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

PaulK · Jan 15, 2004

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 · Jan 16, 2004

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 · Jan 16, 2004

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

(
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
)

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 · Jan 16, 2004

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 · Jan 16, 2004

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_ · Jan 16, 2004

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 · Jan 16, 2004

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 · Jan 16, 2004

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 · Jan 16, 2004

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_ · Jan 17, 2004

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.

jinman · Jan 17, 2004

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. )

Billy, I think you are correct in a round-about way. If I look at a table listing specs for a tractor, it will most often have both horsepower and torque rating listed for the engine. The rpm where the tractor's engine produces the "advertised" horsepower is "rated engine speed." The rpm where the maximum torque is achieved is also "rated engine speed." The engine rpm at which the PTO produces 540 rpm is also "rated engine speed." Therefore, I believe you have to know the rpm's used by the manufacturer when they were measuring horsepower, torque, and 540 PTO rpm to know all the rated engine speeds. I believe the 540 rpm rated engine speed might also be the same as maximum horsepower. That seems to be the case for my tractor.

bigpete · Jan 18, 2004

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

Engineers design ratios through gearing and hydraulic flow volume and pressure that produce specified PTO output speeds at optimal engine speeds. That is, the 2600 RPMs that the BX22 is "rated" at is the optimal speed for that engine to produce the best slope on a torque curve for typical work for that machine. Through the gearing and hydraulic dynamics, the engineers just match up the the implement PTO speeds to optimal engine speed.

As somebody mentioned, torque rises as RPMs are reduced. However, the engineers calculate the best torque curve as RPMs are reduced under load, not as the RPMs are reduced by pulling back on the throttle or running at a lower throttle setting. Running at a lower-than-rated throttle creates a flatter and less-desirable torque curve.

In other words, the rated speed is the speed that produces the best torque rise under load. PTO engineering creates implement speeds that occur at the the same optimal engine speed. Running an engine at a slightly lower RPM than rated speed will produce more torque at that (load-free) moment, but the torque rise will be flatter once the engine experiences a working load at this lower RPM. All of us know this as a point where an engine begins to "lug" or, as I've heard farmers say, the point where a diesel starts "slobbering." Even at the rated speed, an engine will eventualy lug, but it is much later in the increasing load slope than it is at a lower RPM. Engines with greater displacement and HP that are dropped into tractors with lower, torque delivering gearing are said to have better "lugging abilities."

There is plenty of engineering gobbledy-gook published on the the Web on this topic. I certainly am not an expert, but my father is a retired Mechanical Engineer who worked as a JD engineer back in the Waterloo plant in the 50s, and has clarified this exact topic for me. Essentially, you can save some fuel at the lower RPM and put a little higher torque out at the moment of load. However, the torque output degrades faster as the load increases than it would when you are running at rated speed with a well-tuned engine. All of the other tractor specs (HP, torque, hyd. flow rates, and pressures) are then measured at this optimal level and published so we can make educated decisions on the best tractor for our needs.

Here is a definition of Torque Rise that I pulled from a governement bidding glossary:

TORQUE RISE
The percentage increase in torque from high idle to maximum lug. The greater the torque rise, the more "lugging ability" the engine has.

_RaT_ · Jan 18, 2004

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

Excellent explanation BigPete /forums/images/graemlins/wink.gif

bigpete · Jan 19, 2004

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

At a gut level, or by the seat of our pants, most of us find about our torque rise when we are mowing, brush hogging, blowing snow, tilling, pulling a disk or harrow. Usually it's a job where we might start out smoothly and think we don't need to run at rated speed. As we get into the job, we may get into heavy grass, snow, wet soil, whatever, where the load starts to try to work its way back to the engine and the RPMs drop. That's when the governor kicks in to feed more fuel to the engine and torque begins to rise.

If the revs were low going in, we all know to slow down, back up, and throttle up to get through the load. When the going gets tough, the best place to be will always be somewhere near the rated speed, give or take....

A good torque rise slope for most 3-cylinder diesels in the 20-30 HP class with a rated speed around 2,700 RPMs will produce about 90% of the full rated torque when the load pulls the RPMs down to around 1,800 to 2,000 RPMs.

KiotiJohn · Jan 30, 2004

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

I've only had my tractor about a week and I didn't have to read this to find out that I need to run near the rated speed when I'm under load. The tractor has no trouble telling me that I'm not getting the torque I need! When I'm just tooling around doing something light, I keep it lower to conserve fuel, but I just have to run rated RPM (2800 in my case) to keep from "slobbering" all over the place. When I do this, my little Kioti CK20hst just goes all daedong. John

RalphVa · Jan 30, 2004

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

Rated engine speed is often the governed speed on many slower-speed utility engines, like tractors, lawn mowers, etc. Like many have pointed out, this is probably the speed at which the PTO is at 540 rpm the speed at which the PTO attachments were designed to run at.

One thing you have to watch for in some engines (Italian auto engines were notorious for this) is that oil circulation is proportional to engine rpm. Therefore, if you put much load on an engine much below its rated speed, you MIGHT be starving it for lubrication. However, I suspect that most tractors (and indeed most older US engines) are designed with oil pumps that pump enough even just above idle and bypass on pressure control above that point at which they achieve design output pressure.

My dad once bought my grandpa's Plymouth V8 with stick shift. He very quickly ruined the engine because its bearing were weakened from Grampa's eternal lugging of the engine.

Ralph

art · Jan 30, 2004

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

As your engine breaks in the slobbering should go away. If it doesn't you might try injectors or pump work. This is not a good condition as it will wash the cylinders and hinder the break-in process.

keeney · Feb 9, 2004

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

The other thing to remember is that power is proportional to torque times speed. I would imagine that most tractors are designed such that the 540 PTO speed is achieved near the sustainable power peak of the engine. Running at a lower RPM from the power peak will reduce the available power. Even if the available torque increases, it is not enough of an increase to offset the lower rpm factor (by definition, if it was, the power peak would be at that lower RPM, instead).

Some implements require torque, some speed, but most need some combination of both to do a good job, and that means power. Cutting grass and blowing snow are good examples of jobs that require a minimum speed to do a good job, and a minimum torque to carry the load at that speed. Slowing down a rotary mower or snow blower makes them stop working right. They don't clear properly and just bog down even more.

Other tasks like plowing or running a post hole auger are more about torque. Sure, slowing down makes it take longer, but the plow still turns the soil and the auger still continues to dig the hole properly at a wide range of speeds.

- Rick

vikash kumar · Apr 3, 2012

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

how much times torque is at rated irrespective of max torque

patrick_g · Apr 3, 2012

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

_RaT_ said:
Excellent explanation BigPete /forums/images/graemlins/wink.gif

Except for not making it clear that torque vs RPM assumes a load. Running an engine at any RPM under no load (clutch depressed or in neutral) and you are not developing squat for HP or torque.

Pat