Saab's Variable Compression Engine

[Saaber]

Saab engineers have developed an engine technology that improves fuel efficiency by up to 30 percent, reduces tailpipe emissions, and doesn't negatively impact overall performance. Saab calls its new engine technology Saab Variable Compression (SVC). Interestingly, Saab engineers have been thinking about variable compression since 1981, and prototype engines with SVC technology have existed since 1990. At the 2000 Geneva Auto Show, Saab finally felt its SVC technology had advanced enough to show it to the world.

 

The SVC engine shown at Geneva was a supercharged, 1.6-liter, inline five-cylinder engine. With dual-overhead camshafts and four valves-per-cylinder, it looks pretty much like your average internal combustion engine. In fact, an objective in the development work on the SVC concept was to retain as many of the basic components of a conventional engine as possible. The crankshaft, connecting rods, pistons and valves are all of the same type as those of today's engines.

 

What distinguishes the SVC engine is the way it is split into upper and lower parts. Instead of having the cylinders located in the block like all modern vehicle engines, the SVC engine's cylinders are located in the upper section. This upper part is known as the monohead. You heard us right -- the cylinders are built into the head.

 

For those of you who may not know it, this is a very unconventional design. Traditional engine architecture places the cylinders below the head (or heads if the engine is of a V or flat configuration), between the valves and the block.

 

Everything else is pretty "normal" inside the Saab engine. The lower portion of the SVC engine -- the block -- contains the crankshaft, camshaft and pistons.

 

But here's the trick. Since the cylinders are built into the heads, the monohead can be adjusted (moved) relative to the block without affecting engine or valve timing. On the SVC engine, the monohead tilts at up to a 4-degree angle. Rubber bellows seal the monohead to the block.

 

By adjusting the "slope" of the upper part of the engine in relation to the lower part by the use of hydraulic actuators, the Saab engine can vary the volume of the combustion chambers. This, in turn, changes the compression ratio.

 

Compression ratio refers to the ratio between the combined volume of a cylinder and a combustion chamber when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.

 

In all modern engines, the compression ratio is fixed and cannot be altered unless different engine components are installed. Saab's SVC technology allows a variable compression ratio from a low of 8:1 to a maximum of 14:1.

 

Although variable compression is what makes the SVC engine unique, Saab says the fuel efficiency of a conventional, normally aspirated engine would only improve by 4 percent if it were equipped solely with SVC. The potential of variable compression is most effective when combined with a smaller engine design (for better efficiency) and high supercharging pressure.

 

It's hard to argue with that given the impressive results. Saab says its 1.6-liter SVC concept engine generates 225 horsepower and 224 foot-pounds of torque. Maximum boost pressure from the supercharger is 40 psi.

 

These horsepower and torque figures are comparable to a normally aspirated engine of similar displacement (like a 3.0-liter V6 from a Nissan Maxima, for instance). Additionally, Saab says the SVC engine is more fuel efficient at light loads than a standard 1.6-liter normally aspirated engine.

 

Due to its variable compression ratio, the SVC engine can run at an optimum compression ratio of 14:1 at low engine loads to maximize fuel efficiency and lower harmful tailpipe emissions. Increased supercharger pressure at such a high compression ratio would cause detonation, so the compression ratio is lowered to 8:1 at high engine loads to enable maximum engine performance.

 

The SVC engine continuously varies the compression ratio according to driving conditions. This would not be possible, however, without an advanced engine management system. Saab engineers credit their Trionic engine management system with making the whole thing work. The current system, which Saab has been using since 1991 to manage their turbocharged engines, served as the basis for the specially designed SVC system.

 

The Saab Variable Compression technology illustrates yet another innovative success story in the search for more fuel-efficient cars. While manufacturers such as Honda and Toyota have gone the hybrid route, other automakers such as Saab have taken a more traditional route, exploring variations on the theme of the internal combustion engine.

 

The results so far look encouraging.

 

Saab hasn't announced official production plans for SVC technology. The engine is still in the concept stage. However, Saab does say that extensive testing has been performed, including 200,000-kilometer durability tests. Additionally, Saab wouldn't have gone to such marketing lengths at the Geneva Auto Show if it didn't think there was a future to variable compression.

 

Stay tuned, though, as Saab is now owned by GM. Never know what might happen with this technology if it gets perfected.

 

[Andrewkb]

Beside Variable compression, read in one magazine that GM is developing variable displacement engine !!!

 

Wonder how will LTA tax such a car ? ... at it's lowest displacement or highest !!!

 

Guess chances are the highest displacement :-( ...

 

Well .. in R&D stage lah ... probably another 5-8yrs before production ....

[Saaber]

SVC seem more possible to be able to make it into production in the very near future.....

[Andrewkb]

Agree ... the Variable Displacement is still R&D, whereas Saab's SVC had been talking for at least 2yrs in the industry liao ... think probably next model change for 9-3/9-5 should see this in production :-)