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Quick CIS question...

Discussion in 'Technical Q&A' started by bpu699, Mar 25, 2010.

  1. bpu699

    bpu699 F1 World Champ
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    CIS=Continuous Injection System, right?

    But, I presume that the injectors don't flow at all times, right? Does the fuel head allow flow to the injector only during part of the cycle, ie. when the intake valve is open?

    Here is why I ask. I was doing a quick test on some old injectors, using a can of brake cleaner. I know this isn't the Bosch approved method, but its a quick and dirty check for clogged injectors.

    What I noticed is that you could get the entire bottle of brake cleaner through the injector in about 10 seconds. At that rate, with anywhere from 8-12 injectors in our cars, a tank of fuel would last about 5 minutes...at idle...

    So, I assume that the injectors only spray intermittently. And if that so, why is it called CIS?

    Also, If I understand this, with CIS the fuel distributor determines how much volume is sprayed, not the injector. Whereas with electronic injectors, its the pulse lenght/etc. If thats the case, why do some places still volume tests CIS injectors???

    If I am totally off base, let me know... :)

    Bo
     
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  3. Rifledriver

    Rifledriver Two Time F1 World Champ

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    #2 Rifledriver, Mar 25, 2010
    Last edited: Mar 25, 2010
    They spray full time. Delivery quantity is varied by pressure and is controlled by the distributor.

    Volume testing is a test of the distributor, not the nozzle. The only valid test of the nozzle is opening pressure, leakage and spray pattern.


    The only volume tester I am familiar with for CIS was made by Bosch. I own one. It tests by connecting to the injectors for the sake of convienence but is not an injector test, it is a distributor test. There is no way to evaluate the injectors when hooked up to that machine.
     
  4. bpu699

    bpu699 F1 World Champ
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    #3 bpu699, Mar 25, 2010
    Last edited: Mar 25, 2010
    Brian, thanks for the input... can you explain further... what am I missing?

    Seems like a lot of the CIS injectors open at about 50 psi, and I believe the system operates at 90-100 PSI?

    I filled a pressurized cannister with carb cleaner, pressurized to 90 psi, and was able to shoot a heck of a lot of fluid (?8oz) through the injector in several seconds...

    If I look at various websites that show how injectors are cleaned and flow tested, it seems that they can fill up a 16oz beaker in no time. If this is how the injectors flowed in our cars, we would be out of gas in no time...

    I don't know a whole lot about fluid dynamics (disclaimer), but I thought flow=pressure/resistance. Resistance is fixed (size of the cis injector?). So flow should be based on pressure alone. And at 90psi, you flow a ton of fluid, if it flows continuoulsy.

    Now, if opening pressure is 50psi, you would drastically decrease flow if you supplied, say, 55psi. But at 55psi, the injector really doesn't spray anything resembling a nice atomized fuel mist...

    Here is a youtube video of injector testingl Look how fast it goes through fuel...a gallon through 6 open injectors would be out of fuel in 30 seconds...

    http://www.youtube.com/watch?v=pKsX1Lqb5mY

    How does this work differently in the car? I just don't get it...
     
  5. Futureman

    Futureman Formula 3

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    I just so happened to have finished reading the Bosch Tech Brief. It gives a decent overview of what Brian just summed up in a few sentences. I've been trying to get a better understanding of this system myself. It's truly a mechanical system. Pretty clever actually. Brian may correct me on this, but as far as I can tell the only electronics of the system are the ECU for the O2 sensor (for a lambda system, such as mine).

    Here's a link to the tech brief:
    http://www.dmcnews.com/Techsection/Bosch%20K-Jetronic%20Fuel%20Injection%20Manual%20-%20boschtech-12d.pdf
     
  6. bpu699

    bpu699 F1 World Champ
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    #5 bpu699, Mar 25, 2010
    Last edited: Mar 25, 2010
    Thanks guys...according to the site, the system functions at about 75psi? Perhaps when testing the injectors they are run at much higher psi's, accounting for the very high volume flowed...

    I wonder how much psi a can of carb cleaner is at?
     
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  8. bpu699

    bpu699 F1 World Champ
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    Oh, follow up question please...

    CIS injectors seem to clog or at least mis-spray pretty frequently with age... wouldn't this alter the air/fuel mixture? Why don't we see a ton of burned cylinders from running lean?

    I have seen some injectors that only shoot a one sided stream, don't atomize at all, and the motor seems fine... I am sure your mileage may be lower, but why isn't there more engine damage???
     
  9. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #7 Steve Magnusson, Mar 25, 2010
    Last edited: Mar 25, 2010
    Proper atomization is a good thing (especially at low RPM when trying to meet a strict emission limit), but it can be pretty terrible and still work OKish (i.e., the overall A/F ratio really isn't altered as long as the proper amount of fuel is exiting the injector) -- look at a carburettor -- that's like dribbling the liquid fuel out of a spoon into the cylinder and that works OKish ;)

    As Brian indicated, the CIS injector is just a pressure relief valve (and atomizer) that is always open when the engine is running -- it doesn't determine the amount of fuel delivered. It would be rare that an injector problem (or even a fuel distributor problem) would just slightly lean out a single cylinder to where it would still be running (so you couldn't detect it) but cause engine damage. The more likely flaw is no fuel at all, or too little fuel to ignite, reaching the cylinder -- i.e., a bad miss (which isn't great either), but that gets noticed (and fixed).
     
  10. Rifledriver

    Rifledriver Two Time F1 World Champ

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    System pressure and control pressure are two different things and the viscosity of the fluid, even small changes, make a big difference when spray testing nozzles.


    Also, like Steve said, pattern is important but becomes less important as RPM rises and heat increases. Both high velocity with its turbulence and high heat cause fuel to atomize/vaporize allowing a good burn. If you look at a Bosch chart, far less than perfect patterns are acceptable. Leakage is a big problem and any leaky, drippy nozzle should be discarded.
     
  11. bpu699

    bpu699 F1 World Champ
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    Thanks guys...always good to learn...
     
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  13. autowerks9

    autowerks9 Formula 3

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    DIRT in the system is one of the most common problems in the CIS systems. make sue you check all of the in-line filters at the WUR and the Distributor. I have a good write up on cleaning the internal of the Distrubutor and replacing the O-rings. I did mine last fall and replaced the 8 injectors, I found a benz injector that only $23.00 each, not even worth cleaning the old ones. It really woke up the motor. ( 82 928s M28/11 motor 310hp).
     
  14. DGS

    DGS Four Time F1 World Champ

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    #11 DGS, Mar 26, 2010
    Last edited: Mar 26, 2010
    A CIS uses fuel pressure the way EFI uses injection time. The control pressure to the fuel distributor makes up the "injection computer" for a CIS system.

    Where EFI injects at constant pressure (and flow rate) for a variable time, CIS injects at a variable pressure (and flow rate) continuously (a constant time between intake strokes at a constant RPM).
    (Varying RPM varies the "injection time" for a cylinder, which CIS has to account for when varying the injection flow rate.)

    The fuel pump brings the system pressure up to a high value.
    In a lambda system, the frequency valve lowers system delivery pressure in the fuel distributor based on the O2 response.
    The airflow plate (and control regulator) adjusts the control pressure, which regulates the area of the openings to the injection lines. That opening area determines the variable pressure in the injector lines.
    The pressure in the injector lines determines the flow rate through the injector nozzles. ("Choked nozzle" model, for those who studied fluid dynamics.)

    (Pressure difference and opening area determine force through the nozzle, and the fluid characteristics translate that to flow rate.)

    It's mathematically very complex, and it requires a great deal of precision in manufacturing, but then it's a few mechanical moving parts, so there's not a lot to go wrong with the basic system.

    Of course, once you start adding EPA controls, then you're bolting on a lot of extra bits to tweak the basic system. (Like the frequency valve, or more parts in the control regulator.) Each add-on is another failure mode.

    In the early '80s, CIS wasn't deemed the ideal system for Ferrari --- it was considered the euro system that could get certified by the EPA for the US (and would pass other countries' controls, too).
    A lot of euro manufacturers used it in the '70s to '80s, because it was the one that they knew would pass import controls.

    (Remember that the US *required* O2 sensors as of 1980. That disqualified carbs, Spica injection, and other systems you couldn't hook a lambda loop to.)

    EFI in the '80s had the same "parts to go wrong" issue: the more sensors you added, the more likely that one or more of them wouldn't be working at any given time.

    It was, I think, the Japanese who started building self diagnosis into the EFI systems to make replacing the less reliable bits easier.

    My '88 Toyota had EFI with on board diagnostics already. By comparison, my '88 328 has CIS -- which doesn't fail often, but when something does go wrong, it's a @#$% to fix.
    My '81 GTV-6 with Bosch L-Jet had a similar "MTTR" issue: you had to check all the attached bits, one by one, to see which were (and weren't) working.
     
  15. msouza

    msouza Formula Junior

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    David,

    Do you mind sending me a copy of the fuel distributor cleaning and O-ring change. I will PM my email to you.

    Thanks.
     
  16. autowerks9

    autowerks9 Formula 3

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    e-mail sent....
     

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