* Fuel Injection refers to the method of delivering fuel to the engine's cylinders. It's a *delivery system*. Instead of relying on a carburetor to mix air and fuel, a fuel injector precisely meters and sprays fuel directly into the intake manifold (port injection) or directly into the combustion chamber (direct injection). The key improvements are better fuel atomization (leading to better mixing and combustion) and more precise control over the fuel-air mixture.
* Spark Ignition refers to the method of igniting the air-fuel mixture in the cylinder. It's an *ignition system*. A spark plug creates an electrical spark across its gap, which ignites the compressed air-fuel mixture, causing combustion and generating power. This is the most common ignition system used in gasoline engines. (Diesel engines use compression ignition, where the heat of compression ignites the fuel.)
In short: Fuel injection is *how* the fuel gets into the cylinder, while spark ignition is *how* the fuel-air mixture is ignited. You can have a spark ignition engine *without* fuel injection (using a carburetor), but modern engines almost always use both fuel injection *and* spark ignition for better efficiency, power, and emissions control.
