Work exists only in transitional form. Work is equal to the product of the force used to move something and the distance that it is moved. If there is no movement there is no work done.
W = F D
W Work
F Force
D Distance
In a steam engine the pressure of the steam acting upon a piston does work. The force is the product of that pressure and the area of the piston.
F = p A
p pressure
A Area pressure is acting upon
The work done by the piston is equal to the force times the distance the piston moves.
W = p A d
d Distance the piston moves (Stroke)
Since A d is the displacement volume v of the piston. Work can be expressed in terms of the displacement.
W = p v
v Volume displaced by piston.
The above formulas are not too useful as the pressure is not constant throughout the stroke. We can use the following integral:
Work is the energy necessary to move an object over a distance. It is common in this country to measure work in foot-pounds. A foot-pound is the energy required to move a body a distance of one foot when the resistance in the direction of motion is one pound.
As we are interested in heat engines, the conversion of thermal energy into mechanical energy, we need to define heat energy and its measurements. The British thermal unit (BTU) has been defined as 1/180 of the quantity of heat required to raise the temperature of one pound of pure water from 32 to 212 deg. F. at standard atmospheric pressure (14.696 psi). From experimental evidence, it has been found that one British thermal unit is equivalent to approximately 778.26 foot-pounds
1 Btu = 778.26 ft-lb.
Power is a measure of work done in a certain amount of time. Power can be measured in several units ft-lb/sec for example. In the automotive field it is common to use horsepower (HP) as our power unit. 1 HP = 550 ft-lb/sec or 33000 ft-lb/min. 1 Horsepower is equal to 2545 BTU/hour. It is important that we not confuse power with work. Work is not time dependent while power is dependent on the time that it takes to do work.
Another related subject of confusion is torque. Torque while measured in ft-lb, or lb ft to avoid confusion, is a measure of force at a distance from a pivot point. It is a special type of force measured a distance from the center of rotation of a shaft. The force produced by an engine is measured as torque. One foot pound of torque would produce 2 pounds of force at 6 inches from the center of rotation, one half pound of force at 24 inches from the center of rotation. If our engine were to deliver 1000 lb-ft of torque to the driving wheels and the wheels are 24 inches in diameter (2 feet), one foot radius, we would get 1000 pounds of force accelerating the car. How ever if the wheels were only 20 inches in diameter we would have 1200 pounds of force acting on the car. To convert torque to force we must divide the force by the radius, same length units as the torque. At which the force is to be used.