The ground side of the coil is connected to the breaker points. Older distributors with breaker points have another section in the bottom half of the distributor - this section does the job of breaking the current to the coil. This can be the cause of some very mysterious engine problems. Also, the spark-plug wires eventually wear out and lose some of their electrical insulation.
When you do a tune-up, one of the things you replace on your engine is the cap and rotor - these eventually wear out because of the arcing. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder. One of the most common and most accurate methods is by referencing. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. Theres many different methods used in jr drag racing to reference ignition timing.
IGNITION TIMING SERIES
The rotor spins past a series of contacts, one contact per cylinder. The coil is connected to the rotor, which spins inside the cap.
Its first job is to distribute the high voltage from the coil to the correct cylinder. Next we'll go through the components that make the spark. To maximize fuel economy we have to minimize injector pulse width at a given RPM / hp level. Ignition timing is the process of setting the time that a spark will occur in the combustion chamber (during the power stroke) relative to piston position. Not necessarily the highest torque possible with any amount of fuel. So MBT is best torque for a specific throttle position (load) and RPM. Retarding the timing may also eliminate knocking some cars that have knock sensors will do this automatically. The ignition timing is just tuned for this mixture to produce the best torque. Lowering temperatures helps reduce the formation of nitrogen oxides (NOx), which are a regulated pollutant. Ignition timing, in a spark ignition engine, is the process of setting the time that an ignition will occur in the combustion chamber (during the compression stroke) relative to piston position and crankshaft angular velocity. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Other goals, like minimizing emissions, take priority when maximum power is not required. This is called spark advance: The faster the engine speed, the more advance is required. This means that the faster the engine goes, the earlier the spark has to occur. But the speed of the pistons increases as the engine speed increases. The time that the fuel takes to burn is roughly constant. The timing of the spark is important, and the timing can either be advanced or retarded depending on conditions. And because the length of the stroke and the area of the piston are fixed, the only way to maximize work is by increasing pressure.
So when we're talking about a cylinder, work = pressure× piston area× stroke length.
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