An ignition coil (also called a spark coil) is an induction coil in an automobile's ignition system. It transforms the battery's low voltage to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel. Early gasoline internal combustion engines used a magneto ignition system, since vehicles did not have batteries. The voltage produced by a magneto was dependent on the speed of the engine, making starting difficult. A battery-operated coil, however, can provide a high-voltage spark, even at low speeds, making starting easier. When batteries became common in automobiles for cranking and lighting, the ignition coil system displaced the magneto ignition.
Model T Ford Magneto (1908-1927)
Model T Ford
Formerly, ignition coils were made with varnish and paper insulated high-voltage windings, inserted into a drawn-steel can and filled with oil or asphalt for insulation and moisture protection. Coils on modern automobiles are cast in filled epoxy resins which penetrate any voids within the winding. In older vehicles, a single coil would serve all the spark plugs, via the ignition distributor. Drag racers, however, upgraded distributors, coils, points (dual) and condensers with high performance ignition parts from suppliers such as ACCEL, MSD and Mallory.
ACCEL Dual-Point Distributor
Distributor Breaker Plate (Dual Points and Condensor)
So what exactly is an ignition coil and what does it do? Technically speaking, an ignition coil consists of a laminated iron core surrounded by two coils of copper wire. Unlike a power transformer, an ignition coil has an open magnetic circuit-the iron core does not form a closed loop around the windings. The energy that is stored in the magnetic field of the core is the energy that is transferred to the spark plug. The primary winding has relatively few turns of heavy wire. The secondary winding consists of thousands of turns of smaller wire, insulated from the high voltage by enamel on the wires and layers of oiled paper insulation. The coil is usually inserted into a metal can or plastic case with insulated terminals for the high voltage and low voltage connections. When the contact breaker closes, it allows current from the battery to flow through the primary winding of the ignition coil. The current does not flow instantly because of the inductance of the coil. Current flowing in the coil produces a magnetic field in the core and in the air surrounding the core. The current must flow long enough to store enough energy in the field for the spark. Once the current has built up to its full level, the contact breaker opens. Since it has a capacitor connected across it, the primary winding and the capacitor form a tuned circuit, and as the stored energy oscillates between the inductor formed by the coil and the capacitor, the changing magnetic field in the core of the coil induces a much larger voltage in the secondary of the coil. More modern electronic ignition systems operate on exactly the same principle, but some rely on charging the capacitor to around 400 volts rather than charging the inductance of the coil. The timing of the opening of the contacts (or switching of the transistor) must be matched to the position of the piston in the cylinder so that the spark may be timed to ignite the air/fuel mixture to extract the most angular momentum possible. This is usually several degrees before the piston reaches top dead center.
Some coils have an internal resistor, while others rely on a resistor wire or an external resistor to limit the current flowing into the coil from the car's 12-volt supply. Older cars and trucks have spark plug wires that go from the distributor to each of the spark plugs. Every ignition coil system required mechanical contact breaker points and a capacitor (condenser). The contacts are driven off a shaft that is driven by the engine camshaft, or, if electronic ignition is used, a sensor on the engine shaft controls the timing of the pulses. The next generator electronic ignition systems used a power transistor to provide pulses to the ignition coil.
Mallory Pro-Master Coil
The amount of energy in the spark required to ignite the air-fuel mixture varies depending on the pressure and composition of the mixture, and on the speed of the engine. Under laboratory conditions, as little as 1 milli-joule is required in each spark, but practical coils must deliver much more energy than this to allow for higher pressure, rich or lean mixtures, losses in ignition wiring and plug fouling and leakage. When gas velocity is high in the spark gap, the arc between the terminals is blown away from the terminals, making the arc longer and requiring more energy in each spark. Because of this, 30 to 70 milli-joules must be delivered in each spark.
Since the late 1990s, distributer and spark plug wires have been replaced by multiple coils, mounted in a single mounting block with multiple terminals. These coils, regulated by the vehicle’s on-board computer, are called coil packs. Usually, there is one coil for each sparkplug or in some cases, like the modern Hemi engine, one coil for every two sparkplugs.
426 Hemi Engine Spark Plug Wires
Coil Packs (Spark Plug Wires Eliminated)
When creating a spark at the spark plug, the fire must initiate from a high voltage supply, so as much fuel as possible burns in the cylinder. Whenever the combustion process is started, the coil pack builds up the energy, as much as 50,000 to 75,000 volts. When the electronic control module (the car computer) sends the signal to the ignition control, the voltage is released from the coil pack through the spark plug cables to the spark plug.
The biggest reason most manufacturers have turned to coil packs is they are more reliable. Many coil packs don't require service for up to 120,000 to 150,000 miles. Also, distributors are very prone to failure if they become wet or the engine happens to be submerged under water for a few seconds. Coil packs tend to operate even when they have been thoroughly doused with water.
In addition to being more reliable (there are no moving parts and they fire much less often than a distributor), coil packs are generally credited with providing more horsepower and torque than a common distributor. A coil pack generally creates a much more efficient spark and improved combustion, as well as a slight fuel efficiency improvement over vehicles that use standard distributor caps.
Like all automotive parts, sometimes a coil pack can go bad. If this happens, you can tell by a loss of fire or spark in one or more cylinders. Misfiring can cause drag on the crankshaft, and usually results in a very poor performing engine. A faulty coil pack will generally have symptoms similar to a faulty spark plug.
Some of the most common tell-tale signs that a coil may be defective include:
- A rough idle.
- An unexplained louder-than-usual engine sound.
- A noticeable lack of power.
- A significant drop in RPMs while accelerating for no apparent reason.
- A blinking or intermittently activating check engine light.
- An active gas warning light when the vehicle has plenty of gasoline.
- Smoke from the exhaust emitting intermittently, instead of in a steady stream.