RPM vs Longevity

art · Feb 7, 2004

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 · Feb 7, 2004

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 · Feb 7, 2004

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 · Feb 7, 2004

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 · Feb 7, 2004

( I've worked for the big 3 in the diesel engine business )

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 · Feb 7, 2004

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 · Feb 7, 2004

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 · Feb 7, 2004

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 · Feb 7, 2004

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 · Feb 7, 2004

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.

kevinj · Feb 7, 2004

The old Detroits turn about 2100 just like the Cummins. They are 2 cycle so they fire twice as much plus the supercharger screams. I ran an old 318 for a while and it sure was nice when I got a Cummins !!

plaindave · Feb 8, 2004

( ( I've worked for the big 3 in the diesel engine business )

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 assumin.g 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? )

I'll try to answer this - I've given this presentation probably a hundred times and I think it's easier said than written out.

When designing an engine we are concerned about three things, cylinder temperature, cylinder pressure, and bearing load. To a certain degree temperature and pressure are related, bearing loads we can design for. Using the best design tools we have, and a lot of experience we manipulate injection pressure, nozzle patterns, injection timing, and piston design to give us an engine that is the best compromise between life, efficiency, and performance. We try to keep the cylinder temperature and pressure at the point of best flame front in the cylinder without causing the piston (or other components) to self destruct. It's a fine dance figured out by guys with really thick glasses and slide rule calculators on their belts.

Bearing loads are easy to figure out in comparison because we know the forces and the square inches of mating surface we have to work with and the wear rates of metals. Knowing that we design accordingly.

As far as use of appropriate materials, trust me, we try all kinds of things to increase engine performance and life. In the on-highway truck world fuel efficiency is measured down to the thousandth's of lbs fuel used per BHP. Yup, that's right, 3 decimal places. And orders for thousands of engines are based on those numbers combined with life to overhaul. Thermal cycling in engines isn't as big a deal as it used to be.

Most of the thermal problems in the past were coolant and oil leaks caused by the engine expansion and contraction and gasket failure. By switching to O rings and different sealing technology I think we are down to something like 3 gaskets in the engine now, and we don't have leaks.

I'm not a metalurgist, so I can't say why we don't use nickel in the engines, but my guess is that a forged and machined crank and connecting rod are the strongest, most cost effective components we can get for the job they have to do. Sure there is always a tradeoff between cost and performance, same reason I drive a Blazer and not a Hummer - do I want a Hummer, sure it'd tow like a bat out of hell, can I afford it and the additional gas cost? No. Actually in one instance we used one the best bearing technologies available (sputtered bearings-sorry I don't want to describe the process) and they caused more problems than a standard tin overlay bearing we had used for years.

Todays engines are worlds better than the ones I started with back in the mid 80's. They live longer and have much better performance.

nomad · Feb 9, 2004

( Thermal cycling in engines isn't as big a deal as it used to be.
.
Most of the thermal problems in the past were coolant and oil leaks caused by the engine expansion and contraction and gasket failure.
.
I'm not a metalurgist, so I can't say why we don't use nickel in the engines, )

I'm not sure if you really understood my question. My question was about thermal fatique stresses in the engine which I assumed the most important stress for an engine operating at 3000 RPM (50 Hz like electricity) between 20-30 C (60-70F) room temperatures and 1500-2500 C (3500-4500F) flame temperatures.

At such a high frequency (50Hz) operation in between such a large temperature differences (about 4000F), thermal fatique stresses must be really important even though there isn't any mass leakage in the engine. Maybe, you are leaving this part of whole work to another design department, especially to metalurgy dept. and they do analysis when they select the engine body and component materials.

Anyway, I was trying to remind people here of that one of most important problem with high RPM (for same engine) was the thermal fatique stresses.

varmint · Feb 9, 2004

As always, lots of info to process in this thread, and more questions... I get the idea that for long service life, I should be operating my Kubota B8200 (three cylinder, 20 HP) at around 3/4 of full power RPMs, which would mean using around 1950 RPM, if the max. engine speed is rated at 2600 RPM. I suppose I do that, for most things, because the engine just seems "happy" at that speed. Does this make any sense? Of course, I want the tractor to last as long as possible, and I try to take care of it (storage in a barn, etc.)

Since I don't get over 50 hours use a year, I am changing the oil/filter once a year, using Kubota products, though I may switch to synthetic oil, for easier cold weather starting.

nomad · Feb 9, 2004

( the engine just seems "happy" at that speed (1950 RPM, i.e. 3/4 of max. 2600 RPM.) Does this make any sense? Of course, I want the tractor to last as long as possible. )

This "happiness" you feel is due to efficient burning of the fuel in the cylinders and less vibrations in moving components. Actually, in a design process, "a point" state is designed and "a range" is added around this "design point" which can also be called "rated state" to cover the safer operation area. So, I guess "design/rate point/state" may be at 1950 RPM. This is a point where some important quantities like the efficiency is the best, optimum (without considering economical parameters.)

I can recommend you new and especially old tractor users some simple sets of tests to find your "best/optimum" operating RPM. One of them can be measuring carbonmonoxide CO amount in the exhaust gas. Measure it at several RPM values and draw a simple graph. Measure also noise levels your tractor produce at some different RPM values. Draw another graph using these noise levels which tell something about the vibration history of your tractor. Using these two graphs (CO and Noise levels at different RPMs), you can determine "best RPM" for your tractor you want it to last long.

As for thermal fatique; you'll not feel "unhappiness" of your tractor due to thermal fatique stresses. They are the stresses felt later, like heart attack - you don't feel anything in the tractor when it's under big thermal fatique stresses, but, it may suddenly cause some components get broken. High RPMs is one of the reasons of thermal fatique and damage.

Egon · Feb 9, 2004

Why not just look at the manufactures torque and horsepower curves. Lot simpler.

Those high combustion temps and thermal cycling. Is this because the steel changes back and forth between austenistic and martinistic.

Sure hope the aluminum heads and pistons on some vehicles don't turn into a silver coloured crankcase puddle.

Egon

Egon · Feb 9, 2004

Would not metal to metal contact on moving parts be the greatest wear factor??

Egon

nomad · Feb 9, 2004

( Why not just look at the manufactures torque and horsepower curves. Lot simpler. )
You are correct. But I supposed most of tractor users don't have that data and also, older tractors don't have anymore the same curves as they left the factory.

( Those high combustion temps and thermal cycling. Is this because the steel changes back and forth between austenistic and martinistic.)
Color or phase change may not necessarily have to occur at tractors with heavy thermal fatique stresses. But I can't comment on chemistry or metallurgy of engine materials. Are the steel changing between austenitic and martenistic forms in the engines?

( Sure hope the aluminum heads and pistons on some vehicles don't turn into a silver coloured crankcase puddle.
)

Lightness and heat transfer capability of Aluminum are main reasons why it's used in some engines and this is well known. But, use of such a ductile material like aluminum in the engines isn't causing some problems fast contraction & expansions which may block some tolerances between components? I assume that aluminum alloy used in engines has some elements which reduce overall ductility of alloy.

What I'm curious here is ductility is good for thermal fatique stress or not? That's why I asked Dave who is involved in engine designs - but he said he's not involved in metalurgic part of the work.

Ps: yes, metal-to-metal contact is the cause of most wear. Okay, wear is also important at high RPMs problems, but I also reminded thermal fatique problem (at high RPM operation) that can be felt later in time.

Richard · Feb 10, 2004

( This "happiness" you feel is due to efficient burning of the fuel in the cylinders and less vibrations in moving components. Actually, in a design process, "a point" state is designed and "a range" is added around this "design point" which can also be called "rated state" to cover the safer operation area )

Hmm....

My tach has a green span in it. I've never paid MUCH attention to it, but let's presume it's about a 500 rpm wide green band at approximately 2300 rpm.

Does that green band infer the engines (Perkins) peak efficiency area or other? Could it be this design point?

I've seen a redline on a tach before, but can't say I've ever noticed a green band

Richard

slowrev · Feb 10, 2004

One would think that the PTO RPM would be at the peak of the torque curve, but that is not always the case. An example of less than perfect engineering and creating a new model of tractor out of existing engines, trannys, etc without properly adjusting the ratios, etc.