Full RPM or high idle? Bush hogging fuel efficiency

[Domush]

I'm waiting for the president to make a press conference regarding this issue. :laughing: Maybe even form a think tank, diverting some war effort resources for a broad ranged study.

Thanks for the input everyone. I can't say I'm that much closer to an answer, but it sure has been a trip in the process. Honestly, i figured everyone asked themselves this question at one point in time and there would be broad consensus on it. Boy was I wrong!

In the mean time, I think I'll just slow down until the rotary cutter stops cutting cleanly, and idle it up a little when it happens, enjoy the quieter tractor, soak up the day and if I get a little better fuel usage, great, if not, at least it was a little more peaceful in the process. Nobody has mentioned fuel usage will get worse, so at least I have that going for me

As for time for the task, well, we're all here because we love our tractors, so using it a little longer is not a bad thing to me. Kind of like "having" to stay on the sailboat a little longer than expected. Oh gee whiz

Thanks again for everyone's input. Though somehow doubt this thread will become a sticky for most clearly answered question ever, haha :laughing:

Cheers! :drink:

 

[Redneck in training]

Hey Domush I like your tractor. Great job.

 

[JoeBuyer]

Who has a good size field needing mowing that can divide it into two equal sides and do a real world trial topping off and measuring the fuel after each side? Getting equal sides for the high and low rpms might be tough but the results would be interesting. Theory is interesting but real world is proof.

 

[madmax12]

Who has a good size field needing mowing that can divide it into two equal sides and do a real world trial topping off and measuring the fuel after each side? Getting equal sides for the high and low rpms might be tough but the results would be interesting. Theory is interesting but real world is proof.

I don't know... Real world fishing, still results in campfire fish stories.

 

[foreman Etexas]

Who has a good size field needing mowing that can divide it into two equal sides and do a real world trial topping off and measuring the fuel after each side? Getting equal sides for the high and low rpms might be tough but the results would be interesting. Theory is interesting but real world is proof.

I have about 60acres left to cut, more than welcome to come do the test at my place.

 

[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.

All right redneck. Let's start with this one, because this is flawed on many levels. Let's start with your math. Please show us your math where you arrive at 8X power needed to double the speed...

 

[DarkBlack]

Power generated by the engine is always equal to power required to perform the task. Fuel consumption is proportional to power required multiplied by time. Mowing at lower rpm will not save fuel because it takes longer to perform the task. The tractor wear will be also about the same. The pistons in the cylinders travel at lower speed but for longer time. This assumes constant efficiency of the engine regardless of rpm though. The specific fuel consumption curve suggests that the highest efficiency is reached at about maximum torque (PTO speed) so mowing at lower speed might in fact use more fuel. Noise and stress on the mechanical components of the tractor is another issue.
Case study are cars with modern auto transmissions. They are hard to beat in fuel consumption by identical car but with manual transmission because of automatic keeps the rpm in around the region of the best fuel consumption. Manual transmission has theoretically lower losses but most drivers don't keep the rpm at the best fuel consumption (in fact it is impossible unless the trany has many gears).

This one has more incorrect science than I care to correct, but I'll try to get through the first sentence or two:

"Power generated by the engine is always equal to power required to perform the task."

Just plain and simple wrong . Power generated by the engine is equal to
power required to perform the task plus many, many other losses. Ever notice your tractor warms up as you use it?

"Fuel consumption is proportional to power required multiplied by time. "

Nope, same flawed science as before. You haven't factored in all the variable losses. You also don't account for engine/speed efficiency.

"Mowing at lower rpm will not save fuel because it takes longer to perform the task"

That's a total false premise. Nobody said the ground speed wouldn't be maintained.

Let me know if you want me to continue. Please say no.

 

[JDgreen227]

Who has a good size field needing mowing that can divide it into two equal sides and do a real world trial topping off and measuring the fuel after each side? Getting equal sides for the high and low rpms might be tough but the results would be interesting. Theory is interesting but real world is proof.

Too many variables in the real world to make a true comparison possible...maybe one side got more rain, maybe the soil underneath the vegetation varies, the air temperature during the test is going to change a few degrees, and we all know the biggest variable will be the operator, does he have the skill and ability to make each and every turn EXACTLY the same degree, if it's a gear tranny will he make each shift EXACTLY the same? I could go on and on, but why bother.

 

[jejeosborne]

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?

I will bring your original questions back to the front line. As far as finish mowing I run wide open regardless of the grass height. Blade tip speed is very important for a clean cut which is more healthy for the plant. Rotary cutters already have decent tip speed because of the larger diameter and the quality of the cut is less important.

You have basically hit the nail on the head, so to speak, with your pro/con list. Not sure what you meant with the "more torque needed" in the con list, but I like your logic. As long as you are able to maintain the same ground speed with less throttle, it is hard to argue that you are not burning less fuel as long as your governor is not increasing the throttle in the background (lugging the engine). You will feel this happening and just throttle up. Pretty simple concept that everyone SHOULD agree upon. Your mentioning of having the capability to increase your ground speed, but being bounced, assures me that you have some reserve capacity on your engine so reducing rpm is an option.

The charts that were brought up are useful when sizing an engine/gearing combination to achieve the best fuel efficiency when requiring full engine output, creating the most work per gallon of fuel. A constant load is needed and this could be helpful during cutting tall grass requiring all your pto HP. I achieve the best fuel economy in my truck while traveling 70 MPH on the interstate at a lower RPM than the chart would show because it is not requiring the engines maximum output.

As far as the "science" you request, i mentioned
the parasitic loads within the engine, increase in accessory loads (hydraulic pumps, radiator fan, alternator, etc), addition energy required to spin the Rotary cutter faster all as rpms increase. I could explain each of these items in detail if needed. All these items combined equal addition fuel to be burned and it is significant. One quick example, my log splitter as simple as it is , requires more fuel at higher rpms even when the hydraulic pump is in bypass because it is pumping more fluid. A great fuel saving option would disconnect the pump between splits.

As far as getting the president involved........now you are opening a new can of worms and the direction of this thread will take another turn. Will a diesel engine even be legal. Maybe with all kinds of pollution controls or solar or wind assisted. LOL

Hope this is helpful.

 

[Redneck in training]

All right redneck. Let's start with this one, because this is flawed on many levels. Let's start with your math. Please show us your math where you arrive at 8X power needed to double the speed...

Aerodynamic drag equation D=0.5*density*velocity^2*drag coefficient*area This equation calculates absolute drag value. For proportional value throw all constants out. Then the equation can be written as D is proportional to velocity^2.
Total drag of a fast moving vehicle is product of aerodynamic drag and rolling resistance. R is proportional to velocity.
Therefore total drag is TD is proportional to Velocity^2*velocity. Velocity^2 times velocity=velocity cubed.
here is drag and required power calculator Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
Look in the columns HP Aero and HP Rolling at 50, 100 and 200. Then divide value at 100 by value at 50. Then divide value at 200 by value at 100 etc and see what the proportionality constant is and let us know.