Figure 4.24 shows the power strokes in four- cylinder, six-cylinder and eight-cylinder engines. Power is delivered from the top of a stroke as a power impulse and the impulses are transmitted to the crankshaft.
The crankshaft vibrates because each impulse causes it to accelerate a little, followed by a deceleration as the impulse is reduced. The power strokes on six-cylinder and eight-cylinder engines overlap, and this smoothens out the impulses but increases their frequency.
The flywheel helps to reduce this type of vibration because it tries to keep the crankshaft rotating at a constant speed. The flywheel absorbs energy as the crankshaft tends to speed up, and gives back energy as the crankshaft tends to slow down.
A normal flywheel is a single piece of solid cast iron, but some flywheels have two parts and are designed to absorb torsional vibrations. This design is known as a dual-mass flywheel (Figure 4.25). The front part of the flywheel is connected to the crankshaft and the rear part is connected to it through springs.
When a power impulse tries to speed up the front part of the flywheel, the rear part lags behind. This tends to cancel out the force of the impulse. The opposite effect occurs when the impulse ceases, the inertia of the rear part of the flywheel tends to keep it turning.
A torque converter used with an automatic transmission acts in the same way as a flywheel. It can he considered as being a flywheel filled with fluid. The torque converter and the fluid that it contains provide the mass needed. Also, because torque is transmitted through the fluid at lower speeds, less vibration is transferred through to the drive line.
A vibration damper is fitted to the front of the crankshaft. This is usually part of the crankshaft pulley (Figure 4.26). The damper consists of two main parts:
a hub that mounts on the front of the crankshaft, and an inertia ring (a small flywheel) that is attached to the hub by a bonded-rubber insert. The rubber allows the inertia ring to move a little in relation to the hub. Dual- type dampers have two inertia rings.
The inertia ring dampens out crankshaft vibrations in the following way: When the crankshaft tends to speed up, the inertia ring tends to lag behind to oppose the force producing the vibration. When the crankshaft tends to slow down, the inertia ring tries to keep it moving. In this way, the action of the inertia ring on its rubber mounting helps to dampen out crankshaft vibrations.
Inertia is the tendency of a body to resist changes in motion. For example, a flywheel does not suddenly start to rotate as a force applied, nor does it suddenly stop if the force is removed.