Crankshaft and Flywheel

Crankshaft and Flywheel

The crankshaft assembly includes the crankshaft and bearings, flywheel, harmonic balancer, gears, and front and rear oil seals.

The crankshaft converts the reciprocating motion of the pistons to rotary motion of the crankshaft. All the power produced by all the cyliders is transferred to the crankshaft. The crankshaft transmits it to the flywheel or torque converter.

The flywheel or torque converter helps the engine to run smoothly by absorbing some of the energy during the power stroke and releasing it during the other strokes.

The vibration damper (harmonic balancer) dampens crankshaft torsional vibrations that result from the power impulses. As each cylinder fires, it causes the crank throw for that cylinder to speed up. The rest of the crankshaft tends to stay slightly behind, causing a twist. This causes torsional vibrations, which are dampened or partially absorbed by the vibration damper.

The crankshaft is supported by split-type (two-piece) precision bearing inserts that reduce wear and friction.

The front and rear crankshaft seals prevent oil leakage past the rotating crankshaft.

Crankshafts are of forged steel construction and induction-hardened for durability and wear resistance.

A flange at the rear of the crankshaft provides the means for mounting the flywheel or converter drive plate.

The flywheel contributes to the uniform rotation of the crankshaft and helps the engine overcome loads when starting the automobile from rest and also during operation. Even though the power impulses of a multicylinder engine follow each other or overlap, additional smoothing out of the power impulses is desirable. The engine flywheel does this job. The flywheel is a relatively heavy metal wheel which is firmly attached to the crankshaft. Because of its rotation the flywheel acquires kinetic energy; when the flywheel speeds up, it stores additional kinetic energy, and when it slows down it gives back that energy. The amount of energy which a flywheel will store for a given change in speed depends on its inertia, which, in turn, depends on its mass and its effective diameter. The energy which the engine pistons deliver to the crankshaft fluctuates, being greatest when a piston has started on its power stroke, much less on the exhaust and suction strokes, and negative during the compression stroke. These fluctuations in energy to and from the crankshaft.

Cause corresponding fluctuations in its speed; the effect of the flywheel is to reduce the speed fluctuations by storing energy when the crankshaft accelerates and giving it back when the shaft starts to slow down. The heavier the flywheel or the larger its diameter the smaller will be the speed changes.

The flywheel resists any sudden change of crankshaft (engine) speed. Thus, when a power impulse starts (with its initial high pressure), the crankshaft is given a momentary hard push (through the connecting rod and crankshaft). But the flywheel resists the tendency of the crankshaft to surge ahead. Thus, the momentary power peaks are leveled off so the engine runs smoothly.

Since the flywheel also serves to form part of the engine clutch, its rear face is thoroughly machined. In the front face of the flywheel, there is a shallow indentation used to determine the position of the piston in the first cylinder. When this indentation is aligned with a special hole provided in the bell housing, the piston is at top dead center (TDC). In some engines, this indentation indicates the start of fuel injection into the first cylinder. The flywheels of some engines also carry marks indicating the serial numbers of the cylinders where the compression stroke occurs. The flywheel marks and indentation are used for setting the valve and ignition systems relative to prescribed positions of the crankshaft.

In addition, the flywheel has teeth on its outer edge; the electric-starting-moto pinion teeth mesh with these teeth when the engine is being cranked for starting.

A flywheel ordinarily is mounted near the rear main bearing. This is usually the longest and heaviest of the main bearings, since it must support the weight of the flywheel.

The purpose of the flywheel is to assist the engine to idle smoothly by carrying the pistons through parts of the operating cycle when power is not being produced.

The heavier the engine flywheel, the smoother the engine will idle. However, because of its inertia, an excessively heavy flywheel will cause the engine to accelerate and decelerate slowly. For this reasort, heavy, duty or truck engines have large and heavy flywheels. While racing engines or high performance engines have light flywheels.

The rear surface of the flywheel is usually machined flat. This surface is used to mate with one surface of the clutch. With automatic transmissions, where no clutch is used, part of the fluid flywheel or torque converter is attached to and becomes a part of the flywheel.

For an engine of a given horsepower, the energy variations during a complete cycle are greatest if the engine has only one cylinder. Single-cylinder engines, therefore, require large flywheels to keep the momentary speed variations within reasonable limits. In multicylinder engines the energy changes become less as the number of cylinders increases. The reason is that not only are the cylinders smaller but also that their impulses are more frequent and, in the case of engines with many cylinders, one piston delivers power at the same time as another is on compression. Consequently, the required size of the flywheel becomes very small. The cranks, crankpins and large ends of the connecting rods have considerable rotating weight and exert the same inertia effect as a flywheel. So does the rotor of a connected electric generator. Therefore, in some large multicylinder engines, flywheels are not necessary and hence are not used.