Compressor versus Turbo





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  1. #1
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    Default Compressor versus Turbo

    Hi Gents, I beg your indulgence for this discussion. (Please note this is NOT a discussion of petrol versus diesel or anti-turbo)

    Most manufacturers of smaller turbo diesel engines suffer serious failures at some point in time. I ascribe this to any number of reasons such a high stressed motors, irregular servicing, inadequate cooling systems, driving techniques, oil choice etc.
    Although all manufacturers may have designed and tested this type of motors ad nausea, the marketing drew customers predominantly to the fuel consumption and torque advantages. I consider these motors to be “partial” failures of design but do believe that much progress has been made in the new generation common rail engines such as the D4D, DID and D-TECH ? ( Isuzu).

    However, after much deliberation with myself (reminding me of my own intellectual shortfalls), I came to the conclusion that the main enemy of small capacity turbo diesel engines is simply HEAT. Please hear me out, using the “free” kinetic energy of HOT exhaust gasses to turn a turbo, in itself makes sense but it generates much more heat than is desirable in the engine compartment. Although the turbo is oiled and water cooled (in some cases), the back pressure created by the turbo and the restricted exhaust gas flow combined with a high compression engine, just makes for to high temperatures to be disposed of. The proof of this has to be the very high reliability of non-turbo engines, typically the Toyota Land cruiser engine, the 2,4 Toyota diesel etc. (I am sure there are many other manufacturer non-turbo diesels as well). My reasoning is that heat is not the enemy in these engines.
    So, a turbo makes for much better volumetric efficiency and hence much better performance etc but at what price to the consumer?

    I therefore find it very interesting that Mercedes has chosen to go the compressor route (on petrol motors) to achieve the same volumetric efficiencies although at a small energy cost as the compressor is driven off the crank and not by “free” exhaust gasses. VW is now using the compressor to boost the low rev power and turbo for the higher revs.

    My question: Is it not possible to convert the very same small turbo diesel engines to compressor engines? This will totally eliminate the heat enemy as the compression of fresh air combined with an intercooler will not generate nearly as much heat as the turbo. I do except that performances will be slightly less but would also expect reliability to be vastly improved.

    Just for the sake of interest without specific manufacturer debate, what are your thoughts?
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  2. #2
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    Compressor AKA supercharger...

    It's quite a common conversion on NA petrol engines in the US and Australia... If I had the bucks a low boost supercharger is something I would consider, and has the added advantage of no turbo-lag.

    Edit: Hunting my links for a Pajero conversion done in the US
    Last edited by SimonB; 2010/02/19 at 07:17 PM.
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  3. #3
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    Default

    Drove a 4.0lt v6 Prado with a supercharger fitted a few months ago - I must say it is very very strong. Conversion costs around R100 000 - eina.

    I think that the cost of Compressors/superchargers become prohibitive as it means extra pulleys and belts etc
    Low revving diesels may also have a problem getting a compressor to build up enough pressure? just a few thoughts of my own. What say the experts??
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  4. #4
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    A compressor and a turbocharger will both cause a similar rise in the intake compressed air volume temperature - hence a inter / aftercooler will be required to cool the charged intake air down. Thus no real advantage to either of the two types here.

    The supercharger though, as was rightly pointed out, do take some % of engine horsepower - the higher the boost the more the hp drain.

    Turbochargers tuned to provide boost at low rpm's tend to throttle the engines at high speed. Good flowing turbos on the other hand tend to provide poor boost at lower engine rpm's. This is where the later series-turbocharging comes into it's own. Good low-speed boost as well as good high speed boost without the adverse back-pressure.

    Interesting to note that VW decided to let the supercharger blow through the turbo on their TSI engines. Volvo built a truck engine some years back (I do not know if it's in production still) which featured both turbo and supercharger - but with a surprising twist. The turbo fed into the supercharger intake effective turning it into an air motor (whilst supplying the required volume of air to the engine) thereby feeding BACK into the engine through the supercharged geartrain a "claimed" hp gain to the value of 2 to 3.5% - exact value was dependant on the boost and engine speed.

    Would have been interesting to see what the result would be had VW done it this way on their TSi engines. Perhaps some engine tuners somewhere will catch onto this idea and give it a try through some clever plumbing?
    Last edited by Botswana; 2010/02/19 at 08:21 PM. Reason: volume inst of temp....
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  5. #5
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    This was the Pajero supercharger project I was looking for

    http://www.4x4wire.com/forums/showfl...=888365&page=0

    It's a looong read (2000 posts plus)
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  6. #6
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    Quote Originally Posted by SimonB View Post
    Compressor AKA supercharger...
    It's quite a common conversion on NA petrol engines in the US and Australia... If I had the bucks a low boost supercharger is something I would consider, and has the added advantage of no turbo-lag.
    Edit: Hunting my links for a Pajero conversion done in the US
    That would put pumba in the winners circle, good low down usable power without the head snapping boost of a turbo. running .3 - .4 bar boost the reliability would not be affected.
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  7. #7
    AndrieK Guest

    Default

    Quote Originally Posted by Botswana View Post
    Interesting to note that VW decided to let the supercharger blow through the turbo on their TSI engines. Volvo built a truck engine some years back (I do not know if it's in production still) which featured both turbo and supercharger - but with a surprising twist. The turbo fed into the supercharger intake effective turning it into an air motor (whilst supplying the required volume of air to the engine) thereby feeding BACK into the engine through the supercharged geartrain a "claimed" hp gain to the value of 2 to 3.5% - exact value was dependant on the boost and engine speed.
    So if I understand this correctly, Volvo used a small turbo to effectively regain the power consumed by the supercharger's drivetrain...? Interesting concept...

    VW went the TSI route to rather get rid of turbo lag at low rpm. Interesting that they went twin-turbo on the new 2.0 diesel in the Amarok, supposedly for the same reason. I wonder if this decision was based on cost (excessive on the supercharger) alone or if it's got something to do with the fact that it's a diesel. (TSI is only used on the FSI petrol engines.) Come to think of it, I can't recall ever seeing a supercharged diesel engine in production in a passenger car...
    Last edited by AndrieK; 2010/02/25 at 10:09 AM.

  8. #8
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    as mentioned - in a petrol motor, a turbo prevents it from revving, a diesel motor does not rev anyway due to the 'slow' burning speed of the fuel.

    as they don't need to rev, the turbo can be smaller which allows it to spool up more quickly.

    so turbos are way more suitable for a diesel engine.

    Petrol engines either need a HUGE turbo to let the gasses out and rev, which gives lag, or a small turbo to give (much needed with an 8.5:1 compression ratio) low down torque, restricting your rev range.

    at high rpm the supercharger take too much power, also at cruising speed it's pulling power so your economy goes for a loop.. so a hybrid situation is better for a petrol motor... vs. cost, super chargers are expensive to manufacture compared to turbos.

    depends if you want low end torque (like for a 4x4) or high end power ( like a little sports car) both have their places, much like diesel and petrol engines.. horses for courses as they say...

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