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Rule to Calculate Horsepowerthis thread has 20 replies and has been viewed 8193 times


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#1




Rule to Calculate Horsepower
From the John Deere Farmer's Pocket Ledger for 1925: "Rule to Calculate The Horse Power Of An EngineMultiply the diameter of the cylinder in inches by itself; in other words, square the diameter in inches, and divide by four.

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#2




Re: Rule to Calculate Horsepower
Just curious, would that still hold on todays engines? The same horsepower engine of today you can pick up with one hand, a lot smaller piston. Later.

#3




Re: Rule to Calculate Horsepower
I don't know, but I seriously doubt that this rule would work on modern engines.

#4




Re: Rule to Calculate Horsepower
Hello,I am sure that rule can not apply to the modern engine,because an engine with a 4 inch bore would only be rated at 4hp. Ed Kinch

#5




Re: Rule to Calculate Horsepower
I didn't see how it could, but I am weak on math. And I have been suprised several times on this site by things I have learned. Thanks.

#6




Re: Rule to Calculate Horsepower
Hello again,my 9hp. Hercules does come very close but not exact.It is rated 9hp. at 325 RPM if you increase the RPM TO 360 it is rated at 10hp.
Ed Kinch 
#7




Re: Rule to Calculate Horsepower
Bore x bore x stroke x rpm x .7854 divided by 13,400 for a 4stroke and 10,000 for a 2 stroke.
This is the formula I have always used, it is for single cylinder engines and does not take into account the compression ratio. It does do a good job of getting in the ball park. 
#8




Re: Rule to Calculate Horsepower
I am not too smart, but as a child who hated to learn about anything that didn't relate to mechanical motion, I someplace picked up that on a reciprocating internal combustion engine the horsepower was directly related to the size and speed of the flywheels. the cylinder/piston generated torque and delivered it to the crankshaft converting it to rotary motion that is stored by the flywheels as inertia. some of the inertia is required to operate the engine through its cycles, and the balance is stored horsepower. I will not bet on it but somewhere in the back of my brain cell deposit that arrangement exists. please reply this could get interesting. HOW ARE YOUR BRAIN CELLS ORIENTED ON THIS. Al

#9




Re: Rule to Calculate Horsepower
I think I've given this before, but the general calculation for cylinder horsepower is: nxPLAN/33,000
n is the number of power cylinders x is the number of power strokes in the cylinder per revolution (examples: double acting steam x=2, single acting steam or 2 cycle x=1, 4 cycle x=.5 or 1/2) P is average cylinder pressure (examples: for steam assume 1/2 the boiler pressure, many early gasoline engines used an assumed 75 psi) L is the length of the stroke in FEET (example: an 8" stroke would be .667, or 2/3, a 12" stroke would be 1) A is the area of the piston being acted upon in square inches (the entire face, PLUS in double acting engines, the back side minus the area of the rod) N is the number of revolutions per minute multiply all those together and devide that number by 33,000. Why 33,000? That is the number of foot pounds /minute in one standard HP...which is more equivelant to the output of a welsh pony, but that is a digression for another day. First, I'll use my own 20th Century traction engine for an example: It has twin 71/2 x 10 double acting cylinders, and was originally designed for 250 rpm at 150psi. So what does that get us? n=2, x=2, P=assumed at 75lb/sq in, L=10/12 or 0.8333feet, A=44.179 on the face, 42.412 on the reverse for an average of 43.296 sq in, N=250 rev/min The calculated HP comes to 81.98 HP, not bad, but fairly realistic for an engine that was rated at 25HP based on an even more archaic system. Y'all try it, and see how close you come to the factory ratings. 
#10




Excellent information!
Thanks a lot! This is great information.

#11




Re: Rule to Calculate Horsepower
Most of these formulas given are for a specific situation typical at the time the information was published. Many engines way back when were quite similar as far as their insides as to bore, stroke, compression ratio, speed in RPM, etc. Any formula given that does not take into consideration all the measurable factors provides only a good approximation, but maybe that is sufficient for most cases.

#12




Re: Rule to Calculate Horsepower
You fellows are getting much too complex for my simple brain. And besides, I wouldn't want to argue with 'ol John Deere himself. P.S. I don't own a torque wrench.

#13




Re: Rule to Calculate Horsepower
http://www.rustyiron.com/rig/horsepower.html

#14




Re: Rule to Calculate Horsepower
It seems impossible that Horsepower could be calculated using one group of mathamatical fomula. To reinforce this how can an old IHCLB be rated 11/2 to 21/2 hp. It must certainly be the RPM and the energy stored in the flywheel at different speeds, nothing else changes. Horsepower is the amount of work that can be produced at a sustained pace. Without power stored in the flywheels we can only get one power stroke, and that is not sustained output. I don't mean to offend anyone but something still seems to be missing. And or I am a little thick. Al

#15




Re: Rule to Calculate Horsepower
Al,
HP is Torque multiplied by rotational speed. The HP range is for different RPM's and is essentially due to throttle adjustment. For instance, if the engine speed is increased from 500 to 600 rpm (a 20% increase), at a constant torque (actually, there will be a change in torque, but it would probably be small and for the sake of this example, let's say it remains constant), the HP would increase by 20%. The flywheels do store energy, but as you stated, they are mainly there to keep the crank turning during the nonpowered strokes. It also helps maintain a constant rpm when a load is suddenly put on the engine, but doubling the size of the flywheel will not double the HP of the engine. They simply store some of the energy that the engine is producing. By the way, No, you are not "thick". Hope this helps. If not, feel free to come back and ask more questions. Hal Davis 
#16




Re: Rule to Calculate Horsepower
Hal. I think you are exactly right, the variables are the key here. Is there a rule of thumb to calculate the amount of energy stored in a flywheel at a given speed. It seems that some of the old flywheel kickers out there can kick a flywheel and tell the horsepower! AND BE RIGHT. Al

#17




Re: Rule to Calculate Horsepower
Lets assume that some people can kick, or roll the flywheel and come up with a close estimate of the engines horsepower. I imagine that the engineers that designed these early internal combustion engines eventually came up with some general rules for the size of the flywheel as compared to the rest of the engines parts. I have noticed that steam engines for the same rated horsepower, seem to have smaller flywheels as they don't have to deal with a compression stroke.
For a given bore and stroke along with the compression ratio, and engine friction, the engineers understood that it would take a certain amount of weight around the rim of a certain diameter flywheel to insure that at the lowest likely operating RPM, that the engine would still carry over through the compression stroke along with providing power to the load on the engine output shaft. You could make the flywheel heavier with the same diameter within limits, or make it larger in diameter, to increase the available energy from the flywheel. Of course, the energy does not exist in the flywheel until it is placed there by the combustion pressure of the fuel pushing down on the piston through the connecting rod that turns the crankshaft to which the flywheel is attached. If the power was within the flywheel alone, we would not need all these other pesky engine parts. All the flywheel does is to store the intermittent power pulses from the rest of the engines parts. Some of this stored power is used to insure the continued rotation of the engine between power strokes, and some of it is used to supply power to the external load between engine power strokes. That kind of reminded me of a story about how all the human body parts got into an arguement about who was the most important part of the body. I won't go any farther than that here. To make the point, the flywheel is absolutely necessary for operation of an internal combustion engine. You can change the characteristics on any of these parts within some limits and the engine will still continue to run. And by flywheel, I am refering to all the rotating mass that is connected to, or is a part of the crankshaft. The flywheel doesn't make energy, it only stores it for a time, or gives it up for a time. But in any case, its got to be there to make the engine run. They have experimented with busses powered by a spinning flywheel weighing about eight tons that spins up to something like 125,000 RPM. Power is extracted from the flywheel as it slowly runs down through a generator. When the bus stops at a bus stop, the generator becomes a motor when it is temporairly connected to the power mains, and it then spins the flywheel back up un RPM to supply power for the next part of the route. Even in this case, the flywheel does not make any power by itself. It stores and releases it upon demand. They can't get any more power out of the flywheel then they initially first put into it less the system losses. Another long winded thought. A rocket engine produces work that can be expressed in horsepower. Remember, so many pounds of mass accelerated so far in so long, equates to so much horsepower. The rocket engine doesn't use a flywheel, and yet it can produce power when operating as long as it is fed fuel and an oxidizer. So getting back to the original thought. Yes, there are people out there with sufficient experiance to be able to pretty accurately estimate the horsepower of an engine by kicking or rolling over the flywheel. They probably were not quite right in their estimates all by themselves the first few times they tried this, but like all of us, they got better at understanding the subject as time and experiance dictated. At least that is sort of how I understand it. 
#18




Flywheel Energy
In a previous discussion, Al Hettich asked if there was a rule of thumb for calculating the energy stored in a flywheel for a particular rpm. Well the answer is yes, and here goes.
E=WxVxV/64.32 where: E is total energy of flywheel in ftlbs. W is weight of flywheel rim in lbs. V is velocity at the mean radius of flywheel ft/sec The 64.32 is gravity (32.16 ft/s/s) times 2. Say our flywheel has a 2" rim, is 24" in diameter, weighs 50 lbs, and is turning at 200 rpm. Our mean radius is 11" because the outside radius is 12" and our flywheel rim is 2" thick. The circumference at the mean radius is 69.1" or 5.76 ft. We are turning at 200 rpm or 3.33 rev/sec, so the rim speed at the mean radius is 1.73 ft/sec. Plug all of this into our formula above and we get: E = 50x1.73x1.73/64.32 = 2.33 ftlbs. I hope My math is correct. Someone correct me if not. Remember to multiply your answer by two if you have two flywheels. 
#19




Re: Correction
Boy, you guys let me down. Nobody checked my math. I was in a hurry when I did that yesterday. I must have zigged when I should have zagged.
The correct rim speed in my example is 19.2 ft/sec. So... 50 x 19.2 x 19.2 / 64.32 = 287 ftlbs That sounds better. I thought my answer sounded low yesterday. 
#20




Re: Correction
Hal, Thanks for the info. It is A bit over my head, but I am going to try it on A IHC M to see how it comes out. My mathematical prowess is very limited, and I will probably find that the age and condition of the real horse used to compare will need to be factored in. It will also be interesting to see where variations of RPM will take it. Thanks AL

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