what will happen if the gas in a cylinder has a higher pressure than the pressure of the piston on the gas?

Answer 1

Then the piston will retract, and allow the gas to expand until the pressures reach equilibrium*. This is a vital part of the cycle of an engine--without it, the piston would compress the gas, but would never retract with enough force to overcome the internal resistance of the engine (friction, etc.), and thus the engine would stop working. (In a typical gas engine, the ignition of the gas causes an increase in pressure which pushes down the piston and gives your engine its power.)

I'm not an expert on engines, but in a nutshell, during the compression stage the piston compresses the gas (the piston has a higher pressure than the gas), then during the combustion stage the gas pushes on the piston (the gas has a higher pressure than the piston) causing the piston to retract.

Consider our Ideal Gas Law: PV=nRT, where P is the pressure (in Pascals [Pa]), V is the volume (in cubic meters [m³]), n is the number of moles of the gas, R is the gas constant (8.314 J/(K mol), and T is the temperature the gas is at (in Kelvin [K]). (Other units can be used, but will require a different number for the gas constant (R)).

Thus by the piston retracting (increasing the volume the gas can occupy), assuming the number of moles (n) and the temperature (T) don't change, we relieve pressure. Likewise, if this is an engine, the gas probably gained pressure by the change in temperature during the combustion of the gas (since the change in volume is relatively slow, depending on the actual speed at which the piston can retract, the pressure must increase until the volume "catches up"). (In actuality, it's more complex than this, but we'll just assume the gas is ideal, among a variety of other assumptions to make life easier.)

A little bit more in-depth: during the compression stage, the temperature of the gas actually increases. While a gasoline engine still needs a spark to ignite the gasoline, a diesel engine does not--simply adding the diesel to the already hot gases trapped in the cylinder will be enough to ignite it. Diesel will continue to be added to the cylinder for a short interval after that as well, to continue to provide the necessary force and consequent pressure to drive the piston downward. (Remember that Pressure = Force / Area)

*This is assuming the piston has already passed top-dead-center. In normal operation, the piston will already begin retracting (due to the inertia and momentum of the turning engine, and other cylinders) when the gas ignites. However, if the spark plug ignites the gas too soon (namely when the piston is still traveling upward), the combustion will act against the normal movement of the piston due to inertia and momentum. This is very bad and will cause damage to the engine.

Answer 2

This is not possible under normal circumstances. If the gas pressure is increased and the piston retracts, that implies there is net force on the piston, resulting in an acceleration. That is, the piston rod force magnitude on the piston becomes less than the gas pressure times the piston area. This would result in a pressure *gradient* within the gas, but there would still be pressure continuity across the gas-piston interface.

Note I said "under normal circumstances". Now if the gas actually *detonated*, it could cause a shock wave. Shock waves have a pressure discontinuity in the continuum dynamic limit. When the shock wave reached the piston, the pressure in the gas would be higher than in the metal for an instant as the shock entered the metal. This is generally not a good thing to happen in an engine.

Answer 3

you're wording isn't very sparkling, yet once you're declaring that the cylinder rigidity is more advantageous the the piston rigidity then the piston will be forced outward and the cylinder section will enhance.