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This is a Mercedes-Benz 308D van, year 1992, model 602.316. Pre-OBD, no electronics of any sort, everything is purely mechanical.

I would like to retrofit a system which would measure the precise amount of fuel injected into the engine in real time, to be able to feed the data into my Arduino and log it there.

The engine is an OM601.940 - a classic plain naturally-aspirated diesel engine (neither turbocharged nor supercharged). Fuel injection is indirect via a prechamber arrangement. The injection pump is a mechanical fuel injection unit with a 5150 ±50 RPM mechanical governor, automatic altitude compensation, and a 'load sensing' automatic idle speed control.

How could I make a device to precisely (sort of ±1%) measure the amount of fuel going into the engine in each injection and feed the data in real time to an Arduino micro-controller board to be processed there?

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just to understand: does it have injection valves (as opposed to a carburator) ? Does it have an engine control unit (which controls these valves electronically) ? – Andre Holzner Apr 14 '11 at 14:14
@Andre Holzner: For sure it does NOT have an engine control unit which controls these valves electronically - once started this engine will even run the same without any electricity at all! I have no idea if it has injection valves or a carburettor. What I found out is that: "One of the notable features of the OM-601 is the fuel injection, which utilized a pre-chamber design that stayed in production until it was finally replaced by CDI rail-injection. The fuel injection pump was a Bosch PES-4M." – miernik Apr 14 '11 at 14:55
@Andre Holzner: aren't carburetors only used on petrol engines? don't all diesel engines use injection valves? I have found out that the engine uses a DNOSD261 nozzle. Maybe if you Google that model number you can tell for sure is that an injection valve. – miernik Apr 16 '11 at 23:40

If the injectors are fired electrically(which they most likely are), you could wire in a voltage-switch to the injectors, so that it is closed when the injector is fired. This would allow you to measure the time spent open, and do a little math based on the flow-rate of the injector to get a calculated measurement of the fuel injected into the system. If they're fired mechanically, you could try to make some kind of sensor to read the state of the mechanical system, although that might be a little more interesting/complicated.

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@Annath: this engine doesn't need electricity while running, it was reported to run fine with the battery cut-off after starting it, so obviously the injectors can not be fired electrically. Are you sure the injectors can have only open or closed stated, and are not opened partially in which case measuring time would not be enough, as different amounts of fuel can flow in the same chunk of time? – miernik Apr 14 '11 at 16:29
Was the alternator disconnected also? (Or, is there even an alternator?) You might be right about the in between state, so you would just have to tune your sensor(be it electrical or mechanical) to figure out the variation in signal to flow rate relationship and the idea still applies. – Annath Apr 14 '11 at 17:07
@miernik, remember, if the engine is running and turning the alternator, you don't really need a battery connection. If you disconnect the alternator cables (which I don't advise) and the battery connections at the same time, I think you'll have a stopped engine very shortly as the fuel pump stops. – Bob Cross Apr 15 '11 at 1:27
@Bob: it was tested by some guys on a forum with a faulty alternator. The fuel pump is mechanical, driven off through some gearing from the engine - that's sure, confirmed from multiple sources. – miernik Apr 15 '11 at 1:54
Diesel engines do not need electricity to work at all, in fact (as wikipedia states here it's called compression-ignition because the fuel ignition is started when compressing air+fuel gets to the point of making the mixture explode. After starting the engine you may disconnect all electric devices, battery and alternator included and the engine will continue working unless you obstruct air inlet. – Eugenio Miró Apr 15 '11 at 3:12

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