What happens from the time I turn the key to start the car till the engine starts running? --- Kyle, Lansing, Michigan
Marshall Brain Answered:
Let's go back to the 1950s. This is long before the age of engine control computers, fuel injectors, etc. You have a purely mechanical engine with a carbureter feeding gas to the cylinders. Here's how the engine would start:
1) You would pull out the choke knob.
2) You would turn the key to "on" to activate the electrical system, so the ignition system has power.
3) Then you would turn the key to "start". This would send electricity to the starter solenoid.
4) That solenoid is essentially a big relay that sends the massive amount of power needed from the battery to the starting motor.
5) This starter motor has a small gear on the motor to mesh with the larger gear of the flywheel.
6) The starter motor would turn the engine, which would suck gas through the carb into the cylinders.
7) As the engine turned, spark plugs would spark.
8 ) One of those sparks would light the gas in one of the cylinders and the engine would start spinning under gasoline power.
9) You would release the key back to the "on" position and the engine would be running.
10) You would push in the choke knob
The reason you needed to "choke" the engine was to pull more gas through the carb to make sure there was enough gas to start the engine and run it while cold. Later the manual choke got replaced with an automatic choke. Later the carb got replaced by fuel injectors that eliminated the choke.
More on starter motors:
In this video you can see the little gear on the starter motor move out to engage the flywheel and turn it. Then it retracts once the engine is running:
With modern cars, the process is basically the same. There is still a starter solenoid and a starter motor and a flywheel. But a computer is controlling the fuel injectors and mediating the whole process.