Nick Vetter, age 15, of Portland, Ore., for his question:

WHAT IS A STIRLING ENGINE?

A Stirling engine is an experimental source of power that someday may be used in autos and boats. It runs more efficiently and produces less air pollution than most engines.

A Scottish minister named Robert Stirling invented the engine in 1816. The engines have never come into general use because they cost more to build than other engines.

A typical Stirling engine has a sealed cylinder that contains either helium or hydrogen gas. The gas goes through a cycle of pressure changes by means of a process of alternate heating and cooling.

As the gas expands and contracts, it causes a power piston to move back and forth inside the cylinder. A rod connects the power piston to a crankshaft that converts the back and forth motion to the rotary motion of the drive shaft.