Terry Jones, age 12, of Costa Mesa, Calif., for his question:

HOW DOES THE DIESEL ENGINE WORK WITHOUT SPARK PLUGS?

There are two main types of internal combustion engines: the spark ignition engine and the compression ignition.

A spark ignition engine uses electricity and spark plugs to ignite the fuel in the engine's cylinders. The diesel engine is a compression ignition engine. When air in a cylinder is suddenly compressed, the temperature of the air rises. In a diesel, air is compressed into each piston and, when fuel is injected, the heat of the air makes it ignite.

Fuel oils, which require less refining and are cheaper than higher grade fuels such as gasoline, are burned in diesel engines. During the combustion process, the stored chemical energy in the fuel is converted to heat, or thermal, energy.