mechanical knowledge

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Mechanical Knowledge

2007 Edition


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Mechanical Knowledge

2007

Introduction The Under-17 Car Club is not just about learning to drive cars, lorries, fire engines, and anything else we are able to safely arrange to get our hands on! It is also about a greater understanding of all aspects of safer motoring and how to avert problems caused by avoidable mechanical failure. It is assumed that as a Grade 1 going for Grade X, you know where the basics are under the bonnet dipstick; battery; various filler caps; etc, and you have the cockpit check down to a fine art. With this in mind we have attempted to produce a simple guide to some of the main mechanical workings of the motor car, and how to look after it in its natural habitat.

Component layout of Front Wheel Drive Vehicle


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The Petrol Engine The engine, however it functions, is arguably the heart of any vehicle so we will start with diagrams of the basic components of a petrol engine. All the main parts are labelled and this page is designed to refer back to as various functions are explained. Some parts are common to the three main types of car engine. The Cylinder Head The engines cylinder block is usually made of cast iron on to which an aluminium cylinder head is bolted. This contains the inlet valves that let the petrol-air mixture enter the combustion chamber, and the exhaust valves that let gases leave it. These can be moved by pushrods from a block-located crankshaft-driven camshaft. The head more usually uses single or twin overhead camshafts driven by a toothed rubber belt or a chain which runs inside the engine and is lubricated by engine oil as it returns to the sump from the cylinder head.

The Engine Block

crankshaft

camshaft

main bearings

piston

Connecting rod

gudgeon pin engine block

main bearing caps

Piston rings

rod bearings

rod cap Vibration damper

Sump


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The Petrol Engine How Does it Work?

The majority of car engines employ the four-stroke cycle which was invented by

Nicholas Otto way back in 1876.

The Four-Stroke Cycle

A typical four stroke petrol engine, as the name suggests, has four processes. See diagram below.

1. INDUCTION: On the induction stroke the piston is going down and the exhaust valve is closed. The inlet valve opens and a mixture of petrol and air is sucked in.

2. COMPRESSION: On the compression stroke the inlet valve closes. As the piston goes up it

compresses the mixture of petrol and air.

3. POWER: On the power stroke the spark plug creates a spark which ignites the fuel/air mixture causing a rapid burn. This creates great pressure which acts on the piston crown (top of the piston) pushing the piston down the cylinder.

4. EXHAUST: On the exhaust stroke the piston goes back up the cylinder, the outlet valve opens

and the burnt gasses are pushed out into the exhaust system.

N.B. One way to remember this is; SUCK SQUEEZE BANG - BLOW !

1. Induction

3. Power

2. Compression

4. Exhaust

Spark-plug

Inlet valve

Cylinder

Exhaust valve

Piston

Connecting Rod

Crankshaft

Big End


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A Diesel Engine is a type of internal-combustion engine in which ignition of the mixture of fuel and air is achieved by compressing the air first, which causes it to get very hot, before adding the fuel. Hence the name 'Compression Ignition Engine' is sometimes used to distinguish it from the spark-ignition petrol engine. Diesel engines are in general slow-speed engines and run on diesel fuel oil for road use known as DERV (diesel engine road vehicle), rather than petrol.

Although patented by German engineer Rudolph Diesel in 1892, engines working on essentially the same principle were produced by the Priestman brothers in the UK in 1885 and improved by the British engineer Herbert Ackroyd Stuart.

Most diesels are four-stroke engines, but operate quite differently from the four-stroke Otto-cycle petrol engines. Most notably, there are no spark plugs.

The first or induction stroke draws air, but no fuel, into the combustion chamber through an inlet valve.

On the second or compression stroke the air is compressed to a small fraction of its former volume (as little as 4% of its original volume) which causes it to heat to approximately 440 C. At the end of the compression stroke fuel is injected into the combustion chamber vaporising and burning instantly because of the high temperature of the compressed air in the chamber. (Some smaller diesels have an auxiliary electrical glow plug system to assist ignition of the fuel when the engine first starts up and until it warms up. The glow plugs then turn off automatically and play no further part.)

This combustion drives the piston back on the third or power stroke of the cycle.

The fourth stroke, as in the Otto-cycle petrol engine, is an exhaust stroke where the gases are blown out. (See diagram left). So the cycle starts again.

Because diesel engines use higher pressures than petrol engines they must be made stronger and are therefore heavier. This disadvantage however, is counterbalanced by their greater efficiency. The addition of a turbocharger (a form of supercharger) and intercooler can enhance the performance of a diesel engine both in terms of power and efficiency. (see page 11)

The principal drawback of diesel engines is their emission of air pollutants. These engines can typically discharge high levels of soot, reactive nitrogen compounds (commonly designated NO), and odour compared to spark-ignition engines. They were also in the past considerably noisier than their petrol counterparts. Consequently, the use of diesel engines in cars has traditionally been low. Now however, with developments producing quieter more environmentally friendly engines, more modern cars are using diesel power.

Inlet Valve

Direction of Movement

Diesel Engine Parts

Fuel Injectors

Exhaust Valve

Piston

Starter Ring Gear

Camshaft

Tensioner

Cam Cam Follower

Inlet Valve

Timing Belt

Crankshaft

Connecting- rod Big End Flywheel

The Diesel Engine


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The Rotary Petrol Engine In 1954 the German engineer Felix Wankel developed his concept of an internal-combustion petrol engine of a radically new design, in which the piston and cylinder were replaced by a three-cornered rotor turning in a roughly oval (it's actually an epitrochoid) chamber, called the bore. The shape of the combustion chamber is designed so that the three tips of the rotor will always stay in contact with the wall of the chamber, forming three sealed volumes of gas. The rotor has three convex faces, each of which acts like a piston. Each part of the housing is dedicated to one part of the (now familiar) combustion process:-

Induction Compression - Power Exhaust.

11 44 Induction; The fuel-air mixture is drawn (sucked by the rotation of the rotor) into the chamber through the intake port, and trapped between one face of the turning rotor and the wall of the oval chamber. 55 99 CCoommpprreessssiioonn;; The turning of the rotor changes the size of the chamber compressing the mixture and bringing it nearer the spark plugs.

1100 1122 PPoowweerr;; Once the chamber reaches its minimum size, the spark plug fires, igniting the fuel. In newer engine designs, there are two spark plugs, placed symmetrically on either side of the centre line of the epitrochoid. The two plugs fire simultaneously, and reduce the time the combustion takes to spread throughout the fuel, increasing the power of the expansion. As the fuel burns, the expanding gases push the rotor around, driving the cycle (the power stroke). 1133 1188 EExxhhaauusstt;; As the rotor turns, the exhaust is expelled directly through the exhaust port. Just as with the intake, there is no valve, because the turning of the rotor seals and unseals the ports. The cycle takes place alternately at each face of the rotor, giving three power strokes for each turn of the rotor.

The rotor and housing of a rotary engine from a Mazda RX-7: These parts replace the pistons, cylinders, valves, connecting rods and camshafts found in piston engines.

A A

A

A

A A

The letter A tracks the rotor through one third of its rotation


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As the rotor turns, it rotates around the small fixed inner gear as shown. This gear is stationary. The turning of the drive shaft is accomplished using an output shaft (above) with an off-centre lobe, which is mounted inside the rotor. As the rotor turns, it pushes against the lobe of the shaft, causing it to turn. Since 1967 there has been a series of rotary-engine cars, trucks, buses, and even motorbikes. However, production of the engine was discontinued in any great number because of poor fuel economy and high pollutant emissions. Wankel engines are less fuel-efficient than piston engines, due to the shape of the combustion chamber (long instead of small and concentrated), and also as a consequence dirtier than traditional engines. One solution that has been advanced for this problem is

to use two spark plugs, placed symmetrically, so that the combustion does not have to travel as far. However, the limited demand for the engines has meant that not as much research has gone into solving these problems. At the moment, the main

manufacturer of Wankel engines is Mazda, which produces the RX-8 sports car. The rotary engine was also manufactured for use in chainsaws, lawnmowers, snowmobiles and other lighter-duty applications, though for the most part those engines are no longer made.

The Mazda RX8 is the most recent development, a high performance car with truly revolutionary technology. The naturally aspirated two-rotor engine will produce about 250 horsepower; fuel consumption however is still fairly high.

Exhaust Intake

Rotor

Spark Plug

Coolant

Exhaust Cycle

Compression & Ignition Cycle

Induction Cycle

Small Inner Gear


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Crankshaft and Flywheel As you can see from the picture, the crankshaft is not a straight piece of metal. When the piston is pushed down by the power stroke of the petrol or diesel engine, it causes the crankshaft to turn by pushing down on it. Each piston takes its turn to push down on the crankshaft, turning it a bit each time. Do this fast enough and the crankshaft rotates rapidly. Connect the crankshaft to the driveshaft via a gearbox, and we have motion in the car. The flywheel adds both momentum and balance to the rotation of the crankshaft. The geared teeth on the outside of the flywheel is where the starter motor (see Starter Motor) connects to turn the engine over as the ignition key is turned.

Flywheel Ring gear

Connecting rod

Small end bush

Gudgeon pin

Piston Rings

Cylinder Liner

Flywheel Assembly

Big End Bearings

Main Bearing

Piston

Connecting rod Cap

Crank Pin Crankshaft Assembly


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Electrical System The cars engine management system, Electronic Control Unit (E.C.U.), is one of many components to make demands on the vehicles battery. The system is charged by the alternator which is driven by the engine (see Alternator). This charges the battery and in turn provides electrical power for the vehicles ancillaries. A key function of the electrical system is to start the cars engine. This is usually undertaken by a pre-engaged motor (see Starter Motor) in which a solenoid moves a bevel

gear into mesh with the teeth on the engines flywheel.

In addition to providing current for the cars lights and windscreen wipers, modern electrical systems have to service a radio/CD player, cigarette lighter, heated windows, central locking, electric windows, air conditioning, seat adjustment, and more recently, satellite navigation and mobile phones.

Ignition Control

Whether a carburettor (in older vehicles) or a fuel injection system is employed, the petrol/air mixture has to be ignited by a spark-plug. Current is supplied from a high tension coil. The engine management E.C.U. (Electronic Control Unit) controls the timing of the spark from information supplied by sensors detecting the position of the petrol engines crankshaft.

Fuel Injection A carburettor had been used from the earliest days of motoring as a component in petrol engines which the fuel-air mixture was created. The limitation of such an arrangement was that the mixture was unevenly distributed which resulted in incomplete combustion and an undesirable amount of unburnt fuel reaching the atmosphere (pollution).

As a result, the carburettor has now been replaced by an electronic fuel injection system enabling a precise amount of fuel to be delivered, by injectors, to each cylinder. Fuel is pumped, under constant pressure, from the tank through a filter to the injectors. The air/fuel mixture is constantly being adjusted by the engine management Electronic Control Unit (E.C.U.) which receives information from sensors in the engine inlet and the exhaust systems.

The Air Filter The Air Filter on an engine has two main functions: The first is to remove dust and dirt from the air being drawn into the engine; secondly it acts to silence the noise of the air entering the intake. Never run the engine without the air filter fitted, excessive engine wear could result, and back firing could cause a fire in the engine bay.

Starter Motor

Alternator


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The Starter Motor When the ignition key is turned to start, a current is applied to the solenoid which pulls on the actuating arm. This moves the pinion into mesh with the gear teeth on the outer edge of the flywheel (see Crankshaft and Flywheel).

When the solenoid is fully retracted an internal connection in the solenoid supplies current to the starter body from the battery, causing the armature to turn the pinion and from there the engine. When the ignition key is released a one-way clutch in the pinion lets it freewheel until the return spring retracts the pinion out of mesh with the gear teeth on the flywheel. At this point the engine should be running.

The starter motor is designed for high current consumption and delivers considerable power for its size for a limited time, which explains why a battery soon dies if a car is not firing for some reason.

The Alternator Alternators generate electrical power for motor vehicles. All vehicles require a direct-voltage supply for ignition, lights, fans, etc. In modern vehicles the electric power is generated by an alternator that is mechanically coupled to the engine usually by a drive belt. An alternator produces an alternating current (AC) which needs to be converted to a direct current (DC) by a rectifier. A regulator is used to control the field current so that the output voltage of the alternator-rectifier is properly matched to the battery voltage as the speed of the engine varies. Regulator voltage is normally between 13.5 and 14.5 volts. Charging control is necessary because excessive voltage can damage electrical components and cause the battery to overheat producing hydrogen and oxygen gas with potentially explosive results!

OXYGEN + HYDROGEN +SPARK = BANG !

Starter Motor (pre-engaged)

Return Spring

Actuating Arm

Pinion Field Windings

Brushes

Commutator

Solenoid

Wire to Battery

Wire to Ignition switch

Armature

Flywheel

Starter Motor Ring Gear


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Superchargers & Turbochargers A Supercharger, is a compressor used to increase the amount of air admitted to an internal-combustion engine cylinder during the inlet stroke. It enables more fuel to be burnt, so increasing the power output. Superchargers are mechanically driven from the crankshaft by gears or a drive belt. Another form of forced induction is the use of a turbine utilizing the power of the engine exhaust gases. The combination of a directly coupled turbine and compressor is termed a turbo-charger (see diagram), and is widely used in both petrol and diesel engines. The turbocharger is driven by otherwise wasted exhaust gases, it is a small, high-revolution pump that forces air into the cylinders at pressure and is invariably used in conjunction with an intercooler.

It enables the engine to give more power for a given weight and cubic capacity, reduces noise, and maintains power at the higher altitudes encountered in mountainous country. High-performance petrol engined cars commonly use turbo-chargers and/or superchargers.

An intercooler is a radiator to cool air between the turbo and the engine to make it denser further increasing engine power.

Catalytic Converter Catalytic Converter, a device incorporated in the exhaust system of motor vehicles that reduces emissions of certain pollutants. Exhaust gases are passed through chambers coated in such rare metals as palladium and platinum; these metals act as catalysts, encouraging chemical reactions that change pollutants such as carbon monoxide, nitrogen oxides, and certain hydrocarbons into

carbon dioxide and water. Emission standards mean catalytic converters are fitted as standard to all new petrol-engine cars sold in the United States since 1983 and in the EU since 1993. Catalytic converters for diesel engines have also been developed.

In practice catalytic converters reduce emissions less on the road than in test conditions. They can take a 5mile (8 km) drive to work efficiently and require the vehicle to use unleaded petrol. Leaded petrol causes them to stop functioning.

Lubrication All moving parts of a vehicle require lubrication. Without it, friction would increase power consumption and damage the parts. The lubricant also serves as a coolant, a noise-reducing cushion, and a sealant between engine piston rings and cylinder walls. Oil is circulated under pressure from a pump that draws the lubricant from a reservoir contained within the sump at the bottom of the engine (see top diagram - Page 3). It is filtered and delivered under pressure to the main crankshaft bearings from a gallery located in the side of the engine block, and to the appropriately named big-ends of the connecting rods (see bottom diagram - Page 3) via holes drilled in the shaft. Oil reaches the bores by splash although it is pumped to the camshaft and valve gear. It then drains back down into the sump.

The oil level in the engine should be regularly checked via the dipstick (see the Checklist). Not keeping the oil level at the correct level, halfway between Min and Max, can lead to catastrophic failure of the engine! Wheel bearings and universal joints require a fairly stiff grease, unless they are the newer sealed for life ones which only require checking for wear during servicing. Other chassis joints require a soft grease that can be injected by pressure guns. Hydraulic transmissions require a special grade of light hydraulic fluid, and manually shifted transmissions use a heavier gear oil similar to that for rear axles to resist heavy loads on the gear teeth. Gears and bearings in lightly loaded components, such as generators and window regulators, are generally fabricated from self-lubricating plastic materials.


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Cooling System Once any engine starts running the moving parts create a lot of friction which results in heat. Also, the combustion temperature of petrol is 25000c. and although the engine needs a certain amount of heat to run efficiently, too much heat will cause damage. It must therefore be cooled down.

The cylinders and the head of the engine have channels containing a water/anti-freeze mixture which is circulated by a pump. As the engine heats up the water mixture (coolant) gets hot. The pump forces the coolant from the bottom of the radiator around the engine and then back to the top of the radiator. The engine temperature and therefore the flow of coolant to the radiator is controlled by a thermostat. When the engine is cold the thermostat remains closed to speed up the engine warm-up time. When the correct operating temperature for the engine is reached, the thermostat opens allowing coolant to flow through the radiator to maintain a constant temperature.

The Radiator consists of two tanks (top and bottom) joined by a large number of tubes which carry the coolant between them. The tubes have a large number of cooling fins covering them to increase their surface area and maximise the transfer of heat to the air flowing through the radiator. To assist cooling particularly at low speeds, a fan is placed just behind the radiator to draw air through the fins. The Radiator Pressure Cap seals the cooling system but allows access for topping up the water/anti-freeze mixture. As the engine temperature rises and the coolant heats up, pressure builds up within the system raising the point at which it the fluid boils. The cap prevents pressure build-up beyond a safe operating level allowing excess pressure to escape. The cap also contains a vacuum valve, allowing air in again, to prevent a vacuum forming in the system when the engine cools and the pressure drops.

The Water Pump is driven by a belt, which loops around a pulley, which is connected to the crankshaft. The faster the crankshaft goes, the faster the water pump operates increasing the flow of coolant through the system.

The Fan can be operated either by the same belt that drives the water pump, or by means of an electric motor which is switched on to cool the water in the radiator when a thermostatic switch detects the engine temperature has reached its maximum safe level.

Engine Coolant is a mixture of water and anti-freeze, usually 50% of each. Anti-freeze, as well as preventing the coolant from freezing in cold weather, also prevents corrosion of the cooling system components. This is particularly important when different metals are present in the engine, i.e. aluminium and cast iron.

(See Automatic Gearbox)

Valves

Piston

Flywheel

Connecting Rod Sump

(Oil Reservoir)

Crankshaft

Camshaft

Valve Pushrods

Water Pump and Fan

Petrol Engine


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The Clutch The function of the Clutch in a motor vehicle is to disconnect the engine from the road wheels whilst changing gear and then allow the engine to pick up speed smoothly.

In a clutch system a flywheel is bolted to the rear of the crankshaft. The face of the flywheel which touches the clutch plate is very smooth to prevent wear. The clutch plate is a two piece disc about 20cms (8 inches) in diameter. In the centre of the plate is a hole with splines (similar to gear teeth) in it, which correspond to splines on the input shaft of the transmission. The inner splined portion of the plate is connected to the outer friction part by buffer springs which absorb the initial take-up shock. Both sides of the plate are covered with friction material on the outer part of the diameter. This is high friction, low wearing, heat resistant material.

When the clutch pedal is pressed down a release bearing presses down on the centre of the clutch cover and forces the pressure plate away from the clutch plate. This allows the clutch plate to remain stationary between the revolving flywheel and the clutch cover. Gears can now be selected. A slow release of the clutch pedal by the driver gradually clamps the clutch plate to the flywheel allowing direct drive from the crankshaft to the transmission.

Manual Transmission On most front and rear wheel drive cars the gearbox is attached directly to the engine. To change gear, the drive passes through a clutch that must be briefly disengaged by the driver using the clutch pedal. The gearbox usually has four or five forward speeds and a reverse. The changes are effected by sliding dog clutches positioned on the combined first-motion/output shaft. This also incorporates synchromesh cones, which allow silent gear changes.

(see page 14)

Slave Cylinder

Clutch Pedal

Clutch Master Cylinder

Clutch Cover


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Manual Transmission - continued from page 13. Why have a Gearbox?

A car moves at various speeds and so do its wheels, but a car engine always spins much faster. The gearbox matches the speed of the engine to the speed of the wheels for a range of practical conditions: pulling away, overtaking other vehicles, crawling up steep hills and cruising at top speed. That is why a gearbox has four or five gear ratios or speeds.

How It Works

The rotating clutch shaft brings power into the gearbox from the engine; this then permanently drives the layshaft. Pairs of gears on the layshaft and the main shaft convert this motion to a suitable speed. The main shaft then takes this motion out of the gearbox, transmits it to the differential then on to the road wheels.

To engage the correct gear, the driver depresses the clutch and moves the gear lever. This doesnt move the gear wheels around as you may think, they all stay in position on their shafts, and, with the exception of reverse (which works slightly differently) they all mesh continuously with one another. This means they are rotating all the time.

The clever bit is that the gears for first, second and third arent fixed to the main shaft. They are actually free to spin on bearings. Until they are selected and locked into place, they arent actually driving anything.

Gear Selection

With a manual gearbox, the engine is disengaged from the drive during gear changing, and then progressively re-engaged, by means of the clutch. To select a gear (first, second or third) the driver moves the gear lever which, by an arrangement of selector rods, slides a collar along the main shaft. A synchromesh system on all gears aids smooth changing from gear to gear. In a synchromesh system the collar, called a dog, that is fixed to the main shaft rotates with it, is moved by a selector rod to engage with a cone (cone not shown) on the front of the gear wheel to be engaged. Friction between the collar and the cone acts on the freely rotating gearwheel to smoothly bring its rotational speed up to that of the main shaft. When both gear

and collar (see diagrams, left and right showing the principle) are rotating together, dog teeth on the gear engage with an outer toothed ring on the collar locking the two together.

First motion shaft (from engine)

Main output shaft takes motion to the cars differential and then on to the wheels.

To Change Gear Follow the arrows for each gear. The red dog is the one doing the locking. Third (not shown) works by the same principle as first and second. Fourth gear locks the main shaft directly to the clutch shaft. Reverse slides a little idler gear into place.

Toothed collars called dogs spin freely inside the claws of the selector forks, and slide along the splines on the main shaft. The teeth then slip into holes in the gear wheels, locking them onto the shaft.


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Automatic Transmission Cars with automatic transmissions have a gear lever of a kind although instead of numbers they show letters. D (drive), R (reverse), P (park), N (neutral). Most also show 1 and 2, selecting one of these will hold the car in the selected gear preventing it from changing, very useful on steep hills. Automatics have a normal hand brake but also use an additional lock provided by placing the gear lever in the P (park) position (see Brakes). The main brake pedal must be applied to permit shifting the transmission out of P (park).

The engine can only be started with the gear lever in P (park) or N (neutral) to prevent the car from moving when starting.

An automatic unit is much more complicated than a manual one and has at its heart a series of epicyclic gears, which are selected mechanically. Changes are effected automatically by a complex sequence of hydraulically controlled commands. This works in conjunction with a torque converter or fluid flywheel, which transmits the engines power using hydraulic fluid to an automatic gearbox. It therefore does not require a clutch pedal meaning there are only two pedals in the cockpit, an accelerator and a brake.

A simpler system that makes fewer demands on the engine, and is therefore more economical, is continual variable transmission. This initially used rubber belts in conjunction with pulleys that expanded and contracted to alter the engines power ratios. On the newer versions, however, this function is undertaken by a steel belt contained within the transmission casing.

Drive Lines In a front-wheel drive vehicle (see diagram on Page 2 ), power is conveyed by gearing to a differential that is inside the engine/gearbox unit. The differentials function is to permit cornering so that the outer-driven wheel turns faster and further than the inner one. Drive is transferred to each wheel by a constant velocity joint that can also absorb steering forces.

On a rear-wheel drive vehicle, power is transmitted from the gearbox to the rear-located differential via a propeller (prop) shaft. It is then conveyed to the wheels by half-shafts (in the case of a live rear axle) or universally jointed drive shafts (if independent rear suspension is employed).

Prop shaft

Component layout of rear wheel drive vehicle


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Steering

The most popular steering system is rack and pinion. Power-assisted steering, which is hydraulically activated by an engine-driven pump and previously the preserve of expensive cars, is becoming increasingly popular.

Steering on cars is achieved by means of a steering rack. A tube attached to the vehicle body contains a rod with teeth machined into it, known as a rack. A pinion gear connected to the steering wheel by the steering column moves the rack left or right when the steering wheel is turned. (see diagrams below )

At the end of the rack a track rod is attached by a ball joint, this allows for the suspension movement. A rubber gaiter seals the rack to prevent dirt from entering.

At the end of the track rods are the track rod ends connecting the steering to the steering arms.

These are attached to the wheel hubs and move the wheels to left and to the right as the steering wheel is turned.

Details of a steering rack when stationary. Details of a steering rack

when turned. As the steering wheel is turned the movement is transmitted down the steering column to the pinion. As the pinion turns it moves the rack. This movement is transmitted through the steering arms to the wheels which move accordingly.

Coil Spring MacPherson Strut Assembly

CV Driveshaft

Sway Bar Mount Bushing

Gaiter Rack & Pinion Steering Gear

Lower Control Arm & Ball Joint Assembly

Control Arm Bushing CV Boot

Outer Constant Velocity (CV) Joint

Ball Joint

Inner Socket Assembly

Outer Tie Rod End

Strut Rod Bushing

Steering Arm (Attached to the Hub)

Suspension Arm

Track Rod

Track Rod End

Steering Rack Gaiter

Suspension Top Bearing

Coil Spring

McPherson Strut

Brake Disc


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Steering - continued The track rods are adjustable; these allow the tracking of the front wheels to be adjusted. Vehicles with incorrect tracking will suffer excessive tyre wear. Wheels with the front distance between them less than the rear are said to toe in. Wheels with the front distance between them greater than the rear are said to toe out. Correct tracking will extend tyre life and improve vehicle handling.

Power Assisted Steering

Power assisted steering is a system that aids the steering of a vehicle by use of a hydraulic assisted steering rack or by an electric motor driven pump, that amplifies the turning force (torque), applied to the steering wheel by the driver.

Most modern power-steering systems consist of hydraulic boosts applied to the steering rack. Rotation of the steering wheel activates a valve that directs oil, pressurized by a pump driven by the engine, to act on a piston. The hydraulic boost acts only while the steering wheel is moving.

Recently some vehicles use an electro-mechanical power steering system where an electric motor is geared to the steering column shaft. The motor is controlled by an E.C.U (Electronic Control Unit) which receives information from sensors in the steering linkage. This controls the motors direction and amount of assistance applied to the steering column through the gears linking the motor to the shaft when the driver applied force to the steering wheel.

Front

Toe-Out

WHEELS

Front

Parallel

WHEELS

Front

Toe-In

WHEELS


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Suspension Independent suspension is a system in which each wheel is able to act in isolation to the others. One of its most important advantages is that it keeps the wheels vertical and the tyres on the roadway regardless of body roll, and therefore improves the cars road-holding ability (the car sticks to the road better). One version employs two unequal length wishbones, with coil springs and a separate shock absorber providing the suspension medium. In the alternative more common MacPherson strut system (see diagram) there are no wishbones and the coil spring fits over the shock absorber in one assembly.

The function of the shock absorber in the suspension system of a vehicle is to damp out oscillation; 'damper' being the correct term for a shock absorber.

The spring is the means of absorbing shocks due to the wheel passing over holes and bumps in the road, but without dampers the result would be uncomfortable, swaying and bouncing. Originally dampers were solid, of a friction type, which gave poor dynamic control and wore badly. Hydraulic dampers are now used, which consist of a

piston sliding inside a cylinder filled with oil, the flow of which is controlled by valves. In this way a resistance to motion in either direction is provided. One end of the shock absorber tube connects to the suspension; the other end of the rod is connected to the vehicles body. When the suspension spring is affected by an uneven road its nature is to bounce up and down, the shock absorber dampens the springs natural bounce.

If a shock absorber fails or the oil leaks out, the vehicle will continue to bounce after hitting a bump. This seriously affects the handling of the vehicle, making it unstable, difficult to steer, and seriously compromising its ability to stop in a straight line.

Steering Arm (Attached to the Hub)

Suspension Arm

Track Rod

Track Rod End

Steering Rack Gaiter

Suspension Top Bearing

Coil Spring

McPherson Strut

Brake Disc


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Tyres

Tyres are your only point of contact with the road so it should go without saying that they need to be regularly inspected.

Look at the tyre tread to ensure they are wearing evenly (see Motoring Checklist - Tyres). Check they have the right amount of tread depth THE LEGAL MINIMUM REQUIREMENT ON ALL CARS IS 1.6mm, measure if you are unsure; particularly in areas where the tread looks shallower (check the reason for that - there may be a problem with steering or suspension). A tyre with bald patches is not only illegal its unsafe. Look for signs of damage, cuts and bulges are a potential blow-out.

Know the correct air pressure for your car. Incorrect inflation will damage the tyres and seriously shorten their lifespan. If you are going to transport heavy loads in your car (e.g. moving house or going on holiday with a lot of luggage) look in the handbook to se if you need to temporarily increase the air pressure in the tyres.

Tracking (see Steering)

Wheel Balance Tyres should be balanced when fitted to the wheel. This involves spinning the wheel and tyre on a special machine and fitting small weights to the wheel to make them spin true. Occasionally the weights can fall off causing Wheel Imbalance. This can be felt as vibration either through the steering, (front imbalance) or throughout the car (rear imbalance) at a certain speed, typically 50mph. Wheel

balancing should be done as soon as possible by a tyre dealer or a garage because, if left unchecked, there is potential for damage to steering and suspension.

Wear on Both Sides. Under-inflation will cause overheating of the tyre because the tyre will flex too much, therefore the tread will not sit correctly on the road. This will cause loss of grip, excessive wear, and possible sudden tyre failure because of a build-up of heat. Check and adjust pressures. Incorrect wheel camber (wear on one side) repair or renew suspension parts. Hard cornering reduce speed.

Over Inflation. An over inflated tyre will wear rapidly in the centre part of the tread. The grip will be seriously reduced; the ride harsher, and there is a greater danger of shock damage in the tyre casing. Check and adjust pressures. You may sometimes have to inflate your cars tyres to higher pressures specified in you handbook for maximum load or sustained high speed. Dont forget to reduce pressures to normal afterwards.

Wheel Misalignment. Front tyres may wear unevenly as a result of wheel misalignment. Most tyre dealers and garages can check and adjust the wheel alignment (tracking) for a small fee. The suspension may be malfunctioning. Repair or renew suspension parts. Unbalanced wheel. Balance tyres. Incorrect toe setting. Adjust front wheel alignment.

Shoulder Wear Centre Wear Uneven Wear


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Brakes Most cars use Disc Brakes on their front wheels although these are fitted on front and back wheels on the more expensive models. When the brake pedal is applied, hydraulic pressure is applied to callipers that grip the disc slowing and stopping the car. Drum Brakes, that use internally actuated shoes, are often fitted at the rear.

All cars feature a hand or Parking Brake that operates on the vehicles rear brake shoes or discs. Disc Brakes A frictional brake is one where a fixed part is brought into contact with a moving part which has to be slowed or stopped. In a car, each wheel has a hub-mounted disc and a brake unit or calliper rigidly attached to the suspension. The calliper has two friction-pad assemblies, one on each side of the disc. When the brake is applied, hydraulic pressure* forces the friction pads against the disc. The disc is gripped by the two pads in a similar way that the wheel rim of a bicycle is gripped by its brake blocks. * (Hydraulics the transmission of braking force from the brake pedal to the brakes by the use of liquid pressure and a piston. In a car the liquid is brake fluid.) Diagram 1: When the brake pedal is pressed, high pressure brake fluid (red) forces the piston out and at the same time forces the whole calliper back to cause both pads to clamp onto the disc. The calliper floats on the calliper pin. The brake pads themselves are made of a material that withstands the great temperatures generated when braking. However, the disc is open to the atmosphere except where it passes through the caliper, so cooling is very effective. This arrangement is self-adjusting, and the ability of the discs to dissipate heat rapidly in the open air stream makes them practically immune to fading. They grip the metal disc even when wet and will wear away rather than the disc. It is cheaper and easier to replace two disc pads rather than the disc itself.

When the brakes are released, the hydraulic pressure drops. The low residual pressure (blue) allows the seal to draw the piston back freeing the disc from the pads and releasing the brakes. The seal twists and deforms when the brakes are applied and it is the recovery from this deformed state which causes the piston and pads to release. Most systems allow a minimal pad drag even when the brakes are released. However, note that the piston can also slide through the seal providing a self adjusting action to compensate for pad wear.

With disc brakes a larger brake force is needed to operate the system, so the driver/pedal action is therefore usually assisted by a Servo-mechanism. (See Diagram 2) The servo uses the vacuum from the inlet manifold of a petrol engine, or a vacuum pump from a diesel engine, to convert the relatively small amount of force applied when the brake pedal is pressed into the larger force needed to operate the braking system.

Calliper

Disc

Disc

On Off

Vented Discs High Performance Car

Disc

Cross section through disc brakes showing the hydraulic operation.

(Diagram 1)


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Drum brakes. Drum brakes are mainly used on the rear of some small cars, the drum itself being mounted on the wheel hub and rotating with the wheel. The brake components are either mounted on a back plate, fixed to the axle, or the suspension arm.

When the brake pedal is depressed, two pivoted brake shoes (Diagram 3 - 3a & 3b) are forced apart at their free ends to press against the inside of a brake drum. Wheel cylinders ( 9.) are located between the movable ends of the brake shoes, and each is fitted with two pistons that are forced outward toward the ends of the cylinder by the pressure of the fluid between them. As these pistons move outward, they push the brake shoes against the inner surface of the brake drum which is attached and rotates with the wheel. The pivot at the fixed end of the brake shoes contains an adjuster to compensate for the wear on the brake shoes. (see also diagram on next page)

Rear Wheel Drum Brake 1. Brake back (cover) plate 2. Brake shoe adjuster 3a. Brake shoe, rear (trailing) 3b. Brake shoe, front (leading) 4. Bottom return spring 5. Spring plate 6. Tensioning pin 7. Brake shoe return spring 8. Thrust rod 9. Brake wheel cylinder 10. Compression spring 11. Spring plate 12. Thrust pin 13. Brake shoe locating plate 14. Hand brake operating lever (shown as a dashed outline) Much heat is created during braking and may cause drum brakes to fade and lose their effectiveness through reduced friction between brake drum and the shoes because of the high temperatures. For this reason most cars and some motor cycles use disc brakes, at least on the front wheels.

Direction of rotation

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(Diagram 3)

Vacuum Rear wheel Brake circuit

Front wheel Brake circuit

Brake Pedal

Brake servo

Servo vacuum pipe

Tandem brake master cylinder

Front disc brake

Rear drum brake

Brake shoe

(Diagram 2)

This vehicle brake system operates as two independent ones. Should there be a failure in either circuit; some braking is still available to stop the vehicle.


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Hydraulic Brake Fluid Hydraulic brake fluid has a reservoir which is found under the bonnet and constitutes one of the checks to be made along with water, oil, etc. A word of caution, as the brake pads wear, the brake pistons push outwards which means that small reservoir behind the piston fills up with fluid. This means that the level of fluid may need topping up if it goes near the Minimum mark on the reservoir under the bonnet. When the brake pad is replaced, the piston is pushed back which in turn pushes the fluid out of the piston reservoir, back up into the main reservoir. If this is already full, because it looked low and was topped up, the excess fluid will overflow.

(see also Weekly Checks Brake Fluid Level) TTAAKKEE CCAARREE;; brake fluid is very toxic, flammable, and will damage paintwork if spilt on the vehicle. Antilock braking systems (ABS) Antilock braking systems (ABS) became available in the late 1980s and have subsequently become standard equipment on a growing number of cars. ABS installations consist of wheel-mounted sensors that input wheel rotation speed into a microprocessor. When the brakes are applied and the E.C.U (Electronic Control Unit) senses that the wheels are about to lock-up, (tyres start skidding or a loss of traction) the control unit signals a hydraulic or electric modulator to regulate brake line pressure to stop impending wheel lockup.

The brake continues to work as the system alternately releases and applies brake pressure, felt by the driver as a pulsing sensation through the brake pedal. The wheels meanwhile continue to roll, retaining the driver's ability to steer the vehicle and stop in a shorter distance. Hand (Parking) Brake Hand brakes must be mechanically operated, applying force only to the rear brakes (with a few rare exceptions) by means of a flexible cable connected to a hand lever or pedal. Automatic Transmission - Park On cars with automatic transmissions, an additional lock is usually provided in the form of a pawl (a pivoted lever that locks into a toothed ratchet) that can be engaged, by placing the gear lever in the P (park) position, to prevent the drive shaft and rear wheels from turning. The main brake pedal must be applied to permit shifting the transmission out of P (park). This stops the possibility of undesired vehicle motion that could be caused by accidental movement of the transmission control. The engine can only be started with the gear lever in P (park) or N (neutral).


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Motoring Checklist Visual vehicle checks before you get in !

p Are the tyres inflated? p Can you see through all the windows? p Are the lights and number plates clean? p Is anything leaking from underneath?

Weekly checks; Your vehicles hand book will tell you how to maintain your car and keep it in good condition. Below is just a general guide including warnings where appropriate. Various fluids used in cars can be quite nasty and so can the battery, so take the warnings here and in the handbook seriously.

Engine Oil Level Before you start make sure your car is on level ground. Check the oil level before you drive, or at least 5 minutes after the engine has been switched off to give oil in the upper part of engine a chance to settle. Besides that, the engine will be HOT! Check the oil level using the dipstick in the usual way it should be about half way between Min and Max. If it needs topping up make sure you use the correct oil for your vehicle and NEVER OVERFILL IT.

Coolant (Water/Antifreeze mixture) DO NOT ATTEMPT TO CHECK COOLANT LEVEL WHEN THE ENGINE IS HOT, the coolant will be too! There is a great risk of scalding. Adding coolant should not be necessary on a regular basis, if you have to, check the system for leaks. If topping-up is necessary add a mixture of water and antifreeze to the expansion tank up to the correct level taking care to securely replace the pressure cap.

Finally - make sure the antifreeze is safely stored, it is poisonous.

Screen Washer Keep the washer bottle filled up with water and screenwash additive diluted according to its instructions. The additive not only cleans better than plain water it stops the washer system freezing in cold weather. (N.B. NEVER use antifreeze in the washer bottle.)

Wipers Check the wiper blades for damage or cracks. If the glass swept area is smeared change the blades. Your handbook should tell you how.

Tyres First a visual check for any foreign objects caught in the tread. If removal reveals that the tyre has been punctured, replace the object to mark the spot, and change the wheel for the spare. Your tyre dealer will advise you as to whether you need a new one or it can be repaired. Also look for bulges and cuts particularly in the sidewalls, accidental kerbing can damage tyres and the wheels.

Check the tyre pressures regularly with the tyres cold. The vehicle handbook will advise you about the correct pressure for general use and also for when the car is fully laden.

Finally, look at the tyre tread and check they are wearing evenly (see main section Tyres). Check they have the legal amount of tread depth THE LEGAL MINIMUM REQUIREMENT ON ALL CARS IS 1.6mm, measure if you are not sure particularly in areas where the tread looks shallower (check the reason for that).

Brake Fluid Level Make sure your car is on level ground. Always buy fresh fluid, appropriate for your vehicle for the job (see the handbook) and handle it with care, it can damage eyes and paintwork. DONT USE ANY THAT HAS BEEN OPEN for some time, it absorbs moisture from the air which can dangerously affect the braking efficiency.

Refer to the handbook for how to check the level. The fluid level will drop slightly as the brake pads wear but never let it drop below Minimum. If you spill any fluid always replace the cap on the reservoir before flushing the spill away to avoid contamination. If you have to repeatedly top it up, do not use the car until youve had the brakes professionally checked. There could be a leak in the system, NEVER TAKE RISKS WITH BRAKES.


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The Battery Under normal operating conditions the battery requires little maintenance. Do a visual check to make sure all looks normal the terminals are clean and that the battery is still firmly secured in its holder. Your handbook will explain how to service it. Things to beware of;-

1 Do not use an open flame or cause a spark when checking the battery. They give off hydrogen gas; it is flammable and may explode.

1 Do not let battery acid come into contact with skin, eyes, fabric or paintwork. If you get electrolyte on your skin or in your eyes immediately wash with cold water and consult your doctor.

1 Switch off the current before disconnecting battery terminals and always disconnect the earth terminal (negative (-) one first and reconnect it last.

See also HELP section My Battery is flat. Power Steering (where appropriate) Check your handbook for this one. Some vehicles do not require this as weekly check because the power steering is part of the power hydraulic system. Generally, if you do;- Park the vehicle on level ground. Set the steering straight ahead and then turn off the engine. Do not turn the steering once the engine is stopped or the reading will be inaccurate. IT IS ESSENTIAL to use the correct power steering fluid for your vehicle depending on the year of manufacture and type of system fitted. The handbook should guide you, any doubts dont do it, get the professionals. NEVER TAKE RISKS WITH STEERING ! Lights, Bulbs, and fuses The spares should be replaced in your vehicles spares box as and when you use them. Again, your handbook should tell you how to change bulbs and fuses on your type of car. Make sure you check the vehicle lights on a regular basis particularly if there are no warning lights in the cockpit to tell when there is a failure. If you have to check the lights unaided, back up to a wall or garage door and use the reflected light to check if they are working. The horn will be checked for the MOT test, better done by you first! HELP! Ive put the wrong fuel in my car.

DONT EVEN START IT UP. Diesel into petrol / petrol into diesel, either way serious damage will result if you run the engine which neither the vehicle warranty or the insurance will cover. The fuel tank will need draining. Call for help! Im on the motorway and the dashboard has suddenly lit up like a Christmas tree can I carry on to the next exit? NO! STOP as soon as it is safe to do so. Dont assume its an electrical fault. e.g. - If youve lost the oil the engine could seize, not good in the middle of a carriageway. Call for help. Im driving along and smoke or steam is coming out from under the bonnet. STOP as soon as it is safe to do so:-

1. If its smoke you will probably have smelt it already. GET OUT. Phone the fire brigade. If you are tempted to use the extinguisher DONT OPEN THE BONNET, spray it in through a gap in the radiator grill. If in any doubt stay well away and wait for help.

2. If its steam, wait for things to cool down before you very carefully open the bonnet. Chances are something has broken to cause the cloud of steam, and you cant drive very far without coolant. You might as well call for help whilst youre waiting. My cars battery is flat. Modern cars wont push start on a dead battery, electronic fuel pumps need power. No power no fuel ! Neither will automatics no clutch you can play with. An older car may - get lots of helpers and a safe runway. Sit in the drivers seat, car in neutral, handbrake off. Now they push. When you are rolling depress the clutch and put the car in third gear. Ease the clutch in gently and


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hopefully the engine will fire. Keep it running. Only try this couple of times, if it doesnt work give up! You must put right whatever it was that made the battery go flat in the first place. Here are some possibilities;

# The battery has drained because of repeated attempts to start the vehicle. Something is wrong with the engine, get help.

# The lights have been left on for a period of time. An interior light left on overnight can flatten the battery. # The charging system is not working properly (alternator the drive belt the wiring). # There is something wrong with the battery. Low electrolyte, or worn out.

After checking, try recharging the battery, or get a new one. A jump start could get you out of trouble but

WARNING;- Jump starting your vehicle is a last resort after all else has failed because of the risk of severe damage to modern electronic components. When jump starting a car using a booster battery or another car you must take some precautions;-

1 If the vehicles have an onboard computer (E.C.U.) you will need SPECIAL JUMP LEADS WITH A SURGE PROTECTION DEVICE. If you dont have any dont attempt it, you could fry everyones electrics.

1 Before connecting the other battery make sure that the ignition is switched off (both vehicles if youre using another car as a donor).

1 Make sure all other electrical equipment is switched off lights, wipers, heater, etc. 1 Make sure the booster battery is the same voltage as the discharged one in the vehicle. 1 If the dead battery is being jump-started from the battery in another vehicle, the two vehicles MUST NOT

TOUCH each other.

1 Ensure the vehicles are out of gear; in an automatic make sure it is in either neutral N or park P.

Jump Starting a Car (see diagram) Connect one end of the red jump lead to the positive (+) terminal of the flat battery. Connect the other end of the red lead to the positive (+) terminal of the booster battery. Connect one end of the negative jump lead to the negative (-) terminal of the booster battery. Connect the other end of the negative jump lead to a bolt or bracket on the engine block, well away from the battery, on the vehicle to be started.

a Ensure the jump leads will not come into contact with

the fan, drive belts, or other moving parts of the engine.

a Start the donor car, then start the engine. With the engine/s running at idle speed, disconnect the jump leads in REVERSE ORDER OF CONNECTION.

In Conclusion.

Properly looked after, your car should to be of little trouble. However, if you are not considering a career as a mechanical engineer, or even if you are, you may want to join a reliable rescue service. Not all breakdowns can be fixed at the roadside, and these lovely people are paid to crawl out of bed at 3 oclock in the morning in the middle of the winter to get us out of trouble!

Happy Motoring.

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