How do throttle control the speed car?

Answer 1

The throttle does NOT control the speed of the car. On most spark ignition engines (an engine that uses spark plugs to fire) the throttle controls the amount of air entering the engine. The amount of fuel injected is proportional to the amount of air entering the engine. As this fuel is burned, cylinder pressure is increased and in turn moves the pistons. The more fuel, the more pressure, the faster that piston moves. So what the throttle indirectly controls is the speed (RPM's) of the engine. The power output of the engine varies throughout its RPM range. This torque output is the input for the transmission. The torque is increased or reduced as it passes through the gears of the transmission (and the differential) and eventually makes it to the wheels. In simpler terms, more throttle generally means more torque going to the wheels causing acceleration. Less throttle means less torque and the car will decelerate until the speed matches the torque going to the wheels. There are many more factors such as friction, inertia, heat, etc... but this is a pretty close explanation of how this works.