This may have been asked before, but I don't recall seeing it since I've been a member.

Why does the torque and HP always cross on a dyno sheet at 5252 rpms? Does this also happen with a rotary engine or a two stroke piston engine, or is it a 4 stroke piston engine trait only?

Always look forward to learning from what you have to say.

I think it has more to do with the formula for computing hp  than  what type engine  it is..

Here's the answer to your question...

http://auto.howstuffworks.com/question622.htm

As far as I know, the mathematical equation is the same regardless of 4-stroke, 2-stroke or rotary.

Cool. Thanks for that link.  

To some extent the measures of horsepower is accidental and has more to do with ways to determine pricing/cost of the Watt engine rather than the way we think of it as a method to rate power from engines and motors.

James Watt, who was certainly a prolific inventor and truly a shrewd businessman, came up with the measurement not only as a means of comparing and sizing engines for the jobs being done by horses in the mid 18^th century but also charged for his engines on a basis of how many horses they replaced.

Going back to that time we would have found that most mechanical power for factory work was either generated by water wheel or by horses. Since many industries couldn’t be located by a suitable stream, the use of water wheels was quite limited. Windmills having a similar problem as they also would also have to be geographically located where sufficient wind blew. Lacking electricity and its unique ability to be made one place and sent somewhere else to do the work or the internal combustion engine; the horse became the portable power unit of its day. So when Tom Newcomen came up with the condensing steam engine in the early 18^th century, these things were used to replace horses at these remote sites.

A digression for a bit into Newcomen’s engine; this thing is called a condensing engine where low pressure steam is admitted into one side of a cylinder. The steam pushes up on a piston with a rigidly attached connecting rod. The connecting rod has a swivel (think Heim Joint) attached to what’s called a walking beam. The beam is pivoted at its center like a teeter totter such that the piston and rod move their end up and down. The opposite end of the beam is again swivel attached to another ridged connecting rod that is in turn connected the piston of a pump. Thusly configured, the steam engine was used to pump water from mines, pits and to and from canals and sluices for half a century. There being no flywheel to carry the engine over through non power producing cycles, the method employed to get the piston to oscillate up and down the bore was to introduce a shot of cold water when it reached BDC. This would then cause the steam to condense forming a vacuum that pulled the piston back to its starting position.

Mr. Watt first started improving upon this design by moving the condensation function out of the cylinder and into a separate tank. This seemingly simple change doubled the engine’s efficiency. Shortly thereafter, he connected the business end of the walking beam to a flywheel and the world entered into the useful properties of rotary motion. He then realized that with a flywheel it was possible to eliminate the condensation cycle and use boiler pressure itself to supply all the force necessary for the engine to function thru its cycles. The next amazing realization was that steam could be used on both sides of the piston to apply two power strokes per revolution, thus he invented the double acting engine. This almost doubles the engine’s power without increasing its size. This technique is used today with industrial internal combustion engines where space allows for it.

Back to horsepower; Mr. Watt needed a way to relate his engines to the powerhouse of the day which was the horse. Typical then and still seen in third world nations today, rotary motion is generated by draft animals harnessed to a beam where the animal walks a circle. Rather typically these animals work at the end of a 12 foot long beam which means they walk a path of 75 .4 feet. Watt determined that a typical horse could exert a force of 180 pounds on the end of this beam and could circle the path at a rate of 2.4 revolutions per minute, so the horse traveled at a speed of 2.06 miles per hour or (rounded)181 feet per minute. This multiplied by the 180 pounds of force the horse could generate results in one horsepower being equivalent to 33,000 ft.-lbs. per minute, (rounded).

So you’re still wondering about your question of why torque and horsepower are identical at 5252 RPMs. A lot of people have said this is an arbitrary value, but in fact it computes from Watt’s assumptions and calculations. If we reorganize the data of the previous paragraph into the relationships occurring at one pound of force at a radius of one foot, rotated for one revolution you get the following: (3.1416 (Pi) multiplied by two feet of diameter, multiplied by 1 revolution) is equal to 6.2832 foot pounds of work per RPM. When we divide Mr. Watt’s 33,000 foot pounds of torque for one minute which is the equivalent of one horsepower by 6.2832 foot pounds of work per revolution; you get torque and horsepower as being equivalent values at 5252 rpm.

Pretty old stuff coming to us down the centuries, not unlike the distance between railroad rails as being equal to the width of a pair of oxen pulling a Roman chariot.

Yes this calculation applies equally to rotary and 2 cycle engines, electric motors, as well as wind and water mills. Through various conversions it can be equated to BTUs or Watts or anyother measurement of force and energy.

Bogie