All oil burners have the same basic design. An electric motor spins a fan and oil pump simultaneously. Air from the fan is directed into a blast tube. At the end of the tube is a nozzle spraying the oil the pump has put under high pressure. The nozzle resembles a garden hose sprayer, but is smaller and more precise. It has a flow rating ( in gallons per hour ) to size the heat output. It also has a degree angle that determines the width of the spray. A letter stamped on the nozzle identifies the cone formed by the spray as hollow or solid.
Released from the pressure at the nozzle, the oil atomizes into a fog and is ignited with an electric spark. The spark is generated by a transformer that boosts the voltage upwards of 10,000 volts or more. The spark jumps across electrodes placed in front of the nozzle and is blown into the oil spray by the blast of air. Watching the ignition is an electric eye known as a cad cell.
Current generated by the fire on the window of the cell is conducted to a protecto-relay and keeps the relay engaged and the burner motor running. If the fire goes out, the burner will shut down to prevent a build up of unburned fuel and an explosive condition. The protecto relay (AKA burner relay) also has another function. It has a transformer built in that sends a 24 volt current to the thermostat that becomes the means of signaling for heat.
Some manufactures are using increased pump pressure and smaller nozzle size to increase atomization and burn efficiency. Intense testing is done at the factory to determine the best nozzle for each application, so it is important to use the specified nozzle.