An automotive distributor is the part of many older gasoline ignition systems that sends high-voltage spark to the correct cylinder at the correct time. It matters because each cylinder must receive a spark near the end of its compression stroke for the air-fuel mixture to burn efficiently. If the spark goes to the wrong cylinder or arrives too early or too late, the engine may misfire, lose power, overheat, or fail to start.
The distributor acts like a timed electrical traffic director for the engine.
Key Facts
- The ignition coil steps battery voltage up from about 12 V to tens of thousands of volts for the spark plugs.
- The distributor rotor spins inside the distributor cap and passes close to each spark plug terminal in firing order.
- Distributor speed = camshaft speed = 1/2 crankshaft speed in a four-stroke engine.
- For a 4-cylinder four-stroke engine, one spark is sent every 180 degrees of crankshaft rotation.
- Spark timing is measured in degrees before top dead center, such as 10 degrees BTDC.
- Engine rpm relation: distributor rpm = engine rpm / 2 for a four-stroke engine.
Vocabulary
- Distributor
- A timed ignition device that routes high-voltage current from the ignition coil to the correct spark plug.
- Rotor
- A spinning arm inside the distributor cap that directs high voltage toward one spark plug terminal at a time.
- Distributor Cap
- An insulated cover with terminals that connect the rotor output to the spark plug wires.
- Firing Order
- The exact sequence in which the engine cylinders receive spark.
- Ignition Timing
- The crankshaft angle at which the spark plug fires relative to piston position.
Common Mistakes to Avoid
- Mixing up spark plug wires is wrong because the distributor will send spark in the firing order, but the wires may carry it to the wrong cylinders.
- Assuming the distributor spins at crankshaft speed is wrong because a four-stroke distributor is driven by the camshaft and turns once for every two crankshaft rotations.
- Setting timing only by ear is unreliable because correct ignition timing depends on crank angle, engine load, and rpm.
- Ignoring carbon tracks or cracks in the distributor cap is a mistake because high voltage can leak or jump to the wrong terminal and cause misfires.
Practice Questions
- 1 A four-stroke engine is running at 2400 rpm. What is the distributor rpm?
- 2 A 6-cylinder four-stroke engine completes one full engine cycle in 720 degrees of crankshaft rotation. How many degrees of crankshaft rotation occur between spark events?
- 3 A distributor cap has moisture inside and the engine misfires under load. Explain how moisture can change the path of high-voltage spark and why that affects cylinder firing.