Oil & Fuel Fuel usage

[Doug Mac]

Simple question, I think.

Given the same load conditions on the PTO, what percentage of fuel usage would drop if you lowered your rpm's?

E.G. Running a Fertilizer Spreader at PTO of 540, then running it at around 350 rpm... 65%-70% throttle. Would you save 10% on fuel running at the slower engine speed? More? Less? Good idea to gear up and throttle down? Again, a pretty light load... not running a bushog.

Thanks

 

[Lunk]

Simple question, I think.

Given the same load conditions on the PTO, what percentage of fuel usage would drop if you lowered your rpm's?

E.G. Running a Fertilizer Spreader at PTO of 540, then running it at around 350 rpm... 65%-70% throttle. Would you save 10% on fuel running at the slower engine speed? More? Less? Good idea to gear up and throttle down? Again, a pretty light load... not running a bushog.

Thanks

If I'm not in the heavy stuff with my cutter, I'll use the 540E (for economy) setting on my PTO, which drops the engine RPM's from 2600 to 1700, but maintains PTO RPM at 540. If the engine lugs at all, I switch to the normal setting and boost the RPM's accordingly.

I think it would be tough to measure exactly how much fuel you'd actually save, as there's too many variables involved. As long as you're not lugging the engine under load, you should save something though...

Hope that helps.

Lunk

 

[dmccarty]

Best I can tell I burn a gallon per hour when running at PTO speed with my JD 4700. It is a 48 HP/41.5 PTO HP tractor and PTO speed I think is 2600 RPM. I only run PTO speed with the rotary cutter.

Running anything else I figure the tractor burns about 1/2 gallon per hour with the engine at 1700-1800 RPM.

Not sure how that would compare to any other engine.

Later,
Dan

 

[pitt_md]

Best I can tell I burn a gallon per hour when running at PTO speed with my JD 4700. It is a 48 HP/41.5 PTO HP tractor and PTO speed I think is 2600 RPM. I only run PTO speed with the rotary cutter.

Running anything else I figure the tractor burns about 1/2 gallon per hour with the engine at 1700-1800 RPM.

Not sure how that would compare to any other engine.

Later,
Dan

With my 50hp Kubota, running a tiller at PTO speed I think it will use more than 1 gal./hour. Its got a 13 gallon tank and will use most of that in 8 hours. I have not measured it though. I do notice a sharp decrease in fuel consumption if I lower the RPM's to 2300-2400 as compared to the 2700 PTO speed.

 

[3RRL]

My 55hp (50 PTOhp) Kama uses about a gallon an hour at 2300 engine rpm which is 540 rated PTO rpm.
My fuel consumption is just over a half gallon an hour at 1700 engine rpm. It was one of the first things I kept track of when I got the tractor 3-1/2 years ago. So like Dan and Pitt, I see a considerable fuel savings running at lower engine rpm. This test was running the tractor without a PTO implement hooked on though. I have never tested it that way, with PTO implement running, but assume their is a relative fuel savings using lower rpms running a PTO implement that places very little stress on the motor.
Like when doing light rotary cutting. I too have a 2 speed PTO and always use the 720 PTO speed and run the engine rpms at 1725 resulting in 540 PTO rpm. On my engine, the peak torque is developed right about those engine rpms too, so that is another good reason to run mine in that area.

 

[john_bud]

Don't forget that there are 2 types of hour meters. Some measure pure time and will show a drop in fuel usage per hour, even if the time to do the task increases. The better ones actually count engine revolutions and make a time conversion assuming PTO speed. So if you cut engine rpm's down by 1/2 the clock will spin at 2 watch hours per meter hour. Your fuel savings are harder to get a handle on that it may at first appear.

Best way is to gauge the fuel needed to do a job like mow 5 acres. Using full pto speed vs a reduced engine speed. Engine friction does go up with component sliding speed, so reducing rpm's will reduce friction and that will lower fuel consumption. IF you are able to drop rpms AND go up a gear so the ground speed is the same even with the lower rpms, you will see a nice drop in consumption. It's a simple math function of number of combustion firings per distance traveled. At the same ground speed and lower rpm, you will use less fuel if the throttle is at the same point. If you have to increase the throttle to offset the higher gear, the savings will be reduced.

 

[Opti-Mist]

The design target used to be 1/2 pound of fuel per horsepower-hour. If you take the power curve for your machine you can estimate the effect of rpm on fuel consumption. The result will be directional and an order of magnitude, not a precise calculation.

 

[john_bud]

The design target used to be 1/2 pound of fuel per horsepower-hour. If you take the power curve for your machine you can estimate the effect of rpm on fuel consumption. The result will be directional and an order of magnitude, not a precise calculation.

I had heard that with regard to gasoline engines, is that the same for diesel?

jb

 

[SPYDERLK]

Don't forget that there are 2 types of hour meters. Some measure pure time and will show a drop in fuel usage per hour, even if the time to do the task increases. The better ones actually count engine revolutions and make a time conversion assuming PTO speed. So if you cut engine rpm's down by 1/2 the clock will spin at 2 watch hours per meter hour. Your fuel savings are harder to get a handle on that it may at first appear.

Best way is to gauge the fuel needed to do a job like mow 5 acres. Using full pto speed vs a reduced engine speed. Engine friction does go up with component sliding speed, so reducing rpm's will reduce friction and that will lower fuel consumption. IF you are able to drop rpms AND go up a gear so the ground speed is the same even with the lower rpms, you will see a nice drop in consumption. [[[It's a simple math function of number of combustion firings per distance traveled. At the same ground speed and lower rpm, you will use less fuel if the throttle is at the same point. If you have to increase the throttle to offset the higher gear, the savings will be reduced.]]]

Not so simple. The throttle is increased by the governor to hold set rpm under the higher load. More work is being done per revolution at lower rpm but same speed.
larry

 

[slowzuki]

There is more going on here than people realize, diesel engines are at peak efficiency about 1700 rpm but they don't make max power there, so if you need the rpm or the power of a higher engine speed, then you don't have a choice.

At any given speed the engine may be at full power hard against the governer or just ticking over with enough fuel to spin the engine.

Mowing hay with my 52 hp Kubota and a 9 ft moco, the engine consumes 8-10 L / hour. Doing light loader work, it consumes about 1-2 L / hour regardless of throttle setting.

gpm/hr/hp I think is the right english units for fuel use. You'll see that value in ag tractor tests. In metric its usually grams/kWh.

For example one tractor I'm looking at uses 281 g/kWh at 2300 rpm or 246 g/kWh at 1900 rpm, both at maximum output at those speeds. Some other models are as high as 340 g/kWh.