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Car electrical systems question

Question just for my own personal knowledge & expansion.

I know from studying reciprocating aircraft engines that they have “magnetos” which produce electricity needed to fire the cylinders independent of the aircraft’s electrical system.

I’m wondering how a distributor differs in its operation…

Is current supplied to the rotor? Or each cylinder contact node? Or both.

The simple question is this: Disregarding the electric fuel pump, if an alternator fails and the battery is then drained, will a car engine stop running at that point?

With a distributor the voltage is normally supplied to the center terminal at a time specified by some sort of trigger, normally the pickup coil. The rotor is in constant contact with that center electrode and completes the circuit with the individual cylinder posts as it spins around.

If you alternator fails and drains your battery your car will stop running, the ignition needs voltage to spark.

Yes. First You’ll Lose Lights, Wipers, Heater Fan, Radio, Etcetera. Next You’ll Run Out Of Sparks And Have To “Dead-Stick” It.

You can drive pretty far during the day with most of this stuff off, but not with everything draining the battery.


My Old Allis Chalmers “B” Tractor Has A Magneto Ignition. Airplanes Usually Have Two Complete Magneto Ignition Systems, Each Having Its Own Set Of Spark Plugs. As You Know You Check Them Both At Run-Up.


In that case, can a diesel keep running when the Alternator calls it a day?

The magneto rotates a magnet near a coil winding and this is what generates the voltage to fire the plugs. (similar to a lawn mower)
After that the principles about rotors and caps are roughly the same as a distributor equipped engine.

For the sake of argument assume you have a mechanical fuel pump on a carbureted engine.
You can throw the battery and alternator away and the engine will run on the magneto until you run out of gas or the magneto is worn out.
It’s a totally self-contained ignition unit.

Jeff to answer your question about if a diesel loses its alternator can it continue to run? Only if it had a mechanical fuel pump and mechanical injectors like they used to in the old ones. I doubt anyone is making any diesels with that anymore. THey all have electric fuel pump and electronic fuel injectors. Computer controlled. This makes it much easier to control the timing and emisisons. THey are burning a lot cleaner and with more power then the old days.

The car engine will stop running and may backfire through the exhaust pipe when it does. It’s happened before.

In a newer gasoline car (where more is run on electricity) your car will die when you run out of juice. You will lose lights, radio, etc. You will also not be able to start the car as ignition is dependent upon a spark from the spark plugs.
The car electrical systems in older vehicles sometimes use magnetos and until the magneto wears out you would keep running.
Diesel is a different beast. The diesel isn’t ignited by a spark. Diesel is ignited by compressing it. The injectors and stuff are electrical in newer models so, no electricity-no compression-no firing.
Older models are mechanical and would therefore go even if the alternator went out. (But few people make them with mechanical pumps becasue electrical-computer controlled ones run cleaner.)

A diesel will keep running as long as it doesn’t have an electric fuel pump and it would have to have a mechanical injection system.

Magnetos were used on cars, but way back like Model T Fords. Cars with magnetos were rare in the '50’s and no modern car uses a magneto. Now your Briggs & Stratton lawn mower, that uses a magneto.

Your modern car will die when the battery losses enough juice and can no longer power the electronic ignition that makes the “spark”.