Power Loss at High Altitude

[rScotty]

The following quote on was posted today - I don't know if the numbers are right or not...they seem sort of large. But true or not, it certainly leaves the wrong impression. Let me explain why....

"ALTIITUDE - Tractors lose 3% of engine power output for every 1,000' altitude increase, over 1,500' Sometimes twenty posts will be made advising on tractor horsepower, then we find OP is a 6,500 feet altitude and is considering a low power, naturally aspirated tractor."

I live at 7000 feet altitude & have quite a bit of experience with engines here. The numbers in the quote above give the wrong impression about the real effects of power loss. In most cases an operator will simply open the throttle a little farther and run at a slightly higher RPM. He won't even notice a slight loss in efficiency. So although the loss of theoretical maximum power may be real, it isn't something that is normally a factor

Of course an operator who likes to run with the throttle wide open against the stops at all times might feel shorted, but that's not normally how one operates a tractor.
rScotty

 

[Dirk Digger]

In most cases an operator will simply open the throttle a little farther and run at a slightly higher RPM. He won't even notice a slight loss in efficiency. So although the loss of theoretical maximum power may be real, it isn't something that is normally a factor.

Not saying you're wrong, but if an engine is operating at its mechanical limit in terms of rpm (i.e. redline), you can't simply open the throttle more to increase rpm even further to create more power. That's where turbos and superchargers come into play. They allow the engine to operate within its mechanical rpm limits and still achieve the same or better manifold pressure (and subsequent power) as they would at a much lower altitude.

I have a a1960's IH 150 tracked loader with a normally aspirated 282 diesel 6 cylinder engine. They also sold a turbo version which made slightly less power at sea level than the normally aspirated one (72hp vs 75hp), but at altitude the normally aspirated engine's power dropped off significantly, whereas the turbo would maintain power at substantial altitudes all while maintaining the same rpms.

 

[ericm979]

3%/1000ft is the generally accepted number.

If you're running a PTO implement you may not want to set the "throttle" to a higher RPM. Some of my PTO implements aren't supposed to be run at higher rpms.

 

[wagne223]

Jeff is spot on with his quote. No arguing with math and numbers.

What varies is how each owner uses their tractor. If you can simply add throttle to complete your task, then you are probably doing loader and light ground work.

You may also have a tractor with power to spare operating your PTO implements.

In my case, 50 percent of my tractor use is heavy ground work and heavy mowing.
I use every bit of power the tractor has and run at WOT.

This is why Jeff and others ask for so many details as far as tasks and location before making recommendations.

I have disagreed with and been mad at Jeff in the past but on this matter, in my opinion, he is spot on.

 

[jeff9366]

I make my wife mad too.

She usually gets over being mad within six months.

 

[rScotty]

3%/1000ft is the generally accepted number.

If you're running a PTO implement you may not want to set the "throttle" to a higher RPM. Some of my PTO implements aren't supposed to be run at higher rpms.

It works differently than that. If we are at anything less than full throttle, what we are doing by advancing the throttle lever is using the throttle to set the governor so so as to develop more torque against a given resistance. The RPM of the PTO remains the same.
rScotty

 

[rScotty]

SNIP

What varies is how each owner uses their tractor. If you can simply add throttle to complete your task, then you are probably doing loader and light ground work.

You may also have a tractor with power to spare operating your PTO implements.

In my case, 50 percent of my tractor use is heavy ground work and heavy mowing.
I use every bit of power the tractor has and run at WOT.

you're right, I mostly do chores and loader work. We no longer do much PTO work other than bush hog type mowing, post hole drilling, water pumping, and we used to run a 3pt backhoe that had a PTO-driven hydraulic pump. None of those jobs require more than half throttle on a 30/35 hp tractor - although I guess with the exception of the post hole drill, they could all be done at full throttle if I wanted to work that way. But I don't like to work that way; I get more enjoyment out of setting to about half throttle and listening to the way the governor automatically changes the way the engine works to meet the load.
For me that's part of the fun of tractoring.

Having a modern HST tractor that is governed to open the throttle to meet the load and then auto-return to idle is a great help for part-throttle operation. I think that's a big advance for us part-throttle homeowner cowboys. Industrial machines have been like that for a long time...

I wonder how many people run all the time at full throttle versus how many like to run at lower throttle settings?

 

[ovrszd]

It works differently than that. If we are at anything less than full throttle, what we are doing by advancing the throttle lever is using the throttle to set the governor so so as to develop more torque against a given resistance. The RPM of the PTO remains the same.
rScotty

If advancing the throttle does not advance the PTO speed your tractor is overloaded and could benefit from more rated HP regardless how you go about getting it.

I'm with wagne223 on this one.

 

[ericm979]

you're right, I mostly do chores and loader work. We no longer do much PTO work other than bush hog type mowing, post hole drilling, water pumping, and we used to run a 3pt backhoe that had a PTO-driven hydraulic pump. None of those jobs require more than half throttle on a 30/35 hp tractor - although I guess with the exception of the post hole drill, they could all be done at full throttle if I wanted to work that way. But I don't like to work that way; I get more enjoyment out of setting to about half throttle and listening to the way the governor automatically changes the way the engine works to meet the load.
For me that's part of the fun of tractoring.

Sounds like you're running your brush hog at less than PTO rpms. I like to run mine at PTO rpm as it cuts better. On my old Kubota and current Branson I can't sense a difference when the governor adds fuel. Both maintain the rpm I set unless I'm doing something that requires more power than the engine can give at max output, then it slows down.

Having a modern HST tractor that is governed to open the throttle to meet the load and then auto-return to idle is a great help for part-throttle operation. I think that's a big advance for us part-throttle homeowner cowboys. Industrial machines have been like that for a long time...

My Branson has that. They call it auto throttle. It's a simple cable between the HST pedal and the fuel injection throttle lever. It's really nice for driving around or pulling logs out of the woods. I can leave the engine at idle and let the auto throttle rev it up when needed. It gives it an even softer start which is nice when the ground is soft. For loader work I set the throttle at 1500-2200 rpm depending on the loader performance I want and let the auto throttle raise it past that for travelling if needed.

I wonder how many people run all the time at full throttle versus how many like to run at lower throttle settings?

For anything with a PTO I run it at PTO rpm. That's not quite full throttle but it's close.


The 3%/1000ft is for comparing the same power setting. Using PTO rpm for that comparison makes it easy as tractors are rated at maximum output at that rpm. If you can get more power by adding throttle, well you can do that at seal level too, and it'll make more power there.

 

[finn1]

3%/1000 feet is the correct number for a naturally aspirated engine.

Turbochargers don’t have this inherent limitation, with some exceptions. The turbo spins faster at high elevations because the back pressure and inlet pressure are simultaneously reduced, putting the turbo operation on a different part of the operation map. In effect, the turbo naturally spins faster at high altitude (low barometer) to increase air flow, and maintain engine power .

The exception is with modern electronically controlled engines, where the turbo is matched to give improved performance at sea level, and the software is calibrated to protect the tribe from entering surge at altitude. In that case the wastegate and fuel delivery are manipulated electronically and power is reduced at altitude to protect the engine and turbo.