RPM and HP ratings

[Anonymous Poster]

Can anyone explain to me why a BX2200 / BX22 is rated at 22.0 gross HP and 16.7 PTO horsepower at 3200 RPM, but the rear PTO is 540 RPM at an engine RPM of 3068 and the mid PTO is 2500 at an engine RPM of 3125?

Is this typical of tractors in general?

Assuming that the 22.0 HP rating represents peak output, why wouldn't the PTOs be geared so as to operate at their standard nominal speed at the same peak?

Everything else being equal, it appears that running the engine at 3200 RPM for peak power would give a rear PTO speed of 518 RPM- only 5% slow, but why?

What is even more interesting is that looking at the specs ( Kubota Engine Performance ) and torque curves ( D905-E Torque Curve ) for what appears to be the same engine, that 3200 doesn't match either the high speed rating (with an exta 1.5HP) or the standard rating (down a couple of HP). But it does appear that the 22.0 HP rating is had at about the 3068 RPM specified for a 540 RPM PTO!

Realizing that raw engine specs do not necesarilly reflect an engine's performance in a given application, and that the various ways of measuring engine output, or even horsepower, is a significant topic in and of itself, is there a simple explaination for any of this?

Thank you...

 

[rbargeron]

Yes - there is a simple explanation -

It is that two-digit precision is sufficient accuracy with which to describe the very approximate practice of coupling engines, transmissions, pto gearboxes and driven implements. If power supply and power demand are within 10 percent of eachother, the results are almost certainly good enough. Further pursuit of precise matching drives up cost without adding value.

Some engineering endeavors require more precise matching of components - but agricultural machinery does not. All manner of horses have pulled plows, some a little bigger than others, still just one horsepower, and it worked anyway.

My 2 cents. I've been an engineer a long time - not a scientist (big difference) Hope this makes sense.

 

[buzst]

I think its called "marketing". If I recall correctly the engine in the bx2200 is the same as had been sold as a 2600rpm 17 HP,13 PTO hp engine in the kubota b1700. Also I think the bx1800 18HP is currently being sold as a 16hp at 2900 rpm engine in the b7400.