horsepower loss vs. elevation

vjcunr · Aug 7, 2005

For what it is worth...I just bought a B7800 to use on 20 acres at 7400 feet. I had the same concern with the higher altitude, but I have been very happy with the amount of power it has. I've been running a PTO driven chipper, and it has worked great. It hasn't bogged down and still seems to have more than enough power.

Our Kubota dealer offered us a 6 month rent-to-own deal, allowing us to return or exchange the equipment during this time. I thought this was a good way to test the power, and it doesn't look like I will be returning anything. You might see if you can get the same deal.

JeffRey30747 · Aug 9, 2005

Dave,
First off, thanks for the response. I understand that output is a function of the availabilty of the oxidizer agent oxygen for the combustion reaction and that there is no way that there isn't an overall decrease in available hp as altitude increases across a broad spectrum. The ratio of available oxygen is a near linear function and so is the hp available from the reaction.
That wasn't specifically what I wanted to address, though. However, it seems that one of my basic premises (that there was excess air in the diesel system at lower altitudes) just does not hold true. I don't remember where I came across this, but I'm quite sure I read it somewhere. It could have been the infallible internet. /forums/images/graemlins/wink.gif
Since my original post, I have also thought about the impact of excess oxidizers on a combustion reaction such when someone on medical oxygen makes a foolish choice to continue smoking and how that raises the risk of flash fires due to the extreme ratios of oxygen in the environment. From this, it would seem to follow that there isn't a simple point where more oxygen doesn't contribute something.
Anyways, thanks to both you and Skypup as you may have helped me correct a minor misunderstanding with regard to basic diesel mechanics.
Jeff

v8dave · Aug 12, 2005

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However, it seems that one of my basic premises (that there was excess air in the diesel system at lower altitudes) just does not hold true. I don't remember where I came across this, but I'm quite sure I read it somewhere. It could have been the infallible internet. /forums/images/graemlins/wink.gif
. . .
Jeff )</font>

What you read may well be correct--as far as it goes. At less than full power, Diesel engines have an excess of air. At full power they are using up all the oxidizer and fuel. At less than full power there is an excess of oxidizer (air).

Which is one of the reasons why Diesel automotive engines get good mileage. At partial throttle Diesel engines have more air than needed to fully burn all the fuel (Diesel) squirted into the cylinder. Gasoline engines run at a constant stoichiometric fuel/air ratio. Which doesn't allow them to cut down on fuel as much as a Diesel when running partial throttle. What the link doesn't say is that running near the stoichiometric ratio is needed in a gasoline engine in order to ignite the fuel with a puny spark ingnition system.

Although, electronic ignition systems have increased the power available in the spark a bunch. Enough so that the air:fuel ratio has moved from the ~12:1 of the 50's, 60's and 70's to the better ratio of ~14:1 today. But still, a Diesel will ignite almost anything that's flamable in it's high temperature combustion chamber, due to the high compression ratio, and still run at partial throttle air:fuel ratios of better than 20:1. Way better at idle where it may be as high as 40:1.