DYNOMOMETERS

Allstate's Of America, Inc. 2595 N.W. 37TH. ST. MIAMI, FLORIDA 33142 USA 800 223 2573, 305 635-4525 Fax 305 635-5353 e-mail Bob@allstates.com http://www.allstates.com

What is a waterbrake?
Some Dynamometers use water as a load mechanism during engine testing. Water flow proportional to desired applied load is used to create resistance to the engine or motor. A controlled flow of water is directed at the absorber center, and via centrifugal force, is expelled towards the outside of the dynamometer and into the stator. As the water hits the stator, it decelerates, causing an applied load to the input device. Through the water's continual acceleration and deceleration, load may be controlled and measured. This is a very effective and safe way to measure the performance of an engine running under a controlled load. The dynamometers feature water as a braking mechanism, (or absorber), hence the name waterbrake.

How do we measure horsepower?

this is the simple formula: Horsepower = torque x RPM

5252

How do you convert engine torque to horsepower?

Have you ever looked at the specs of an engine in a magazine and seen something like "this engine makes 300 pound-feet of torque at 4,000 RPM," and wondered how much power that was? How much horsepower are we talking about here? You can calculate how many foot-pounds of horsepower this engine produces using a common equation:

(Torque x Engine speed) / 5,252 = Horsepower

The engine that makes 300 pound-feet of torque at 4,000 RPM produces [(300 x 4,000) / 5,252] 228 horsepower at 4,000 RPM. But where does the number 5,252 come from?

To get from pound-feet of torque to horsepower, you need to go through a few conversions. The number 5,252 is the result of lumping several different conversion factors together into one number.

First, 1 horsepower is defined as 550 foot-pounds per second The units of torque are pound-feet. So to get from torque to horsepower, you need the "per second" term. You get that by multiplying the torque by the engine speed.

But engine speed is normally referred to in revolutions per minute (RPM). Since we want a "per second," we need to convert RPMs to "something per second." The seconds are easy -- we just divide by 60 to get from minutes to seconds. Now what we need is a dimensionless unit for revolutions: a radian. A radian is actually a ratio of the length of an arc divided by the length of a radius, so the units of length cancel out and you're left with a dimensionless measure.

You can think of a revolution as a measurement of an angle. One revolution is 360 degrees of a circle. Since the circumference of a circle is (2 x pi x radius), there are 2-pi radians in a revolution. To convert revolutions per minute to radians per second, you multiply RPM by (2-pi/60), which equals 0.10472 radians per second. This gives us the "per second" we need to calculate horsepower.

Let's put this all together. We need to get to horsepower, which is 550 foot-pounds per second, using torque (pound-feet) and engine speed (RPM). If we divide the 550 foot-pounds by the 0.10472 radians per second (engine speed), we get 550/0.10472, which equals 5,252.

So if you multiply torque (in pound-feet) by engine speed (in RPM) and divide the product by 5,252, RPM is converted to "radians per second" and you can get from torque to horsepower -- from "pound-feet" to "foot-pounds per second."

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