13-08-2012, 04:25 PM
Power and Its Measurement
Power.ppt (Size: 797 KB / Downloads: 83)
Objectives
Define terminology related to power and its measurement, i. e. energy, force, work, torque, horsepower, watt
Interpret information on power ratings of agricultural power units to select appropriate power units.
Measure horsepower with a dynamometer and plot horsepower / torque curves
Interpret dynamometer test results and make recommendations for improving power performance.
Terms and definitions
Energy - capacity or ability to do work, can not be created or destroyed, only changes from one form to another
Potential energy
Position- water stored behind a dam
Chemical - motor fuels
Condition - a spring compressed
Kinetic energy - energy in motion,
Water falling over a dam
Rotating power shafts
A spring released
Energy
Example: A heavy weight suspended on a rope contains energy potential. When the rope is cut, potential energy turns into kinetic energy as the weight falls.
When the weight hits the ground, the kinetic energy is dissipated in three ways: sound, heat (absorbed by the soil, and compaction of the earth
Engines convert a fuel’s potential energy into useable kinetic energy. The process generates tremendous heat. Two thirds of this heat is lost to exhaust and engine cooling and only 1/3 of the heat generated is used to power machinery components.
Force
action that attempts to move of an object from a state of rest or causes a change of speed or direction of an object in motion
expressed in terms of weight, e.g. pounds, tons, kilograms
Work
results when force (pounds, tons, kilograms) overcomes a resistance and causes movement of an object over a distance (feet, yards, meters).
The formula to calculate work is:Work = Distance X Force
Work Calculation
If an object that weighs 330 pounds is moved a distance of 100 feet then 33,000 foot pounds of work has been completed
100 ft X 330 lbs = 33,000 foot pounds of work
Note: if no movement results upon the application of force, no work is accomplished.
Motion must occur for work to be calculated.
Horsepower
Standard unit of engine power associated with an amount of work accomplished over specified time intervals and can be expressed in several ways.
Horsepower was originally defined by James Watt in 1765 when trying to compare steam engines with the known power source of that era (the draft horse).
Watt Takes Notes at the Dock
James Watt (1780’s) observed that a draft horse could move a 330 pound weight a distance of 100 feet in a time of one minute.
This is equal to 33,000 foot pounds per minute or one horsepower
This fact has been used as our standard for horsepower measurements ever since.
Horsepower Formulas
There are many formulas for computing horsepower (Hp) but they are derived from the following expression that:
One Hp = 33,000 ft lbs of work per minute
several other mathematical formula have been derived from this basic formula to calculate horsepower