Tachometer for four plug, two cycle motor

Hello all,

I have a use case for a deluxe tachometer system.

  • two cycle engine
  • four plug wires
  • inductive signal detection (wire wrapped around plug wire) tinytach.com
  • 12v powered tachometer
  • one digital display per plug wire
  • small four digit digital displays for a small footprint.

Details: This will be a backup system for the main engine instrument. In this case the primary rpm is being derived by the stator plate turning on the engine it's self. I need to have independant rpm from the spark being sent to each plug, not how fast the engine is turning. There are inductive tachometers available (tinytach.com) but they are large and I would need four. At $50ea that's $200 in tach's plus the mounting would look bad, also the tiny tach's use a sealed battery and I want somethign that just runs off of the native 12v battery. I have limited space to mount these displays but a lot of room behind the panel for the "guts".

I have not used arduino before however I am a network engineer but am not comfortable with writing programs from scratch but am good modifying other peoples code. I have been looking for a project to use an arduino and this may be a great first with real practical use case.


Thanks guys!

Looking for advice on which arduino to buy and what other bits i'll need for this project.

Is this 2 cylinder or 4 cylinder (aircraft engines for example use 2 plugs per cylinder with dual ignition systems)? why do you need 4 individual ones instead of just one? (and what are you looking for - a tachometer shows engine speed but you say you don't care how fast it is turning).

I would use existing code for an Arduino. See http://www.instructables.com/id/Arduino-Based-Optical-Tachometer/?ALLSTEPS

The only thing that would be different is the electrical interface (spark plug wire) vs. the optical sensor, and use the calculation he mentions, rpm = 60*1000/(millis() - timeold)*rpmcount; because your 2 cycle engine fires on every revolution, while the example used a sensor that triggers twice per revolution.

Unsure why you want a tach for each spark plug, since both fire at the same time, maybe just to verify a spark actually occurred? One tach should give the same result as 4 tachs if the engine is operating normally.

Yes aircraft engine and I want to know that the spark actually happened on each plug. The engine rpm is not derived from spark, just how fast the stator plate is spinning. As you might imagine, on an aircraft if you have a mag degrading you would want to know in the air if it’s an instrumentation problem or if it’s an actual problem with the spark system requiring an expedited course to a runway. I want to know what each mag is doing on each plug as I explained in my original post.

So, on to the next part. I’m a total arduino noob.
Is Arduino the right thing to use?
If so which one would be appropriate to take 4 pulse sensor sources and then output to four discrete digital 4 digit display’s?
Also what would I need to do the induction sensing?

You may have a problem picking up the signal from aircraft plug wires - they are usually shielded. Another issue is determining if the plug actually fired correctly. You can tell from looking at the waveform of the impulse to the plug just what is happening, and I suspect the risetime of the pulse will be significantly different for a fouled plug than a clean firing one.

Your comment "I want to know what each mag is doing on each plug as I explained in my original post." is also wrong - I just happened to familiar with aircraft engines - you never indicated dual ignition - you said it was "a two cycle engine" and had "4 plugs". Two cycle is NOT the same as two cylinder.

You also may have another issue - depending on exactly what the category of the aircraft is, the FAA (or whatever you have in your area) is very sensitive about "modifications" to any kind of aircraft without correct documentation and testing.