C172 Magneto question

[WMUchickenhawk]

In a Cessna 172 are the magnetos driven by the engine? I say they arent, but a friend says they are. Who is right?

 

[Pedro]

Your friend is right, buy him a beer!

 

[WMUchickenhawk]

If they are driven by the engine, how do they work if the engine quits?

 

[bugchaser]

In a Cessna 172 are the magnetos driven by the engine? I say they arent, but a friend says they are. Who is right?

Did you really not learn that while getting your private licinse? Just wondering what do you think drives them if not the engine?

 

[NYCPilot]

Your friend's right.

The magnetos are driven off of the engine and are independent of the electrical system.

 

[NYCPilot]

If they are driven by the engine, how do they work if the engine quits?

They don't.

 

[Pedro]

If the engine quits but keeeps turning (prop doesn't stop) then they keep turning and working. If the propstops moving then the magnetos stopworking too.

The magnetos are an accesory, just like the alternator, engine driven fuel pumps, etc.

 

[WMUchickenhawk]

They don't.

I was thinking backwards, I was thinking alternator. Just messed up my logic. Whats the point of magnetos if you dont have an engine working anyways. Thanks for the good replies, that cleared it up. and yours too bugchaser.

 

[Geronimo4497]

engine ignition 101

I was thinking backwards, I was thinking alternator. Just messed up my logic. Whats the point of magnetos if you dont have an engine working anyways. Thanks for the good replies, that cleared it up. and yours too bugchaser.

Fuel, air and spark. That is all you need to make a piston engine run.


Fuel, which is gravity feed or pumped in by an engine driven/electric fuel pump. This fuel goes from the pumps into the carbureted or fuel injector. If using a carburetor, the fuel is atomized and sucked into the intake pipes (more on this later). If using a fuel injector, the fuel is run through independent fuel lines to each cylinder intake port (each cylinder has one intake port and one exhaust port).



Air, which is sucked in through the air filter and then onto the intake pipes/manifold. On a Lycoming, these are the silver colored pipes next to the exhaust pipes. The intake takes the fuel/air mixture (in the case of a carbureted engine) and brings it up to the intake manifold and goes into the cylinder where the piston is, at the right time. I could write about firing order of the engine, but not now, maybe later if you are interested.



Now the best part.



Spark, which is created by the magnetos, fires this fuel/air mixture at the appropriate time. There are 2 magnetos because each cylinder has 2 spark plugs. The reason is that if one plug fails OR a magneto fails, each cylinder can still produce power. The mags are located on the back of the engine. They are usually black, look identical, and in the case of the 172, have 4 wires coming out of the back of the. Magnetos are great because they are independent of the electrical system (unlike a car) so if you ever have a alternator failure, the engine will not quit. Basically, a magneto is an AC generator that produces 10,000+ volts that go to the spark plug. They are very reliable and usually not the cause for engine failure alone.



Do yourself a favor, and go bother some of the mechanics after one of your lessons. Just be honest and say that you would like to learn about the working of your engine. Most mechanics are great (myself included ) and will take a few minutes to show you the basic components of a 172 engine. It is pretty simply to understand the operation of the engine once you know what each component does.



DON’T BE AFRAID TO ASK QUESTIONS! There really is no stupid question, especially when starting off in this field. We all started off knowing very little, and most will be happy to pass the knowlege on to others.

 

[GravityHater]

If the prop stops moving then the magnetos stop working too.

Aha! His suspicion was right!!!... it's the prop that drives the magnetos!

Or is that magnetoes? Tomatoes Tomatos; Potatos Potatoes? Dang. Anyway.. I need to get back to my wine.