Yes, you are right. So far, the best and more economic device I have found is a version of the 58 bucks TinyTach but devoted for diesel engines, which costs a little more:
New Diesel Engine Tiny Tach Hour Meter Tachometer w 6mm Tansducer MDS 6 | eBay
It lists for USD 104 app.
This tool senses the diesel pulses, count them, but throw away the pulse duration information. But, I think it processes it, as to count the real pulses.
There is a voltage for the beggining of the pulse, and a voltage to consider the pulse ended. As in this graph:
https://www.flickr.com/photos/oldbeaver/16098018915/
which is not mine but has been used in this and other sites for its clarity.
One alternative is to use the TinyTach and try to take out the info from that device.
Another approach may be what I read somewhere (maybe you wrote?): to take the electric signal of a clamp piezo transducer on a diesel injection pipe, and use it as the input on a simple PWM circuit. It will produce a pulse which should mimic the injection pulses. I think this idea is very clever. You don't know to measure pulses duration, the PWM will do. Simpler is always better.
I am afraid it may require some signal conditioning that can hopefully done in the PWM circuit itself. Luckily.
What do you think?
Oldbeaver