measuring rpm 4-stroke 1 cylinder engine - how?

[Frank Stacey]

My lawnmower has a single cylinder 4-stroke engine. The ignition is by a magneto. I have a general "auto-engine meter" which can measure lots of parameters of internal combustion engines. The rpm measurement involves connecting a lead to the "ignition coil".

I connected this meter to my engine and hoped it would give me a reading which was 1/4 (1cyl not 4) of the true rpm - which it seemed to do. Unfortunately I have no independent check other than the fact that the rpm seemed about right..

On trying to think through whether or not I was being sensible about this I came up with 2 puzzles.

1. The magneto is not as simple as I thought because it has a second lead which connects it to the internal electrics controlling the machine. Is this an input or an output and what does it do? This is the lead to which I connected my rpm meter.

1. The magneto is "fired" from the perimeter of the fly-wheel (not the toothed starter motor rim , of course). This wheel is smooth metal except at

2 diametrically opposite places. One of these places has just a depression the other has a strong magnet attached (judging by what is does to spanners and screwdrivers). All that seems reasonable until it occurred to me that a 4-stroke only fires once for every TWO rotations of the crankshaft. At this point my understanding and surmising of magnetos fails.

Can someone enlighten me? Especially, can I measure rpm as I am trying? The end of the crankshaft is easily accessible at the centre of the flywheel. Isn't there a mechanical rpm meter that can be used by simply by pressing a small shaft against the crankshaft? Can such a thing still be obtained in this electronic age?

Frank

[mrcheerful]

there is probably a wasted spark. you can connect a decent strobe and it will show the rpm. you can still buy rpm mechanical gauges, they are still used for lathe work. you can buy a rev counter that reads a white mark as it flies past, these are used by model engineers and places like maplins sell them.

mrcheerful

[Christian McArdle]

It is easier and cheaper to simply spark twice in the cycle than having timing gear that knows if it is has just finished the compression or the exhaust stroke. There's no harm in sparking between exhaust and inlet strokes. There shouldn't be any unburnt fuel in there.

Christian.

[Set Square]

When used with a conventional (i.e. car rather than mower) engine, does your device connect to an LT or HT connection on the coil? What do the instructions say about interpreting the reading depending on the number of cylinders - bearing in mind that a car engine might have 3, 4, 5, 6, 8, 10, or 12 cylinders? (and possibly other numbers!)

You certainly used to be able to get little hand-held tachometers which came with an assortment of little rubber wheels and cones for pressing against the end or side of a rotating shaft. I used one at work about 30 years ago, but can't remember the make. Typing "hand-held tachometer" into Google gives about 1200 hits - some of which may be relevant.

You can also measure rotational speed with a stroboscope - adjusting the flash frequency until the shaft appears to be stationary.

[Gordon Henderson]

You basically save on the distributor. Citroen 2CVs do this IIRC. They just connect both plugs to the one lead and get it to spark on every cycle.

Gordon

[Set Square]

I've just remembered, they were made by Hasler - [Swiss, I think.]

If you're interested, there's one for sale on Ebay now. See: