Gas systems need either a hot surface or a pilot light to get started, but not oil systems. Why the difference? It boils down to the fuel being fundementally different. Fuels like propane and natural gas enter the combustion chamber in a gaseous state. Once part of that material is lit on fire, it will conduct the heat and light the entire fuel stream. This doesn’t work for oil.
The Major Difference
Oil enters the combustion chamber as a liquid and it requires extreme heat to light. This makes a pilot light nearly pointless, as it would be just as easy to light the main burner as the pilot. A hot surface igniter would be a viable option except it needs to be in the path of the fuel stream, where it would endure the direct-heat of the flame throughout operation. Oil as a fuel source is a completely different beast, unrelated to gas heating.
These problems all stem from heating oil’s chemical make up. It’s a cousin of diesel fuel used in over the road trucking, but thicker. It shares some of diesel’s inherent safety. Heating oil and diesel require either extreme heat or extreme pressure. This is why diesel trucks don’t have sparkplugs. At least for an engine’s needs, a spark wouldn’t work well in the long run. They generally remain in a liquid state as well, rather than readily becoming gasses.
A Really Big Spark
Natural gas can be lit with even a tiny spark, perhaps from slamming to rocks together. Oil can be lit by a spark, but you need something closer to Zeus’s lightning to get the job done. High voltage sparks are one way to create absurd amounts of heat. This works great in a furnace where there’s a constant supply of AC Service (it works horribly in trucks, where this would kill the battery in a heartbeat).
In most configurations, this is a two part setup. There is a huge transformer mounted on the burner. This will increase the voltage, perhaps into the tens of thousands of volts, while lowering the amperage. During furnace ignition, power is passed into the transformer, boosted up, then fed into electrodes sitting just out of the fuel nozzle’s path. Fuel is blasted into the chamber, there’s an enormous spark, the fuel is heated to thousands of degrees, and ignites.
The main burner will continue shooting fuel into the chamber as long as necessary. The electrodes can shut down, as the flame will be self-sustaining as long as the fuel injection continues.
Still Not Viable for Gas
So, this is great for oil, why not gas? It can’t do big burners. Gas systems already use small, electric igniters on intermittent pilots. These make tiny sparks over short distances and in many cases can be considered a compact mile of an oil ignition system. The problem comes in with lighting a large burner. Even the immense voltage in an oil igniter can only make an inch-long spark. Electrically igniting a large gas burner, without a pilot or hot surface would be difficult under the best of circumstances, due to issues with the fuel mixture, directing the spark, and avoiding explosions.