Originally Posted by Wally
Forget all the tripe. Fuel is rated in joules and the when burnt at a rate of one joule per second releases a watt of power. How that power is applied whether it be for cooking steaks or to make a wheel turn is a product of that power.
Without power torque cannot exist, but power can exist without torque.
You cannot have more or less torque than power simply because they are different measurement, like oranges and lemons although they may share common elements. Just because someone decided to scale a graph to conveniently correlate with a 5252 derived constant does not mean one is greater than the other. The Torque (ft-lbs) = HP x 5252/rpm equation can be rearranged whichever way you like. If you use the same nonsense internet mechanics do by rearranging the the equation to prove hp is dependent on torque, you could equally argue power is dependent on rpm, which we all know is not true.
Just like in any math, you can rearrange the equation many ways. But that isnt' what we are talking about here and you should know it.
Torque is tangential force * the distance from the fulcrum. Power is defined as work per unit of time.
Applying 1 lb of force 1 ft from the fulcrum for a complete revolution will lead to;
W = F*2*pi*r = 1 lb * 2*pi * 1 ft = 2*pi lb-ft = 6.283 lb-ft
If it takes one minute to complete this revolution, then the power is;
P = W / time = 6.283 lb-ft / min
1 hp is defined as 550 lb-ft / s = 33,000 lb-ft / min
Therefore, applying 1 lb-ft of torque in one minute (1 rpm) = [6.283 lb-ft / min] / [33,000 lb-ft / min] = 1 / 5252 of 1 hp.
From this you can then calculate the number of hp from any given torque and rpm:
hp = torque (lb-ft) * rpm / 5252
No one "scaled a graph conveniently" to make that happen.
Where does the equation HP=TORQUE * RPM / 5252 come from? We will use Watts observation of one horsepower as 150 pounds, 220 feet in one minute. First we need express 150 pounds of force as foot pounds torque.
Pretend the force of 150 pounds is "applied" tangentially to a one foot radius circle. This would be 150 foot pounds torque.
Next we need to express 220 feet in one minute as RPM.
The circumference of a one foot radius circle is 6.283186 feet. ft. (Pi x diameter 3.141593 * 2 feet)
The distance of 220 feet, divided by 6.283185 feet, gives us a RPM of 35.014.
We are then talking about 150 pounds of force (150 foot pounds torque), 35 RPM, and one horsepower.
Constant (X) = 150 ft.lbs. * 35.014 RPM / 1hp
35.014 * 150 / 1 = 5252.1
5252 is the constant.
So then hp = torque * RPM / 5252
"The word horsepower was introduced by James Watt, the inventor of the steam engine in about 1775. Watt learned that "a strong horse could lift 150 pounds a height of 220 feet in 1 minute." One horsepower is also commonly expressed as 550 pounds one foot in one second or 33,000 pounds one foot in one minute. These are just different ways of saying the same thing. Notice these definitions includes force (pounds), distance (feet), and time, (minute, second). A horse could hold weight in a static position but this would not be considered horsepower, it would be similar to what we call torque. Adding time and distance to a static force (or to torque) results in horsepower. RPM, revolutions (distance) per minute (time), is today's equivalent of time and distance. Back to horses, imagine a horse raising coal out of a coal mine. A horse exerting one horsepower could raise 550 pounds of coal one foot every second."
Of course, the problem, as someone else pointed out in another thread, is that "Watt was simply observing that the horse lifted the 150 lb. object by 220 feet in one minute. We can't change that. Had Watt simply observed a bigger (say, a Clydesdale) or smaller horse (like Ford used to measure the '99 Cobra!) the definition of HP would be different."
The reason we dont measure internal combustion engine horsepower directly is that engies make a rotary motion, not linear motion. And unless the engine is geared down, the speed at which they do work (time and distance or RPM) is too great for practical direct measurement of horsepower. It seems logical then that the solution was to directly measure torque (rotational force eventually expressed in pounds at one foot radius) and RPM (time and distance, i.e. distance in circumference at the one foot radius) and from these calculate horsepower. Torque and RPM are easily measured directly.
Therefore when discussing internal combustion engines, torque IS the measured force, and hp is ALWAYS the calculated amount.