The distributor both sends the HT current to the correct sparkplug and ensures
that it arrives at the best time for maximum efficiency.
For an engine to work at its best, the fuel/air mixture in each cylinder must fire just as the piston reaches top dead centre (TDC).
It takes a certain time for the spark-plug to ignite the mixture and for the combustion to build up. This time stays roughly the same no matter how fast the engine is running.
The timing mechanism is set to fire the plug a short time before the TDC. But because the mechanism is worked by the motion of the engine, this time would normally decrease as the engine ran faster, and the plug would fire too late.
So a mechanical device is fitted to advance firing - make it happen earlier - with increasing engine speed.
The load on an engine - whether it is pulling hard or cruising - also affects the timing.
A lightly loaded engine works best if the ignition is advanced an extra amount. A second vacuum-operated device controls this independently of the first.
The centrifugal advance mechanism responds to engine speed. It is usually in the bottom of the distributor body under the contact-breaker baseplate.
Two steel weights are attached to a revolving plate on the distributor shaft by pivots, and held in the closed position by strong springs.
As the engine speeds up, centrifugal force throws the weights outwards.
They turn on their pivots, twisting the contact-breaker cam forwards so that the points open earlier, and the sparkplug fires earlier as the speed increases.
The vacuum advance mechanism responds to the vacuum in the engine inlet manifold, which is caused by the suction of the moving pistons. When the engine is lightly loaded the vacuum increases.
A narrow pipe runs from the manifold to a vacuum chamber on the distributor, inside which there is a flexible diaphragm.
As the vacuum increases, the diaphragm bends, moving a rod connected to its centre which causes the contact-breaker baseplate to swivel slightly. This moves the contact-breaker heel relative to the distributor cam and advances the ignition.
When the engine is under load, vacuum decreases, the diaphragm springs back and the ignition is retarded to suit the changed conditions.
Many newer cars have an electronic ignition system which times the spark more precisely than a mechanical system.
In this electronic ignition system, steel ridges on a rotor create a small voltage
as they pass through the magnetic field of the triggerhead.
Magnetic strips embedded in the rotor create the voltage in this system each time
they are opposite the triggerhead, which triggers a power transistor.
It also wears less, so that it is always at peak efficiency, and it overcomes one problem of a mechanical system: at high engine speeds a mechanical system does not work at peak efficiency.
Electronic systems may be of the inductive discharge or capacitive discharge type.
An inductive discharge system is the type usually fitted as original equipment on cars with electronic ignition. It produces high-tension (HT) current in the normal way: by switching low-tension (LT) current off and on in a coil.
In the simplest inductive discharge system, the transistor-assisted contacts (TAC) type, there is also a normal contact breaker.
It carries only a very small current, which is fed to a power transistor which switches on and off the heavier LT current to the coil.
The contact-breaker points are not eroded by the small current, so they stay clean for longer, and the gap seldom needs resetting.
More advanced, fully electronic systems may not have points. Instead, the distributor contains another form of triggering device for the power transistor which relies on electrical pulses instead of a mechanical make-and-break method.
In one type there is an electromagnetic coil and a revolving spiked rotor with one steel spike for each cylinder.
Every time a spike moves past the coil it creates a small voltage which triggers the transistor.
Some other types may have optical or magnetic triggers - they all perform the same function.
A capacitive discharge (CD) system - used in some do-it-yourself kits produces HT current in the coil by sending a large pulse from a capacitor through the primary winding.
The capacitor is an electrical storage device which can be charged and discharged very rapidly.
The secondary windings of the coil produce HT current both at the moment when the LT current in the primary windings is switched on, and at the moment it is switched off.
Because a capacitor can give a very large pulse very fast, there is always a strong spark, irrespective of the speed of the engine.
The timing in this system may again be fully electronic or it may use the contact-breaker points.
The usual way of adjusting the timing is to slacken the clamping bolt of the distributor and turn the whole unit slightly.
The amount by which the two advance mechanisms change the timing is not adjustable.
Some earlier distributors have a knurled nut on the vacuum advance mechanism, by which you can alter the timing as a whole (not just the action of the mechanism).
This article explains how the ignition system of a car works to start the engine.
The purpose of the ignition system is to generate a very high voltage from the car's 12 volt battery, and to send this to each sparkplug in turn, igniting the fuel-air mixture in the engine's combustion chambers.
The coil is the component that produces this high voltage. It is an electromagnetic device that converts the low-tension (LT) current from the battery to high-tension (HT) current each time the distributor contact-breaker points open.
The distributor unit consists of a metal bowl containing a central shaft, which is usually driven directly by the camshaft or, sometimes, by the crankshaft.
The bowl houses the contact-breaker points, rotor arm, and a device for altering the ignition timing. It also carries the distributor cap.
The distributor cap is made of nonconductive plastic, and the current is fed to its central electrode by the HT lead from the centre of the coil.
Inside the cap there are more electrodes often called segments to which the sparkplug leads are connected, one per cylinder.
The rotor arm is fitted on top of the central shaft, and connects to the central electrode by means of a metal spring or spring-loaded brush in the top of the distributor cap.
The current enters the cap through the central electrode, passes to the centre of the rotor arm through the brush, and is distributed to each plug as the rotor arm revolves.
As the rotor arm approaches a segment, the contact-breaker points open and HT current passes through the rotor arm to the appropriate sparkplug lead.
The contact-breaker points are mounted inside the distributor. They act as a switch, in synchronisation with the engine, that cuts off and reconnects the 12 volt low-tension (LT) circuit to the coil.
The points are opened by cams on the central shaft, and are closed again by a spring arm on the moving contact.
With the points closed, LT current flows from the battery to the primary windings in the coil, and then to earth through the points.
When the points open, the magnetic field in the primary winding collapses and high-tension (HT) current is induced in the secondary windings.
This current is transferred to the sparkplugs through the distributor cap.
On a four-cylinder engine there are four cams. With each full rotation of the shaft the points open four times. Six-cylinder engines have six cams and six electrodes in the cap.
The position of the points and the distributor's body in relation to the central shaft can be adjusted manually.
This alters the timing of the spark to obtain an exact setting (seeHow engine timing works).
Further changes occur automatically as the engine speed varies according to the throttle opening.
In some modern ignition systems, micro-electronics ensure the optimum ignition timing for all engine speeds and engine load conditions (see How engine timing works).
The sparkplugs are screwed into the combustion chambers in the cylinder head.
HT current passes from each segment on the distributor cap down the plug leads to the plug caps.
It then passes down the central electrode, which is insulated along its length, to the nose of the plug.
A side electrode connected to the plug body protrudes just below the central one, with the gap between the two usually set from 0.025 in. (0.6 mm) to 0.035 in. (0.9 mm).
The current sparks across this gap, flows along the side electrode, through the plug body and the engine, then back to the coil, completing the circuit.
A typical fixed-jet carburettor. This is a Solex, and air flow is from top to bottom.
This is called a down-draught carburettor.
The fixed-jet carburettor resembles the simpler variable-jet type (See How variable-jet carburettors work) in having a venturi - a constricted neck - through which air flows on its way to the engine.
The partial vacuum caused by increased air speed through the venturi sucks fuel through a jet to mix with the air.
Similarly, air flow is controlled by a throttle flap linked to the accelerator pedal, to regulate engine speed.
Above the throttle a choke flap partially blocks the air flow, to give a richer mixture for starting. As in all carburettors, a float chamber provides a steady supply of fuel.
The fixed-jet carburettor has open jets to regulate fuel flow through them. Consequently there must be several jets of different sizes to provide the different amounts of fuel needed at any moment.
When the engine is idling, very little fuel is required. There is not much air flow through the almost closed throttle - too little to draw any fuel through the main jet in the venturi.
But there is a high vacuum underneath the throttle flap, where there is a tiny slow-running jet that forms part of the often complex slow-running (idling) circuit. The vacuum pulls a trickle of fuel through this jet to keep the engine idling.
When the throttle is opened, the air flow suddenly speeds up. An accelerator pump linked to the throttle provides a brief squirt of extra fuel to enrich the mixture temporarily to prevent a flat spot - a momentary hesitation - which is the inability of the carburettor to provide the correct mixture to meet the sudden power demand.
The pressure to supply this squirt comes from a rubber diaphragm open to the air on one side. Normal air pressure, higher than the partial vacuum inside the carburettor, pushes the diaphragm inwards against a piston, which pumps fuel.
Afterwards, the fast air flow sets up a vacuum in the venturi which draws fuel from the main jet. The faster the flow, the more fuel is sucked out. Most carburettors have one or more non-return valves, usually a small ball seating on a conical hole. This prevents wasted flow-back of fuel.
By itself the main jet is not accurate enough to supply exactly the right amount of fuel over the full range of engine speeds. It tends to provide too much at high speeds.
There are several devices for avoiding an over-rich mixture. Depending on type, a fixed-jet carburettor may have one or more of them.
In the compensation system, the fuel supply from the float chamber is split in two. One branch leads straight to the main jet. On the other branch, air leaks into the fuel through a small jet. The faster the fuel flow, the more air leaks in and the weaker the final mixture.
In the air-correction system all the fuel goes through the main jet, but instead of going directly into the venturi it first passes through a vertical well containing a perforated emulsion tube.
At the top of the emulsion tube is a small jet, open to the air. It allows air to bubble into the fuel through the holes in the tube.
When the car is cruising, engine speed is high but the throttle is not wide open. Some carburettors have an economy device with a rubber diaphragm connected on one side to the venturi and open to the air on the other.
The increased vacuum under the throttle in these conditions makes the diaphragm bulge inwards, opening a valve to blend extra air into the fuel and weaken the mixture slightly.
Some high-performance engines do not have carburettors. Instead, fuel is injected through nozzles directly into the air passages ahead of the cylinder inlet valves.
The SU is the simplest type of variable-jet carburettor, the other main type,
the Stromberg, has a rubber diaphragm instead of a piston.
A carburettor mixes fuel and air in the proportions and quantity the engine needs at any time.
It does this by spraying the fuel into the moving air stream through a jet, so that the fuel vaporises and makes an explosive mixture.
The faster an engine runs, the more air it sucks in. The air passes through a narrowed neck inside the carburettor (called a venturi), which speeds up its flow at that point.
As air flows faster its pressure drops, so there is a slight vacuum inside the venturi. The fuel jet opens into the venturi, and the partial vacuum sucks fuel through the jet into the air stream.
The speed of the engine is controlled by a throttle, a movable circular flap linked to the accelerator pedal which partly blocks the venturi to admit variable amounts of air.
There has to be some way of governing the fuel flow through the jet so that the mixture is right so that the fuel is in the right proportion to the air. The simplest way of doing this is with a variable jet.
The fuel jet is partly blocked by a tapered needle, which can be raised progressively to unblock it.
The needle is fixed to a piston, which is free to slide up and down in a chamber above the jet. The top of the chamber is linked to the inlet manifold through a narrow passage.
When the engine is idling, low manifold depression and a light spring causes the piston to sit at the bottom of the chamber, and the needle almost completely blocks the jet. Little fuel flows.
As the throttle opens, air flow to the engine increases. The engine speeds up and sucks in yet more air.
This suction creates a partial vacuum in the inlet manifold, and therefore also in the top of the chamber, which is connected to it.
The vacuum is more powerful than the slight vacuum in the venturi under the piston, so it draws the piston up, unblocking the jet and letting more fuel flow.
A sudden burst of acceleration causes a sudden rush of mixture into the inlet manifold, so that the vacuum there lessens for an instant.
That would allow the piston to fall, closing the jet and weakening the mixture;
but the problem is avoided by having an oil-filled damper attached to the piston,
which prevents it from moving quickly. Therefore, the mixture does not suddenly
become too weak for smooth combustion.
An engine needs an extra-rich mixture more petrol, less air for starting from cold.
On some variable-jet carburettors, this is provided for by the jet being lowered a short way, so that it is less blocked by the needle and dispenses more fuel than usual.
On others, neat fuel is atomised by the rotation of a disc of progressively larger-sized holes.
On a fixed-jet carburettor (See How the fuel system works - fixed-jet carburettors) the opposite happens: instead of more petrol being supplied to the carburettor, the air supply is partly blocked off by a choke flap above the throttle.
However, both systems are referred to as 'choke' mechanisms, or cold-start enrichment systems.
On some cars you have to set the choke yourself before starting, usually by means of a pull-push control in the dashboard, or steering column or floor pan.
Other cars have an automatic choke which uses a bimetallic coiled strip a strip made of two different metals brazed together attached to the choke lever.
When the engine is cold, the choke is 'on'. As the engine warms, so does the strip.
It expands with heat, but one of the metals expands more than the other, so that the coiled strip bends and uncurls and progressively moves the choke lever to 'off.
For a carburettor to maintain a steady fuel flow, it needs to draw on a supply of fuel which is always kept at the same level.
That supply is provided by a float chamber attached to the carburettor. The float chamber contains a pivoted float which bears against a needle valve through which fuel enters the chamber.
When the float drops, the needle valve opens. As the fuel level in the chamber rises so does the float, until, at a pre-set level, the needle closes.
This fuel system has both supply and return pipes along which petrol circulates continuously;
the carburettor draws off whatever it needs. Single-pipe systems are more usual.
A car engine burns a mixture of petrol and air. Petrol is pumped along a pipe from the tank and mixed with air in the carburettor, from which the engine sucks in the mixture.
In the fuel-injection system, used on some engines, the petrol and air are mixed in the inlet manifold.
For safety, the petrol tank is placed at the opposite end of the car from the engine.
Inside the tank, a float works an electrical sender unit that transmits current to the fuel gauge, signalling how much petrol is in the tank.
The tank has an air vent - usually a pipe or a small hole in the filler cap to allow air in as the tank empties. Some of the latest systems have a carbon filter, so that fuel fumes do not escape.
A fuel pump draws petrol out of the tank through a pipe to the carburettor.
The pump may be mechanical worked by the engine - or it may be electric, in which case it is usually next to or even inside the fuel tank.
A mechanical fuel pump is driven by the camshaft, or by a special shaft driven by the crankshaft. As the shaft turns, a cam passes under a pivoted lever and forces it up at one end.
The other end of the lever, which is linked loosely to a rubber diaphragm forming the floor of a chamber in the pump, goes down and pulls the diaphragm with it.
When the lever pulls the diaphragm down, it creates suction that draws fuel along the fuel pipe into the pump through a one-way valve.
As the revolving cam turns further, so that it no longer presses on the lever, the lever is moved back by a return spring, relaxing its pull on the diaphragm.
The loosely linked lever does not push the diaphragm up, but there is a return spring that pushes against it.
The diaphragm can move up only by expelling petrol from the chamber. The petrol cannot go back through the first one-way valve, so it goes out through another one leading to the carburettor.
The carburettor admits petrol only as it needs it, through the needle valve in its float chamber (See How variable-jet carburettors work).
While the carburettor is full and the needle valve is closed, no petrol leaves the pump. The diaphragm stays down, and the lever idles up and down.
When the carburettor accepts more petrol, the return spring pushes the diaphragm up and, by taking up the slack in the loose linkage, brings it back into contact with the lever, which again pulls it down to refill the pump chamber.
An electric pump has a similar diaphragm-and-valve arrangement, but instead of the camshaft, a solenoid (an electromagnetic switch) provides the pull on the diaphragm.
The solenoid attracts an iron rod that pulls the diaphragm down, drawing petrol into the chamber.
At the end of its travel the iron rod forces apart a set of contacts, breaking the current to the electromagnet and relaxing the pull on the diaphragm.
When the diaphragm return spring raises the diaphragm, it also pulls the rod away from the contacts; they then close so that the solenoid pulls the rod and diaphragm down again.
Most mechanical and electrical systems pump fuel only when the carburettor needs it. An alternative system has a complete circuit of pipes, from the tank to the carburettor and back again. The pump sends petrol continuously round this circuit, from which the carburettor draws petrol as it needs it.
Both petrol and air are filtered before passing into the carburettor.
The petrol filter may be a replaceable paper one inside a plastic housing in the fuel line. A pump may include a wire or plastic gauze filter, and sometimes a bowl to catch sediment.
The air cleaner is a box fitted over the carburettor air intake, usually containing a replaceable paper-filter element.
Some older cars are fitted with an oil-soaked wire-gauze element, which needs washing from time to time in petrol or paraffin, and re-oiling.
