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Lawn Mower Ignition System,Understanding the Current Path, Grounds, and Transformer

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tsa256

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Hello everyone,

Recently I began fixing up a neighbors lawn mower. It runs but I can not quite wrap my head around the flow of current withing the ignition/spark system.

I know the following:
The system consists of a magnet located on the flywheel.
The magnet spins around and induces a current on a coil.
The coil then ups the voltage by going to the secondary winding just like a transformer.
The current from the transformer flows into the spark plug past a very small air gap and to ground on the mowers engine block.

Now knowing the above gave me knowledge to fix it, but I am having some real trouble visualizing the flow of current in this system, the many grounds of the engine, spark plugs, and transformer are confusing me.
Confusing parts are mainly;
current flow,
What the proper circuit diagram is
how all the grounds such as screws on the coil work
Why is the secondary circuit ( spark plug, to engine block) grounded why wouldn't it just work like a 'normal transformer' circuit a shown in the picture


Below is some attached images that may help.

Thanks!
 

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  • normal_transformer.gif
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  • diagram_of_circuit_from visual ( may be incorrect).jpg
    diagram_of_circuit_from visual ( may be incorrect).jpg
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  • ignition_coil.jpg
    ignition_coil.jpg
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Don't get confused by the grounds. It's all just one big fat ground. Ground (or common) is just a term for the common electrical return points on a circuit. It can be a wire or a piece of metal (anything that's conductive). Anything connected, screwed, or bolted to the engine becomes part of the ground. Thus the ground current flows wherever it need to. The primary current flows back to the primary ground point and the secondary current flows back to the secondary ground point.

The main reason the transformer output is grounded is because the sparkplug is grounded. They actually commonly use ungrounded coil outputs in modern "wasted spark" ignition systems for multi-cylinder engines, where each end of the secondary goes to a separate spark plug with both plugs firing at the same time (the current flows through one plug, through the engine ground between the two plugs, and then through the second plug to the other side of the coil). The engine timing is such that one plug fires at the top of the compression stoke igniting the fuel, but the other plug fires at the top of the exhaust stroke which does nothing (hence wasted).
 
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