measure rpm with spark plug inductor

[alexl2014]

Hello all,

I am working on a project where i would like to measure the engine RPM of my motorcycle using a simple inductor wrapped around a spark plug wire. Is this possible to do with the NI MyDaq? If so, what would be the best way to go about this? I have had one instrumentation class using the mydaq and labview so i have a basic understanding but am definently still a novice. If possible, be detailed with your explanation-like you are explaining it to a well trained monkey :). I appreciate it!

Alex

[SnowMule]

Shouldn't be too hard.  Good news is there's not a lot of danger involved with an inductive coupling into a DAQ.  MyDAQ should be able to pull a signal in.

I'm assuming you're trying to do something like an hour-meter pickup (pictured on my generator), just going into a DAQ instead of an hour meter.  You might need to terminate both ends of the coil into the DAQ, play around with either differential or single-ended measurements.  Not sure which one would work the best.

Have to look at the signals you get back, might be able to trigger at some voltage level and get a usable number out of that.  Might have to do some filtering if the signal's not real clean. 

Also keep in mind you may have to multiply or divide by two to get the actual RPM.  A lot of motorcycle systems are "wasted spark" systems, where the ignition will fire with the piston(s) at BDC or on the exhaust stroke.  A two- or four-stroke engine will be different too. 

My snowmobile's ignition (twin-cylinder two-stroke) fires both plugs simultaneously, so when one piston is at TDC the other is at BDC.  Obviously the spark in the cylinder at BDC isn't doing anything.  For every revolution of the crankshaft, there's two sparks on each plug, so in order to get the actual RPM you'd need to divide that number by two.

My dirt bike and generator (both single-cyl four-strokes) fire once per revolution, but since power comes from the ignition stroke and not the exhaust stroke, only the spark after the compression stroke actually ignites the mix.  The spark in the exhaust stroke is "wasted".  In this case, the SPM (sparks/minute) equals the RPM since the pickup is on the flywheel.

If the ignition pickup is on the camshaft you'd need to multiply by two since the cam rotates once for every two rotations of the crank.  ECU can also control which plugs fire and when in an engine like my truck.

Just poke around and see what you get.

[johnsold]

Spark signals are of fairly short duration, so consider that a sample rate of twice the maximum spark rate (~RPM) may miss these short pulses.

As SnowMule said, poke around and see what you get.

Lynn

[SnowMule]

Sampling at twice the RPM gets you one reading per 180° revolution. I'd say you'll want to read at at least 100/revolution.

MyDAQ can do 200kS/Sec.
1000 RPM = ~17 rps.
at 200kS/Sec, you're getting 6120 readings per revolution.
That's about 32 readings per degree. More than enough to catch a spark pulse.

[AnalogKid2DigitalMan]

Just don't poke through the high volatge insulation 🙂

You may be in for a really nasty waveform with a lot of transients and ringing. You may need to condition your signal first instead of trying to deal with it in software.

See the first circuit on this page (I think their terminology is a bit off):

http://www.sportdevices.com/rpm_readings/index.htm

For the probe, use a few inches of coaxial cable.

Link taken from this discussion:

http://www.eng-tips.com/viewthread.cfm?qid=320604

Maybe a better option is to tap off the signal that controls the spark firing, it is probably low voltage and a cleaner signal.

-AK2DM