Originally Posted by A_Harman
Originally Posted by RayCJ
Originally Posted by DGXR
I understand what each means but I don't understand the mathematical equation that all the engineers use:
HP = (TQ x RPM)/5252
How could this equation be true for every engine? There are SO MANY different engine designs: different bore x stroke, different compression ratios, 2 large valves vs 4 small valves per cylinder, different camshaft profiles... all these factors will affect the output of torque vs HP at different RPMs and different throttle settings. Specifically I am thinking of the long-stroke Harley-Davidson motorcycle engines that produce loads of torque off-idle vs an equivalent displacement Honda 4-cylinder motorcycle engine that produces comparatively little torque off idle but way more peak HP than the Harley. In this scenario the Harley has high TQ at low RPM, which would generally equal a high HP number. And the Honda has a low TQ at a low RPM, which would generally equal a low HP number but the Honda ultimately produces more peak HP. So again it does not make sense to me.
What am I missing here? Someone please explain, preferably in the simplest terms possible.
Thank you!
In the simplest terms without using much math...
A long time ago, folks settled on the idea that a horse could pull a rope connected to a pulley and lift a 550 lb weight at constant rate of 1 foot per second. Since there's 60 seconds per minute, that converts to an equivalent of 33,000 pounds per minute because 550 x 60 = 33,000. This calculation is for things moving in a straight line. Engines have shafts that rotate so, conversions must be done to convert linear speed to rotational speed. The circumference of a round shaft is 2 x Pi x radius (Pi = 3.1416).
When all is said and done, the 5252 comes from 2 x Pi / 33,000 (because 2 x 3.1416 / 33,000 is roughly equal to 5252). Think of it as a conversion factor from linear motion per second to rotational motion per minute.
Thus: Horsepower ends-up equaling (Torque x RPM) / 5252.
Taking it a little further, Torque = Force x Distance. ( Example: Think of a 1 foot long wrench with 5 lbs of weight at the end. The torque will be 5 ft lbs).
Horsepower is the rate at which you can apply torque and is: (Force x Distance) / Time.
In other words, Horsepower = Torque / Time.
Your train of logic goes off the rails when you get here. You are equating torque to work done, and that is incorrect. To convert torque to work done, multiply torque by 2*pi. For example, if an engine is producing 100 lb*ft torque, and rotates through 2 revolutions, then it has done 100 lb*ft x 2 revolutions x (2*pi) radians/revolution = 1256 lb*ft of work
The definition of Horsepower was originated by James Watt when he was developing steam engines to pump water out of coal mines in England. He put a scale on a horse pushing a capstan on a pump and measured 181 pounds of force while covering 180 feet per minute. Multiplying, the work done per unit time was 32,580 lb*ft/min, which Watt rounded up to 33000, and called a Horsepower. The creators of the Metric system honored James Watt by naming its unit of power the Watt, which is a Newton*meter / second.
I guess it depends on which history book you want to believe. Watt had a very hard time selling the notion of horsepower and it's quite possible there are different explanations of why it came about. In your example, you've gone from arbitrary revolutions to radians. In my example I'm showing that the constant 5252 is the rounded value of (33,000 ftâ‹…lbf/min)/(2Ï€ rad/rev) but I'm not showing the details of conversion because most folks have a hard time understanding radians (as evidenced by the countless freshmen and sophomore college kids I taught this class to).
https://en.wikipedia.org/wiki/Horsepower
Horsepower (hp) is a unit of measurement of power, or the rate at which work is done. There are many different standards and types of horsepower. Two common definitions being used today are the mechanical horsepower (or imperial horsepower), which is about 745.7 watts, and the metric horsepower, which is approximately 735.5 watts.
The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, as well as turbines, electric motors and other machinery.[1][2] The definition of the unit varied among geographical regions. Most countries now use the SI unit watt for measurement of power. With the implementation of the EU Directive 80/181/EEC on January 1, 2010, the use of horsepower in the EU is permitted only as a supplementary unit.[3]