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KnifeEdge_2K1
08-28-2004, 12:55 PM
saab prototype claims that their variable cylinderhead can vary the compression ratio anywhere between 8:1 to 14:1 allowing supercharger to run at 2.8 bar
...

i have no doubt that they can vary the compression ratio but once you tilt the cylinder head dont you affect the geometry of the piston/crankshaft movement?

and 2.8 bar on an 8:1 compression ratio ... wow

Ferrari Tifosi
08-28-2004, 01:00 PM
Interesting stuff here, never heard of anything like this before. I just don't see how they would be able to vary compression. Do you have any information on how this system works?

Matra et Alpine
08-28-2004, 01:12 PM
Saab have ben playing with this for years.
Some crazy prototypes were talkd about yearsa go with multiple combusiont chambers and valves to introduce these 'swirl' chambers when needed.

But the SVC works by moving the head up away from the crank, thus decreasing the compression ratio. Thhe cylinders are part of the head, so nothing too hard to engineer.
See http://www.edmunds.com/news/innovations/articles/43027/article.html
They're claiming 30% improvement.
Interesing idea - but somethign else to breaka dn can't see that it woudl be viabel with a performance engine where the combustion pressures are significantly higher.

The current engine can vary from 8:1 to 12:1 compression and has already completed 200,000 km durability tests.

If you need pictures to grasp the concept, see Saab USA ( I'm saying nothing :) ) http://www.saabusa.com/main/GLOBAL/en/vepsilon/

biggysmalls
08-28-2004, 06:33 PM
Very interesting thing. We need imaginative people to create that kind of invention (which works and reliable of course... ;) )

KnifeEdge_2K1
08-28-2004, 09:03 PM
i still think the 2.8 bar supercharging at 8:1 compression is bogus

but the theory is sound, an engine which is capable of varying it's compression ratio would have the same impact of a good CVT but applications are hard, many current CVTs arnt much better than a standard automatic because of mechanical inefficiency (my info is kinda out of date but i hvnt heard anythign ground breaking in a while)

KnifeEdge_2K1
08-28-2004, 09:12 PM
Interesting stuff here, never heard of anything like this before. I just don't see how they would be able to vary compression. Do you have any information on how this system works?
http://autozine.kyul.net/technical_school/engine/tech_engine_4.htm#SVC

Misho
08-29-2004, 05:09 AM
can we start this thread by a simple explanation of what exactly is "compression ratio". what does it represent and how significant are its values.

just a quick explanation of the ratio, no need to make as complicated as that OTHER ratio !! (you all know what im talking about!) :D

Matra et Alpine
08-29-2004, 06:26 AM
Compression ratio = combustion chamber volume at BDC / volume at TDC

So piston at bottom of cylinder say 250cc
Piston at top of cylinder say 25cc
Compression ration = 250/25 = 10:1

:)

Matra et Alpine
08-29-2004, 06:30 AM
i still think the 2.8 bar supercharging at 8:1 compression is bogus
Explain ?

many current CVTs arnt much better than a standard automatic because of mechanical inefficiency (my info is kinda out of date but i hvnt heard anythign ground breaking in a while)
http://www.transrevolution.com/images/graph---IVT,%20manual,%20auto--smaller.JPG
The thing with CVT is that a good one enables the engine to be kept at maximum efficiencey so the overal improvement can be huge.
The drawback is that people don't feel comfortabel driving fully variable CVT. So we end up with computer controlled CVVT which varies the engine revs to make it sound liek a "normal" gearbox and have stepped changes to make it feel as if it's actually changing gear - the Citroen one actually has paddles to switch gears on their CVT !! :) !!

paul
08-29-2004, 09:18 AM
Compression ratio = combustion chamber volume at BDC / volume at TDC

So piston at bottom of cylinder say 250cc
Piston at top of cylinder say 25cc
Compression ration = 250/25 = 10:1

:)
ok need some educating

you said 250/25 = 10:1 how do you get 10:1

could someone explain please?
and yes i am very dense :confused:

Matra et Alpine
08-29-2004, 09:47 AM
ok need some educating

you said 250/25 = 10:1 how do you get 10:1

could someone explain please?
and yes i am very dense :confused:

250 / 25 is 250 divide by 25. so that gives 10.
So the CR is 10 to 1 commonly written 10:1

KnifeEdge_2K1
08-29-2004, 11:11 AM
basically the higher the compression ratio the more you get out of your "bang", but the higher the ratio the easier the engine will misfire, knock ect
thats why you use higher octane fuel in high compression ratio cars, higher octane means lower chance of detonation

on turbo charged cars the turbocharger compresses the air before going into the piston which bassically accomplishes the same thing as a high compression ratio NA engine, but the drawback is since a turbo already compressed the air before hand you cant have as high a CR because it would again be too high causing knocking detonation misfires ect

so to summerize it, a turbo is like using brute force, while NA is more precise, but since the power and torque gains you get from a turbo are so huge, and the potential is everything but limitless, the tiny drawbacks it has are inconsequential

matra will prolly cover anything i've missed or mistaken

henk4
08-29-2004, 11:21 AM
then of course there are the diesel engines which have ratios of 20 or more, without this misfiring etc.

KnifeEdge_2K1
08-29-2004, 05:45 PM
but then you're talking about a different source of fuel :P

Matra et Alpine
08-29-2004, 06:16 PM
but then you're talking about a different source of fuel :P
:confused:
diesel -- gasoline -- different source.
I think I've lost the thread somewhere.
Can you explain please.

KnifeEdge_2K1
08-29-2004, 09:03 PM
nm man

megotmea7
08-30-2004, 07:14 PM
nm man
good answer :rolleyes:

fpv_gtho
08-31-2004, 03:31 AM
then of course there are the diesel engines which have ratios of 20 or more, without this misfiring etc.


well diesel engines though are engineered to fire off detonation rather than a spark, even if they use glow plugs to get started

henk4
08-31-2004, 06:12 AM
well diesel engines though are engineered to fire off detonation rather than a spark, even if they use glow plugs to get started

but they can still misfire :)

paul
09-01-2004, 11:02 AM
can they ?? in what context do you mean missfire becuse there are many different reasons for misfire? am i right?