The name pencil coil is derived from the shape of the coil. The pencil coil is situated above the spark plug and is normally clamped down with a bolt. The pencil coil is connected to the Engine Control Unit (ECU) via a special wiring harness and can generate electrical energy of up to 35 000 volts.
Placing individual ignition coils directly over each spark plug eliminates the need for long, bulky, expensive and high-voltage spark plug cables. This reduces radio frequency interference and eliminates potential misfire problems caused by the deterioration of spark plug wires due to the under-bonnet heat generated by the engine. It also reduces resistance along the path between the coil and plug.
The pencil coil also enhances engine performance. This is because each cylinder has its own pencil coil which gives each coil more time to recharge between cylinder firings. With single coil distributor systems, the coil must fire twice with every revolution of the crankshaft in a four cylinder engine and four times in a V8. With a multi-coil system, each coil only has to fire once every other revolution of the crankshaft.
This provides more saturation time for a hotter spark, especially at higher rpm when firing times are reduced. The result is fewer misfires, cleaner combustion and better fuel economy.
According to the original equipment suppliers who make multi-coil ignition systems, having a separate coil for each cylinder also improves the engine’s ability to handle more exhaust gas recirculation. This reduces oxides of nitrogen emissions which is important with today’s low-emission vehicle standards.
A hotter spark also makes spark plugs more resistant to carbon build-up and assists spark plugs to go the distance prescribed by the OEM. The pencil coil ignition system also improves idle stability and reduces idle emissions.