RPM vs Longevity

[MJB]

You don't want the machine to run right at the torque peak. A properly designed unit will have the rated speed at a significantly higher RPM that where the torque peak is. This is so that as the load on the engine increases and the RPM is pulled down the torque output actually increases to pick up the load. This is called "torque rise". If you run right at the torque peak and then add a load to the machine the resulting decrease in RPM would cause a lower torque output, which would further reduce RPM, futher lowering torque, and on and on until the engine stalled.

 

[slowrev]

I was not talking about maximum RPM, Just PTO RPM. I know the maximum RPM should always be higher than PTO RPM. You have to have a bit of headroom there. Course I am no real expert here either /forums/images/graemlins/smile.gif

 

[art]

On many of the newer ag tractors they are only 200 rpm away!

 

[keeney]

I do agree with the post that talked about engines changing with time and wear. I used to work on engine-dyno software at Toro and one of the things the design engineers frequently did was compare the curves from a new engine to one with a lot of hours on it to make sure the machine would "age" well and last its intended life.

They would always be running dozens of engines 24/7 in loud and surprisingly well-ventilated test cells. The test rooms even had fuel on-tap from a big tank on the roof so they didn't have to stop to put gas in. The loads were carefully tailored to match real-world histograms. It takes quite a while to get thousands of actual hours on an engine!

Another interesting thing they always measured was "governer droop". If the governer design has too much droop in it, you can loose RPM's under load not due to the actual engine curve, but due to lack of sensitivity of the governer. In other words, how many RPM's did the engine have to drop before the governer had opened the throttle valve to maximum?

For many applications, you could just set the governer set-point a little higher. But for lawn mowers, the blade tip speed is limited by safety standards. You can't exceed the max tip speed under no-load conditions so you end up running at less than the allowed max under full load due to governer droop.

To measure the engine curve without the governer droop they would servo the dyno to maintain a set RPM and wedge the throttle valve on the engine wide open. The dyno would servo the torque load to keep the RPM's at the set-point. Then the test would sweep through the RPM range.

I would hope that modern electronic engine controls would minimize governer droop, but on the old air-vane and spring governers on lawn-mower small gas engines, it was actually a significant concern.

I would be curious to know what modern CUT's use for governer's these days. Is it mechanical or electronic?

One thing I did learn is that running an engine at the high end of its RPM range under zero load is not all that rough on them. Its the full-load condition that wears them out. Of course, RPM's above the red-line start to be a whole different story.

- Rick

 

[slowrev]

Art,
Yes, but on some of the CUT's they are not so close. Not a major issue, just not quite right in my opinion. Maybe it is due to the lower torque engines in CUT's, and the governor needs more headroom ? Not sure just a thought.