Variable Compression Engines Saab
and others have done it.
Variable Compression Engine <<< Animation
Variable Compression 
2000 February: Saab showed a prototype
variable compression engine at the Geneva Motor Show. This engine tries to solve the problem of being both a small, fuel-efficient engine
and a large,
powerful engine by changing its geometry.
Devised by Per Gillbrand 20 years ago, the Saab Variable Compression (SVC) engine can vary its compression ratio between 8.0:1, for wide throttle, high-power settings and 14.0:1, for light throttle, fuel efficient operation. The trick is made possible by tilting the "monohead" (head and cylinders) by up to 4° about an axis on the left of the engine (in the diagram). This changes the distance of the
cylinder head from the crank by a few millimetres - enough to change the cylinder volume at top dead centre by nearly a factor of two.
By itself, lowering the compression ratio would not increase power at all but it enables the super-charger to switch to a higher boost pressure without any danger of pre-ignition. Squeezing more air into the cylinders allows more fuel to be burned - more power.
The 1.6 litre, 5-cylinder prototype delivers 168kW of power and 305Nm of torque. Boost pressure varies up to a maximum of 2.8 bar. In effect, the engine can behave like a 1.6-litre motor out of a shopping trolley or like a 3+ litre engine on demand. (e.g. The 3.2 litre V6 in the 1999 Mercedes
ML320 produces 160kW.)
- <LI itxtvisited="1">Saab Variable Compression engine, 1.598 litres, 5-cyl, c.r.: 8:1 to 14:1 variable, <LI itxtvisited="1">Max. monohead tilt angle: 4 degrees, Max. super-charger boost: 2.8 bar
- power: 168 kW, torque: 305 Nm, bore: 68mm, stroke: 88mm
Other have far different designs:
Buzz up!
Increasing Engine Efficiency and Power
Most of us are probably anxious to get rid of internal combustion engines (ICE) and move on to electric motors. Unfortunately, refining a technology until it is affordable and performs well enough for the mass market takes a while (read up on the history of computers or cell phones), and we might not have electric cars for a few more years. In the meantime, engineers are trying to squeeze more life out of ICEs (see
5 technologies to improve ICEs).
One promising technique is the use of variable compression ratios. Read on for details.
How Variable Compression Ratio Engines Work
In an internal combustion engine, the compression ratio tells you what the ratio is between the biggest and smallest volumes of the combustion chamber in the cylinder. The concept of modifying that ratio is very old (around the 1920s), but very few cars have actually used it (SAAB was working on it before
GM
took over, but that project has been shelved as far as I know).
The benefits of having a variable compression ratio is that you can control much more precisely combustion and adjust the variable to get the best performance for each situation (acceleration, cruising, deceleration, etc). It becomes especially potent - in theory - when combined with
other technologies like turbochargers, direct injection, variable valve timing and lift, etc.
From a green point of view, this mean you could downsize an engine quite a bit while still getting enough power and achieving high thermal efficiency. It might even help with the use of (cellulosic) ethanol (which has different characteristics from gasoline), or to reduce emissions by optimizing combustion.
MCE-5's Prototype Variable Compression Ratio Engine
From Green Car Congress:
At the upcoming 79th International Motor Show in Geneva, France-based MCE-5 Development will showcase its first vehicle application (a Peugeot 407) of a prototype 1.5-liter MCE-5 VCRi (variable compression ratio) gasoline engine. The four-cylinder 1.5L VCRi, equipped with a two-stage turbocharger, develops 220 hp (164 kW) of power (comparable to that of a 3.0L V6 engine), and 420 Nm (310 lb-ft) of torque at 1,500 rpm (comparable to a V8 gasoline engine). Fuel consumption on the NEDC is 6.7 L/100 km (35 mpg US) with 158 g CO2/km. The technology can be applied to smaller displacements, MCE-5 notes.
What's interesting is that this prototype is
not "equipped with GDI (gasoline direct injection) or optimized combustion chambers." The 2010 version of the engine should have these improvements and bring fuel economy to 6.0 L/100 km (39 mpg US) while producing 50 hp more!
Imagine a small car that
doesn't need 200+ hp. A 0.8-liter version (or whatever) could probably get very good MPG, possibly running on cellulosic ethanol made from agricultural waste. Not a silver bullet, but this technology would certainly be an improvement on the engines that we have now.
Is all this just to complex to be mass produced, cost effective and reliable?
Variable Compression Engines 
