Air Risistance and Altitude

[widman]

I remember that an internal combustion engine loses 3% of it's power for every 1000 ft altitude. So at 12,000 ft I lose 36% power. Can anyone tell me how much this is offset by reduced friction and resistance from the air?
In other words: As I cruise along at 140 kpm at 12,400 feet above sea level (getting 2 km per liter better mileage than 110 kpm at 1200 ft), how much easier is it on the front of my pickup?

 

[Neil Womack]

Widman try asking a question that requires a little though will you?

Just kidding.

I'm sure some bright guy will have an formula to answer this. But I suspect you know through real world experience what the answer is.

Using Kentucky windage and venturing a flat landers guess I would say you can not make up the loss of engine efficiency with the lower frictional forces on the body of your vehicle.

 

[Bill J.]

Turbocharger Triumphs, page 62 - "The Gothenburg Bible" by Paul Grimshaw

If parasitic drag and charge air heating is minimized, turbocharged engines can produce amazing power. An additional benefit of turbocharging is its ability to maintain power output at increased altitudes such as those found in the European Alps and North American Rocky Mountains. A normally aspirated (non-turbo) car will lose 3% of its power for each 1000ft/330m that it is operated above sea level. This occurs because air density drops a corresponding amount with an increase in altitude. However, modern turbocharged cars with electronic engine management and waste gate control can overcome power drop-off with altitude by increasing boost to maintain absolute manifold boost.

There does come a time when reductions in ambient air density overcome even a turbo's ability to keep up. This occurs after about 6000ft/2000m. Beyond this altitude turbo engines begin to lose power at the rate of 3% per additonal 1000ft/300m climbed. Despite this eventual power drop-off, the advantage that turbocharged engines demonstrate at altitude can be readily appreciated when compared with outputs of normally aspirated engines.

 

[widman]

Bill J.... that explains why my 4Runner (turbo diesel) performs well at 6600 ft, climbing anything I can find at decent speed with any load, but is reduced at 12,000 and has nothing in the way of power from a dead start at 12,000 ft until the turbo kicks in (sometimes have to use low range to start up hills).
Unfortunately the pickup is a straight gasoline 2.7 efi, which suffered from power loss but had no problem with high speed.

 

[RobY]

Widman, the equations for friction loss in air show the total loss is a function of geometry (area times length) times the density of air times the velocity squared.

The density is a only a first order effect while the velocity effect is squared. Slowing down should have a greater effect on friction losses than the density, all other things being equal.

I am not sure if power reduction can be equated with fuel economy, especially with a fuel injected engine. Even at reduced atmospheric pressure you should still get an optimum fuel to air ratio for economy, but will not have as much charge in the cylinder reducing power. I also seem to remember that an engine is more efficient at a more open throttle than at nearly closed throttle (engine breathes better), but may be mistaken

If so, then at altitude you would need more throttle to maintain speed due to the reduced power.