Distributor
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Image:Distribcap.jpg Image:Rotor.jpg Image:Breakerpoints.jpg The distributor in the ignition system of an internal combustion engine is a device which routes the high voltage in the correct firing order to the spark plugs.
It consists of a rotating arm or rotor inside the distributor cap, on top of the distributor shaft. The rotor contacts the central high voltage cable from the coil via a spring loaded carbon brush. The rotor arm passes close to (but does not touch) the output contacts which connect via high tension cables to the spark plug of each cylinder. Within the distributor, the high voltage energy is able to jump the small gap from the rotor arm to the contact.
The distributor shaft has a cam that operates the contact breaker. Opening the points causes a high induction voltage in the system's ignition coil.
The distributor also houses the centrifugal advance unit: a set of hinged weights attached to the distributor shaft, that cause the breaker points mounting plate to slightly rotate and advance the spark timing with higher engine rpm. In addition, the distributor has a vacuum advance unit that advances the timing even further as a function of the vacuum in the inlet manifold. Usually there is also a capacitor attached to the distributor. The capacitor is connected parallel to the breaker points, to suppress sparking and prevent wear of the points.
Around the 1970s the primary breaker points were largely replaced with Hall effect sensors. As this is a non-contacting device and the primary circuit is controlled by solid-state electronics, a great amount of maintenance in point adjustment and replacement was eliminated. This also eliminates any problem with breaker follower or cam wear, and by eliminating a side load extends distributor shaft bearing life. The remaining secondary (high voltage) circuit was as described above, using a single coil and a rotary distributor.
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Direct ignition
Modern engine designs are tending to do away with the distributor and coil, instead performing the distribution function in the primary circuit electronically and applying the primary pulse to individual coils on top of each spark plug (Direct Ignition or coil-on-plug). This avoids the need to switch very high voltages, which is very often a source of trouble, especially in damp conditions.
Distributor caps
These are used in automobile engine to cover the distributor and its internal rotor. The rotor switches a high sparking voltage to the spark plugs so that these fire in correct sequence.
The distributor cap is a prime example of a component that eventually succumbs to heat and vibration. But even if its bakelite housing has not broken or cracked, carbon deposits and eroded metal terminals can cause distributor-cap failure. However it is a fairly easy and inexpensive part to replace.
The distributor cap has 3 to 9 posts on it. One post is for the coil voltage coming into the distributor. The other posts go to each spark plug respectively.
On the inside of the cap there is a terminal that corresponds to each post. The plug terminals are arranged around the circumference of the cap. Some distributors have the outside terminals in a straight line.
The rotor head (usually called "the rotor") is attached to the top of the distributor shaft which is driven by a gear on the engine's camshaft and thus synchronized to it.
The rotor is pressed against a carbon brush on the center terminal of the distributor cap. A spring is used to keep tension on the carbon point. On the inside of the cap, the coil terminal is in the center. The rotor is constructed such that the center tab is electrically connected to its edge, so the voltage coming in the coil post will travel through the carbon point to the center of the rotor, then to its edge. Some rotors have an integrated resistor in the center tab for suppression of radio interference.
As the rotor rotates, its edge passes each of the plug terminals that are arranged around the inside of the cap, according to the firing order, sending the secondary voltage to the proper spark plug.
Patents
- Template:US patent - Ignition apparatus - Arthur Atwater Kent
- Template:US patent - Ignition apparatus - Arthur Atwater Kent
- Template:US patent - Ignition apparatus - Arthur Atwater Kent
- Template:US patent - Ignition apparatus - Arthur Atwater Kent
- Template:US patent - Ignition apparatus - Arthur Atwater Kent
- Template:US patent - Ignition apparatus - Arthur Atwater Kent
- Template:US patent - Ingnition system - Frank Conrad