A contact breaker (or "points") is a type of electrical
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
, found in the
ignition system
Ignition systems are used by heat engines to initiate combustion by igniting the fuel-air mixture. In a spark ignition versions of the internal combustion engine (such as petrol engines), the ignition system creates a spark to ignite the fuel-ai ...
s of spark-ignition
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...
s. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch is set so that a spark is produced at the right time to ignite the compressed air/fuel mixture in the cylinder of the engine. A mechanism may be provided to slightly adjust timing to allow for varying load on the engine. Since these contacts operate frequently, they are subject to wear, causing erratic ignition of the engine. More recent engines use electronic means to trigger the spark, which eliminated contact wear and allows computer control of ignition timing.
Purpose
The purpose of the contact breaker is to interrupt the
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (hydr ...
flowing in the primary winding of the
ignition coil
An ignition coil is used in the ignition system of a spark-ignition engine to transform the battery voltage to the much higher voltages required to operate the spark plug(s). The spark plugs then use this burst of high-voltage electricity to ig ...
. When the current stops flowing, the resulting collapse of the magnetic field in the primary winding
induces a high voltage in the secondary winding. This causes a very high
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
to appear at the coil output for a short period—enough to arc across the electrodes of a
spark plug
A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air ...
.
[Horst Bauer, (ed), "Automotive Handbook 4th Edition", Robert Bosch GmBh, 1996 ISBN 0-8376-0333-1, pp. 445-449]
Operation
The contact breaker is operated by an engine-driven
cam
Cam or CAM may refer to:
Science and technology
* Cam (mechanism), a mechanical linkage which translates motion
* Camshaft, a shaft with a cam
* Camera or webcam, a device that records images or video
In computing
* Computer-aided manufacturin ...
. On an engine with a
distributor
A distributor is an electric and mechanical device used in the ignition system of older spark-ignition engines. The distributor's main function is to route electricity from the ignition coil to each spark plug at the correct time.
Design
...
, the contact breaker can be found beneath the distributor cap. The position of the contact breaker is set so that it opens (and hence generates a spark) at exactly the optimum moment to ignite the fuel/air mixture. This point is generally just before the piston reaches the top of its compression stroke. The contact breaker is often mounted on a plate that is able to rotate relative to the
camshaft
A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition syst ...
operating it. The plate is most typically rotated by a centrifugal mechanism, thus advancing the
ignition timing
In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
The need ...
(making the spark occur earlier) at higher revolutions. This gives the fuel ignition process time to proceed so that the resulting combustion reaches its maximum pressure at the proper point in the
crankshaft's rotation.
[
Many engines are also fitted with a ]manifold vacuum
Manifold vacuum, or engine vacuum in a petrol engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.
Manifold vacuum is an effect of a piston's movement on the induction stroke and the airflow thr ...
-operated servomechanism
In mechanical and control engineering, a servomechanism (also called servo system, or simply servo) is a control system for the position and its time derivatives, such as velocity, of a mechanical system. It often includes a servomotor, and ...
to provide additional rotation of the plate's position (within limits), in order to advance the timing when the engine is required to speed up on demand. Advancing the ignition timing helps to prevent pre-ignition Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated ...
(or pinging).[
]
Materials
Disadvantages of contact breakers
Since they open and close so often (several times with every turn of the engine on distributor-equipped engines), contact breaker points and cam followers can suffer from wear—both mechanical and pitting caused by arcing across the contacts. This latter effect is largely prevented by placing a capacitor
In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
parallel
Parallel may refer to:
Mathematics
* Parallel (geometry), two lines in the Euclidean plane which never intersect
* Parallel (operator), mathematical operation named after the composition of electrical resistance in parallel circuits
Science a ...
across the contact breaker—this is often referred to by the more old fashioned term condenser by mechanics. As well as suppressing arcing, it helps boost the coil output by creating a resonant LC circuit
An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together. The circuit can act ...
with the coil windings.[
A drawback of using a mechanical switch as part of the ignition timing is that it is not very precise, needs regular adjustment of the dwell (contact) angle, and at higher revolutions, its mass becomes significant, leading to poor operation at higher engine speeds. These effects can largely be overcome using electronic ignition systems, where the contact breakers are ]retro
Retro style is imitative or consciously derivative of lifestyles, trends, or art forms from the past, including in music, modes, fashions, or attitudes. It has been argued that there is a nostalgia cycle in popular culture.
Definition
The term ...
fitted by a magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...
ic (Hall effect
The Hall effect is the production of a voltage, potential difference (the Hall voltage) across an electrical conductor that is wikt:transverse, transverse to an electric current in the conductor and to an applied magnetic field wikt:perpendicul ...
) or optical sensor
A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal.
In the broadest definition, a sensor is a devi ...
device. However, because of their simplicity, and since contact breaker points gradually degrade instead of catastrophically failing, they are still used on aircraft engines.
See also
* Ignition magneto
An ignition magneto (also called a high-tension magneto) is an older type of ignition system used in spark-ignition engines (such as petrol engines). It uses a magneto and a transformer to make pulses of high voltage for the spark plugs. The old ...
* Contactor
A contactor is an electrically controlled switch used for switching an electrical power circuit. A contactor is typically controlled by a circuit which has a much lower power level than the switched circuit, such as a 24-volt coil electromagne ...
* Relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switc ...
References
{{Automotive engine , collapsed
Automotive ignition systems
Automotive electrics