How it Works: Crankshaft and Flywheel

Crankshaft and Flywheel

When a piston engine is used to propel a vehicle, one of the first engineering problems encountered is how to gain the rotary power necessary to drive the wheels from an engine which produces linear power as its pistons rise and fall. The problem is solved by means of a crankshaft. This shaft, connected to both the piston assemblies and the transmission, converts the vertical movement of the pistons to rotary drive so that it can be transmitted to the road wheels. Apart from transmitting the drive, the crankshaft also performs several other functions. These include carrying the camshaft drive, the fanbelt pulley and the gear which is engaged by the starter motor.

Crankshaft Form and Function

The form which the crankshaft takes varies depending on the type of engine to which it is fitted. All designs are, however, basically the same, as the function they have to fulfil is always the same. The crankshaft carries the piston assembly, or assemblies in multi-cylinder engines, and is fitted at the base of the block (fig. 4) in all but horizontally-opposed engines, where it is at the centre. In single-cylinder engines the crankshaft has only one piston assembly bearing on it and hence has only one crankpin. The crankpin is offset from the axis of the shaft (fig. 6) to allow the con-rod room to move up and down at the same time as it rotates. The offset crankpin is called a 'throw'.

On either side of the crankpin are the main bearings which locate the shaft in the engine block as well as allowing it to rotate. Thus the shaft has a characteristic U-shaped design. This U-shaped design enables the crankshaft to convert the essentially linear motion of the piston assembly to rotary drive. For as the piston moves down the cylinder, the big-end of the con-rod pushes the crankpin, to which it is attached, down and to one side (fig. 3), rotating around it at the same time. When the piston begins its upward travel the big-end is pushed round and up by the crankpin, again rotating around it at the same time. The result of this is the rotation of the crankshaft in its main bearings and the transmission of circular drive to the vehicle's wheels.

Bearing and Throw Arrangement

The crankshaft in a single-cylinder engine is the most simple in form. Most vehicles, however, have four cylinders or more and consequently the crankshafts in these engines are much more complex. The two main variables in the form of multi-throw crankshafts are the location and number of main bearings, and the orientation of the throws. A certain number of main bearings are essential in order to securely mount a multi-throw crankshaft to prevent it whipping or distorting under the loads imposed on it by the piston assemblies. Traditionally this usually meant bearings being placed on either side of each crankpin, thus giving five main bearings on the shaft of a four-cylinder engine.

However, with the use of stronger alloy steel shafts and lighter piston assemblies, sometimes two con-rods bear on two adjacent crankpins which have no bearing between them. The shorter shafts possible in flat and V engines also allow bearings to be omitted without losing the necessary rigidity. The way the throws are sited is an essential part of engine balancing. This is because to smooth out the lumpiness of the four-stroke cycle the piston assemblies do not rise and fall together, but are staggered. This has the effect of balancing the weight of one piston assembly against that of another. Consequently the crank throws have to be sited in a pattern consistent with the engine's firing order.

In a four-cylinder in-line engine, the crank throws are arranged with the centre two throws at 180 to the end two throws. This arrangement usually has five main bearings, giving a very rigid shaft, although three-bearing arrangements do occur, particularly on smaller-engined vehicles. The horizontally-opposed four-cylinder layout has a crankshaft design similar to that of the in-line engine, but because the pairs of pistons face each other the shaft is shorter and more rigid and often has only three bearings. The crankshaft in a V-four engine is again shorter than its in-line equivalent. The throws are usually at an angle of about 90 to each other, as are the two cylinder blocks. Six cylinder engines follow the same general pattern as four-cylinder engines and consequently the crankshafts are similar - simply longer.

With a six-cylinder in-line engine the crankshaft throws are arranged so that the two end pistons rise and fall together, as do pistons two and five, and three and four. This results in the throws being spaced around the shaft at angles of 120 to each other. As these shafts are long they usually have a full seven bearings to give a maximum of support. Because it is hard to accommodate the width of a flat six-cylinder engine in a car, they are almost never used. Instead, the alternative to the in-line six-cylinder is the V-six layout. The V-six has the advantage over the in-line arrangement of being much shorter. It is sometimes supported by four bearings, but seven is more common as this arrangement offers a much greater degree of rigidity and less vibration. The throw arrangement is at 60, which is the usual angle at which the cylinder blocks are aligned.

The V-eight has similar advantages to the V-six in the sense that a relatively short and rigid crankshaft can be used for a large engine. Two cylinder banks at an angle of 90 are used, and thus the throws are arranged around the shaft at 90 or 180 intervals. This effect is in fact doubled up because there are two pistons on each crankpin, each piston on one crankpin entering a cylinder on facing sides of the block. This results in the V-eight crankshaft having only five main bearings.

Crankshaft Balancing and Damping

While the firing order helps to even out the vibration inherent in the four-stroke cycle, it does not do the whole job. Some of the lumpiness is removed by the provision of a fly-wheel at the back end of the shaft. This is simply a heavily weighted disc which, once started spinning by the engine, tends to keep going and thus evens out the power delivery. Apart from engine balancing, the crankshaft itself needs balancing too. This is because the throws on the crankshaft are offset from the axis of the shaft and their motion creates an out-of-balance effect. Further vibration is caused by the con-rod big-ends rotating around the crank journals.

This vibration is controlled by the casting of webs opposite the crank throws. The webs are of a weight which matches the thrust effect on the crankpin as the piston assembly drives it round. Their size and weight thus varies depending upon engine size and type. The webs on a shaft in a flat engine are usually quite small, for instance, as these engines are so well balanced that they do not vibrate much. As well as the out-of-balance effect, the crankshaft also suffers from torsional or twisting vibrations. This is due to the force of the piston assembly forcing round the crank throws, and the shaft's tendency to twist back again as the pressure ends at the end of the power stroke.

In order to prevent these vibrations from becoming too severe a vibration damper is fitted to the end of the shaft. A typical damper consists of a small fly-wheel bonded to the pulley on which the fanbelt runs by an insert of rubber about 6 mm thick. The rubber flexes as the shaft twists, effectively dampening any vibration. While most crankshafts have both webs and vibration dampers to deal with vibration, another method of balancing is used only on particular engines. This method uses a weighted counter-balancing shaft which is attached to the crankshaft through gears. It is standard on V-four engines but is also used on some twin-cylinder engines such as those used on motor-cycles.

Crankshaft Bearings

While crankshafts themselves are made from tough alloy steels, usually nickel chromium molybdenum steel, and the journals are specially hardened, the bearings are made of a softer metal. This means that the cheaper and more easily replaced part wears out first. The bearings take the form of shells inside both the big-ends and main bearings. All are basically similar except one of the main bearings, which is called the thrust bearing. While the other main bearings prevent the shaft from moving up or down, the thrust bearing stops the shaft moving forwards or backwards. In order to do this it is U-shaped while the others are flat. It bears not only on the shaft journal but also on the crank flanges as well. The position of this bearing is variable, but on large V-engines it tends to be the centre bearing, while on in-line engines it is usually at the rear end. Where there is no thrust bearing as such, thrust washers are fitted alongside a main bearing. These fulfil an identical function.

Bearing Lubrication

In order to prevent excess wear of the journal and bearings, they must continually 'float' on a thin film of oil. So the shaft has a carefully designed oilway drilled through it, and the bearings are sometimes grooved. The oil pathway through the shaft has to be specially shaped to avoid oil sludge building up and clogging the channels under the influence of centrifugal force. The bearings are grooved to allow oil to pass all the way round, and to flow regardless of the rotational position of the shaft. The oil also has two other functions: to cool the bearings as it passes over them, and to flush away any deposits or swarf that may be present.

Additional Functions of the Crankshaft

As the first part of the drive shaft, the crankshaft fulfils other functions than simply passing on the drive. The front end of the crankshaft protrudes through a main bearing and oil-seal and carries the sprocket which drives the camshaft which in turn operates the valve gear. Also at this end is the pulley on which the fanbelt runs and this drives the generator, fan and water pump. At the rear end of the shaft is another main bearing and oil-seal and the shaft again protrudes to end in a flange, to which is bolted the flywheel. While with better engine balancing, the size of the flywheel in modern engines has been reduced, it does have other uses. It forms an integral part of the clutch mechanism, with the face away from the engine driving the drive plate of the clutch. This plate, essentially a friction disc, allows the power of the engine to be passed progressively to the transmission system to allow the vehicle to pull away from rest.

An equally important function of the flywheel is its part in the starting of the engine. Around its perimeter is a toothed edge. This cast, toothed ring is engaged by the pinion on the starter motor to turn the crankshaft and start the engine. The ratio of the teeth on the flywheel to those on the starter pinion is high enough to enable the starter motor to turn the engine over fast enough to start it.