# Building a tachometer for a turbojet engine (100,000+ RPM)

I have a buddy here at school who has built a turbojet engine from an automotive turbocharger. He wants to get a reasonably accurate indication of the rotor speed, and I volunteered to help him build a tachometer for his engine. It has to have the following characteristics:
[ul]
[li]It must be permenantly mounted to his instrument panel (no handheld tachs or multi-meter solutions)[/li][li]It cannot involve mechanically mounting anything to the turbine wheel[/li][li]It has to read up to about 120,000 RPM[/li][li]It has to be cheap![/li][/ul]
Now, I have thought of a couple of ways to do this, but I wanted to run the problem past you guys in case there might be a better solution I haven’t thought of.

1. Use an optical sensor with a mark on the turbine wheel. At 100,000 rpm, this will produce a 1666Hz signal. I could then use an IC such as a Philips ECG995 to convert the frequency to voltage, and display the voltage on an analog voltmeter marked in RPM instead of volts. This is my favored solution at the moment.

2. Use the optical sensor, and then use a binary rate multiplier to bring the frequency down into a range where it can be displayed on an electronic motorcycle tach. For instance, divide the 100,000 RPMs signal of 1666Hz by 5, resulting in a 333Hz signal that the tachometer would read as 10,000 RPM. We could simply change the face marking from RPMx1000 to RPMx10000.

Any comments or suggestions would be appreciated.

{Proving you get it working the way you want and installed in a vehicle to test it, …

Where are you gonna test it?

Have you asked on the DIY-GT group on Yahoo? It’s the big one, and the tach question has been addressed many times. Don’t forget the 1450/1450 rule, or 27 million if you’re in metric.

Duckster, Chris Krug mounted his turbocharger turbojet with afterburner on a go-kart, and just ran it on the street. 100 lbs of thrust from a tank turbocharger.

Mark Nye used a commercailly built gas turbine and built a jet boat around it. The thing kicks ass.

Both were the teams’ experts on an episode of Junkyard Wars to be aired in the spring.

The jet is already running. He taking it apart to add better instrumentation and more reliable fuel/oil/electrical systems, etc. He runs it at the airport, in the same spot we run our JT-12s, J-33, etc. It isn’t on a vehicle, but may end up on a go-cart.

Or were you asking about how we were gonna test the tach? I was planning on first trying it on a known frequency. Holding the sensor up to a fluorescent light should produce a 3600 rpm indication. After that, I would hook both the tachometer and an oscilloscope to the optical sensor and use the oscilloscope to calibrate the scale on the tach.

Not that I would (or could) build something like this jet, I’d love to know where I could see the plans/instructions for building one. Or should I just wait for the Junkyard Wars version?

No, i haven’t checked out the Yahoo group, but I will.

What is the 1450/1450 rule?

It’s pretty much a seat-of-the-pants thing.

A simple one can be really simple, just some pipe fittings bolted onto a turbocharger. It might not last very long, if it leaks or runs at too high a temperature, but it will work.

Mike Early and Mark Nye both offfer plans, if you really want to go that way. Most people just see what everyone else has done, and then they do their own however makes most sence to them out of whatever material they can get easily.

You’d be amazed what one can do with some exhuast tubing and duct tape…