Flywheels

The flywheel is a large and heavy carefully balanced disc, bolted onto the rear of the crankshaft to act as a reservoir for mechanical energy created by the power strokes of the engine.

The large mass of the flywheel assists in maintaining the crankshaft turning at a steady rate in between power strokes in order to assist smooth running of the engine. This is especially important at low running speeds when the time between strokes is longer. The mechanical energy stored by the flywheel is also used to assist each piston back up the bore and during induction, compression and exhaust strokes, when additional forces are imposed onto the crankshaft.

Road cars typically feature flywheels made from cast iron however those converted to racers commonly use a lighter billet steel alternative with aluminium generally being reserved for out-and-out racers.

Pictured below is a standard flywheel and alternative lightened version both from the BMC A-series engine;



The flywheel must also assist in carrying drive from the engine to the gearbox, via the clutch, which on a manual transmission vehicle is bolted to it and provide a flat, true surface on which the clutch friction plate can engage and disengage. Provision is also given to carry the ring gear around its circumference upon which the pinion of the starter motor engages in order to turn and start the engine.

Larger engines with a higher number of cylinders are not as dependent on the action of the flywheel owing to the increased number of power strokes per crankshaft revolution. Without the flywheel, the engine would have to reach a considerable amount of RPM in order to pull away without stalling.

Provision is given so that the flywheel may only be fitted in one position and therefore will not run out of true with the crankshaft, which would set up a heavy vibration placing components under much undue stress. However, a very small amount of imbalance is tolerable. The flywheel and crankshaft are ideally balanced as one, but this is not always possible. 

A more recent feature on today's modern engines, with their higher torque and power outputs, especially diesel engines is the dual-mass flywheel (DMF). The DMF features two flywheels, the input, bolted onto the rear of the crank as per a standard flywheel, and an output driven by and linked to the input flywheel by means of very strong springs, rubber, or a combination of the two in order to further dampen any vibrations from the engine and thus provide a smoother take-off of power upon moving off.

However, failure rates of dual-mass flywheels are much more common than the single-mass flywheel, which provided is not abused, is virtually indestructible!. For this reason, several owners of vehicles featuring a dual mass flywheel have where possible, converted to a single-mass flywheel.

Assembly of a typical dual-mass flywheel

Flywheels may be lightened by either turning material off the back face or by drilling holes around the periphery. Alternatively, a lightweight alternative to the standard article may be fitted.

A lightweight flywheel is a definite advantage for competition use as it improves the acceleration and rapidity of the gear changes, however, this is hardly a necessary modification unless competition work is envisaged. If it is carried out it should not be overdone or the engine will lose some of its low-speed flexibility and smoothness, and if properly done,this can reduce stresses in the crank.

Careful dynamic balancing of the crankshaft and flywheel is undoubtedly the most valuable modification that can be carried out on engines that are perpetually driven and high rev/min.,

A lightened flywheel must of course be rebalanced afterwards and the work is best left to experts as haphazard lightening and any resultant severe weakening of flywheels can be extremely dangerous, especially given that on some cars, the driver is sitting directly in line with it!

Crankshafts

The crankshaft is responsible for transmitting engine power, created by the pressure on the pistons and connecting rods during power strokes, to the gearbox and on to the road wheels. This is achieved by converting the linear motion of the pistons into a rotational motion. A typical road car engine will revolve up to around 6000 times per minute. However,on some race engines this figure can be doubled!


Although some crankshafts seem to stand up to very high rev/min indefinitely, breakage is by no means unknown on a tuned engine. Examples of damage from prolonged over-revving being the destruction of timing wheels and chains. Torsional vibration can also be very destructive and cause fatigue to the crankshaft itself. With this in mind, undoubtedly the most valuable modification that can be carried out on engines that are perpetually driven at high rev/min is careful dynamic balancing of the crankshaft and flywheel, preferably as an assembly however this is not always possible.

The shaft is either cast or forged, usually in one piece, however on certain engines including many pre-war Bugattis, the crank is built up in sections. The shaft consists of the journals, which rotate in and are supported by the main bearings, and the crankpins which rotate in the big end bearings of the connecting rods. These in turn connect the crankpins to the pistons.

The crankpins and journals are joined together by the webs, which on many engines also act as counterweights creating a counterbalance mass to that of the crankpin. The excessive loads would otherwise overload the main bearings at high rev/min. This goes a long way in maintaining smoother engine running. 

Crankshafts are of utmost importance when raising the rev limit of an engine, although in race applications, it is usually the last component to receive attention. With this in mind, cast iron crankshafts are normally substituted for a forged steel alternative as these tend to be stronger and less prone to wear and breakage. A more expensive option is the use of a billet crankshaft.

A fillet or radius is formed around the area where the webs and journals join in order to eliminate sharp corners, which would be a point of weakness that could lead to fatigue, or even breakage of the crankshaft. This is of particular importance in crankshafts that are subject to extremely heavy loads or significant stresses i.e race engines. The distance between the centre of the crankpin and that of the main bearing journal is referred to as either the "crank radius" or occasionally the "throw".

Pictured below is a diagram showing the basic construction of a crankshaft from an in-line four-cylinder engine.

Oil travels under pressure from a pump and/or by means of a "splash system" (which will be described later in a separate chapter on engine lubrication). Oil flows through drillings in the crankshaft running from the centres of the main bearing journals to those of the crankpins, in order to provide continual lubrication to the bearings and relevant surfaces. Some crankshafts feature hollow journals and crankpins to save weight, and these must obviously use seals in the form of caps or plugs bolted to their ends to prevent oil leakage.

Oil is retained within the crankcase by means of seals or/and retainers at either end of the crankshaft housing. Two commonly used methods are lip-type seals and scroll-type retainers. The former is used on most modern engines and has been in use for many years. It consists of a circular steel strip encased in a synthetic rubber which fits into a recess between the shaft and crankcase (rear) or timing case (front) and is held into place using a "garter" spring and sometimes retro-fitted to older engines in place of, or in addition to, the traditional scroll and flinger ring oil retainer. Scroll-type oil retainers use a thin metal ring, formed around the shaft, and therefore rotating with it. Any oil that reaches it is flung off by its rotational motion and drained back to the sump. Behind the flinger ring is the scroll, a helical groove machined into the crankshaft and housed inside the stationary rear housing, with a corresponding helix. This works very much like the groove of a thread or screw, bringing back into the crankcase any oil that should reach it as it turns in the stationary housing.

Pulleys and/or vibration dampers, along with gears or sprockets, are usually fitted to an extended section of the front end/nose of the crankshaft in order to drive the camshaft and components such as cooling fans, alternators/generators, power steering pumps and air conditioning compressors via belts, or in the case of the camshaft this can also be achieved by means of chains or gears. Pulleys and gears are generally located onto the end of the shaft by means of a tapered seat or a woodruff key and held in place by a large nut, or bolted onto the shaft, depending on the application.

A vibration damper typically consists of a metal disc with a ring of rubber bonded onto it. This helps to control torsional vibration, which is a slight twisting and untwisting of the crankshaft caused by the downward thrusts of the pistons during power strokes, which in turn causes sudden thrusts on the crankshaft.

Crankshafts with a longer throw/increased radius can be used to increase engine capacity. However, the connecting rod lengths must be reduced in order to make this work. This method is known as "stroking" and increases torque outputs considerably but can sometimes slightly reduce the safe rev limit of an engine. Many years ago, engines featuring very long strokes were the norm and excessive piston speeds were known to exert heavy loads onto both main and big-end bearings, with the final result being their imminent failure. However, the big-end and main bearings on today's high-revving modern engines tend to normally only fail due to lack of lubrication caused by poor maintenance, rather than the excessive loading previously mentioned.

The flywheel is a large and heavy carefully balanced disc, bolted onto the rear of the crankshaft to act as a reservoir for mechanical energy created by the power strokes of the engine. This is achieved by maintaining the crankshaft turning at a steady rate in between power strokes in order to assist smooth running of the engine. Provision is given so that the flywheel may only be fitted in one position and therefore will not run out of true with the crankshaft, which would set up a heavy vibration placing components under much undue stress. However, a very small amount of imbalance is tolerable.

The clutch shaft spigot bearing or bush is also included in the rear end of the crankshaft and in simple terms, its job is to locate the gearbox input shaft, carrying the clutch driven plate.

A more modern method of driving ancillary components is to do so from the rear end of the crankshaft, by means of a READ (rear end accessory drive) system, using gears from the crankshaft to drive components such as the alternator, from a common power take-off. One such manufacturer to employ this technology, among others, is Volvo and its primary focus is to save space by creating a more compact unit.

Popular materials used in the construction of crankshafts include steel and iron and they are manufactured by means or either forging or casting. Forging is a process where the material is formed and shaped by means of heating and then compressing or beating into shape, making for a stronger construction. Casting involves pouring heated liquid metal into moulds and has the added bonuses of stiffness and a more lightweight construction that requires considerably less machining than the forged counterparts and produces a stiffer end product. This is of great importance when we consider the short throws and large journals found on modern crankshafts. Upon completion, the shaft is then ground in places to a very smooth and accurate surface (sometimes an accuracy of less than one thousandth of an inch/0.025mm) with high-performance crankshafts being machined all over, further reducing oil drag. 

Nitriding and induction hardening are methods used to prevent both torsional oscillation and shaft flexing due to constant load. Nitriding is where steel is mixed with nitrogenous metals and heated to around 500° for several hours in an ammonia filled atmosphere, which helps to prevent corrosion and increase the hardness of the shaft. This is achieved as the nitrogen from the ammonia is absorbed by the steel shaft, forming a hard iron nitride surface.

The electrical process of induction hardening involves the hardening of a steel surface by means of exposing the material to a magnetic field, causing it to heat, and then cooling it rapidly by quenching, usually done via a water spray.

Engine Balance

When talking in terms of engine balance, there are two main states, primary and secondary balance. In it's simplest form, primary balance refers to counter-acting the inertia forces created by the sudden change of direction when a piston passes both TDC and BDC as the piston momentarily attempts to carry on in the previous direction. The amount of force exerted on the piston increases with engine speed.

On an in-line four-cylinder engine, pistons work in sets and the downward (negative) inertia of one set, counter balances and therefore cancels out the upward (positive) inertia of the other. Counterweights are added to reduce twisting action imposed onto the crankshaft by opposing forces and to reduce stress on it's centre main bearing although most engines nowadays use five instead of three crankshaft main bearings in order to provide a stiffer construction.

Single-cylinder engines obviously are unable to use this method as there is only one piston moving up and down and therefore the force, for this reason a counterweight is used on the crankshaft that rotates in the opposite direction of the piston however, the crank and piston assembly will not be balanced horizontally as both the crankpin and connecting rod are travelling in the same plane.

In-line three cylinder engines have the large force of the piston at TDC or BDC cancelled out by the smaller forces of the other two pistons. This also applies to both in-line and vee six cylinder engines.

Secondary balance refers to forces that occur twice during the same revolution as the primary force, which only occurs once per revolution. The distance travelled by the crank and piston assembly during the same amount of time is greater from the top of the rotation (TDC to 90° and 270° to TDC) than it is between  the bottom of the rotation (90° to 180° and 180° to 270°) meaning that the piston is moving faster during the top parts of the rotation. This is what creates secondary imbalance as there is a stronger upward force at TDC and a weaker downward force at BDC. These forces can, depending on engine layout, be cancelled out by adding extra piston or counterweights. Some engines employ a harmonic balancer for this purpose although soft rubber engine mountings are usually used due to cost.

Diagram displaying engine balance

First used in 1911 by Frederick Lanchester to balance out secondary imbalance on in-line four cylinder engines, Mitsubishi also produced a secondary harmonic balancer in 1975 not dissimilar to that of Lanchester, which is now being made and sold under licence to a variety of vehicle manufacturers and engine builders including Saab and Porsche. V4 engines generally tend to incorporate a harmonic balancer in order to keep vibration at an acceptable level. 

The harmonic balancer itself consists of two counter-weighted shafts both taking drive from and timed to the crankshaft. These shafts turn clockwise and anti-clockwise respectively to one another and exert a downward force at TDC. The opposing force of the balancer must be exerted only when needed i.e, when at TDC the shafts must be pointing downwards and at times of low force they must provide a neutral effect as the engine is already in a satisfactory state of balance. 

Operating principles of a secondary harmonic balancer

Aside from primary and secondary balance, all rotating components/masses must also be balanced in order to minimise any untoward vibration such as the complete crankshaft,flywheel and clutch assembly which although ideally dynamically balanced as one assembly, are usually balanced individually due to reasons of cost and locating devices used to ensure the flywheel and clutch run in-line with the crankshaft axis. Removal of metal from the opposing side of a component or/and drilling holes to reduce weight at the heavy point are common methods of correcting imbalance. Pistons and connecting rods can be balanced by ensuring all of their individual weights are equal.

Power strokes of multi-cylinder engines must be regular in order to reduce vibration and the more power strokes in any given 720°  four-stroke cycle, the smoother the engine, torque output and delivery of power from the engine to the road wheels.

Sources of power for motor vehicles

Although the internal-combustion engine is by far the best and most efficient way of powering vehicles, both steam engines and electric motors have been used although the steam engine is an external-combustion engine and therefore requires a huge boiler in order to produce steam along with the actual engine. Also, the steam engine is rather bulky and comparatively inefficient due to considerable heat losses from both the engine and boiler, not also to mention that this type of engine continues to consume fuel quite for quite some time after it has been stopped in order to maintain steam pressure, whereas, the internal-combustion engine only uses fuel when running.

Electric motors are increasingly being used to insist the internal-combustion engine such as in hybrid applications although we are also seeing the increase in cars that are ran solely by electricity. However, this has actually been around for quite some time although only in common use for the last 15 years or so as technology has improved and associated components have became considerably lighter and less bulky.

Other drawbacks of electric cars are that they have a limited operating range and therefore require fairly regular recharging, hence the continual increase in "plug-in points" for these vehicles and also the fact that they are limited on speed

A drawback of the internal-combustion engine is that it can take considerable time for it to run at its peak temperature and hence performance whereas the external-combustion engine is still emitting a small amount of steam in order to relieve pressure in the boiler and by opening of a small valve, it will be able to work straight away as like an electric motor can simply just be switched on and used.

Motor Car Evolution

Right back in times immemorable, humans had no alternative other than to carry heavy loads themselves which was not particularly pleasant on the body and both speeds and load handling abilities were severely limited. The only alternative to this was the use of domestic animals such as Horses and Donkeys, which were more often able to carry more heavy loads at a faster pace than humans were able to achieve, with people travelling in comparative comfort.

Theories have existed that sledges and even rounded tree stumps had been used as a basis to transport loads before the invention of the wheel for example; how were the ancient rocks of Stonehenge not only erected on site but how did they get there? Studies have been carried out and the rocks are believed to be native to an area of Wales and some believe they may have been placed on top of cylindrical shaped tree stumps horizontally and pushed!

Fast forward several years and we had wheeled chariots and carts which were obviously able to carry considerably more weight than their predecessors with a drawback being that the wheel needed - and still does - a relatively smooth surface on which to run without getting into ruts and abnormalities in the surface etc. Therefore, as the vehicle has evolved, so have roads.

Initially, steam engines were used to power wheeled vehicles although only a small handful were successful due to weight, low speeds and legislation, however, one successful application was in traction engines which although very slow, noisy and polluting, were able to carry more substantial loads than any road vehicle before it. One of the greatest engineers and evolutionaries of these type of engined vehicles was Cornishman Richard Trevithick 1771-1833, an inventor and mining engineer of whom produced many "steam carriages" several of which were used to assist the mining industry and process in Cornwall amongst other things.

Richard Trevithick

Cylinder Layout & Firing Order

Over the years, there have been several engine designs including different layouts for the cylinders with the most common being of the in-line variety. However the cylinders may also be opposed either in a Vee, W or horizontally.

The pistons of any engine must have their power strokes in succession, this is referred to as the firing order of the engine, which is determined by two main factors including the crankshaft design, which will determine all the possible firing orders and the webs, which are designed in such a way as to provide the best possible balance and to ensure that regular firing strokes occur. The cams on the camshaft must also be arranged in such a way as to adhere to one of the possible firing orders.

Power strokes in an in-line four cylinder engine occur at 180° intervals and the pistons move in pairs with one and four forming one pair and two and three forming the other. For instance,in a four cylinder in-line engine with a firing order of 1,3,4,2, if piston two is on induction stroke, piston three will be on its power stroke, and pistons one and four on their compression and exhaust strokes respectively.

The power strokes of an engine must be spaced at uniform intervals with every interval being equal to the number of degrees per engine cycle divided by the number of cylinders. For example, the calculation used to determine the number of degrees between strokes on a four cylinder is as follows;

Number of degrees per engine cycle/number of cylinders

(720°/4) = 180°

On an in-line four-cylinder engine, the two possible firing orders are 1,3,4,2 or 1,2,4,3 as found on some engines. If the firing order were to be 1,2,3,4, the crankshaft and engine mountings would be subject to such high levels of vibration and stress that it would be unbearable to the vehicle occupants and the engine components would very soon wear out under such high levels of fatigue.
The firing order can be found in workshop manuals and various manufacturers literature. It may also be marked in a prominent place on the engine itself. This my also be defined by turning the engine in its normal direction of rotation with the rocker/camshaft cover removed and watching the order of which either the inlet or exhaust valves operate or to note the order by which the cylinders create pressure on compression stroke by means of turning the engine with the spark plug holes either covered by thumb or finger or appropriately plugged. This test can also be performed using a cylinder pressure gauge.

The cylinders of a straight engine are formed in a straight line parallel to one another and may be opposed either vertically  or slanted at an angle such as those found in early Saabs, the Hillman Imp and certain Triumph models. Owing to the reasonably spaced power strokes, this type of engine is relatively smooth in operation and any vibration or harshness is largely unnoticed by the vehicle occupants as rubber engine mountings damp out much of this.

In the most common cylinder layout found in the motor vehicle, the aforementioned in-line, the number of cylinders varies from design to design with four being the most popular however many manufacturers are now moving to three-cylinder engines for reasons of reduced emissions and compaction. In the past however, some designs have used as little as two cylinders and as many as eight as the straight eight engine was much cheaper to manufacture than its V8 counterpart. But this design was superseded many years ago. Other numbers in common use are five and six cylinders but again, the straight six engine has largely given way to the V6 although it is still in use by certain car makers.

How the number of cylinders effects engine behaviour

In its simplest form, an engine has a single cylinder, although this is not suitable for motor car applications as the torque would be very uneven due to the fact that there is just one power stroke for every two crankshaft revolutions and the vibration that would occur as a result. This design is however commonly used in motorcycles and some microcar applications. One example of which is the Villiers engine as shown below;

The minimum amount of cylinders required to provide acceptable levels of vibration and harshness in motor cars is two, giving one power stroke per crankshaft revolution although, compared to engines with larger numbers of cylinders, the vibration at low speeds is still very much noticeable. In the case of a single-cylinder engine, even a large flywheel designed to store this energy would be inefficient in giving smoother running at lower speeds. 

Motor vehicles today use typically anything from 2 to 12 cylinders, all of which can be arranged in a number of ways. The average torque value of a four-cylinder engine will be greater than that of a single-cylinder engine of the same displacement however, the maximum torque value generated by each individual cylinder would be significantly lower than that of a single cylinder engine of the same displacement.

Torque delivery is much smoother with multi-cylinder engines owing to that fact that the more cylinders an engine has, the more power strokes per crankshaft revolution. Multi-cylinder engines also have the added advantages of being safer at higher speeds, a greater ability to develop more power and have longer lives due to less uneven torque and vibration. Although multi-cylinder engines are essential for smooth running in motor car applications, there is the obvious drawback of the fact that they are more complicated in their design due to the increased number of parts and overall cost of manufacture.

Traditionally, more cylinders meant more power however nowadays, manufacturers are continually moving towards the trend of smaller engines due to emissions laws. Thanks to modern technology, smaller engines can now produce much higher power outputs and it is not unheard of for these aforementioned engines to produce more power than older engines of more than twice their size or capacity.

In certain cases, high-performance supercars have now taken to using W-engines, incorporating up to 18 cylinders

A W16 engine of the Bugatti Veyron