Diesel engines do not have which component, since they ignite fuel via compression rather than a spark?

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Diesel engines ignite fuel by compression, not by a spark. The air in the cylinder is compressed to a very high temperature, so when diesel fuel is injected it auto-ignites. Because of this, a spark plug isn’t needed. Diesel injectors deliver the fuel, piston rings seal the combustion chamber, and valves control the intake and exhaust. In cold starts, some diesels use glow plugs to preheat the air, but that’s different from spark plugs. So the component not used for ignition in a diesel engine is the spark plug.