Refurbish antique diesel engines with modern parts

Sensing Engine Speed – Various Approaches

Some turbo controllers don’t look at engine speed, only boost levels and maybe throttle position. I’d like to know how fast the engine is spinning in relation to TPS, boost, EGT and EGP. A VNT turbo can build boost very quickly but you don’t want that boost too early in the RPM range.

One of the things that’s been puzzling me a bit as I’ve thought about this project for the past couple years is the best way to sense engine speed. My Toyota diesels happen to have the stock variable reluctance sensor on the bell housing that was used to drive the stock tachometer. The tach was an option on most of the 40 and 60 series diesel Toyotas, and a fairly rare option to find these days. What I’m not sure about is if all of the IDI diesel Toyotas with inline mechanical injection pumps came standard with the sensor installed? I’ll need to research that a bit more. I started a topic @ in the diesel section to see if maybe someone there can help answer this question.

Interfacing to a variable reluctance transducer is fairly straightforward using the Maxim MAX9924 series chip. It would be fairly straightforward to use this engine speed sensing mechanism for my trucks but maybe not such a great idea for other applications. I’ve prototyped it on a spare Geo Metro flywheel I had laying around on the garage. Interfacing the MAX9924 to a CPU’s counter circuit is very easy and seems that it could be reliable too.

I’ve also thought about using a piezoelectric transducer clamped to an injection line. This approach would be fairly universal and could work on many different engines much like the diesel TinyTach. The problem with these sensors is cost. I haven’t found a source for them at a reasonable price point. If I go this way I’ll likely end up telling customers to buy the transducer direct from TinyTach for $65. Most other sources for such a component are well over $100 each. The benefits of a piezo solution is that it’s more generic, easy to install and will work on more engines. The downside is cost. I’ve looked at the clamp-type transducer’s output on an oscilloscope and I think a good signal conditioning circuit will take a little bit of effort to design, but the supporting circuitry should be fairly inexpensive.

A third approach would be to use a Hall Effect sensor. This would involve making a bracket and installing a toothed wheel or magnets on the crankshaft pulley. Cost is reasonable but installation becomes much more cumbersome. And if the installer doesn’t do a good job lining up the sensor at the proper location and distance from the trigger it’s not going to work very well. Back in the late 90s I had a turbo 2.3 Ford sandrail with the SDS EM4-F engine computer system. (They are on the 5F version now.) The system worked great and I highly recommend their products but the install process for setting up the Hall Effect sensor was a royal PITA. This is the least appealing option for me. Cost-wise it’s reasonable. Installation-wise it’s barely acceptable.

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