Why do we need to hit the Stirling engine to start it? We are experimenting on Stirling engine. We are using an alpha-type Stirling engine. Once one starts heating the one chamber, the piston will not move immediately. You can wait for long, but it wouldn't start until you give it a push for the first time. What's the reason for this?
 A: Consider the description of the four stage process, quoted from Wikipedia:


*

*Power piston has compressed the gas, the displacer piston has moved so that most of the gas is adjacent to the hot heat exchanger.

*The heated gas increases in pressure and pushes the power piston to the farthest limit of the power stroke.

*The displacer piston now moves, shunting the gas to the cold end of the cylinder.

*The cooled gas is now compressed by the flywheel momentum. This takes less energy, since its pressure drops when it is cooled.


Notice in step 4, the compression is achieved by flywheel momentum. If the flywheel isn't spinning, (or is moving too slowly) there may not be sufficient momentum to compress the cooled gas. "Hitting the engine" provides this initial momentum.
A: This is due to stiction (static friction) which is a very common problem in piston/cylinder devices that use air to move the piston. When at rest, the piston sticks in the cylinder because the coefficient of static friction is greater than that for dynamic (sliding) friction. When the pressure rises enough, there comes the breakaway point where the piston suddenly jumps into motion. If the pressure does not reach the breakaway point, the piston stays stuck until you hit it to break it loose.
Pneumomechanical actuators called air cylinders are used in mechanical engineering machinery to move cams and levers and to prevent the pistons in them from getting stuck it is necessary to install a spring return which preloads the piston with a force that helps it not get stuck.
The legend in the mechanical engineering world is that everyone's first complex mechanism is powered by air acting in air cylinders, and that none of the tools one designs after that is.
