Full RPM or high idle? Bush hogging fuel efficiency

[PapaPerk]

Not to add another curve to this thread, but what about those of us who have 2 speed PTO's (my Kioti is 540/1000). Does using the 1000 rpm setting at a lower engine speed help or hurt wear/fuel economy?

Likewise, does it damage an implement to run it at a higher PTO speed? Dealer told me it shouldn't, and so far he seems to have been right, but I'm concerned about long-term.

You should operate implements at PTO speeds they are designed for. In other words only operate 540 rated bush hog at a max of 540 rpm. You would not want to run it up to 1000 rpm. :shocked:

 

[madmax12]

You should operate implements at PTO speeds they are designed for. In other words only operate 540 rated bush hog at a max of 540 rpm. You would not want to run it up to 1000 rpm. :shocked:

Well, that's not the first time I've heard that. Pto generators usually bring that out. If 540 pto reguires "x" engine rpm, then setting Pto to 1000 should require roughly one half "x" engine rpm...thus replicating the proper Pto speed for the implement. The question does not usually imply running the Pto implement above 540 actual rpm.

Using your example of less rpm = less fuel consumption (and less noise, wear?)... Fuel and noise and part wear, r usually the reasons I've heard the question asked, in the first place.

I'm not shocked by the question, as it is an "old" one.

While I do not know the correct answer, I do not think it is a dumb question.

It is also not a challenge I face, since my Pto only has one option: 540 rpm...unless, of course, I vary from "x" engine rpm.

 

[PapaPerk]

Well, that's not the first time I've heard that. Pto generators usually bring that out. If 540 pto reguires "x" engine rpm, then setting Pto to 1000 should require roughly one half "x" engine rpm...thus replicating the proper Pto speed for the implement. The question does not usually imply running the Pto implement above 540 actual rpm.

Using your example of less rpm = less fuel consumption (and less noise, wear?)... Fuel and noise and part wear, r usually the reasons I've heard the question asked, in the first place.

I'm not shocked by the question, as it is an "old" one.

While I do not know the correct answer, I do not think it is a dumb question.

It is also not a challenge I face, since my Pto only has one option: 540 rpm...unless, of course, I vary from "x" engine rpm.

That's a good point about running at half the rpm. However I would be concerned about "lugging" the engine... especially in a high load situation like bushhogging or roto tilling. The engine needs to operate in the appropriate power band. Half engine speed would most likely kick you out of the powerband... especially on a diesel.

Like I said... I believe 1000 rpm PTO is for operating implements that require high shaft rpm... and not for reducing engine rpm of the tractor.

 

[Redneck in training]

My example is not flawed. I agree with your numbers. I understand the reason for the additional fuel consumption, I am a pilot and an airframe and power plant licensed mechanic and have studied aerodynamics.

The point I was trying to convey was that fuel consumption is not fixed to complete a given task. A tractor example using the same ground speed would include more wheel slip with a larger implement, more gearbox parasitic drag using a larger implement, etc. Using the SAME tractor to pull a larger implement to get the job done quicker can cost more in fuel.

The possible improvements in a larger implements is less overlaps on your passes are required. Today's modern tractors display ground speed and wheel slip so the operator can make adjustments to hit a predetermined sweet spot for fuel consumption.

The thread title is based on engine rpm though so back to that example. The charts are basically showing how to complete the most work using the least fuel. To accomplish this, large loads have to be put on the engine. I agree that when mowing thick brush, the opportunity is there to gain from these charts. But if the engine load opportunity is not there, such as in light to no brush similar to what I have in my forest, these charts are not helpful due to the parasitic drag being a higher percentage of the total engine load. Try this experiment: run your rotary cutter in your driveway using the sweet spot rpm your chart suggests with the emergency brake on and see how long a tank of fuel last. Are you telling me it will last longer than a rpm setting just above idle?

I see your point. Since the blades of the rotary cutter also work like a prop (especially the high lift) there will be difference in fuel consumption when idling or running at PTO speed, assuming no grass cutting takes place. Once you start cutting grass most of the power is used to cut and discharge the grass. Therefore I think that fuel consumption is more or less proportional to mass of the grass mowed.

 

[madmax12]

That's a good point about running at half the rpm. However I would be concerned about "lugging" the engine... especially in a high load situation like bushhogging or roto tilling. The engine needs to operate in the appropriate power band. Half engine speed would most likely kick you out of the powerband... especially on a diesel.

Like I said... I believe 1000 rpm PTO is for operating implements that require high shaft rpm... and not for reducing engine rpm of the tractor.

Thank you for your explanation. That made sense to me.

 

[jejeosborne]

I see your point. Since the blades of the rotary cutter also work like a prop (especially the high lift) there will be difference in fuel consumption when idling or running at PTO speed, assuming no grass cutting takes place. Once you start cutting grass most of the power is used to cut and discharge the grass. Therefore I think that fuel consumption is more or less proportional to mass of the grass mowed.

I am with you and agree. There is a point in which the density of the grass will apply enough load to the engine to make the parasitic drag become a much lower portion of the total load. I think most people who own a tractor can tell the load on their engine by the seat of their pants and can adjust as necessary. The quality of the cut needed also comes into play per operator and job. If I bend over and knock the leaves off a plant in the woods, it will die due to the poor growing conditions it is trying to grow in.

There are too many variables to make a blank statement on which rpm is correct. If the engine is struggling or the cut quality is too poor, throttle up. Simple as that.

 

[jejeosborne]

Not to add another curve to this thread, but what about those of us who have 2 speed PTO's (my Kioti is 540/1000). Does using the 1000 rpm setting at a lower engine speed help or hurt wear/fuel economy?

Likewise, does it damage an implement to run it at a higher PTO speed? Dealer told me it shouldn't, and so far he seems to have been right, but I'm concerned about long-term.

I have thought about this in the past. I used to mow with a 1980's vintage kubota with a three speed pto. I could see it saving you fuel if your rotary cutter is much smaller than the capacity of the tractor. For example, a 5' cutter on a 50 HP tractor. Just run at half the engine rpm an see if the engine can support it without lugging. Cutter will still be running 540.

 

[JoeBuyer]

Dealer told me it shouldn't, and so far he seems to have been right, but I'm concerned about long-term.

I would not buy anything from a dealer as irresponsible as this, nor should equipment manufacturers allow that dealer to sell their equipment. Would anyone run a 540 rpm hay baler at 1000 rpm? Equipment can fail at "twice" the rpm they were designed for and people can get hurt or killed when it happens. Farming accidents are often serious and fatal.

 

[DarkBlack]

Your example is about the car is flawed due to aerodynamic drag difference at 30 and 60 mph. Aerodynamic drag increases with square of speed while rolling drag increases linearly. Therefore doubling the speed of the car will require eight times as much power. The same apply to tractors but since the speed they are moving is slow we don't need to take aerodynamic drag into account. .

Wrong again.
I think the redneck in training, and sounguy log-on posters are the same guy. posting the same made up science. Am I right? It's not just the bs made up science you pulled from your lower cavity on the previous pages, but "you" both try to morph the issue when you post total bs science and your statements from previous posts contradict what you post later to try to save face.
Oh, and again, what you posted above is wrong as usual.

 

[DarkBlack]

You should operate implements at PTO speeds they are designed for. In other words only operate 540 rated bush hog at a max of 540 rpm. You would not want to run it up to 1000 rpm. :shocked:

I agree you should not over speed an implement If you're an experienced operator and understand machinery you can run a 1k pto at half engine rpms to get the same pto shaft speed to save fuel when the load is less than max. A good example would be a post hole digger.