Archive for the 'My Car From The Road Up' Category

Having covered engine fuel control in the last article we now move on to the ignition system.
 
Prior to the acceptance of ignition electronics the generation and distribution of the high voltage required for the spark plugs was primarily the role of the distributor in conjunction with the ignition coil. By and large it did a reasonable job back suffered from three main drawbacks, 1) high maintenance – the contact breaker set (points) had to be checked and/or replaced at regular intervals, 2) damp – the exposed high voltage contacts in the distributor cap were always in close proximity to earth points and a small build up of moisture guaranteed a misfire or at worse a non-start, and lastly, 3) ignition timing control – to allow the ignition to respond to engine demand two mechanical systems were built into the distributor body, both were susceptible to wear leading to poor performance.
 
With the advent of electronic ignition drawbacks 1) and 3) disappeared as there were no longer any mechanical parts requiring service and the need for ignition timing adjustment through various engine speeds was addressed by programs on a microprocessor. Until the advent of distributorless systems, drawback 2) was always present, however, because engine bays became increasingly full weather resistance also increased reducing the effects of a damp atmosphere.
 
So what do we require of the ignition system? The main requirements are to produce a high enough voltage to generate and maintain a spark at the spark plug sufficient to set the combustion process going and secondly, to provide a system of ignition timing adjustment depending on engine speed and load.
 
Unfortunately, the spark created by the use of 12volts is of no use whatsoever in the extremely harsh environment of the combustion chamber so a much higher energy is needed for an effective burn to take place. The joint efforts of the ignition control unit and the ignition coil work to produce the high energy required generating voltages in excess of 40,000volts. The generation of such a high voltage relies on the transformer principle; basically if two coils of wire (known as the primary and secondary) are wound around a central core any change in the magnetic field generated by the passing of a current through the primary will induce a voltage in the secondary. The change in the magnetic field of the primary is simply achieved by switching the supply on and off. If the number of turns in the secondary winding is greater than those in the primary then a higher voltage than the primary supply will be generated. With this principle in mind, three main factors will have a direct effect on the secondary voltage produced; 1) the primary current, 2) the ratio of turns between the primary and secondary windings, and 3) the speed at which the magnetic field changes. Points 1 and 3 are usually controlled by the ignition control unit or engine management ECU and point two is the result of the collaboration between the vehicle manufacturer and the manufacturer of the ignition components. Obviously the presence of the voltage required is of little use unless it is channelled to the spark plugs at the correct time to meet the fresh fuel/air charge in the combustion chamber; again this is the job of the control unit which will take into account engine operating conditions.
 
The most effective burn process producing the most efficient combustion pressure inside the combustion chamber occurs at approximately 10 degrees after top dead centre. As the engine speed increases the time taken by the piston to cover the same distance reduces so the spark has to be created earlier (ignition advance), under higher load conditions more fuel is introduced to produce more power, this richer mixture burns quicker so the presence of a spark can occur later (ignition retard). Ignition control is much like a balancing act, constantly changing to give the most efficient burn and this, along with fuel control has led to the adoption of electronic engine management.
 
With thousands of volts being produced at the ignition coil and the same being required at the spark plugs a special wiring requirement exists possessing insulating properties sufficient to prevent leakage to earth which will cause misfiring, a construction keeping radio frequency interference to an absolute minimum which can have a detrimental effect on radio reception and on the sensitive workings of some engine management components plus an outer protective covering to enable long service life and a high resistance to the harsh under bonnet environment. One of the most common types of ignition lead comprises six elements; a central non-conducting fibre surrounded by two layers of a conductive material e.g., latex and silicone, a layer of insulation covered by a braid and finally an outer jacket.
 
Once the high voltage passes through the ignition lead it reaches the spark plug the device screwed into the cylinder head just protruding into the combustion chamber. Basically the plug is designed to initiate combustion by forcing the ignition voltage to jump a carefully engineered gap to earth causing a spark thus starting the burn process. The construction of a typical spark plug usually consists of a terminal which connects to the ignition lead followed by a conductive core which includes a resistor to provide interference suppression and finally the exposed centre electrode. Surrounding this core is a ceramic shell which provides a high degree of electrical insulation and these two areas are sealed by a special packing into a gas tight assembly which is then encased in a steel alloy shell the outer surface of which carries the threads enabling the plug to be screwed into the cylinder head. There are literally hundreds of variations on this basic theme but they all exist to provide an effective spark over a wide range of engine and combustion temperatures and pressures. For those interested, a spark plug working for 20,000 miles will produce well over 20 million sparks, will endure voltages of around 30,000v, pressures at certain points of the four stroke cycle of 100bar (1470 psi) and have to work in an environment made up of hot fuel vapour, combustion products and fuel/oil residues.
Briefly returning to the ignition leads which have been the bane of many a motorist over many a year, high voltage running through the cabling close to good earth points has always been a recipe for misfires as electricity, like water, will always take the easy route, so why not get rid of the ignition leads altogether? In recent years manufacturers have utilized the good old primary/secondary coil technology but used one coil per plug and with many spark plugs being so deeply recessed into the cylinder head this provides a very sheltered environment for the high voltage and also allows each coil/spark plug assembly to be individually controlled by the engine management system.

It is common knowledge that petrol and CI engines require fuel and that this fuel needs to be stored for a period of time and also needs to be introduced into the combustion chamber in the correct form for the engine to run properly. For the most efficient burn four main requirements need to be met; pressure, flow, timing of injection and period of injection. Please note that as the vast majority of vehicles today use fuel injection of one type or another, carburettor systems will be ignored.

In the average petrol engine car fuel from the tank is fed under pressure through a fuel filter by the fuel pump, this pump can be mounted externally onto the body or internally within the tank. The pump unit itself is controlled by the fuel/engine management ECU providing a current supply via an electrical relay, fuel then passes up to the engine bay inside high pressure solid and/or flexible pipes where it joins the pressure regulator, this unit governs the fuel pressure available to the injector(s). The regulator usually has three connections; 1) supply from tank, 2) feed to injector(s) and a vacuum line from the inlet manifold which allows changes in fuel pressure depending on engine load. Under full load conditions the regulated fuel pressure can reach 40psi (pounds per square inch) or 2.7bar. The supply now reaches the injectors which are small electro-magnetically operated valves mounted close to the back of the inlet valves which, when energised, allow the pressurised petrol to flow through a nozzle producing a mist (atomisation) which is directed into the inlet air stream. For those interested, the injector valve moves approximately 0.15mm and this occurs over a time period of between 1.5 and 10 milliseconds completely governed by the ECU. This time variation allows for a very high degree of fuel/air mixture control depending on engine operational requirements.

As the quantity of fuel delivered by the pump is far in excess of that required by the engine a return line is provided to allow this excess to return to the tank. This aspect of the system ensures that there is a constant supply of continually filtered fuel at a constant supply pressure.

The next factor to be decided is the amount of fuel delivered via adjustment of injection duration; simply, longer injector open time equals more fuel injected. The fuel/engine management ECU takes information from the various sensors around the engine notably, engine speed, coolant temperature, inlet air temperature, engine load, etc and calculates the fuel requirement. For example, a morning cold start will involve a longer injection period than when starting from warm and the main sensor input will be from the engine temperature sensor as the engine load will be very low as will the engine speed until the vehicle is driven off. As the engine temperature increases the requirement for cold start enrichment decreases with the shortening of the period of injection, the injection duration only then increases with the increase in engine load.

With the CI engine, many similarities with the petrol engine exist in so much as the fuel must reach the combustion chamber in the same form. From the fuel tank fuel is drawn by a primer pump through a filter and delivered to the injection pump. In the modern motor vehicle two main types of injection 1) rotary pump, and 2) common rail; in the first an engine driven pump produces pulses of high pressure fuel delivered to the injectors through thick walled steel pipes and in 2) an engine driven pump supplies a fuel rail in turn connected to every injector, hence the term common rail. This is slightly different to the rotary system in that the pressure at the injectors is not pulsed it is kept constant. It is this pulsed supply which builds up behind each injector until the pressure is sufficient to lift a needle arrangement clear of its seal against spring pressure, fuel then flows through the injector nozzles producing a fine mist. As the pressure at the injectors is constant in the common rail system the injectors are opened electrically under the control of the engine/fuel ECU. The overriding advantage with the common rail system is that as the injection process is under computer control far more accurate fuelling is available throughout the engine speed range.

To summarize, fuel control exists to provide the engine with the correct amount of fuel at the correct time taking into account all engine operating conditions.

In the last article, “Body Electrics�, I mentioned the term electronics alongside electrical, this was done as the two areas are inseparable in so much as electronic systems tend to control electrical systems in the modern motor vehicle. Good examples of this are the electronic timer for intermittent wiper operation and the memory function found on quite a few electrically adjustable seats.

The advance in vehicle electronics has been almost meteoric with high end vehicles fairly bristling with as many as fifty or more micro-processors controlling everything from suspension settings to cabin temperature.

So why have all this electronic wizardry? In most instances the greater the number of systems installed by manufacturers which have a range of adjustment the greater the scope for high end electronic control, this leads to the reason for the subject of this article; because of the number of computer systems fitted their efforts affect the vehicle has a whole rather than just one particular area. Take for example a car being driven along a road which twists and turns, it starts to rain so the wipers operate by themselves regulating their own start/stop cycle, the driver presses on with his journey increasing speed, a road speed signal is fed to the suspension computer along with information from other sensors indicating vehicle pitch and yaw, this then alters the stiffness of the suspension enabling better handling, the processors involved with steering reduce the degree of power assistance which increases the “feel� available to the driver. All along, the engine and gearbox ECU’s are communicating to give the best response in relation to driver input selecting the best gear option for the engine speed; the driver accelerates ignoring the deteriorating road conditions and starts to “attack� the on-coming bends, inevitably the driver starts to lose control and in a split second the engine power has been greatly reduced, the correct gear has been selected, suspension adjusted selectively and the brake computer has started to organise brake control as appropriate to bring the vehicle back into a stable condition. Without the driver making a move away from the steering wheel, a whole raft of operations have been carried out within a very short period of time, non of this would have been remotely possible without the intervention of microprocessors communicating with each other at a phenomenal rate. 

Of course, this is only the tip of the iceberg as far as future developments go. We have all witnessed the meteoric advances in home computing technology over the past few years and exactly the same thing is happening with automotive electronics, multiple systems collaborating to give control over the whole unit whether that be a 44 tonne truck, a family saloon car or a multi user home PC. It is almost impossible to predict what the impact will be over the next twenty years of these advances but many venture to say that the driver will become more and more a passenger leaving the negotiation of point A to point B to a small bundle of silicon chips!

Aside from the electrical systems already described in starting the modern day motor vehicle, they are now fitted with an ever increasing number of electrical components. These can further split into even more electrically controlled devices which seem to increase in complexity with every single model change. An example of this is in the latest models from the Ferrari owned Alpha Romeo car parts.

As the number of electrical systems is so large this article will be restricted to general principles only.

One of the main advantages of using a low voltage system is that there is virtually no chance of a person being on the receiving end of an electric shock. With this in mind all the system designer has to do is put together a collection of wires (the loom) from a power supply to the components concerned via the correct switching and circuit protection devices, the return circuit does not need to be wired as the vehicle body can serve as the return path to the negative terminal on the battery. Please note that this “no risk� factor does NOT apply to parts of the ignition system nor parts of the latest gas discharge type headlamps. With so many wires in the vehicle some method had to be devised to identify different circuits from each other, hence the advent of colour coding incorporated into the PVC insulation covering each wire.

The size, or more correctly, the diameter of each wire is dictated by the current it is expected to carry in normal use, note that current is the deciding factor and not voltage. Compare two lengths of wire, one feeding the starter motor and the other feeding the CD player, both carry 12vDC but the starter feed may carry hundreds of amps and the CD supply little more than one amp. For the technically interested wire sizes are denoted by the number of strands and their individual diameter contained within the insulation, e.g.

         9/0.30   means 9 strands of 0.30mm diameter

         37/0.90 means 37 strands of 0.90mm diameter

Typical application for the 9/0.30 would be a sidelight circuit drawing 5.75amps, the 37/0.90 a starter feed drawing 350amps.

Already mentioned is the requirement for circuit protection, this is normally provided by a fuse, this fuse is a metal alloy wire of a size that has limited current carrying capacity, should the current flow exceed the design level the fuse element will melt away breaking the circuit thus saving the circuit. For various current loads several fuse ratings are produced usually being colour coded for ease of selection. It is important to note that the fuse rating specified by the vehicle manufacturer is not exceeded; a circuit which is continually blowing fuses has a fault, pure and simple. The risk of serious wiring damage or even fire caused by an electrical fault in a circuit fitted with a fuse of too higher rating is very real.

With the voltage fixed, in the majority of cases, at 12vDC and a brief explanation of the applications for high and low current what other factor does an electrical circuit have that can vary between two extremes? The answer is resistance, if you were to take two pieces of material of the same dimensions, one plastic and the other copper and place them in turn into an electrical circuit you would quickly find that the plastic component will break the circuit and the copper one would make little or no difference to the flow of current. The plastic component has such a high resistance that it becomes an isolator; copper on the other hand has an extremely low resistance therefore offering a clear path through the circuit. When powering devices around the vehicle the electrical path must be of low resistance so that there is little drop in power between supply and load hence the widespread use of copper wiring. There are, however, situations where a certain level of high resistance is required, e.g. heating elements and filament bulbs. Both of these applications rely on a carefully calculated degree of resistance, with the heated rear screen the passing of current will cause a small heat effect impossible to see unless the glass is misted or covered in ice, the filament bulb on the other hand reacts to the passing of current by glowing white hot.

From the previous article on starter motors we can see that the motor can demand quite a large current from the battery via the supply cable. Obviously, numerous start cycles will deplete the battery power to the extent that it will no longer be able to start the engine. In order to recharge the battery vehicle manufacturers provide a generator system driven by the engine usually via a rubber belt.

Until the late sixties this generator could have taken one of two forms, the dynamo or the alternator. Gradually, over a fairly short period of time, the dynamo fell from favour and the job of battery charging and supplying electrical power has been satisfied by the alternator. As the dynamo is, to all intents and purposes, a dead issue this article will be confined to the far superior alternator.

The main reason for the rise of the alternator is its ability to provide a relatively high output without being physically large, an alternator with an output of 160 amps will be little different in size to one with half that figure. The potential for high output has become particularly relevant for today’s vehicles as the demand for power has risen many, many times, between 1960 and 1980 average current demand rarely exceeded 50 amps, today that requirement can be as high as 200 amps and it is still rising this is especially true in bigger make cars like in Alpha Romeo car parts where a bigger battery is required due to the extra size and weight of the vehicle. This factor alone has led many authorities to say that the days of the 12 volt system are numbered, current demand can be halved by simply doubling the system voltage to 24 volts and after all trucks and buses have been using this system for many years

The alternator generates power by revolving a magnetic assembly (the rotor) inside a cylinder made up of many loops of wire (the stator), this loop construction is made up of three separate phases each producing an output in turn when the rotor revolves about its centre line. The first hurdle is to change the alternating current (AC) output into direct current (DC) compatible with the battery and other vehicle systems, this is the job of the rectifier. Without going into the intricacies of rectifier construction, conversion of AC to DC is achieved by a series of one way electrical valves (diodes) wired so that a process called full wave rectification occurs. The next hurdle is one of voltage regulation, too high and battery/component damage may well occur, too low and the battery charge state will not be maintained leading directly to a non start situation. Put simply, the voltage regulator manages output by interrupting the magnetic field in the rotor. With output voltage set at around 14.2v, should output rise above the set maximum the rotor magnetic field will be switched off, dropping below this and it will be switched on. This process happens within a few milliseconds giving a very accurate voltage output. Causing the alternator to turn is a drive arrangement comprising a pulley secured to the end of the crankshaft and a similar, but much smaller, pulley at the end of the alternator with a suitable belt wrapped round the two. The difference in pulley sizes allows the alternator to be driven at a higher speed than the engine.

The whole charging system effort is to maintain the car battery and provide power for vehicle systems when the vehicle is running.

In the previous two articles on basic engine operation it was explained how the four stroke cycle worked, but how do you initiate the process? With the engine running, the up and down movement of the pistons is transferred to the crankshaft to produce rotation, to start an engine we need to rotate the crankshaft by external means which will then allow the engine run cycle to take over.

The most basic method would be to use a suitably cranked handle temporarily engaged in the end of the crankshaft and turned by some athletic individual. Better still would be the fitting of an electric motor under driver control – a starter motor.

The most common starter motor found on modern vehicles is the pre-engaged type which uses an electro-magnet to operate switching and engagement functions, this differs from the older inertia type which requires a separate heavy current switch and engages by throwing a drive gear into engagement. This engagement with the engine is achieved by the use of a toothed ring secured to the outer rim of the flywheel (the ring gear), when the starter is engaged drive is transmitted from the motor drive gear onto the ring gear and the engine turns over.

For the average engine to start, a minimum speed of 100rpm is required, less than this will not produce the levels of compression and fuel vapour mixing required for proper combustion. During the cranking phase the starter motor will demand around 150 – 200 amps from the battery, however, under extreme climatic conditions even the average family 2 litre hatchback can ask for 450 – 500 amps to overcome the initial resistance to rotation and friction found within the engine. To give some idea of the torque required for this freezing cold morning scenario, the average wheel nut is tightened to around 90 – 110Nm, the amount required to start the engine moving can be as much as 450 – 500Nm dropping to 160 – 200Nm to maintain the 100rpm minimum crank speed. It must be noted that cranking should be restricted to a maximum of 10 seconds at a time, extended crank times, more than 3 seconds, will cause the motor and associated wiring to heat up leading to increased fatigue and a much shorter service life.

So that’s what the starter motor does, but how does it do it and how is it controlled?
As the pre-engaged type is by far the most common the following will be restricted to this type. The two main parts of the starter motor are the motor itself and the solenoid which is usually mounted on the top of the motor casing, the motor is devoted purely to rotating like most other electric motors, the solenoid has two functions 1) to handle the high current switching, and, 2) to push the drive gear into mesh with the ring gear. It would be impractical to run the large high current cables from the battery to a switch mounted in the cab then down to the starter, so the pre-engaged system uses the solenoid to handle the heavy current switching required. This is done under the control of a much lower current circuit using far smaller wires from the starter switch which can now be incorporated into a combined ignition/start switch assembly. The second job for the solenoid is to facilitate the engagement of the starter drive gear into the engine ring gear. Both of these tasks are carried out by an electro-magnet making up the solenoid assembly. The high current feed from the battery is connected to one of the large terminals of the solenoid; the other large terminal feeds the motor itself. In the at rest position this circuit is open (no current flow), when the solenoid is operated by a low current feed from the starter switch the centre plunger of the solenoid pulls the drive gear into mesh with the ring gear and closes the high current circuit so powering the motor. As the engine starts, the driver releases the ignition key from the crank position thereby cutting the low current feed to the solenoid which in turn breaks the supply to the motor and brings the drive gear out of mesh assisted by a return spring.

For those interested in engine history Dr Rudolf Diesel took out patent 7241 in 1892 which detailed an engine design relying on the induction of coal dust for fuel. Prior to this, the English engineer Herbert Ackroyd-Stuart filed patent 7146 in 1890 which contained a design for an engine which ran on the timed, high pressure injection of fuel oil. Whichever camp you decide to pitch in, it cannot be denied that the work of Ackroyd-Stuart preceded that of Diesel. To avoid being dragged into the argument the term “compression ignition� or “CI� will be used throughout.

The basic four stroke cycle of induction, compression, power, exhaust, used in the CI engine is exactly the same as that used in the petrol engine of such makes as Alfa Romeo. Two main differences exist; 1) engine construction tends to be heavier and more robust to handle the much higher compression and combustion pressures, and 2) ignition of the fuel/air mixture does not require an external source (a spark).

So we have an engine which will run without having any external form of starting the burn process, how can this happen? Put simply, pressure; cast your mind back to when you last used a bicycle pump, notice the fixed end of the barrel becoming warmer, this was not entirely due to the warmth of your hand it was caused by the air in the pump tube getting hotter as its pressure increased. Now imagine this scenario magnified many, many times and instead of generating 30-45 pounds per square inch (psi) or 2-3 bar you generate 500psi or 34 bar, at this pressure created over a very short period of time temperatures usually reach well over 600 C. Just before top dead centre, fuel oil is injected at very high pressure, around 2500psi or 175 bar, via an injector which produces a very fine mist. The fuel mist will self ignite at around 400 C so its introduction into this cylinder will produce almost instant combustion thus producing the power stroke.

So why use a CI engine over a similar sized petrol engine? Until relatively recently the CI engine was the sole power unit of choice for heavier duty applications, trucks, buses, taxis etc, the greater fuel efficiency over the petrol engine was guaranteed  to give the operator lower fuel costs and its better power characteristics produced high power output at lower engine speeds thus promoting a longer service life. The flip side to this was the CI engines higher noise and harshness levels plus its ability to belch black smoke under heavy acceleration. In more recent times however, CI engine development has come on in leaps and bounds offering noise, harshness and exhaust emissions levels virtually identical to its petrol driven cousins whilst still delivering the old attributes of excellent power output at relatively low engine speeds.

The basic principle behind the petrol engine and for that matter, the diesel engine, is one of internal combustion, although the two differ in their respective fuel systems and their method of initiating the combustion process.

With the petrol engine used in such makes as Alfa Romeo, introduce an explosive mixture (petrol vapour and air) into a virtually sealed cylinder, compress it by moving one end of the cylinder toward the other then ignite the mixture with a spark. The resulting very rapid burn will force the moveable end of the cylinder (the piston) away from the fixed end (the cylinder head) and via a connecting rod and crank arrangement rotation will result.

For the above scenario to take place effectively other conditions must be present both before and after the burn (combustion). The process starts with (1) a piston moving downward within a cylinder drawing in a charge of petrol vapour and air past an open inlet valve which then closes, at the bottom of this downward stroke the piston, via the connecting rod/crank arrangement, will change direction and start to move up. As the piston rises (2), the fuel/air mixture will be compressed and in doing so will increase in temperature, just before the top of the upward travel a spark will be introduced into this now highly volatile mixture and it will start to burn very rapidly, by this time the piston will have past the top of its travel and is now (3) being forced down by the expanding gases of combustion. The piston again reaches the bottom of the down stroke and (4) starts to rise pushing out the burnt gases through the now open exhaust valve which will close just before the top of the piston travel. Just past the top of piston travel the inlet valve opens ready for the process to be repeated. This is the four stroke cycle explained and it is referred to in engineering circles as induction, compression, power, exhaust; it is also known by the shorter version; suck, squeeze, bang, blow!

Obviously this process is now duplicated across all cylinders with each cylinder carrying out a different task in the cycle relative to its neighbour. This phasing is achieved by crank and valve position and will ensure an even power output. As a general rule, the more cylinders an engine has the smoother its power output will be, this is mainly due to the shorter time between each power stroke. However, there will still be the situation involving Newton’s third law which involves equal and opposite reactions to any action, to counteract, or at least minimise its effect, the crankshaft has strategically placed counter weights incorporated into its casting. This has a very pronounced effect in evening out the severe direction changes experienced by the piston/connecting rod/crankshaft assemblies.

Mentioned above is the inlet/exhaust valve arrangement and its relevance to the correct operation of the engine. It is vital that these valves are opened and closed at exactly the right time for the engine to “breathe� properly. To achieve this a camshaft is mounted such that its rotation permits the valve operation relative to the piston position. To achieve this a drive arrangement is incorporated in the engine design so that as the crank turns drive is transmitted via a chain or toothed belt to the camshaft. It is most important that the drive between the crank and cam is correctly set at production and at replacement intervals during the life of the engine, this is usually achieved by the use of specific marks on the engine allowing the correct positioning of the shafts relative to each other before the drive is fitted. Should the timing of piston and valve come adrift there is a distinct possibility that the two will meet catastrophically resulting in a very large repair bill.

There are, of course, many other systems and components which enable the engine to run and these systems will be dealt with in later articles.

As engine power is transmitted through the gearbox, the gears themselves will have a tendency to stay in mesh making gear changing very difficult. Even more difficult, once a gear has been disengaged, is the selection of another. The difficulty arises from the fact that the speed of the two gears being introduced to each other is very different. To make this gear changing process smoother and more gradual a device is required that will interrupt the engine output before it reaches the gearbox internals. This device is the clutch.

The most commonly used clutch today is the single plate diaphragm spring type. Put simply, a circular plate (driven plate) with a splined centre boss and a ring of friction material fixed to either side is slid onto the input shaft of the gearbox, one side of this plate will press against the working surface of the engine flywheel which is, in turn, bolted to the engine crankshaft, the other friction lined side of the driven plate will be pressed against another working surface mounted onto an assembly called the pressure plate. With the clutch in the at rest position this friction sandwich is clamped firmly between the flywheel and the pressure plate and will, therefore, rotate at the same speed as the engine. As the driven plate is splined it can move back and forth a small amount when required. At the centre of the pressure plate is a circular spring steel diaphragm which is slightly conical in the at rest position. Pushing on the centre of the diaphragm, flattening it out, will have the effect of reducing the clamp effect produced. The last part of the clutch assembly is the release bearing which is mounted around a tube over the input shaft being able to slide quite freely and in turn indirectly attached to the clutch pedal via a cable or a system of hydraulics.

When the clutch pedal is depressed, the release bearing moves forward pressing against the diaphragm spring unclamping the driven plate and interrupting the power supplied to the gearbox, with the drive gone, selecting or changing gear is easily achieved. The gradual release of the clutch pedal by the driver once a gear is selected will reinstate the clamp force on the driven plate and power will once again pass to the gearbox and on to the driven wheels. Even with the engine running and the clutch not being operated, no vehicle movement will occur until a gear is selected.

Although various mechanisms are incorporated in the driven plate to facilitate smooth power take up, the main advantage of the friction material on the driven plate is that it will slip for a very short time greatly assisting this power transfer.

Various other permutations on this basic theme exist e.g.; paddle clutches, multi-plate clutches, electro-mechanical clutches, damped flywheel clutches, centrifugal clutch, etc; but they all exist to interrupt engine power to the gearbox so that gears can be selected and the vehicle moved away smoothly.

Engine power in the form of rotation is delivered to the final drive from the gearbox. The gearbox is normally in the form of a cast metal casing which not only contains the individual gears, the selector mechanisms and the means of securing all the bearings necessary to allow the gears and accompanying shafts to rotate freely but also a reservoir for the lubricating oil vital to ensure a long service life and smooth, quiet operation.

Why do we need a gearbox? Without going into the depths of gear ratios and the like, which may be the subject of a later article, imagine jumping on a bicycle and trying to pedal away in the highest gear, huge amounts of effort are required just to keep moving without falling off and acceleration is extremely difficult. With the huge effort comes excessive strain on the entire drivetrain; pedals, chainrings, chain, rear block and the rear wheel assembly, all in all very inefficient. Now repeat the exercise starting off in the lowest gear, moving away and accelerating becomes easy no longer requiring the huge amounts of lung and leg effort. The whole outfit also becomes much more stable and further acceleration through the gears increases the speed far more comfortably without loading the components beyond their normal limits. Now instead of accelerating through the gears see the rider staying in the lowest gear, acceleration and top speed will be limited by the speed the rider can spin his legs and there is a definite limit to this. Using a selection of gears permits the progressive acceleration up to the required speed and the facility to choose a gear to suit prevailing road conditions.

The effects felt by the rider are very much like the effects experienced by a motor vehicle when used without a selection of gear ratios being available. The high gear scenario will require the extensive slipping of the clutch, greatly shortening its service life, and the general fuel efficiency will suffer. A vehicle fixed in a low gear will obviously move away from rest with no problem but further acceleration and the maximum speed will be greatly limited by the noise from the engine just prior to it failing in a most catastrophic and expensive manner!

The specification of gear ratios and the number eventually offered by the gearbox will largely depend on the vehicles intended use. This is the reason for large commercial vehicles having, in most cases, twelve to sixteen forward gears. The requirement to overcome initial resistance to movement caused by a much larger vehicle coupled with a sizeable load means that the laden truck must have a greater number of low ratio gears. This also applies throughout the acceleration process with a larger number of intermediate gears being required before reaching the higher ratios and the desired final speed. As this process is a much more drawn out affair in a commercial vehicle the driver will endeavour to make the most efficient use of engine power, this is usually achieved by taking much greater notice of engine speed and therefore making greater use of the power range indicated on the dashboard engine rev counter, the so-called “green band�.

A much smaller vehicle will have no such requirement even if it is asked to tow a trailer or caravan of an approved weight, so the provision of four, five or even six forward gears will suffice.

To summarize, the gearbox is fitted to provide a range of ratios allowing easier acceleration and the maintenance of a required road speed without either under or over revving the engine thus promoting greater fuel efficiency.

One last, but very important feature, the gearbox offers the facility to go backwards!