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Old 08-04-2008, 07:30 AM   #1 (permalink)
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high voltage injectors (HDi diesel)

I'm very interested in building one of these.

My car has a Peugeot/Citroen HDi engine. The injectors open at 70V, way above a conventional petrol injector. What additional circuit could I add to a) not interfere with the car and b) not fry the MPGuino's microprocessor!

Would a simple "potential divider" do the trick with suitable resistor values?

What voltage does the MPGuino want to see on the injector connection?

70V---
I
R
I
-----------MPGuino
I
R
I
----------GND

There is some introductory info on the engine at
autospeed.com/cms/A_109357/article.html
for those interested.

Thanks in advance,
Simon

PS Just realised that as my injection pressure can vary from 200 to 1350 bar (that's 1350bar) then I need to add this into the equation. I have a fuel pressure sensor. How hard would it be to add in another variable?


Last edited by ecoxantia; 08-28-2008 at 04:23 AM.. Reason: PS
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Old 08-04-2008, 08:42 AM   #2 (permalink)
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It will probably work as is. You are looking at 0.7 milliamps (tiny) with the standard 100k resistor on the mpguino schematic. The processors are pretty cheap ($4.00) so don't over engineer it too much, except to make the processor replaceable


re: fuel pressure, do you know what causes it to change? The other variable pressure schemes I've seen were to keep the pressure drop across the injector consistent and thus did not need accounting for.
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Old 08-04-2008, 09:32 AM   #3 (permalink)
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Thanks.

regarding fuel pressure, as I understand it the high pressure is there to enable pre-injection, for knock reduction; high atomisation and so efficient burn.

Of course generating such pressures takes power and this engine is aimed at economy- so the pump has an ECU- switchable extra piston for high pressure. So the fuel pressure isn't just engine speed dependent, the ECU will drop it at low loads.

I'll try the circuit as-is to start with.
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Old 08-04-2008, 09:40 PM   #4 (permalink)
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More importantly, there are three injections per single combustion cycle.
  • pre-injection
  • main injection
  • post injection
The current MPGuino program can't measure these pulses accurately.

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Old 08-04-2008, 10:44 PM   #5 (permalink)
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Thanks Yoshi, that's a tricky pattern.

Sorry eco, didn't notice the link the first time: http://autospeed.com/cms/A_109357/article.html
"Pre-injection is designed to reduce combustion noise, and involves the injecting of a very small quantity of fuel (less than 1 milligram) prior to the main injection. This early injection allows the gradual increase in the temperature of the combustion chamber, slowing down spontaneous flame-spread and reducing the amount of fuel burned at the beginning of combustion. This reduces idle noise output by over 3dB. Pre-injection occurs only if the engine speed is less than 3200 rpm.

The start and duration of the main injection depends on the amount of load and whether or not a pre-injection process has already occurred. Main injection is cut if the engine exceeds maximum speed, or if there is insufficient fuel pressure in the injector rail (ie less than 120 Bar).

Finally post-injection can occur in engines equipped with a cat converter - presumably to quickly bring the cat up to operating temperature."

So it may work, it depends on the duration of that initial squirt. Sounds like it would work if you only drive above 3200rpm with a hot cat I won't be able to sort it out for you entirely, but of course you are welcome to do whatever you need to to the code. You *might* need to tap into the crankshaft position sensor to determine if the last pulse was a pre-ignition or what.
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Old 08-05-2008, 01:20 AM   #6 (permalink)
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Quote:
Originally Posted by dcb View Post
So it may work, it depends on the duration of that initial squirt.
Do you have any clue how much the "Injector DelayuS" value would be?

add note:
The MPGuino program assumes the fuel pressure is constant thus the fuel usage is proportional to the injector ON time.
The article says...
Quote:
Unlike a petrol injection engine where fuel pressure is held a constant headroom above intake manifold pressure, in the HDi engine, fuel pressure varies with engine speed. Thus, at idle quite long injector opening times are used, because the lower fuel pressure means that there is less flow for a given pulse width. At higher engine speeds the time available to inject the fuel is less and so the injector opening times must be shorter. To squeeze the greater amount of required fuel through the injectors, the fuel pressure must be high. In fact, the 3D fuel map of the ECU uses these axes: fuel pressure, engine speed and fuel flow. In addition, other 3D ECU maps exist for the:
(snip)
I believe we have to have a kind of "3D fuel map" inside of the program to see an accurate fuel usage.

Yoshi

Last edited by Yoshi; 08-05-2008 at 01:41 AM..
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Old 08-05-2008, 05:54 AM   #7 (permalink)
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3D maps certainly are used by the CPU, see
http://www.christiantena.pwp.blueyon...operation.html

It seems to me that to use the MPGuino on this engine there are too many variables to make it work - I think an OBD approach might make more sense.

Simon
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Old 08-05-2008, 09:14 AM   #8 (permalink)
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Quote:
Originally Posted by Yoshi View Post
Do you have any clue how much the "Injector DelayuS" value would be?
No clue, I'm guessing it is a small amount of time. But I never even contemplated HDI till yesterday, have not hunted down injector specs, and don't know how long it takes to squirt out a milligram at unknown pressure at unknown flow rate. That's a measure it and see problem IMHO.

Quote:
Originally Posted by ecoxantia View Post
It seems to me that to use the MPGuino on this engine there are too many variables to make it work
The atmega in the mpguino is most certainly capable of performing accurate fuel consumption monitoring on an HDI with the right programming strategy. It may come back to monitoring the fuel pressure like you said, and applying that and the injector open time to a 2d map formula. It isn't that there are too many variables though, technically speaking.

But it might not be a very practical exercise if a $160 scangauge will tell you what you want to know. You would have to really want the additional precision an injection monitor can provide to go through the effort.
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Old 08-05-2008, 01:57 PM   #9 (permalink)
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You could also build some sort of flow meter that you put in the fuel line. I know the guys that are doing the "pre-fuel injection" stuff have been looking at a solution doing something like that. The nice thing is that if you can get a flow meter, it should work on any vehicle, and you wouldn't have all the complexities. It may not be as accurate, but it would be doable in the short term.
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Old 08-05-2008, 04:59 PM   #10 (permalink)
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Thanks for all these replies, its a great forum.

The flowmeter probably wouldn't be viable - as most of the fuel is in fact returned to the tank! I guess 2 flowmeters might...

Good to hear that you reckon it could be made to work, dcb. Time for me to look at some code I think.

BTW the scangaugeII won't work according to the european distributors. As I understand it, the CPU is only partly EOBD compliant. It's a "Bosch special". In Europe, only 2004-on diesels have had to be OBDII compliant.

Simon

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