How Does a Car Start?

Please tell me what happens from the moment you turn the key until the engine starts. When you turn the key, are you simply making electrical contact to complete a circuit off the battery serving to energize a solenoid (ignition coil?) which causes the turning of the starter-motor which, in turn, moves the pistons (or fly wheel, perhaps?) enough to start the engine idling on its own?

And, is the term “starter” short for “starter-motor”?


  • Jinx

Engine Subsystems

The solenoid also forces a gear on the starter motor to engage the engine’s flywheel. This ensures that the starter motor doesn’t spin all the time the engine is spinning.

I’d also quibble with the term “electronic switch” used above. It’s an electro-mechanical switch, i.e., a relay.

And yes, starter = starter motor.

Hi Jinx, heres a good page from

When you turn the key, you close an electrical switch which energizes the solenoid. This does two things. First, it starts spinning the starter motor, which is just a DC electrical motor. Second, it pushes out the starter motor shaft so that the gear engages the flywheel. The flywheel just happens to be a convenient place to grab a hold of the engine’s moving parts. There’s no law that says you have to turn the flywheel, and in fact (IIRC) the early hand crank on cars turned the main crankshaft. The real main purpose of the flywheel is just to be a big hunk of metal. It’s large mass means that it doesn’t want to change its motion very quickly, so it helps to smooth out the engine’s jerkiness.

There is also another electrical switch which is turned on when you turn the key. This one turns on electricity to the ignition coil. The ignition coil is just a coil of wire. When you energize it, the changing amount of electricity (going from off to on) causes a large magnetic field to develop. As the engine spins, it also spins a thing called the distributer. All the distributer does is connect the coil to the various spark plugs as it spins around. In other words, as the distributer spins, it’s going to momentarily connect the coil to spark plug 1, then it will spin a little bit further and connect to spark plug 2, etc. When it connects, the energy in the coil is discharged through the spark plug, making a spark. This ignites any gas in the cylinder.

Mechanically, the engine will open valves to let gas into the cylinder, then close the valves as the piston starts to move up. Once the gas/air mixture is compressed, the spark will cause it to burn very quickly. This miniature explosion forces the piston down and turns the shaft. Then another valve opens and as the momentum of the engine pushes the piston back up, it pushes the exhaust gasses out. Then the gas valves open back up, and as the piston goes back down (still from the momentum of the engine) it sucks gas into the cylinder, and the whole thing starts all over again.

The tricky part to making it all work is the timing. There is a thing called the timing chain, which rotates a cam shaft above the engine. This shaft opens and closes the valves at just the right time, so that gas goes into the cylinders and the exhaust comes out of the cylinders, and also that all the valves are closed when the cylinder fires so that all the energy goes into pushing the pistons down.

Once you get the engine going, the momentum of the engine will keep the pistons moving up and down, so at that point it will run forever on its own and doesn’t need any help. You then let go of the key and the starter motor is disengaged (the little arm with the gear retracts away from the fly wheel) and the starter motor stops spinning. The second electrical contact to the ignition coil is still connected, which makes the sparks as the engine rotates. When you turn off the key, all you do is remove power from the ignition coil. Without spark, the gasoline doesn’t explode, and the engine stops fairly quickly.

Real cars these days aren’t quite so simple. There are things like electronic ignition and fuel injection and such, but that’s the basic idea of an internal combustion engine. Your lawn mower, weed wacker, etc. all work on basically the same idea.

engineer_comp_geek [nitpick] When the starter turns the flywheel, it is turning the crankshaft, just like a hand crank since the flywheel is bolted to the back of the crankshaft. This did not seem clear in your post

As engineer_comp_geek mentioned in a modern car a lot more happens, let me see if I can run through the list for you. IN addition what has been mentioned
(this list is on the cars I work on, YMMV)
[li]When you place the key in the igniton and start to turn it, an electrical field querries the transponder chip in the key[/li][li]The chip modifies the electrical field[/li][li]If this modification is recognised, the starting procedure continues, if not a message is displayed “Start prevented, try again”[/li][li]When the key gets to position II (the on postion) relays engage to power up the Engine control module computer[/li][li]The cars central computer then sends a password request to the engine control module[/li][li]The engine control module returns a password, if both both passwords agree, starting continues, if not starting is prevented[/li][li]When the key is turned to position III (start) the starter is engaged[/li][li]When the speed of the engine (detcted by the rpm sensor) reaches about 40 RPM the fuel pumps are started, the injectors start to trigger, and spark is sent to the plugs. It should be noted that many modern engines do not use distributors any longer and have indivudal coils for each plug (or pairs) the ECM selects which plug to fire[/li][li]when the engine speed reaches about 500 RPM start mode is over and run mode is selected by the ECM injectors go to sequential injection (from simulationus) and car runs[/li][/ul]
All of the above happens in just a second or two.

The point I was trying to make is that the main purpose of the flywheel is not to start the engine. The purpose of the flywheel is to provide mass for a smoother rotation once the engine is running. It just happens to be a very convenient thing to attach the starter motor to. Anything that can turn the crankshaft can be used to start the car, as anyone with a stick shift and a dead battery knows (the old push-start technique).

If for some reason you didn’t have a starter, as in the case of some race cars which use a portable external starter to save weight, then you’d still have a flywheel. The flywheel is not part of the starting system, it’s just something that’s already there that we can easily use to get the engine spinning.

Sorry if that wasn’t clear.