The SI is known to be a Spark Ignition Engine, while CI is known to be a compression engine. SI engine will be effective in making sure that there will be enough spark so that the fuel can be ignited, and it will begin. Take note that this would require a spark plug so that it can work well.
For the CI engine, what it will do is the air will be compressed within the cylinder. The heat coming from the compression will be in charge of igniting the fuel. There are a lot of ways that fuel can be ignited depending on the engine that you currently have.
The SI engine can be the type of engine that will be used by those who are using premium and unleaded fuel tanks, while those who would use CI may have a diesel fuel tank.