RPM vs Longevity

[art]

Interesting reading the feed back here, The same engine just turned up is going to give the lower RPM engine an advantage to life. Not as much as receiving the proper maintenance and probably not even able to put an hour figure on. Some engines are of older design with design flaws still incorporated into it that other engine manufacturers are building around. Some diesel engines have been built with the latest of designs and metals and can handle 5000 rpm from the factory! Look at the new Honda engine in cars at 9000 rpm! Good quality fuel,oils and filters and proper operation will improve the longevity even of a poorly designed engine!

 

[plaindave]

I've worked for the big 3 in the diesel engine business (Cummin, CAT, Detroit Diesel) for the last 20 years, all in the industrial engine business and I can tell you without question that the RPM of the engine does not have a significant affect on the life as long as you are operating within the design parameters of the engine.

I've been part of design teams that have built 1200rpm compressor engines, 2100rpm industrial engines, and 2800rpm automotive engines. All of them would exceed 10,000 hours operation before overhaul - as long as they were run as designed. Misapplication of the engine kills life far faster than high RPMs.

Generally I'd agree that a long-stroke, slow speed engine will last longer to overhaul, but that's how those engines were designed. But to get the same effective HP from a slow speed engine it needs to be much larger in displacement. That's part of the reason my grandfathers old Massey weighed in at 3000 lbs compared to my Kubota at somewhere around 2000. His engine had a max RPM of 1800, mine is 2500.

But all things equal, today's high speed engines are far more fuel efficient, lower in emissions, and have a longer life than previous models.

For what it's worth I run my engine at about 80% throttle. That's where you will be getting the best fuel efficiency and most complete combustion in the cylinder and are usually in the upper end of the power/torque curve. Much faster and you are out of the torque curve.

 

[Bird]

I think that's a very good explanation, plaindave. And in the case of our compact tractors, I just don't think many of us will ever put enough hours on one to be concerned about that wear. I just got a phone call this morning from an old friend from Pennsylvania. He bought a slightly used L245DT Kubota with a FEL in the late '70s; still has it, doesn't remember just how many hours he's put on it, but said he replaced the front tires, and had to have some welding repairs done on the rear wheels because of corrosion caused by the calcium in them, and otherwise only changed oil & filters now for 25 years.

 

[John_Mc]

Of course you're assuming that the person with the TC33 actually runs at a higher RPM than the one with a TC29. Since I rarely run around at the max RPM, I'd say that the engine longevity has more to do with how it's run and obviously hhow it's maintained) than what the max rated RPM of the engine is. I'd agree that RPM may be a significant factor for a tractor rated at a max of 3600 RPM vs one rated at 2500 (since they are probably geared differently, and you wouldn't get much mowing done running your 3600 RPM tractor at 2300, for example). The difference between a 2600 and a 2800 RPM rated engine is going to be dwarfed by operator technique and how hard you work the tractor.

Incidentally, the TC29 and the TC33 are both rated at 2800 RPM. The HP difference comes from 81.2 vs. 91.3 cu. in. displacement (I don't have the specs for the new TC33DA). Perhaps you were thinking of the TC25 vs the TC29 which had the same engine, but 2600 and 2800 RPM ratings, respectively. (Probably one of the reasons the TC25 was discontinued... why derate an engine that will run just fine at the higher rating?

John Mc

 

[nomad]

</font><font color="blue" class="small">( I've worked for the big 3 in the diesel engine business )</font>

Since you are involved in the design processes of engines (I've not) may I ask you that what type of stress is the most important stress, most important determining factor for the life span of the engines? Without thinking much, I'm assuming time-dependent thermal stresses, i.e. termal cycles as called in the public, will play the major role in the engine life. Right?

But I don't see the use of appropriate materials that will resist thermal stresses much in engine components like pistons, cranks, etc. For ex., as far as I remember, Nickel, a relatively expansive material, isn't more than 2-3% in some engine components like the crank which are under heavy thermal cycle stresses. Why is this so? I think, in the engine designs, the lower cost criteria comes first before the longer life criteria? Am I missing anything here? Is Nickel percentage low because its relatively higher ductility that won't be so good against the thermal cycles? Or, is it low just to reduce the cost of engines?

 

[JerryG]

Maybe the TC29 vs the TC 33 was a bad example. I should have used the BX series. But there are several by several different manufactures. The point being, if a increase in hp is obtained only by the manufacture increasing the rpm, the higher rpm of the two will not have the longevity that the originally rated engine will have given everything else is the same. Anytime that a engine has more cycles on it there is more wear.

 

[art]

Jerry how about when an engine has a long stroke which is low RPM but high piston speed? Maybe far higher then an engine turning twice as fast? It is better to have a well designed engine for longevity then to have a slow turning engine of poor design. There are differences and you might never find the best but you are not quite right with the rpm's as a basis of facts here.

 

[JerryG]

No Art that is why I have been stating the same engine with only a rpm change to increase hp most often to just add a new model in the line up. The reason that I would like a long stroke engine better aside from the above discussion is that a long stroke engine will develop its max torque at a lower rpm and also have a higher torque for the same cubic inch.

 

[GrayBeard]

Back to the low idling, carbon/condensation build up problem.
Many years ago I worked evenings and weekends at a truck stop.
At the time, it was common practice to let the trucks idle for hours or even days in very cold temps.
They were mostly Cummins , Cat or Detroit and some with 300-400,000 miles with no obivious carbon problems.
So, what was different about those engines ? Is it just the smaller diesel engines that may suffer ?
Another question, does anyone know the rpm's on one of those old Detroits.
They always sounded like they were turnin' a bunch.
GrayBeard

 

[Bird]

Graybeard, there's no harm in letting it sit and "idle" for long periods of time as long as it's either a very fast "idle" or one that fluctuates up and down periodically.