Saab, variable compression engine - Michael
Does anyone know the status of Saab's new variable compression engine? Is it in production? Is it any good?
Re: Saab, variable compression engine - Michael
does no-one know anything about this engine? It was in the news earlier this year. It had a variable compression ratio by utilising a gizmo to raise or lower the cylinder head in relation to the block. I have not seen anything since. Has anyone else?
Re: Saab, variable compression engine - steve paterson
Sounds impossible! Can't see a cyl head moving up and down, think of the problems involved. Maybe something moves in and out of the combustion chambers, sounds unlikely though! Some steam Loco's had a sort of variable crankshaft and a constant piston stroke. I suppose if things worked in reverse, piston stroke could be altered, changing compression ratio's.
Re: Saab, variable compression engine - Andrew Moorey (Tune-Up Ltd.)
It works quite simply. Imagine putting a hinge along the bottom of the block parallel with the crankshaft on say the left hand side of the block. Then with the crank held in the "sump" part tilt the block part over on the hinge . This then moves the head further away from the centre line of the crank. I understand that the "tilt" is only a few millimteres. Long term I don't like the idea from an engineering point of view, but then, what do I know except it's probably yet another over-complication, complete with its own ECU, to go wrong!
Re: Saab, variable compression engine - John Slaughter

That's it exactly. As you say sounds like overcomplication for the sheer hell of it. I could be wrong, but doesn't restricting the amount of air drawn in to the cylinder have a similar effect - in as much as it varies the maximum compression pressure?


Nightmare!! - David Lacey
What a complete and utter nightmare!!

Glad I don't work in a SAAB dealership!

I don't want to think about the durability side of things, perhaps in 10 years or so......

We get enough problems these day with head gaskets, oils leaks etc etc...

Re: Nightmare!! - David Woollard
Nightmare indeed.

Interesting that to get this to work at all there is no split between the cylinder head and the bores, they are one casting so no head gasket. The split is below the cylinders with a kind of lower crankcase, the sealing is by a bellows to allow for the movement that changes the compression ratio.

I think Andrew said but it tilts up to just 4 degrees and is moved by a hydraulic actuator controlled by the ECU.

There have been other ideas to achieve the same result including an extending con rod on a type of rack system.

How to make a device complicated for no great benefit in one easy lesson.

Mind you when I had one of the early Saab 99 Turbos all the guys at work said they'll never catch on for production cars!!

Re: Saab, variable compression engine - Bill Doodson
I can see how this can work for a production engine, but why do it? I follow your train of thought on this one.

Value my car