RPM vs Longevity

[JerryG]

Life expectancy of our tractor engines comes up here pretty often. I would like to say that the life of any engine is decreased as the rpm is increased. I am not saying that you are to run 1800 rpm on a engine designed to run 3600 rpm. What I am saying is that, if the rpm is increase to reach a higher hp, then the engine will not last as long. Example: TC 29 vs TC 33, same engine but rpm increased to raise hp. All manufactures have examples. The more the rpm is increased, the shorter the life expectancy. Given two tractors that are identical except for the rpm limit, the tractor with the lower rpm will last longer.

 

[Henro]

<font color="blue"> Given two tractors that are identical except for the rpm limit, the tractor with the lower rpm will last longer. </font>

And if they have tach-driven hour meters, the hour meter readings of the two will reflect this... /forums/images/graemlins/smile.gif ...Well at least the lower rpm tractor will show less hours.

Still, I wonder if it is true. Perhaps there is a maximum-life-point/rpm that is somewhere in between idle and maximum rated speed.

Complicated subject! How about same speed on two identical tractors but different engine loading... /forums/images/graemlins/confused.gif

So many questions...so little time... /forums/images/graemlins/grin.gif

 

[JerryG]

This is an over simplification, but a piston moving at 3600 strokes a minute is putting on 50% more wear per minute than a piston moving only 2400 strokes a minute.

 

[slowrev]

Yes this has been discussed several times before. IMHO it has to decrease the engine life.

 

[nomad]

Yes, you are correct.
The more RPM; the more wear, the more heat, the more dynamic & thermal cycles which will cause the fatique in materials to happen, etc.

 

[Egon]

I'd think there would be more to it than just RPM. Most likely keeping the rpm in the proper torque band will result in the best life span as this should be the rpm range the motor is designed for.

There may also be vibrational harmonics which change with the rpm making a certain rpm band more desirable than others.

And many more factors such as bore/stroke/compression/ flywheel weight and ?????? to take into consideration.

Egon

 

[JerryG]

Egon,
I know that if given a choice, I would always take a long stroke, low rpm engine.

 

[Bob_S]

What about carbon deposits I keep hearing about when running at lower RPMs? Is this a different concern? Just trying to lean more about somethin I know little about /forums/images/graemlins/crazy.gif

 

[JerryG]

Carbon deposits are formed when an engine is in what I call the lugging range. I was only referring to the practice of manufactures raising the rpm to increase hp so that they can add another model to their line up.

 

[RoyJackson]

"What about carbon deposits I keep hearing about when running at lower RPMs"

This results from excessive idling at lower RPM. Carboning the heads results from unburned fuel. Dirty fuel cause it too...I just had my Explorer's heads decarbonized. In a gasoline engine, heavily carbonized engines can actually increase the compression ratio, resulting in pre-detonation (pinging) which is not good for the pistons.
Sometimes you'll see a diesel generator or other usage where the idle suddenly increases for a minute, then returns to low RPM.

Another thing to be concerned about with low RPM's from idling for longer periods of time is pounding the main bearings. Instead of a nice even rotation of the crank shaft, very low RPM's can result in excessive wear on the bearings. Cranks have counterweights for a reason...to maintain the momentum of the rotation.

Diesels have thicker bearings (at least, what I've seen) then gasoline engines, so "pounding" the bearings in a diesel isn't as harmful.

At a maximum RPM of 3000 or so, I just cannot see worrying too much about high RPM's.

Anyway, who of us sets their hand throttle to higher RPM's unless we need PTO speed?

 

[RalphVa]

If you look at cars, look at typical miles possible on 4 cylinder Volvos or 4 cylinder Benzes or 4 cylinder Toyotas or Honda that run probably closer to 3,000 rpm vs. big V-8s that lumber along at around 1,500 to 2,000 rpm. Both will go 300 to 400k miles before overhaul (possibly a valve job on either). The Volvo 4s will run near 300k before they even start to use any oil.

Most wear on an engine occurs at startup, if it is maintained properly. To keep an engine running properly keep it revved up. On a tractor or other low-speed-governed engine, this means running it at maximum speed most of the time. The diff between 2,500 and 3,500 rpm is nothing.

Ralph

 

[KubotaBling]

Higher RPM does decrease life expectancy tho too low an rpm will too having too much load will decrease life expectancy also.. so if u have a high load on medium rpm could be worse than high rpm and capable of sustaining the load with less wear. This equation must be figured as also.. tho it's likely higher rpm is 95% more likely..

 

[Egon]

Ralph:

[The diff between 2,500 and 3,500 rpm is nothing.]

For my truck that little difference may mean a new engine.

But I know what you mean.

Egon

 

[BIGUN]

</font><font color="blue" class="small">( In a gasoline engine, heavily carbonized engines can actually increase the compression ratio, resulting in pre-detonation (pinging) which is not good for the pistons. )</font>

I had a twin cylinder Kohler 17 hp Ariens that I mowed two acres with for 20 years. Kohler recommended removing the heads and cleaning the carbon off of them every 100 hours which I never did. After all those years I tore down the engine after it threw a rod. The heads definitely had a lot of carbon on them and had literally beaten the top edges of the pistons down to the point that the rings were stuck in the smashed aluminum. This engine was run full throttle 90% of the time.

 

[OkeeDon]

I'm not sure of the basic premise involved. According the the NH website, the TC29D and the TC33D are both rated at 2800 RPM. Also, I'm not sure, but I believe the PTO RPM on both is the same, yet the TC33D has more PTO HP. If your premise is correct, then the 33 would have to run at a higher PTO RPM in order to achieve the higher PTO HP.

It seems to me I read somewhere else on the forum that HP on the same engine is increased by increasing fuel pressure or some such.

One way to check would be to look at the red line on both. Is the TC33D red line higher than the TC29D? I don't know the answer; perhaps owners of both could check and post the max RPMs shown on their tachs.

Now, getting down to the discussion at hand, one theory has been proposed that an engine has a shorter life span at 3600 than it does at 2400. This would depend on how you define life span. In terms of hours alone, this may be a valid theory. But, there is no reason to increase the RPMS to do the same work on both. The higher horsepower tractor has the potential to do more work than the lower HP tractor. If both are used to their maximum capability, the higher horsepower tractor will do more work in a given number of hours than a lower HP tractor. Thus, in terms of work accomplished, both will have the same life span.

To put it another way, assume the 3600 RPM tractor has a shorter life span in terms of hours, but accomplishes the same amount of work in those hours as the 2400 RPM tractor does in it's longer number of hours.

Perhaps a better way to look at it would be to compare two engines of the same HP, but achieved in a different way. Sticking with the 33 HP example, you could have a higher revving 3 cylinder engine or a lower revving 4 cylinder engine, bit rated at 33 HP. The larger, lower revving engine may have a longer life span in terms of hours and work accomplished, but at a higher cost. In other words, the only way you get the longer live span is to pay for it. The ratio of life span:dollars may actually be the same.

 

[nomad]

</font><font color="blue" class="small">( The diff between 2,500 and 3,500 rpm is nothing.)</font>

Not always so. Especially at such high speeds, you are in the vicinity of critical speeds where sudden increase in some vibrational wave amplitudes may appear and that may shock your tractor. At least, the probability of occuring such shocks is much more in the vicinity of the critical speed where overall net force balance is critical. BUT, according to the science philosophy today, you get the best efficieny/performance at the critical speed. So, it's like doubled-edged sword. You take the risk of damaging your tractor (due to the wear, heat and dynamic&thermal cycles causing fatique, etc) and you MAY get the best performance from your tractor.

Things in the technology is analogy to human life. We have a saying "hizli yasa, cesedin yakisikli olsun" that is said by young boys "living fast" to defend their lifestyles- Its literatlly translation is that "live the life fast (bars, discos, etc etc), let your dead body be handsome."

 

[JDgreen227]

"Kohler recommended removing the heads and cleaning the carbon out every 100 hours..."
That's really frequent, on my '89 JD with 18 hp Onan the manual recommends that service every 250 hours if using leaded fuel and 500 hours using unleaded fuel. I did mine at 900 hours and there wasn't a lot of carbon to remove.

Could anyone tell me why these type of engines build up carbon deposits? I have never known of an automobile engine that requires this type of service.

 

[CTyler]

</font><font color="blue" class="small">( Things in the technology is analogy to human life. We have a saying "hizli yasa, cesedin yakisikli olsun" that is said by young boys "living fast" to defend their lifestyles- Its literatlly translation is that "live the life fast (bars, discos, etc etc), let your dead body be handsome." )</font>

We used to joke about that as kids/young adults.
Live fast, die young and leave a good looking corpse.


I think the premise of the question is basicaly this.
If I can do the same job/work with less RPM will My tractor live longer?

 

[Egon]

Its not the suppossedly high teck higgly piggy gobbldey gook that cause the problems. It's usually the valves hitting the top of the pistons that make for self destruct because the valve train is not designed for that RPM in order to limit camshaft wear and reduce engine power loss.

There was a time floating points limited RPM unless modifications were made.

Don't know if they were in Real or Integer mode.
Egon

 

[BIGUN]

After posting that I was thinking it was more like 300 hours to clean the carbon off the heads. My memory ain’t what it used to be. /forums/images/graemlins/confused.gif I know the rubber valve seals on mine were hard as glass when I tore it down. The new gasket/seal kit didn’t have new exhaust valve seals. When I ask where they were, the answer is that they get so hot that they are ruined in a short time, so they just don’t bother to replace them. That doesn’t really explain the carbon buildup though since a lot of overhead valve engines have leaking valve seals and don’t require routine carbon removal.