Full RPM or high idle? Bush hogging fuel efficiency

[Domush]

I was hogging a field the other day and, in being cheap, was wondering if hogging the field at a lower-than-rated RPM would be more efficient or not. I have a neighbor who finish mows with his tractor barely above idle, and his lawn still gets mowed and his tractor is downright peaceful to listen to as a result. Whether or not it's a fuel savings or good for his tractor, I'm undecided on.

My thinking is this with mowing with a lower RPM:

PROs
- Quieter
- Less fuel per hour burned
- Less engine heat (== higher efficiency?)

CONs
- Slower blade speed
- More torque needed (is torque being wasted at a higher RPM?)

A notable absence in my Pro/Con list is speed, as I can easily compensate by mowing/hogging in a higher gear. It is possible to simply bush hog in a higher gear at the rated RPM, but then I'd get bounced to death and possibly tip the tractor careening over those hidden stumps nobody tells you are there.

What I couldn't decide on was if the lower RPMs would cause more engine wear, as the load would be higher.

Anyone know the science behind this?

 

[RDrancher]

Interesting question! On paying jobs I almost always run at 2400 rpm with a low ground speed, but they're typically overgrown, thick lots. On my own property (which gets mowed regularly) I run in between 1800 and 2100 rpm and scoot along. Looking forward to the scientific opinions as well.

 

[JDgreen227]

I mow 5+ acres with a Deere 4210 and a 60-inch 7-Iron mid-mower. For 4 inch high grass, I run the engine at about 2100 rpm, and for higher grass at 2500 rpm. Fuel consumption is pretty much the same although the higher rpm burns more fuel, but I can also travel faster so I finish in less time. My mower does seem to cut much better at the higher rpm setting, and although I can mow at 1700-1800 rpm and get a decent cut, my mower deck manual says "mow at full throttle" but I have never done that, as the noise level is much higher.

Interesting thread, Domush !! :thumbsup:

 

[KennyG]

My understanding is that all cutting attachments are designed to work best at the PTO rated speed. This probably isn't too critical on finish mowers, but I think a bush hog should always be run at high speed. You want the inertia of the blades to be working for you and to minimize the impact on the shear pin/clutch.

 

[RedNeckRacin]

Well to chime in, the Deere's have a 540E pto option. you run at 1800rpm IIRC and full pto speed. I personally run at 2400? rated rpm for 540 pto if not a little over just to not stall the tractor out while brushhogging. Then again, a 10 footer is not the norm and sure takes out a large area of dirt mound in a hurry if your not quick on the 3pt.

 

[Jerry/MT]

If you can find a curve of brake specific fuel consumption (BSFC)vs. brake horsepower for your tractor you could detemine the optimum operating point. Usually it's about 200-400 rpm above peak torque rpm (you can usually get that from the tractor spec) and many times it's around the engine the rpm that will give you 540 pto rpm.

If you don't have that data, fill the tank up and try starting at the peak torque rpm and work the tractor for a 1/2 hr or so and refill the tank to the same point measuring the added fuel as accurately as possible and record the results and the operating time. Calculate fuel burn per hour. ( This assumes that you have a constant avergae load on the mower., so going up and down hills or working on the flatswill make the results vary.)You can also try different gears to vary ground speed if you want to make a project out of it.You'll find the minimum fuel burn point. Some folks do it by "feel". The engine purrs and the grass gets cut and it doesn't take much fuel.

Sometimes you just can't operate at the min fuel burn point because of field conditions, obstacles, etc.

 

[V1Rotate]

It's important not to lug the engine. If you are putting a pretty good load against the engine while hogging, you'll want to run plenty of RPMs. This is better for your engine for a number of reasons including; more airflow through the radiator, the engine coolant will circulate faster and the oil will circulate faster. All of the things will make the engine run cooler.

 

[JDgreen227]

From my understanding, a diesel engine is much less prone to being stressed from lugging than is a gasoline engine, partly because the internal componts are much stronger, and because a diesel doesn't use a throttle in the sense that a gas engine does, they are not as likely to be subject to lugging.

Feel free to correct me if I am mistaken.

 

[PoorPlowboy]

Lugging a diesel engine is just as bad (if not more expensive to repair later) than a gasoline engine. Lugging the engine increases EGT to dangerous levels, frying the turbo if you have one. It also contributes to high levels of soot/carbon buildup. Carbon buildup will gum up your valves causing leaks, buildup in the compression chamber can increase your compression ratio which can lead to predetonation. When that occurs, and you continue running the engine, be prepared to replace crank and rod bearings. If you keep running it after it knocks, then get out the check book and the credit cards for a broken crankshaft or bent connecting rods.

The above scenario is lugging, which is a high load, low rpm scenario. Simply running the engine at a lower RPM at a light to medium load is not as dangerous, but it's cumulative effects can build up over time. i.e. the engine not operating at full operating temperature. Moisture accumulating in the engine oil, etc.

 

[Soundguy]

i am continually amazed at the people afraid to run their tractor at it's rated work rpm?

it's a tool!! use it.

when a wear item wears.. replace it.

if a durable part breaks.. fix it.

keep on going.

this weekend I was out hogging 8 ac of 7' to 8' ragweed, dog fennel and something that looked like goldenrod at the back of my pasture along a causeway strip. stuff was so tall I couldn't see anything but the treeline in the distance to judge my position.. was on my old clapped out 1955 ford 950 trike running at 7/8 throttle the entire time... only stopped when i ran out of gas and the rain set in sideways ( both at about the same time )...

use that tractor.. it's what it was meant for!! you don't swing a framing hammer softer to 'save' it do you?

sheesh