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Thread: Common Rail vs Direct Injection *Diesel*

  1. #1
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    Common Rail vs Direct Injection *Diesel*

    Hello all:

    Could one of you 'tech heads' please explain the diff; pros; cons; of common rail and direct injection vw diesel engines. I know only the basic stuff, but i see that some new models will have common rail.

    thanks,

    vw50

  2. #2
    brackie Guest

    Direct Injection and Common Rail

    Wow....This could be a long one! In a nutshell:
    • Most large truck/machinery diesels are, and some early passenger diesels were, direct injection
    • "direct injection" means just that...the diesel is atomised by the injector and sprayed directly into the combustion space offered by the ascending piston
    • direct injection diesels are generally noisier (more "diesel knock") than IDI (InDirect Injection) diesels
    • indirect injection was adopted by engine developers who saw it as a way of offering more petrol engine-like characteristics to buyers of diesel engined passenger cars and light commercials... eg less noise, smoother running and a more compact engine
    • direct injection is generally more fuel efficient than indirect injection as the fuel isn't forced to "swirl" in the pre-combustion chamber, a process that saps some of its potency
    • glow plugs to preheat the fuel in the pre combustion chamber for easier starting aren't so important in direct injection diesels (although they are still used to make the car more petrol engine-like)
    • direct injection became applicable in passenger car diesel only after computerisation of the fuel system. This combined with modifications to the pump(s) and injectors solved all of the problems of earlier direct injection engines
    Common Rail
    • until VW brought in the PD engines of the later TDIs, all diesels had a separate injection pump to provide pressurised diesel to each injector. These were either multi-element (in-line) pumps or distributor pumps like the Bosch VE used on VWs until PD was introduced
    • injection pumps are expensive and complex with very fine tolerances and are only serviceable by people who know what they're doing (having said that they are extremely durable and should outlast the engine if the fuel is kept clean and water-free)
    • common rail injection involves a high pressure fuel pump delivering extremely high pressure fuel to a pipe (the "common rail") that delivers fuel to the injectors. Unlike other systems, the fuel is under a constantly high pressure.
    • Each injector is computer controlled in order to deliver the exact amount of fuel to the cylinder at the exact time it's needed. This negates all of the "guesswork" that evolves no matter how well the fuel injecting system is designed or maintained in a mechanical system
    There's a whole lot more to this than this brief account, however it may help your understanding.

    I stand fully prepared to edit and amend any inaccuracies in the account above

    I'll move this thread to the "Diesels" forum
    Last edited by brackie; 26-06-2007 at 05:07 PM.

  3. #3
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    well done brackie,

    vw50, you should understand that diesel technology is actually developing now, and so there are no cut and dried answers like which is better. different diesel fuel injection and combustion systems are suited to different applications, the fuel you want to use, how you want the engine to behave etc etc etc at length.

    it is interesting to note that many companies started using common rail injection before vw did, and vw are very clear about this - they feel/felt that common rail is not up to the performance and reliability standards of PD (pump duse) (someone correct my spelling).
    Last edited by phaeton; 27-06-2007 at 09:10 AM. Reason: corrected deuse to duse ;)
    '91 Cabby 1.9 TD - Garrett T3, FMIC, stainless 2.5" exhaust, Bilstein shocks, tuned injector pump, big nozzles, BIO-DIESEL - 5.1l/100km @110km/h
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  4. #4
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    Quote Originally Posted by gldgti View Post
    well done brackie,

    vw50, you should understand that diesel technology is actually developing now, and so there are no cut and dried answers like which is better. different diesel fuel injection and combustion systems are suited to different applications, the fuel you want to use, how you want the engine to behave etc etc etc at length.

    it is interesting to note that many companies started using common rail injection before vw did, and vw are very clear about this - they feel/felt that common rail is not up to the performance and reliability standards of PD (pump deuse) (someone correct my spelling).
    Isn't it the other way arround?
    PD system is not able to meet future emission requairments.
    Common rail is.
    VW aknowledges that and starts fitting common rail system to their engines.
    Temporarily they see window of oportunity with DPF fitted and after exhaust treatment.
    Unit injector system is not as flexible as common rail and I don't think it is more reliable either. Common rail system allows multiple injections and pre injection or after injection. Maximum pressure today is 2000 bars. Verry close to max pressure in PD system. There is less black smoke (soot) from the exhaust of the common rail than PD TDi-Before fitting DPF or after exhaust treatment.
    I've had MB Vito van which was common rail it was reliable and exhaust fumes were not as black as from VW Transporter T5 PD TDi.
    I was driving them for six years I never saw black exhaust smoke in the rear view mirror. It is not a case of VW Transporter. I can make the car behind me disapear in the clouds of black smoke if I accelerate from lights little bit faster than normall driving requaires.
    But I like driving my PD TDI's.

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  5. #5
    brackie Guest

    Great info

    Thanks for that info. We dieselers are always hungry for knowledge and up to date information.

    This sort of thread should really be archived so I'm going to make it a "sticky".

    What do you guys think about a FAQ page?

  6. #6
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    Already added one (while you were away)

    EDIT Thanks for all the info guys
    Last edited by phaeton; 27-06-2007 at 09:11 AM.
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  7. #7
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    Quote Originally Posted by vmq6695 View Post
    Why bother? No need.

    Wikipedia does it all better (up to date, no typos, articles that are planned and therefore well structured and well written, compared to a dynamic group dialogue).

    Point your browser to:

    http://en.wikipedia.org/wiki/Common_rail (the Wikipedia version doesn't mention Lucas, so the diesel engineering teachers and textbooks I had are obviously worthless fools!)

    http://en.wikipedia.org/wiki/Pumpe-duse (hard to understand why the article doesn't mention VAG, Audi or VW; but then the manufacturer of my car, my diesel engineering teachers and texts are obviously worthless fools! Not to mention the questionability of representing u-umlaut with 'u' instead of 'ue'!)

    http://en.wikipedia.org/wiki/Turboch...rect_Injection (at last a mention of VAG - even if it refers to the 'Volkswagen Group" and not the *correct* Volkswagen Audi Gesellschaft.)
    So Wikipedia STILL does it better then........<ha ha ha> .......forgive my cynicism.

    But gee this comment hardly adds to this thread......does it? Oh well.

    Lets add something then........a new diesel injection concept with delivery pressures of 160,000psi, developed in Australia and can apparently be retrofitted. Currently seeking investment http://www.greendieselcorp.com/

    ..and the use of LPG in conjunction with diesel to make a bigger bang and reduce fuel use http://www.dieselgasaustralia.com.au/

  8. #8
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    Wink

    Quote Originally Posted by vmq6695 View Post
    Why bother? No need.

    Wikipedia does it all better (up to date, no typos, articles that are planned and therefore well structured and well written, compared to a dynamic group dialogue).

    (at last a mention of VAG - even if it refers to the 'Volkswagen Group" and not the *correct* Volkswagen Audi Gesellschaft.)
    Wikipedia is more correct than most encylopedias but not always 100% accurate as its free for anyone to edit the pages

    VAG actually means Volkswagen Aktiengesellschaft (stock trading company )
    - Ben -
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    Quote Originally Posted by phaeton View Post
    Wikipedia is more correct than most encylopedias but not always 100% accurate as its free for anyone to edit the pages

    VAG actually means Volkswagen Aktiengesellschaft (stock trading company )
    Or Volkswagen Audi Grupe

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  10. #10
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    Quote Originally Posted by MRL T5 View Post
    Or Volkswagen Audi Grupe
    that would be Gruppe...

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