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Variable compression ratio engine

  1. Sep 24, 2007 #1
    I m making a project on Variable Compression Ratio (VCR)engine and hav got some ideas for attaining Variable Compression Ratio in an engine.
    i studied ur SAAB Variable Compression (SVC) engine,i found it quite intrewsting and innovative,but i wud like to have some more information about that engine that ll be helpful in my project.my area of intrest is:

    1.how u are changin the Compression Ratio?
    2.can it be done in a running engine?
    3.how are u locking it in one Compression Ratio position?

    i ll be highly glad to receive a reply from ur side n looking forward for the d same
     
  2. jcsd
  3. Sep 24, 2007 #2

    mgb_phys

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    Interesting engine - it tilts the cylinder head by a few degrees to change the volume without requiring any changes to the mechanics of the cylinder/head valve assembly or the pistons.

    No details on how they manage the sealing but on a small capacity engine it gives a big range in compression ratios.
     
  4. Sep 24, 2007 #3

    Danger

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    Welcome to PF, Shreesh.
    I must admit that I've never even heard of a VCR engine until now. I share Mgb's curiosity about how SAAB managed to seal that split-block design. Since GM shelved the project upon acquiring SAAB, I assume that there were some serious problems.
    There are a couple of other ways to achieve VCR that I can think of, but both would be pretty difficult to implement. One would be to use telescoping rods with internal hydraulic cylinders fed from a pressure supply piped through the crankshaft. The other would be to tweak the cam timing so that the compression stroke would force some of the fresh charge back through the intake valve into the manifold.
     
  5. Sep 24, 2007 #4

    brewnog

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    Have a search for Miller cycle engines. It's not quite what you're after but it'll be interesting for you nonetheless; it effectively lets you have unequal compression and expansion strokes.
     
  6. Sep 24, 2007 #5

    mgb_phys

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    The advantage of the tilt is that it doesn't significantly change the shape of the combustion chamber so all the fluid dynamics of the burn are the same.
    I think introducing extra cylinders etc into the chamber to reduce the volume would lead to an inefficent burn.
    I would have though of having the valves seal against the walls of the a valve cylinder rather than a seat then you could retract the valves slightly to change the volume - and you could use the same mechanism as for variable valve timing.
     
  7. Sep 24, 2007 #6

    Danger

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    I think that you misunderstood what I meant about the hydraulics. My idea was just to have a system whereby you can lengthen or shorten the con rods. The hyraulics would be inside the rods. There would be no effect upon the combustion chamber shape; the pistons would just extend a different distance into them.
    I don't quite get an image of what you mean about the valves. Can you elaborate a bit, or maybe post a picture? It sounds interesting, but I'm just not sure about what the plan is.
     
  8. Sep 24, 2007 #7

    mgb_phys

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    I see - I thought you meant a sort of plunger piston to occupy some of the volume of the chamber.

    Instead of the valves pushing out into the cylinder and sealing flush against the valve seats - I imagined having them as miniture pistons with piston rings. They would go forward to allow the fuel in but instead of sealing flush they could retract into mini piston bores. The further they retracted the more volume added to the combustion.
     
  9. Sep 25, 2007 #8
    c wat method i chose 2 attain this was by changin d conn rod length as said by danger..
    i wud like 2 kno how conn rod needs modification,n wat changes it requires...as i was thinkin of havin a slidin mech either at d crank or near d gudgeon pin so dat d effective rod length is alterd...
    at present i wud appreciate comments on its designin,stress analysis n dyanamic analysisof such a method..

    and for further info on VCR engine u can visit
    http://www.mce-5.com/vcr_strategy/index.htm

    http://www.gomecsys.com/uk/go-engine_how_does_the_goengine_work.html

    also i hav seen yamaha havin engine of this type,link i ll b sendin shortly..
     
  10. Oct 25, 2007 #9
    Yikes, on the expanding / contracting connecting rod! Its tough enough to build a really good, tough connecting rod without any fatigue issues, let alone implementing that...

    I did see an interesting idea put forth by bruce crower in this latest issue of "automotive engineering". He mentions a "cushion" system built into the cylinder head that would simply absorb excessive cylinder pressure, and release that energy as the pressure starts to fall again... Pretty smart actually! His concept was to build the engine to a 15:1 or 16:1 or similar compression ratio, which the engine would operate at under low load... Under high load, it would soak up some of that excessive cylinder pressure, in a way creating a variable compression ratio.

    The article did note he has already applied for a patent, although that doesn't mean much as far as anything actually working.

    I do quite a bit of work with forged steel connecting rods for the aftermarket vw scene, I think you will find that anything adjustable in that system will cause masses and costs to skyrocket, or the MEP and peak cylinder pressures of your system to have to fall dramatically, as well as the operating speed of the engine. It may still work, but personally I'd keep it out of the rotating assembly! How about moving the whole crankshaft up and down? Same effect, but at least your massive control system does not need to reciprocate or rotate, which is a very good thing.
     
    Last edited: Oct 25, 2007
  11. Oct 25, 2007 #10
    As soon as I got in bed last night, I realized how stupid my suggestion in the above post is- obviously the crankshaft can't be moving, if you want to couple anything to it directly. What would work though and would be neat would be a eccentric bushing insert inside a relatively standard connecting rod. Granted, you would end up with a slight quick return action, but its possible that may not be a bad thing. The real trick there would be setting up some sort of control system that can actually hold the bushing where it needs to be, when the forces start getting really high. It also has to be very fast acting or the whole thing will be a waste of time.
     
  12. Oct 25, 2007 #11

    brewnog

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    Or, achieve the same effect by clever cam timing.
     
  13. Oct 26, 2007 #12
    actually d exact waqy of locking d conn rod only is giving me sleepless nights.
    its true dat its a rotatin n reciprocatin part,bt i think if i hav some concrete idea, n some better materiall with gr8er strength,stress wont come in its way..

    hey i wud like u all 2 comment on its balancing, at different positions...
    also it ll giv a quik return effect if i dont lock d rod,bt i m not gettin ur idea of quik return,i mean i dont hav much idea of it,.

    and 2 ur surprise , i was alongside thinkin of puttin a quik return mech instead of normal slider mech,which i think ll boost up d power output,bt it ll give serious jerks n vibrations..

    wats ur say??
     
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