18.3. Using Thermal Energy http://www.ck12.org
FIGURE 18.14
A refrigerator must do work to reverse the normal direction of heat flow.External Combustion Engine
An external combustion engine burns fuel externally, or outside the engine. The burning fuel releases thermal
energy that is used to turn water to steam. The pressure of the steam is then used to move a piston back and forth
in a cylinder. The kinetic energy of the moving piston can be used to turn a turbine or other device.Figure18.15
explains in greater detail how this type of engine works. You can see an animated version of an external combustion
engine at this URL: http://science.howstuffworks.com/transport/engines-equipment/steam1.htm.
Internal Combustion Engine
An internal combustion engine (seeFigure18.16) burns fuel internally, or inside the engine. This type of engine is
found in most cars and other motor vehicles. It works in these steps, which keep repeating:
- A mixture of fuel and air is pulled into a cylinder through a valve, which then closes.
- The piston is pushed upward, compressing the fuel-air mixture in the closed cylinder. The mixture is now
under a lot of pressure and very warm. - A spark from a spark plug is used to ignite the fuel-air mixture, causing it to burn explosively within the
confined space of the closed cylinder. - The pressure of the hot gases from combustion forces the piston downward.
- When the piston moves up again, it forces the exhaust gases of combustion out of the cylinder though another
valve. Then the process repeats.