Heat: is motion of molecules

Pressure and Temperature are measures of aggregate motion of vapor molecules. They allow the conversion of motion on a large scale to and from motion on a microscopic scale. The effect is that energy can be stored in or released from a vapor.

A given pressure in a given volume represents an exact amount of force. It is independent of the type of vapor, such as air, or steam, independent of temperature or density. Equal pressures in equal volumes is equal force. (Heat storage varies some between gases, so equal force is not quite equal energy, but for vapors with similar specific heat's, it is approximately equal energy. If you put the same energy into a lower specific heat vapor, it will have a higher pressure/temperature before expansion, and lower after expansion, but you will get the same total energy back for any specific heat.)

Work of engines is often shown as an engine cycle plot on a pressure vs volume plot. Area on a pressure volume plot is a measure of energy or work.

It can be quantified and converted to other forms of energy. 1 Atmosphere is 15 psi (pounds per square inch). It is also almost exactly 10 Newton's per square centimeter. One atmosphere (average) pressure expanding 1 liter in an engine, is 10 Newton's/square centimeters times a liter (which is 10 meters times 1 square centimeters) or 10 * 10 = 100 Newton-Meters aka Joules, aka Watt-Seconds. So 1 atmosphere, expanding one liter, each second, gives exactly 100 watts, or enough energy to light a 100 watt bulb.

At 7+ times per second, 700+ watts or about 1 horsepower is produced.

At 10 times a second 1000 watts or 1 kilowatt is being produced. And so on.

Volume, average pressure, and rate all multiply to get power. For example, instead of 1 atmosphere expanding 10 times a second, 10 atmospheres at 1 a second is still a kilowatt.