lower pressure differential), so the explanation for observed increases in mileage must be external to the engine.

 

[Langolier]

Octane

At its most basic level, octane is nothing more than a rating given to gasoline to describe its “anti-knock” performance. That is, its ability to resist uncontrolled combustion within the cylinders of an engine. Gas is supposed to be ignited by the spark plugs, but when it ignites from compression it causes a “knocking,” which is an explosion of air and gas that can damage the engine. Lower octane levels combust from compression at lower levels of pressure.


Importance of Octane

Octane levels are important because they help prevent these explosions from happening. Over time, prolonged knocking—more commonly referred to as pinging—can actually cause damage to an engine and even result in power loss over time. Different engines are built with different octanes in mind, and your user’s manual will let you know which octane you are supposed to use in your vehicle.



Engine Compression

Engines work by compression, and the compression ratio of your engine determines the level of octane required by your particular car. By increasing the compression ratio of an engine you can actually increase the horsepower of a vehicle, although it will burn more fuel this way. By lowering the compression ratio you lower its output, but also increase the fuel efficiency.


Effects of Air Pressure

Altitude decreases an engine’s octane requirements because of the change in air pressure. The higher elevations have a lower level of air pressure, which means an engine needs less octane to properly fire due to the lower ambient pressure. In certain states, such as Colorado where the altitude is generally above 5,000 feet in the mountain regions, 85 octane gasoline is sold, while in lower elevations 87 octane is the lowest sold.


End Result

When an engine requires lower levels of octane to “fire,” the overall efficiency of the engine is also lowered, which means the engine works harder to achieve the same results. However, less fuel burns, which means the overall fuel-per-mile efficiency is increased because the engine is working at a reduced compression rate. In short, you can get higher miles-per-gallon at higher elevations, but the power of the engine is also somewhat reduced.


I should add: When a cylinder of air lets in air (at any given throttle setting) Its a matter of how many molecules of air that are within the given volume. If your engine is supercharge or turbocharged it's a whole different ball game. Also the theory of traveling thru thinner air has merit (at what % I could only guess). A reason why jets travel at higher altitudes.

 

[Karson]

I have nothing technical to add other than what's already been said to confirm/deny any ideas, but y'all in the altitude must feel like you're on a completely different Tenere when you get down to 1-2k altitude.

Riding two-up last year in Ouray was quite the eye opener ???

 

[Don in Lodi]

I wouldn't have thought air density/resistance would have played that big a role. I was leaning more to the reduced speed part of the equation. Hmm, 777 Wide body at 33,000 feet, SR71 at 70,000 feet, I guess air density may have more to it.

 

[tomatocity]

Just got back from a 3,450 mile trip with many elevation changes. My thought has been the gas you save when at full or partial off throttle is greater than the gas you use while riding uphill at partial throttle. I prove this while riding mountains in the crappy gas state of California and usually get 45 mpg. I normally get 41-43 mpg.

While on this trip, out of California, I got as high as 58 mpg and never below 46 mpg. One of the pluses was Sinclair has 91 octane without ethanol (rocket gas to a Californian). The other plus on this trip was traveling with a KLR with a 16T counter sprocket. When I was at 4,000 rpms he was at 5,000 rpms. So we usually traveled at 68-73 mph though at times I pushed the 100 mph button.

My first tank of gas back in California was 38 mpg but that was 4,600 rpm at 82 mph while crossing the Extraterrestrial Highway.

 

[markjenn]

All your theory looks good to me, although I don't think the effect of reduced air density on drag is quite as dramatic as you say. I think the best explanation is that one tends to ride a bike more sedately at altitude, both because the engine doesn't have as much power to use, and because the roads don't support the same higher speeds.

Most riders on tour get better mileage in general just because they're spending less time in town. It would be interested to look on someplace like fuelly and see if, for example, Denver riders report generally higher mileage than Houston riders. This would tend to remove the effect of riders using their bikes differently when they're riding at altitude.

On any individual bike, there may be some tuning things going on too....FI systems aren't perfect and I believe the S10 uses a "canned" lookup table to convert throttle position, air temperature, and ambient air pressure to a mass air flow rate. (Some engines use a mass airflow sensor that directly measures the mass of air flowing in the intake - these systems are generally thought to be much more accurate.) It's possible that the FI system is off a little and tends to run leaner at altitude which probably hurts power but helps fuel economy.

- Mark

 

[advswede1981]

::017::

 

[jbrown]

The air drag is proportional to the square of velocity, so it quickly becomes the dominant fuel user on level ground at a steady speed at anything above in-town speeds.
Air drag is also directly proportional to the air density, so half the air density means half the drag, everything else being the same.
Air density variation as altitude changes is complicated, but at 6000 feet elevation the air density is about 80% of that at sea level, so you would expect 80% of the drag. That would equate to a 20% decrease in the power required to overcome the air drag at the same speed at 6000 feet.

I'm pretty convinced that drag due to air density change is a significant factor.
Certainly different riding characteristics could swamp all other changes, and different fuel would require evaluating the energy content and efficiency of the engine for each fuel to draw any conclusion about that effect.

 

[Langolier]

::017::

LOL I know .... I know .... density, altitude, molecules, Oil ........

 

[markjenn]

The air drag is proportional to the square of velocity, so it quickly becomes the dominant fuel user on level ground at a steady speed at anything above in-town speeds.
Air drag is also directly proportional to the air density, so half the air density means half the drag, everything else being the same.
Air density variation as altitude changes is complicated, but at 6000 feet elevation the air density is about 80% of that at sea level, so you would expect 80% of the drag. That would equate to a 20% decrease in the power required to overcome the air drag at the same speed at 6000 feet.

I'm pretty convinced that drag due to air density change is a significant factor.
Certainly different riding characteristics could swamp all other changes, and different fuel would require evaluating the energy content and efficiency of the engine for each fuel to draw any conclusion about that effect.

This is all basically true for steady-state high speed riding, but this is not the only thing that uses fuel - there's also driveline losses, rolling resistance, etc. And there is the fuel required to accelerate the vehicle - in non-steady state conditions, this can be very significant. So you can't just say 20% less drag = 20% better fuel mileage, at least not in typical riding. I do agree with you that it is "significant" though.

- Mark

 

[Chuck B]

My driveway is at 7000ft (Flagstaff, AZ) and typically ride up hill from there. I don't pay much attention to fuel economy but dropping down to 1500ft (Phoenix, AZ) results in significant butt dyno improvement. I'll take the lower elevation 'snap' over minor increase in mpg.

 

[autoteach]

tire pressure? just look at an unopened bag of chips at altitude. Yeah, that is it... tire pressure.

 

[Twisties]

I've thought about this for years. First of all, no doubt in my mind, every internal combustion vehicle gets better fuel economy and less power at altitude. I think it's all of the above: Lower drag, less fuel injected, cooler ambient temperatures, less humidity, and lower speeds.

When you go up there are fewer molecules of air per unit volume. The volume per cycle is fixed by the cylinder dimension. You can only burn as much fuel as there is oxygen for. You'll have less of that altitude. Without EFI your engine will run rich, and eventually you will start smelling gas, maybe foul a plug. In an EFI system the amount of fuel is cut to match what may be burned. Either way, you just can't burn as much fuel with a fixed volume of air at altitude as you can at sea level.

I think the drag is important too. Definitely less of it at altitude.

 

[jaeger22]

A lot of good points here guys. ::012:: I agree that the rider has more influence on MPG than any other factor, and that the higher roads often force us to slow down. And also that it is not just one thing, but several factors combined. However I do maintain that drag is by far the big dog. As I said in the beginning is seems counter intuitive, especially to those that don't happen to have a scientific or engineering back ground and even to some that do. And I do not mean that in any way to be condescending. I am an an engineer and it was not immediately obvious to me either. My first thought was along the lines of an EFI table a bit lean as mentioned above. And that still COULD be a factor. But I think we have to accept several things as fact. First, aerodynamic drag is the major cause of fuel consumption at steady speed at any altitude, as much as 90% depending on speed. ??? Second, as air density decreases, drag decreases proportionally. Simple physics. Third, at high altitudes, (7000 ft for example) air density is only around 80% of what it is at sea level and therefore drag is 80%, or 20% less. So for all else being equal, the fuel consumption at steady speed MUST be less by those two percentages multiplied together, or around 16 to 18% less. It has no choice.
Acceleration, especially up hill, will of course use much more fuel and dwarf the drag factor. But most of us do not accelerate hard for a major portion of our rides. And even then the reduced drag is still helping some.
The other factors all contribute, but if not for the reduced drag, my guess is that we would hardly notice.
As always YMMV.