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

[jinman]

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

</font><font color="blue" class="small">( I believe "rated engine speed" is the RPM of the engine, when the rear PTO reaches 540 RPMs. )</font>

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]

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_]

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

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

 

[bigpete]

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]

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]

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]

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]

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]

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

how much times torque is at rated irrespective of max torque

 

[patrick_g]

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

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