Physics Photo of the Week

April 6, 2007

Antique Engine Flywheels
These are antique internal combustion engines on "exhibit" at a mining museum in Jerome, Arizona, southwest of Flagstaff.  They had been used in the mining industry - pumping, lifting, conveyor driving, and other chores requiring mechanical energy.

The large flywheels and the concentration of mass near the perimeter of the flywheels illustrates the physics of the photo.  Flywheels are important in any engine to keep the engine moving during the non-power strokes in the cycle.  The large inertia keeps the engine running in a coasting mode until the next power stroke adds kinetic energy.  With wheels and rotating objects, the rotational inertia depends on the location of the mass relative to the axle.  The flywheels posess much more rotational inertia if the most of the mass is distributed on the rim of the flywheel as opposed to near the axle.  The rim of a rotating wheel moves faster than the parts near the center.  By concentrating most of the flywheel mass near the rim, the wheel possesses much more rotational inertia than if the mass were concentrated towards the cneter.

All reciprocating engines need flywheels - lawnmowers, cars, motorcycles, weed eaters, chainsaws, etc.  In practically all modern engines, the flywheel is enclosed in a protective housing, so they are not visible as on the antique engines.

Also notice the hopper on top of the engine.  The hopper holds a reservoir of cooling water.  These engines do not generally have a radiator.  The large amount of cooling water as well as continuing repleshment by the
operators precludes the need for radiators and cooling fans.

In the similar antique engine on the right one can also see the connecting rod that connects the reciprocating piston to the crank shaft of the flywheel.  These connecting rods and crank shafts are all enclosed in modern reciprocating engines as well.  The enclosures ensure protection to the operators of the equipment as well as better lubrication, especially at the much higher speeds of modern engines.

Collectors of antique engines enjoy restoring these machines to working order.  A future Physics Photo of the Week will eventually show videos of similar engines operating. 

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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