shane
Gold Member
Why are tractors now days designed to run at higher rpm's? Such as 2600 rpm to get 540 rpm at the pto.
High RPM's
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755inNY
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Shane,
I'm not sure what you are comparing to when you say "now days". My 1987 JD 755 had to run at about 3200 rpm to get the 540 PTO rpm. I think it is probably due to the smaller diesel engines needing to run at a higher rpm to get the rated horse power.
Jeff
I'm not sure what you are comparing to when you say "now days". My 1987 JD 755 had to run at about 3200 rpm to get the 540 PTO rpm. I think it is probably due to the smaller diesel engines needing to run at a higher rpm to get the rated horse power.
Jeff
RoyJackson
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- John Deere 4052R Cab,, Deere 855D UTV, Z920A Zero Turn Mower and assorted implements
shane said:
Diesels are built to run at a constant RPM for maximum efficiency. Look at marine diesels or industrial engines the run at a steady RPM for days (weeks..months) at a time. Yanmar (who built many CUT's for Deere) builds an incredible number of marine and industrial diesels. One of these engines is in my Deere 790...which also requires 2600 RPM for a 540 RPM PTO speed.
An engine has to produce power (Horse Power) and torque (which is the more important number for our machines). Due to the relatively small displacement of CUT engines, the engine must run at a higher RPM. It's built for this...and will last longer at a constant higher RPM then it would if ran below 1000 RPM.
shane said:
Shane,
Good question. The old tractors, like a Ford 841 with a 172 diesel ran at 1414 rpm to get 540 pto. It also had a max rpm of 2200. Peak torque was below 1400 rpm. "Modern" diesel engines typically run almost 2x that rpm.
Part is that they are physically smaller engines. A Kubota 3 cyl with 100 cid will get about the same peak torque as the old ford with 172 cid. That has to do with how Hp is calculated.
HP = (torque X rpm)/5250.
That means that the higher you rev, the more HP you make -- up to a point.
The newer smaller engines were designed to take advantage of that and run at higher RPM to make up for having less torque.
30 HP at 2800 rpm = 56 ft-lb of torque
30 HP at 1400 rpm = 112 ft-lb of torque.
This does not mean that one is better than the other. Just designed differently. My 3 cyl kubota L3410 can flat out work my 4 cyl Ford. The Ford is 43 years old and just was rebuilt after 8500 hours. Will the K last as long? Dunno, I hope to still be using it in 38 more years to see!
jb
Soundguy
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shane said:
I think you will find that the rpm at which your pto comes up vs engine rpm will vary by design. My 2002 NH 7610s develops 540 pto rpm at about 19xx engine rpm.
Soundguy
herringchoker
Platinum Member
Another thing is that a shorter stroke engine can turn at a higher RPM than a longer stroke engine. For every turn of the crankshaft, the piston, wrist pin, con rod, etc. accelerates from a standstill (say at TDC), reaches maximum lineal speed, then decelerates to a standstill at BDC, accelerates and decelerates again to get back to TDC, all in 1 revolution. The longer the stroke, the faster the maximum piston speed and the greater the acceleration and deceleration forces acting on the piston assembly (for a given RPM). There would be an absolute maximum piston speed where something would fail, but the more practical limitation on speed would be to have a longer service life (less stress = longer life).
Wow, that was a long - winded bit of philosophy.
Wow, that was a long - winded bit of philosophy.
While Herringchoker is accurate, it is not relevant for diesel engines due to flame propagation speed being much slower than a gasoline flame. After 4400-4600 rpm, the time in the combustion chamber is not long enough to allow all the fuel to burn and still unburnt fuel will be exhausted. That is why the maximum rpm for diesel engines is so low.
Thus for diesel engines, longer strokes are benefitial as they increase the time around TDC, increase the combustion pressure and help to ensure full and complete burn. All that is why diesels make more torque and are more efficient, well that and the high static compression ratio.
Thus for diesel engines, longer strokes are benefitial as they increase the time around TDC, increase the combustion pressure and help to ensure full and complete burn. All that is why diesels make more torque and are more efficient, well that and the high static compression ratio.
PineRidge
Super Member
Don't you just love the hum of a high pitched working diesel? Man that's music to my ears.
jimmysisson
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I must say brush-hogging with my old Massey 135 was a quieter operation than with the NH 2120, mainly due to about 300 rpm more being needed to get to 540 PTO. Plus of course the 135 was 3 cylinder and had a low exhaust.
Still, I don't need the power, just the tip speed. Seems like a 2-speed PTO would be a help, then you could run the engine at 1500 or so and still get a good cut.
Jim
Still, I don't need the power, just the tip speed. Seems like a 2-speed PTO would be a help, then you could run the engine at 1500 or so and still get a good cut.
Jim