accelerator of a motor vehicle work

Actually two types of fuel supplies work in a motor vehicle.  One uses carburettor in petrol engines and the other is fuel injection pump in diesel engines.

Carburettor gives air and petrol mixture and fuel injection pump gives a spray form of diesel using a nozzle to the engine input side.

The carburettor of the two wheeler consists of a small fuel opening port at the place where the atmospheric air flow pipe has a smaller cross section area .

When we press the accelerator, a small plate like opening allows the carburettor to draw more air from atmosphere, and also this action draws more fuel(petrol) from the  fuel tank by venturi  action (venturi  action means a fluid or gas flowing through a constricted section of a tube undergoes a decrease in pressure, so due to pressure difference fuel comes from the tank ).

Finally this will give more air fuel mixture to the engine input.  So due to higher combustion inside the engine the vehicle gets more power and goes in a faster manner.

It is the same in diesel engine cars, but in the diesel cars the fuel injection pump (which runs by taking some output of the rotational movement of the engine) gives more fuel to the ignition chamber of the engine in the spray form.

So finally, we are quantitatively changing the volume of the fuel to the engine by pressing the accelerator.  

When we use the choke option in the vehicle during starting problem, it is qualitatively changing the ratio of air and fuel for easy starting during winter or cold condition of the engine.