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Variable compression ratio engine

Vary the compression ratio of turbo-charged petrol engines to improve low-end torque
 
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This idea aims to improve the low-end torque of turbo-charged vehicles without sacrificing the boost. It seems to me that turbo-charged petrol engines reduce their compression ratio to achieve greater amounts of boost without the risk of detonation. This makes the car difficult to drive in the low-rev range, and results in a weaker idle (correct me if I'm wrong).

So, how about if we could use a high compression ratio in the low rev range (e.g. 11:1 below 2000rpm) and then change back to a lower compression ratio when the boost comes in (e.g. 8.5:1 above 2000rpm)?

This could be achieved by modifying the crankshaft, so that the bottom bit is not square (where the con rod fits), but "trapeziodal". Then, the compression ratio would be continuously variable, by sliding the crankshaft so that the con rod slides to the "bottom" end (for higher compression ratio), or vice versa. (Not sure if I have the terminology all correct...)

toby, Nov 18 2002

Mayflower http://www.mayflowe...tory/mn_eureka.html
Variable compression & capacity [8th of 7, Oct 04 2004]

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       This is Baked, or at least in development - Mayflower rings a bell.....
8th of 7, Nov 18 2002
  

       Damn! So it is a good idea :-) That Mayflower arrangement is really impressive...
toby, Nov 19 2002
  

       Saab showed an engine off with variable compression. The head was attached to the cylinder walls, but not to the rest of the block including the crank, rods, and pistons. In their case the engine was an inline and they just rocked the head on the block a few degrees to change the compression ratio.
SublimeGTP, Dec 10 2002
  

       If instead of tradional poppet valves you had overhead piston valves, it'd be simpler to vary compression by adjusting the motion of these valves. Low compression doesn't really hurt low end torque. The engine is less torquey when off boost, its just some engines have camshafts and induction/exhaust designed for working best on boost. Many older engines that ran on lower octanes had compressions right down to 7.5:1 and were quite happy to idle around. Modern variable valve timing and variable lift/duration systems are good enough that it has some of the same effect as variable compression, as the dynamic compression ratio varies with the engine speed and valve opening and closing profile.
venomx, Feb 09 2003
  

       A friend and I were discussing this some months ago,the idea we came up with was to mount the main bearings in eccentrics,which could then be turned to raise/lower the crank, altering the compression ratio but not the swept volume.But as venomx says,variable valve timing can be used to give the same end-result,so why complicate things?
Antman, Apr 29 2003
  

       first time posting. I think what you would want to do is to attach the compression adjuster to the anti knock sensor. when it knocks compression is backed off a little so you end up always running at max possible compression. atleast thats how i would try making it
davidcreede, Jul 20 2003
  

       good ideal that has already been done by saab they just spit the block above the crank and hinged it on one side so by lifting up on the one side of the block you decrese compression the block was sealed with a rubber skirt and the timming chain was on a tensioner   

       the unit was electro hydrolicaly controlled by the on board engine managment microcompressor   

       no need to mess around with the crank by adding extra weight and dynamic stresses on the engine block. Not to mention the extra power (fuel) needed to rotate the greater mass
matthewq4b, Aug 15 2003
  

       good idea,but unpractical. all that work for torque under 2000 rpm. heres athought rev it to 2000 pop the clutch and keep it above four till the next stop light . problem solved
mini1, Oct 13 2003
  

       Nice idea, it may also have a use for dual fuel engines. You do need to consider the physical effect of changing the compression ratio and hence the torque output whilst the engine is running, which will most likely result in a shock being felt through the car. Perhaps Saab have already resolved this?
jamesdawson, Dec 09 2003
  

       I think you should do a search on a website called www.autospeed.com for what you are talking about. It will give you a detailed article on how it works there.
Anatomy_expert, May 21 2005
  

       I've recently come across some impressive French research by MCE - 5. They claim a variable compression from 7:1 to 20:1 - now that is variable!   

       They have a very comprehensive site at 'mce-5.fr' with a full english version - who says they don't like us?   

       First impressions are of a pretty complicated piece of engineering, but their technical rationale is well impressive.   

       Does anyone know or can anyone find out which car manufacturers are working with them?   

       Also, what chance linking this ICE approach with the British Torotrak IVT gearbox (see 'torotrak.com')?   

       philr
philr, Jul 20 2005
  

       hey guys baked idea guys the old duel fuel tractors from allis used this in the form of a piston in the head to change compression ratios for petrol and diesel running thanks Nick
randylandy666, Jul 24 2007
  
      
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