What is the exact meaning of "rated engine speed"?

[PaulK]

What is the exact meaning of \"rated engine speed\"?

I'd like to learn more about the parameters involved when a company designates a specific engine RPM as the "rated speed".
Specifically, my Kubota B21 has a rated engine speed of 2600 RPMs. That speed is used for tests of other components in the system. The hydraulic pump test is performed at "the rated speed of 2600 RPM" pressure of 1991 PSI and oil at 122F. Tests for the backhoe are calibrated at 2600 RPM. The literature indicates that the backhoe can be run at rated speed and slowed to 2000 RPM if noise is a problem, while 1800 RPM is the slowest permissible engine speed for the backhoe.
If rated speed is the standard operating speed for implements, is it the most efficient speed? I usually run my machine at 1800-1900 RPMs because the power I get at 2200 RPMs is not that much more. I know that there have been numerous discussions on this topic, i.e. "what RPMs are you running"?
My gut feeling when in the drivers seat and listening to 2600 RPMs is that it is very high. Obviously, since I am not conditioned to that rev, it will seem high. My naive and wimpy assessment of "rated speed" is that it is the highest sustained speed that the engine can take. Could it be that the engine is actually "happier" to be running at rated speed and could withstand a few hundred more RPMs if needed and as long as it isn't done in hot conditions or to the point where it overheats?
I'm interested in the history of the term "rated engine speed" as a calibration tool. Our car tires have a weight rating that is probably half of the weight that they can actually withstand. Does an engine mfgr find a speed that it determines as "most efficient", or "highest RPM without breaking", etc. and then regard this as a rated speed? Just curious about the procedure and rating parameters.
Paul

 

[mahlers]

Re: What is the exact meaning of \"rated engine speed\"?

My understanding is that the rated speed is where they rated the engine power. The governor will usually be set so that the engine doesn't exceed the rated speed, and the engine's power is measured at the rated speed. Most engines can handle much more than the rated speed, but for long-term use, the lower the rated speed, the better. I've seen the exact same engines with different rated HP, and the only difference was the rated speed.

I can't remember the exact formula, but I think the engine horsepower is equal to the rated speed times torque at that speed, divided by 5280. On many engines, since the torque curve is very flat, all you need to do to increase rated HP is to increase rated speed.

It's been too many years since I've done those calcs, so if someone knows what the real formula is, please let me know.

 

[nomad]

Re: What is the exact meaning of \"rated engine speed\"?

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I'm interested in the history of the term "rated engine speed" as a calibration tool. Our car tires have a weight rating that is probably half of the weight that they can actually withstand. Does an engine mfgr find a speed that it determines as "most efficient", or "highest RPM without breaking", etc. and then regard this as a rated speed? Just curious about the procedure and rating parameters.
Paul
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a "rated x" in practical field can be considered as "designed x" - and the designs are usually made at "optimum" points that many factors/parameters such as efficiency, power, safety, etc etc as well as economy are considered. It doesn't have to be maximum efficient, it doesn't have to rotate at highest power, etc etc - it has to be designed at "optimum" that has to consider also non-technical things like the health/safety, economy, etc. Also, for ex., PTO of a tractor will not always rotate at the same speed, say, at 540 rpm even if you try to do so. Maybe, it will have a speed of 539 rpm maybe 541 rpm. But what will you do if you are about to design an attachment which will use PTO? You just take a fixed RPM value of PTO, say, 540 RPM and you will call this "designed or rated rpm". Actually, this is usually also an optimum RPM because you are also taking some other factors such as economy/safety/etc into consideration when designing an attachment.

 

[Mosey]

Re: What is the exact meaning of \"rated engine speed\"?

"At Rated" is the RPM where the engine produces the maximum torque. A "torque curve" has the torque on the "Y" axis and the RPM on the "X" axis. It starts out at the lower left of the graph and goes up as RPM goes up, then it will top out at some RPM and then actually go down at higher RPM. So, maximum RPM does not produce maximum torque. On a tractor, 2600 RPM is where the engine produces the most torque as well as where the PTO rotates at 540 RPM. It's OK to run at lower RPM if you want (you'll use less fuel), but not safe to run at higher RPM if you're using the PTO. If you're not using the PTO, it's OK to run at higher RPM.

 

[BillyP]

Re: What is the exact meaning of \"rated engine speed\"?

I believe "rated engine speed" is the RPM of the engine, when the rear PTO reaches 540 RPMs.

 

[_RaT_]

Re: What is the exact meaning of \"rated engine speed\"?

I believe Billy is most accurate. At rated engine speed, you will find the PTO RPM at close to 540. Torque is not optimum at this point, it is at a much lower RPM and thats what you want. Rated engine speed is not a governed speed, most tractors can go beyond rated engine speed in RPM, mine can. The torque curve typically is bell shaped. The reason you want more torque at a lesser RPM say 1800 RPM is so that at rated speed as the load increases the torque does also to help maintain the engine RPM. This is how it was all explained to me some time ago.

 

[zuiko]

Re: What is the exact meaning of \"rated engine speed\"?

Yes my tach has rated and maximum speed on it. Maximum is several hundred RPMs higher than rated. In practice I keep it to rated or less but I have no qualms about running at at rated indefinately since that is what it's designed for.

 

[art]

Re: What is the exact meaning of \"rated engine speed\"?

Billy is right as it is the speed that they rate the engine at. On some it is done at 540 and the larger tractors pto system rates it at the 1000 rpm on the pto. There are some variations of that but in a nut shell that does answer the question.

 

[PaulK]

Re: What is the exact meaning of \"rated engine speed\"?

Everyone,
Thanks for the answers. Billy's answer seems dead solid perfect. I've never used the PTO on my TLB but I viewed it as standard like the Bobcat quick-attach bucket, etc. system. My hunch was that since so many implements from such a diverse group of mfgrs were run off the PTO, it had to be a standard which the engine could easily handle. To paraphrase Billy, if you're going to use a PTO implement that requires 540 RPMs, you have to know how fast to run your engine.
Thanks everyone.
Paul

 

[_RaT_]

Re: What is the exact meaning of \"rated engine speed\"?

Art, my International 454 has 2 PTO's. The top one runs at 540 and the bottom at 1000. The 540 is a 6 spline, the 1000 is a 12 spline if memory serves me correct. I have run my 540 RPM 84" finish mower on the 1000 RPM by cutting the engine RPM in half and through use of an adapter. That means mowing at about 1200 RPM. That little 3 cylinder diesel on the International just hums right along except in 4th up a hill.