The primary winding of the oil ignition coil needs to be energized to form a magnetic circuit through the air at both ends. This ignition coil is also known as the open magnetic circuit ignition coil. The magnetic circuit of the open magnetic circuit has relatively high magnetic resistance and magnetic leakage loss more. Therefore, the energy conversion efficiency of the primary winding of this ignition coil is not high, about 60%.
Dry ignition coil
There is no insulating oil inside the dry ignition coil. The core of the dry ignition coil is usually shaped like 日or 口. The magnetic field produced by energizing the primary winding forms a closed magnetic circuit. So, this ignition coil is also called a closed magnetic circuit type ignition coil, as below:

In order to reduce the magnetic hysteresis of the core, a small air gap is left in the core of the closed magnetic circuit ignition coil. The magnetic circuit of this type’s ignition coil is composed of the core with negligible magnetic resistance, so the magnetic leakage is less and the energy conversion efficiency is high. Therefore, mass magnetic circuit ignition coil is used more and more in electronic ignition system.
How does ignition coil work?