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I stumbled into a single cylinder engine tachometer, which looks identical to this one but mine says it was made by Dixson:
https://www.amazon.com/Wireless-Tachomet...tachometer
I figured I'd test it on my 2-stroke Lawn-Boy and got about 3000-RPM.
But then I got to thinking, it doesn't have a 2-stroke/4-stroke switch. So how does it know which I'm measuring? A 2-stroke fires the spark every revolution, where a 4-stroke fires every other revolution.
So I'd think there would be a conversion necessary if it is just telling me the # of sparks per minute? Because you can't get to RPM from sparks per minute w/o knowing the strokes.
Am I overthinking this? Maybe it isn't reading spark but something else?
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Wouldn't it fire on every stroke with a magneto system, the four stroke would also fire on the exhaust stroke so if it measures the electric field it would be the same for 2 or4 cycle engines. Roly
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(09-15-2016, 06:31 PM)Roly Wrote: Wouldn't it fire on every stroke with a magneto system, the four stroke would also fire on the exhaust stroke so if it measures the electric field it would be the same for 2 or4 cycle engines. Roly
Oh that is interesting, I didn't consider that the plug is firing on the exhaust stroke. I had thought the plug would only fire after compression.
The only other theory I had was that the unit was sensing the magnet on the flywheel, and that on the 4-stroke, that a points system (I don't understand how points work) was sending a current to the spark plug every other rotation of the flywheel.
Buy your answer would make the engine easier to build (do small lawnmowers have points, and what do points do?) and would explain why my tach is universal.
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The plug in your single cylinder fires on each rotation of the flywheel, 2 stroke or 4. A simple flywheel mounted magnet crosses a wire wound core and induces a voltage which is fed to the spark plug. A 2 cylinder engine will have 2 magnetos, usually 90 or 180 degrees apart depending on the geometry of the cylinder arrangement. More cylinders will have some form of spark distribution system which may include points and a distributer to get spark to the cylinders at the correct point in the piston's travel.
Blackhat
Bad experiences come from poor decisions. So do good stories.
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On a automobile that uses a distributor, it does not fire on the exhaust stroke as the distributor runs at half of engine rpm. The older small engines used points to get a quick disconnect from the magneto coil.
The tach senses the high voltage field from the spark to determine rpm. Roly
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Got it, thank you gentlemen.
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(09-15-2016, 08:16 PM)Roly Wrote: On a automobile that uses a distributor, it does not fire on the exhaust stroke as the distributor runs at half of engine rpm. The older small engines used points to get a quick disconnect from the magneto coil.
The tach senses the high voltage field from the spark to determine rpm. Roly
Nor does it fire on the exhaust stroke on many later vehicle engines, such as the case of my F150.
The computer reads the camshaft sensor to fire each coil pack (coil over plug) only on compression, but if the camshaft sensor should fail, the computer resorts to using the crankshaft sensor- it will then waste fire, throw a code, then you can limp on home.
