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.

 

[Lead Sled]

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

Wrong. You forgot MONEY.

'Sled

 

[Pedro]

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.

No, the prop doesn't drive the magnetos, the engine does, and also drives the prop. If the propstops turning is an indication that the engine stopped turning, which is alsoavery goodindication that the magnetos stopped turning.

Back to your wine!

 

[erj-145mech]

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

The prop is just a cooling device, it keeps the nut behind the yoke cool.

Want proof? When the prop stops, watch the nut sweat.

 

[Geronimo4497]

Wrong. You forgot MONEY.

'Sled

How foolish of me. Money first, then all of those thing.

 

[bertengineer]

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.

This really could have been answered if he had opened up the book that came with the private pilot package, and he is an instrument student? Good luck with that instructor.

 

[Crossky]

No, the prop doesn't drive the magnetos, the engine does, and also drives the prop. If the propstops turning is an indication that the engine stopped turning, which is alsoavery goodindication that the magnetos stopped turning.

Back to your wine!

Of course he knew that, you couldn't see that in his smarta$$ remark? Though, in a windmilling, no power situation airborne, the prop does drive the engine and the thus the magnetos, until you get slow enough for the prop to stop. That's the situation he and the original poster are referencing.

Peace.

 

[FN FAL]

There is no "I" in "Magneto"...it's all in the "O"!

 

[NYCPilot]

Here's some trivial information about your engine too.

Engine designations might be something like IO-360, O-320 and so on. You might have wondered what these numbers and letters represented.

I = Injected
O = Opposed (as in horizontally opposed cylinders)
360 = 360 cubic inch displacement


Also, if you take half that figure, you'll get an approximation of the HP of the engine.

360 = 180 HP

320 = 160 HP

 

[WMUchickenhawk]

Here's some trivial information about your engine too.

Engine designations might be something like IO-360, O-320 and so on. You might have wondered what these numbers and letters represented.

I = Injected
O = Opposed (as in horizontally opposed cylinders)
360 = 360 cubic inch displacement


Also, if you take half that figure, you'll get an approximation of the HP of the engine.

360 = 180 HP

320 = 160 HP

Its a textron Lycoming IO-360-L2A, but it also says the hp is 160.

 

[Geronimo4497]

I take it that you are flying a late 1990s 172 then? The early new production 172s had a IO-360 engine that was derated to 160HP. Kind of stupid, but whatever.


What is the max RPM in your airplane? I ask because some OEMs limited the HP by derating the max engine RPM to get the desired HP.