Re: How to explain Horsepower?

Posted by Pooh Bear on November 24, 2014 at 17:44:54 from (66.11.243.192):

In Reply to: How to explain Horsepower? posted by jon on November 23, 2014 at 17:10:17:

I did this write-up for another forum several years ago.

Everyone has provided a lot of great info about horsepower and torque,

but nothing as where it is derived from in an engine.

It's actually quite simple. From a 19th century steam engine book:

Horsepower = PLAN/33000

P = Pressure (Mean Effective Pressure, MEP)

L = Length of Stroke

A = Surface Area of Face of Piston

N = Number of (Power Strokes) per minute

The formula may have originally been for steam engines

but it also works for internal combustion engines.

Some additional info:

Mean Effective Pressure (MEP).

At the top of the stroke a pressurized gas is introduced into the cylinder.

The pressurized gas can be steam, or it can be pressurized by sealing an

air/fuel mixture in the cylinder and igniting it. If steam is used, a valve is

opened momentarily to admit steam, then closed. The steam pushes the

piston and expands the volume of the cylinder. The piston pushes the

crank rod which pushes the crank which rotates because of the torque

produced by the crank angle and the force of the crank rod.

If an air/fuel mixture is used in an internal combustion engine the same thing

happens but the pressure is generated inside the cylinder by the burning

fuel. I'll get to flame speed in a bit as it relates to gas or diesel engines.

If you inject 100psi steam into a cylinder and then close the valve, the

piston will move and the volume inside the cylinder will increase. As the

volume increases the pressure will drop. MEP is sort of an average pressure

as the piston moves from the top to the bottom. MEP is also affected by

the crank angle as the crank rotates it changes the moment arm between

the crank rod and the crank shaft. This also affects torque in the stroke.

It is the same thing in an I.C engine. The fuel ignites and expands to create

pressure. Then this pressure does the work the same as in a steam engine.

Now this is also where flame speed comes in. Someone once said that time

is nature's way of keeping everything from happening at once. This is so true

in the timing of an engine. Everything has to happen in its proper time.

In a steam engine, the crank reaches top dead center (TDC) and a valve

opens to admit steam. But even as the steam is being admitted the piston

is already moving down the shaft increasing the volume. If steam is

admitted at a rate so that the MEP is constant thru the stroke then the only

H.P and Torque being created is being used to overcome friction losses.

You have to admit steam fast enough to keep cylinder pressure up so that

there will be extra H.P and Torque to do useful work from the engine.

In I.C engines where the pressure is generated inside of the cylinder

The fuel will burn at a certain rate (flame speed) until it is consumed.

As it burns the gaseos vapors will be expanding (creating pressure) and

the piston will be moving down the cylinder. When the fuel is consumed the

resulting gaseous vapor will be at its maximum volume and the rest is up to

MEP in the cylinder. Gasoline has a high flame speed so most of the

combustion takes place at the top of the stroke where the moment arm is

smallest. The rest of the stroke is dependant on MEP so torque is lower and

must be produced at higher RPMs burning more fuel. With diesel engines the

flame speed is slower and the pressure is generated thru a great portion of

the stroke. At half stroke the moment arm is greatest and the most torque is

produced. Because of the slower flame speed the torque curve is shifted into

this section of the stroke giving the diesel engine a higher torque rating and

at a lower RPM, making it more fuel efficient.

Horsepower is a unit in the English Standard system and is defined as

33000 foot-pounds per minute, or 550 foot-pounds per second.

Any permutation of these numbers can be used. 1100 ft-lbs/sec = two H.P.

15500 foot-pounds per 30 seconds = one horse power.

It is a derived unit based on amount of work done per unit of time.

Around 1780 James Watt defined the unit arbitrarily by watching horses

power a water pump. Four horses were hitched to a device that walked

them in a circular path. Their walking rotated the device and the rotative

energy was used to lift water from a deep well. Watt studied this process

and kept notes of the volume of water lifted and the speed of the horses.

From this data he calculated one average horse could lift 33000 pounds of

water a distance of one foot in one minute. Or 550 pounds in one second.

Since it is a unit of power per unit time it can be converted to other similar

units. It may sound strange to hear it but my 23hp 8N tractor could also be

said to be a 17151.097 watt 8N tractor. A 200 watt light bulb =

.268204418 horsepower. And one HP = 745.699872 watts.

A watt is a unit in the S.I system (metric) named in honor of James Watt.

Units of power can also be expressed as joules per second, or it can be

(((kilogram*meter/second^2))*meter)/minute, or newton*meter/second, etc.

And horsepower don't mean nothing if you can't put it to the ground.

You can have 3000HP but if you just sit there and spin your tires in the mud

then it don't mean anything. Putting the power to work is what counts.

You can put your power in traction, or operating a hay baler, or brush hog,

or basically anything that requires energy input to recieve a desired action.

How you can use it is what counts. Just like the man that couldn't pull the

heavy boat out of his barn with the 300 hp truck, but the 20 hp tractor

could do it, it's all in how you put the power to use.

And the reason you see the inflated HP ratings on things today is horsepower

sells tractors. Salesmen are counting on you to not know what it means.

Then you get into real horsepower vs developed horsepower.

Developed HP is an imaginary number and basically be ignored.

It's what you can actually use that is what counts.

Pooh Bear (aka Fluff for Brains)

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