horsepower loss vs. elevation

[nedhed]

Hello Everyone! My first post. I have already gotten a lot of great information and insight on Kubotas. Thanks.

I am considering the purchase of a B series. I have 3.6 acres of trees and grass, but no mowing required. The CUT will be used for snow removal, grading and maintanence of 310' of a gently sloping gravel driveway, landscaping, log dragging, etc. The elevation is 8300 ft. I want to know how much extra horsepower I should get to compensate for the elevation. The only info I can find is for LP generators which are derated about 4%/1000ft of elevation above sea level. Is it similar for normally aspirated diesel engines?

 

[v8dave]

I just looked up the derating for altitude of Diesel generators. The recommended derating varied between 3% and 4% per 1,000'. Although, more gensets used 3% that 4%.

Looks like you'll lose 25% of your power at that altitude. I'd check with your local dealer to see what they recommend for continuous use at that altitude. Kubota doesn't appear to make a turbo tractor in the B-series. You might look at some other mfrs and see if they make a turbo CUT tractor. A turbo will make up for a lot of altitude pressure (and resulting power) loss.

 

[bandit67]

I run a Subcompact, 24 hp Case, and my neighbor runs a 22hp BX22, and both machines seem to have plenty of power. Years ago, I ran a 14 hp diesel at sea level, and that had plenty of power, so I figured 24 hp at 5600' would be fine, and it is. I think you'd be perfectly happy with a B7610 (24hp), and certainly anything up to 30hp. These diesel machines are very powerful, even up at altitude.

By the way, I'm sure you're talking to Longmont Farm Supply (they always seem to be asking too much for their machines), so I'd recommend contacting B&G Equipment in Greeley - I found better pricing and the service has been very good - they sell a lot of machines in the Denver Metro area, and especially in Boulder County. PM me if you want any additional info or specifics.

 

[earlw]

I'm at 7,300' here in Peyton, Colorado and had the same concerns as you about the loss of HP at this altitude. From what I have read, 3% loss per 1,000' seems to be the standard I see the most (as someone else already mentioned). But I have seen articles that said the first 1,000' is not included, and others that say it's included, so in your case of 8,300', you could be looking at a loss of 21.9% (7.3*3), or 24.9% (8.3*3).

There is a calculator out there which takes into account other factors like humidity and temp as well. Here is one I found:
Relative Horsepower Calculator . You might want to use WeatherUnderground to get the weather values you'll need to use the calculator.

Oh, just to make sure things are more confusing /forums/images/graemlins/tongue.gif, I called Kubota's division office (not the local dealer), and they said that you use 1% per 1,000' of elevation. I think that is a mistake, I have never seen 1% in my research. They also said to expect more smoke from the engine because of the lack of air density into the engine. Kubota doesn't offer a high-altitude kit or turbo-charger for these compact tractors.

 

[nedhed]

Thanks. It all pretty much confirms my suspicions. Can anyone comment on who manufactures CUTs with turbocharged engines? I'm not expecting there to be any since it adds significant cost to the engine, and it would likely be cheaper just to buy a higher displacement engine. Thanks for the link to the horsepower calculator.

 

[SkyPup]

As far as altitude goes, every single foot is included, including the first thousand simply because the barometric pressure is less as your elevation increases. Whoever thought that the first thousand feet of elevation was equilivant to sea level must have had altitude sickness....

There are some 45-50HP turbocharged tractors, both John Deere and Kubota make turbo models once you get over the mid 40's in horsepower. They all use wastegated turbos so your turbodiesel will perform virtually the same as it would at sea level, ie the 3% loss for each 1,000 foot elevation is negated.

 

[SkyPup]

BTW, the reason the engine power decreases 3% for every 1,000 rise in height is simply because the air pressure decreases by that exact amount and a naturally aspirated diesel is a simple air pump.

We had a barometic reading of 27.9 inches of mercury here at our farm last year (65 feet about sea level) when the eye of Hurricane Jeanne went right over us, making it seem as though we were at about 2,700 feet for a couple of hours..... /forums/images/graemlins/blush.gif

 

[AlbertaKubota]

I am at 4000ft and use a B7800. It works great and I only want more power when I use the tiller. I don't have any problem with other implements. If I follow the calculations I should have 30*(100-4*4) = 30*.84 = 25HP.
In you would use the tractor at 8000ft you only have 20HP. That's a little bit low

 

[JerryG]

<font color="blue">Can anyone comment on who manufactures CUTs with turbocharged engines? </font>
John Deere and Massey Ferguson has small turbocharged tractors. I can't tell you what size the JDs are, but the MF is a 40 hp.

 

[JeffRey30747]

I'm going to go off on a hopefully related tangent here and probably show how little I know, but here it goes.
My understanding has always been that a gasoline engine ignites a metered amount of air/fuel mixture in a confined space to facilitate internal combustion. A diesel on the other hand, injects a metered amount of fuel into an overabundance of air in a similar confined space to acheive the same end result. As a result, it would seem to me that the elevation change would have little to no effect until that overabundance of air was diminished. This probably depends on a number of design factors but with designs where hp increases are acheived by simply tweaking the injector pump settings, I would venture a guess that models on the low hp end of that common design platform would be less affected by altitude change than those on the upper hp end as the upper end models would probably come closer to utililizing the full potential of the air intake design for that model series.
Does this make sense to anyone else?
Jeff

 

[SkyPup]

Nein, the lower air pressure producing less power at altitude is exactly the same in both gasoline and diesel engines no matter what the compression ratio is.


Question: How does a naturally aspirated diesel or gasoline engine manage to fill its combustion chamber with the same amount of air at 8,000 feet as it does at sea level?

Answer: It Doesn't.


Now, did you also know that it is more difficult to start a diesel engine at altitude than at sea level? Simply because there is less air present and the heat of compression produced to start it up is therefore less, making it more difficult to even ignite the fuel charge.

 

[JeffRey30747]

Skypup,
First off, I am not saying that what I detailed above is correct. I don't know. You are correct that there is no way that a naturally aspirated engine of any type to fill the combustion chamber with as much air at 8K feet as it does at sea level.
My point was that if a 27hp Kubota and a 34hp Kubota both use the same engine and acheive their hp gain merely by injecting more fuel into the combustion chamber, would the 34hp unit need more air to combust the fuel? If so, the air intake would be designed to ensure the higher volume was supplied at the nominal altitude (1K ft maybe). The 27hp unit uses all the same parts, so it has enough air to function to full potential at maybe 2K ft where the 34hp unit would already begin to suffer because the air volume is not sufficient.
My reasoning that a diesel engine would behave differently than a gasoline engine is not because of the compression ratio but because the gasoline system regulates the air to fuel mixture and diesel only regulates the fuel. Air flow is only regulated by the constraints of the intake on the engine.
However, your starting at altitude comment did make me think. If there isn't really an overabundance of oxygen present for the combustion of the fuel, then every little bit of additional oxygen would help and this whole idea goes down the crapper.
Now, back to thinking about something a little less theoretical,
Jeff

 

[SkyPup]

Still no good.

There is no way any naturally aspirated engine is NOT going to produce less horsepower at higher altitudes than at sea level.

If it produces 34hp at sea level or 27hp at sea level, both engines would decrease by the same amount at whatever higher elevation they were operated at. The same amount of fuel is being injected at both elevations but the higher elevation has less air to combust that fuel and the output is always less.

BTW, it does not matter if it is a diesel engine, a gasoline engine, an air compressor, the hemoglobin in your blood cells, or a pot of boiling water, they ALL have less air at higher altitude. That is a fact.

 

[BillyP]

I think it would be better understood if you would use the word oxygen, instead of simply air.

 

[SkyPup]

The actual weight of air in the Earth's gravity field is comprised mostly of nitrogen atoms (about 78%), oxygen atoms (about 20%), carbon atoms (<2%), and traces of other atoms.

It doesn't make any difference if you state oxygen or just plain air, since they both decrease equally at higher elevations, ie the air pressure declines on a constant slope plotted against altitude. If there is less air, there is correspondingly less oxygen and vice versa.

 

[BillyP]

Yeah, I remember the 80/20 rule from grade school. Just thought it would be better understood using a word that was needed for combustion to take place.

 

[BillyP]

The JD 3520 is a 30 PTO HP turbo charged CUT.

 

[v8dave]

It may be easier to understand the altitude loss if you take it to extremes.

Would the diesel engine work in outer space? Of course not. How about 100,000 feet? Still wouldn't. How about 50,000 ft? Hmmm, not likely. How about 25,000 ft? Maybe, but surely not very well.

Ok, just to pick a point for the argument lets say it barely runs at 25,000' and runs fine at 0' (sea level). Is there a magic elevetion above which it won't work and below which it will? In other words is there a point where 10' higher and it won't run? That doesn't sound likely either.

So, assuming there isn't a light switch effect where some places diesels work and above which they won't. There must be a ratio where the performance decreases gradually as you go up in elevation--the 3% the manufacturers say you lose per 1,000'. So, by their numbers it won't run at 33,333'. Actually It'll probably give up long before 33,333', say 25,000'?

 

[earlw]

I understand the loss of HP at higher altitudes, but what about the hydraulics? My thinking is that the pressures within the hydraulic system would be the same, but the fluid flow rate (GPM) would be lower since there is less HP to drive the pump. That would just mean the Loader would be slower, but would have the same lifting power. Does this sound correct?

 

[v8dave]

The hydraulic fluid flow is set by the engine speed. If the engine has enough power to run at rated speed, you have the same hydraulic flow as at sea level. Therefore, the same implement power as you had at sea level--if the engine doesn't slow down.

The time you'd notice the power loss at elevation is: on steep grades where the tractor slows down, utilizing any ground breaking implements where you're loading down the tractor and when using any 3pt or PTO implements that load down the engine (bush hogging, a generator on the PTO, etc).

On a CUT your backhoe will probably have the same dig power because it doesn't use up all the engine's power. However, your loader will probably seem less powerfull as the engine slows down when you force your way into a pile, but, you were used to that anyway, it'll just happen sooner.