Full RPM or high idle? Bush hogging fuel efficiency

[Redneck in training]

If you can find a curve of brake specific fuel consumption (BSFC)vs. brake horsepower for your tractor you could determine 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 average 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.

That is exactly the point Specific fuel consumption is the lowest at peak torque. PTO speed is usually designed somewhat past the torque peak. Reason being if you start loading the engine and it starts bogging down torque will increase. In other words it is harder to bog the engine down.

Actual fuel consumption is proportional to mass of the grass mowed so rpm have only small effect on the fuel consumption. If the deck loads the engine the speed governor will inject more fuel to the engine to maintain the selected rpm.
Wear is less at lower rpm but for all practical purpose it makes little difference. Tractor engines are way underpowered so they will last long time even when running at top speed.

 

[scrappy isb67]

That is exactly the point Specific fuel consumption is the lowest at peak torque. PTO speed is usually designed somewhat past the torque peak. Reason being if you start loading the engine and it starts bogging down torque will increase. In other words it is harder to bog the engine down.

Actual fuel consumption is proportional to mass of the grass mowed so rpm have only small effect on the fuel consumption. If the deck loads the engine the speed governor will inject more fuel to the engine to maintain the selected rpm.
Wear is less at lower rpm but for all practical purpose it makes little difference. Tractor engines are way underpowered so they will last long time even when running at top speed.

but is the wear less when you are hammering the bearings because you are lugging it? or how about overheating the hydro because the hydraulic flow is too low to operate everything AND cool the hydro?

 

[jejeosborne]

you may have a reading comprehension problem.. or you didn't read my post before replying.

I SPECIFICALLY stated that if a wear item wears.. replace it. ( that's why they call them wear items!!! brakes, tires, clutches, bushings, bearings.. etc. )

you can continue to use your tractor and baby it as much as you want.. it's your tractor.. free world. likewise I'm free to wonder why someone buys a tractor to baby it.

soundguy

I can tell you I don't have a reading comprehension problem and but now comprehend what type of person would make that accusation.

I would bet that you too would not run your rotary cutter through my wooded trails at pto rated rpm cutting what little vegetation is there unless you have a tractor with a cab and air filtration system.

Babying is different than being logical. My tractor is quite opposite of babied as I mentioned in my post. It has 500 hrs on it but it looks like it has been through a war. I run the piss out of it and work it beyond its capabilities but do use the throttle as it was placed there for a reason. My post hole digger says 540 rpm input too but I don't run it there. The only implement that requires 540 is a generator and anything else is up to the operator. I will not make anyone feel like less than a man for choosing to, as many times it is a logical option.

 

[jejeosborne]

It all boils down to engine loading and output efficiency. You certainly do not want to bog the engine. However if the PTO chore is light, there is no reason to run the machine at full throttle... or anything close.

An operator must use experience when operating a tractor... listen to the engine or watch the tac when the bushhog etc begins to load the tractor. Keep the engine running at a throttle setting where it's not bogging the engine but also not racing it. Like I said this requires some "knack". And is why manufactures say to run the machine at 500 rpm, etc. That's a good general recommendation to follow for inexperienced operators.

If you are running in a high load situation like very heavy grass or roto tilling... then you run at max rated PTO speed for implement... and then slow tractor ground speed till engine is not bogging. Again... this takes a bit of time and experience to learn. You certainly do not want to bog the engine! Keep the revs up. However if the mowing, etc is easy then you can back the throttle off to save fuel, reduce noise and wear/tear.

Have fun! :thumbsup:

Amen, I agree.

 

[Soundguy]

Should we be worried.

none of the major bodies of water have froze over yet and no fire is raining from the sky.. so I'd say we are still in the green..

 

[Soundguy]

I can tell you I don't have a reading comprehension problem and but now comprehend what type of person would make that accusation.

the kind of person that overlooks a piece of a post and says I didn't factor in wear.. when I specifically did!

I would bet that you too would not run your rotary cutter through my wooded trails at pto rated rpm cutting what little vegetation is there unless you have a tractor with a cab and air filtration system.

i'll take that bet. I have no tractors with a cab.. most don't have a canopy or rops.. just plain open station. I am down here in florida. it's either raining on me sideways.. or it's hot and dry and dusty. occasionally it's hot and raining with the sun out. This past weekend i was cutting vegitation that was a few feet over my head when setting.. when standing it was head level.. bugs flying everywhere.. pollen laden weeds swinging.. dust cloud like a tornado coming out of my rotary cutter... running at full rated speed i might add.

so yeah.. I'll take that bet. I'm used to looking like the swamp thing after a good day of mowing.. only thing clean is what's under my glasses and ear plugs.

My post hole digger says 540 rpm input too but I don't run it there.

I don't have your PHD.. but I'd bet the 540 rpm marking is a gearbox max rating.. not a sugested run rating. and yes.. i agree.. PHD's are one of those items that you run as low as you can get away with. this however is not a comparison to a mower application.. where you have blade tip speed, toruque and suction to deal with.

I will not make anyone feel like less than a man for choosing to, as many times it is a logical option.

like I said.. free world. we're all entitled to an opinion.. My idea of buying a tool is to use it so that i have to work less, personally.. or work less time. that means I'm generally using my machiens at their rated hp and or speed or capability to get the most out of them. I DO realize that with use comes wear. I'm quite prepaired to deal with worn tools and fix them as needed.. vs not using them as much to prevent them from wearing out.

My guess, and only a guess, is that some people don't use an item hard so that it lasts longer before wearing out, perhaps because they lack the skill to renew it.. or don't want the hassle of rebuilding it.. or paying to get it rebuilt. The catch there is work=wear. more work=more wear.. less work = less wear.

work a tool less it lasts longer.. but you get less work out of it. time's the big variable in that. You might well get more life (time) out of a lightly used tool, vs a more heavilly used tool. but if you look at work/wear.. did they both do the same amount of work maybee?

not making an assumpton there.. just throwing it out.

like i said. feel free to use your tool they way you like. I'm using mine the way the manufacturer rated it and built it for

 

[Jerry/MT]

This post has sort of taken on a life of it's own! I wonder if the OP is still looking at it?

I don''t want t get into a p****** contest but it's actually above peak torque as I stated and it's generally caused by a decrease in the volumetric efficiency due to increasing losses in the induction system (intake and valves).

 

[Domush]

I am, but I'm not sure why. There seems to be some confusion between opinion and science. That torque chart was helpful.

I don't see the point of the big debate as to PTO speeds, as if a mower still mows at a reduced tip speed, does it really matter what the speed is? The end goal is to do the job.

What is rated and what does the job are not always mutually exclusive. If using a mallet on a tack, you can afford to slow down your swing a bit. Not every bush hogging experience is eight foot tall saplings.

I'm still wondering if the fuel difference is enough to justify the slowdown, and the science behind wear is still not solid.

So, any science behind this?

Sent from my SCH-I500 using TractorByNet

 

[DarkBlack]

My vote assuming you have a feel for engines,is run the rpm's as slow as possible without loosing implement results, or unburned signs of black smoke, or below around 1,200rpms( my engine). There is a huge differnce in fuel consumption on mine when you run higher rpm's doing a light chore
Your car engine is designed to make max power at 5-6k rpm's. Do you keep it in first gear to keep the rpm's up threre? I didn't think so.

 

[Jerry/MT]

Wear is piston speed( ~ rpm) and peak gas emperature (load)/ Load is ~ depth and density of grass, grade, and ground speed. Ground speed may be a f( roughness, trees and brush, obstacles, etc)
Without some data, there's no single answer to your question. The best advice I can give is to use the minimum rpm that gives you a satisfactory cut for the terrain you are on at the max ground speed you can handle. I would not hog with visible smoke out of the stack, but then I don't make our Ford 4610 blow black smoke.(Black smoke on a diesel in reasonable mechanical condition is an indication of max power-not enough oxygen to burn all the fuel being injected. That's generally how the power curve is defined on pre -emmission tractors-visible smoke)