The complicated process of a vehicle's ignition system requires precise timing on the part of the various systems involved. Starting a vehicle involves much more than just turning a key in the ignition switch; it requires each system to work in unison to start a vehicle. After turning the key, the process to ignite the fuel and power the engine begins. If a problem occurs anywhere along the path, the engine won't turn over and the vehicle owner must have it repaired.
A Matter of Timing
Every system in an engine is set up to work at a precise time in the combustion process. When this process does not work properly, the engine becomes subjected to misfires, decreased power, and lower fuel efficiency. After turning the key, the starter solenoid engages, allowing the power surge from the battery to reach the spark plugs via the spark plug wires. This allows for the spark plug to fire, igniting the fuel/air mixture in the chamber, which moves the piston down. The ignition system's part in this process takes place way before producing the spark and involves a selection of systems designed to facilitate the spark-producing process.
Spark Plugs and Wires
The electric charge from the battery, via the starter solenoid, ignites the fuel/air mixture in the combustion chamber. Each chamber holds a single spark plug that receives the electricity to spark through the spark plug wires. You need to keep both the spark plugs and wires in good shape or the vehicle can suffer from misfires, poor power and performance, and worsening gas mileage. You must also make sure a mechanic properly gaps the spark plugs before installing them into the vehicle. A spark is produced as the electric current jumps the gap. Improperly gapped spark plugs cause engines to run poorly.
Other problem areas when it comes to spark plugs include the buildup of deposits on the electrode area. The make and model of the vehicle helps determine whether it uses cold or hot plugs. Hot plugs burn hotter and thus burn off more of these deposits. Cold plugs come into play in high-performance engines.
A good way to identify a spark plug wire that needs replacing is to start the vehicle in a dark area. While the engine runs, examine the wires running from the spark plug to the distributor cap. The dim lighting will allow you to see any out of place sparks in the system; tiny electric arcs commonly jump from the cracks and breaks in worn spark plug wires.
Raising the Voltage With the Ignition Coil
Electric voltage from the battery first passes through the ignition coil on its way to the spark plugs. Amplifying this low-voltage charge is the main purpose of the ignition coil. Current flows along the primary coil, one of two sets of wound wire found within the ignition coil. Also, wrapped around the primary coil, you find the secondary coil, which contains hundreds more turns than the primary coil. Breaker points disrupt the flow of the current through the primary coil, causing the magnetic field in the coil to collapse, and produce a magnetic field in the secondary coil. This process creates a high-voltage electric current which feeds into the distributor and on to the spark plugs.
The Function of the Rotor and Distributor Cap
The distributor uses a cap and rotor system to distribute the high-voltage charge to the correct cylinder. The rotor spins, distributing the charge to each cylinder as it passes the contact for each. Current arcs across the small gap between the rotor and the contact as they pass one another.
Unfortunately, the high heats produced during the passage of the charge can cause a distributor to wear out, especially the rotor. When performing a tune-up on an older vehicle, a mechanic will usually replace the rotor and distributor cap as a part of the process.
Engines Without a Distributor
Newer vehicles forego the use of a central distributor and instead utilize a coil on each spark plug. Connected directly to the engine computer, or Engine Control Unit (ECU), this affords the vehicle control system more accurate control over the timing of the spark plug. This system eliminates the need for a distributor and spark plug wires since the ignition system applies the charge at the plug. This setup gives a vehicle better fuel efficiency, reduced emissions, and more overall power.
Diesel Engines and Glow Plugs
Unlike a gasoline engine, diesel engines use a glow plug, instead of a spark plug, to preheat the combustion chamber prior to firing. The tendency of the cylinder block and head to absorb the heat produced from the compression of the fuel/air mixture sometimes prevent ignition, especially in cold weather. The tip of the glow plug provides heat as the fuel enters the combustion chamber, spraying directly upon the element, allowing it to ignite, even when it's cold outside.
This article originally appeared on YourMechanic.com as How a Car Ignition System Works and was authored by Cheryl Knight.