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  1. #21

    Default Re: What is common rail technology

    yes it is, but only cuz the pump's running, like jfm said. The 'reservoir' has no means of maintaining pressure once fuel is removed from it. Not like say, the pressure-vessels used on a cheating system or boaty pressurised water.

    so yeah, in theory ( and likely practice ) the pressure in the err rail will be constant & at some elevated prss, but simply because the designer hopefully specced a pump that can deliver fuel at that pressure, whether at neutral/idle or full chat.

  2. #22
    jfm's Avatar
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    Default Re: What is common rail technology

    The pressure in the common rail is always the 30000psi becuase of the pump. I dont know how the retrun is plumbed. I dont know if any of the fuel that was at 30,000 is returned to tank (spannerman?) but if it is it will have its pressure reduced before being sent down the return line.

  3. #23
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    Default Re: What is common rail technology

    The injectors in a D4-6 rail system have a continual high pressure applied to them from the common rail, and they are also connected to a leak off rail which has a pressure control valve in it to maintain the req'd pressure other wise the fuel would just rush back to the tank. Diesel that has passed through this valve is no longer under high pressure, it meets bleed off fuel from the main rail and is returned to the tank.
    The pressure valve on the main rail opens at 1900 bar and closes at 600 bar, so it cycles, opening less frequently with higher demand as then most fuel is going through the injector nozzles.
    The same principle has been on petrol engined cars for a couple of decades as they use a rail to supply fuel to the injectors and a pressure control valve to maintain a stable pressure.
    I think what is confusing people is that the old systems needed the pressure pulse from the injection pump to lift the needle off its seat and inject fuel, what didn't go through the nozzle bleeds off as low pressure fuel.
    Whereas the Volvo system has continual high pressure available and it uses an electromagnet to open a servo valve in the injector which then lets the high pressure fuel open the needle further, when the ECU has decided that enough fuel has been injected it switches off the electromagnet which then shuts the servo valve forcing the high pressure fuel back into the leak off port and thereby closing the main needle and terminating the injection cycle.
    I am quoting from the Volvo workshop manual in front of me here.
    The pump injector on a D9 or D12 has a gallery supplying fuel at 450-500 kpa to fill up the injection pump part of the injector, then when the cam lobe pushes down on the injector plunger it pressurises the fuel at up to 2000bar, the electronics then determine how long and therefore how much it injects.
    Hope this clarifies the theory behind these modern systems.

  4. #24
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    Default Re: What is common rail technology

    Thanks,

    Stating the obvious here but I would imagine it would be pretty dangerous to be around or the the high pressure side of the rail in the event of a leak!!!!

  5. #25
    Oldhand is offline Registered User
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    Default Re: What is common rail technology

    [ QUOTE ]
    If you let out a tiny amount of fuel the pressure would drop substantially, becuase of the near zero elasticity of the rail itself and liquid diesel fuel. In other words, there is no pressure reservoir really. To have a pressure reservoir that would maintain its pressure as the fuel was let out, you would need an elastic component (either the fuel itself or the container, ie the common rail, in which is is held), and there isn't one. The reason the pressure stays high in the common rail is that the pump is running the whole time the injector is leaking fuel into the cylinder. I think...

    [/ QUOTE ]

    I agree the pressure would drop appreciably if there wasn't a high pressure pump continuously operating, the volume of the common rail alone will act as a pressure change damper. The pump itself is likely to produce a pulsating pressure, however my common rail car engine does appear to have an "accordian pleated" chamber as well whcih I presume is to further dampen pressure changes in the supply to the injectors by changing the volume of the system. This engine's torque and efficiency far exceed that of previous mechanically injected models. I haven't examined CR boat diesels but I wouldn't like a boat engine that depends on electronics to function.

  6. #26
    jfm's Avatar
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    Default Re: What is common rail technology

    Ah, ok, the accordian pleated bit sounds like it is to give the chamber elasticity, which makes it act like a bit of a pressure reservoir

    I wouldn't worry about electronics at sea. They seem to have got the watertightness sorted. I have had 4 years/800hours of electronic diesels and would never go back to mechanical. The clean exhaust, fuel efficiency, easy starting, quietness and fab instrumentation are huge benefits, and I have never had a water-in-the-electronics problem. Most systems are duplicated anyway in a twin engined boat so you can usually get home even if you lose an engine. All imho though, and touch wood etc!

  7. #27
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    Default Re: What is common rail technology

    I agree with jfm
    We had some early D12s on our last boat.
    Never a problem and are over 6 years old now.
    Always ran like a dream - never any electronic problems either.

  8. #28
    DavidJ's Avatar
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    Default Re: What is common rail technology

    As much as I am pleased that marine electronics doesn't cause too many problems for most people, I have KAD43's and the only bit of electronics is the speed sensor electronic module for the compressor. You've guessed!....the only problem I've had in 5 years is with the electronic module.

  9. #29
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    Default Re: What is common rail technology

    Can only concur. D4 two seasons use, 250hrs. Hate to tempt fate but have to say all excellent. Very clean and very economical. Never any hint of smoke even when starting from cold or 'flooring' throttle. Slight trace of black on transom after many hours of running. burns about 25 l/hr at 25 knots, yep an Nm/litre.
    Old Chinese proverb 'Man who sail boat into rice field, soon get into paddy'

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