Working of an engine?

no it is not a device bit in layman words its the accelerator. So when you press the accelerator the throttle valve opens alowing the air-gas mixture into the cumbustion chamber above the piston, where it gets ingnited by the spark from the spark plug and burns, producing energy. Pressing the accelerator is also called opening the throttle in other words as it opens the throttle valve alowing the mixture into the chamber

Is the fuel injected into throttle?

No. Fuel is mixed with air in the intake manifold (after the throttle). This mixture is then passed into the individual cylinders of the engine for combustion.