horsepower loss vs. elevation

[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.