# Inertia forces in an ic engine

## Main Question or Discussion Point

when they say the inertia forces in the engine,what does it refer to ?
this is with respect to the fact that in a diesel engine the point of maximum pressure is at combustion as there are no sufficient inertia forces to cancel out load forces..

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Whenever you have mass moving in a dynamic manner (accelerating, rotating) you encounter inertial forces. Changing the momentum of an object requires that you input a force and that change in momentum is what we call the inertial force. In any engine, there are lots of rotating and translating masses and in order to move them and the load you have to input energy. I think in this case, the point of combustion is at top dead center (ideally) which is, at least conceptually, where the inertial forces are the smallest. If you fired the cylinder once, you would develop some momentum in the piston head, which may be consumed by the load and friction, and any shaft compliance etc. but the engine would quickly come to a halt as all that momentum and kinetic energy stored in your piston head has already changed forms.

But generally speaking the inertial forces would be higher in a diesel engine due to more parts and heavier parts involved than an gasoline engine ,am I right ?

Dotini
Gold Member
The flywheel plays the major role in determining and adjusting the moment of inertia of the reciprocating engine. The amount of the inertia stored is proportional to the square of the rotational speed.

Racing engines will typically have light flywheels so that they may rev up more freely. Trucks will have heavy flywheels in order that they resist stalling at low rpm's.