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Automotive Industry

Saab Unveils Radical Variable-Compression Engine - Brief Article - Statistical Data Included

Automotive Industries, April, 2000 by Gerry Kobe

Nothing at the recent Geneva Auto Show created as much of a buzz as Saab's variable compression engine. Called SVC for short, the Saab concept is aimed at producing the power of a 3.0L engine from just 1.6L of displacement, with 30 percent better fuel consumption (and thus CO2 emissions) than a conventional 3.0L.

While variable compression alone won't make that kind of difference, Saab is combining the SVC design with a high boost (2.8 bar) supercharger, to extract a claimed 225 horsepower from the tiny 5-cylinder.

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Still in the research phase, the engine is calibrated to run close to full load conditions, maximizing fuel use and taking advantage of the lower pumping losses in a smaller engine. The supercharger simply boosts power beyond that of a normally aspirated design, and the ability to lower compression ratio on the fly permits extremely high boost pressures without detonation.

"The SVC can alter its compression ratio from a high of 14:1 to a low of 8:1," says SVC technical project leader Hans Drangel. "It uses ion knock sensing through the spark plug, as on the normal Saab Trionic system, to give us maximum possible boost. If it knocks, first it changes ignition timing because that is instantaneous. Then if it is still going to knock, it alters compression ratio and bumps the timing forward again."

To vary each cylinder's compression ratio, the joint face of the head and block "hinges" on one side, like a clamshell. The head can tilt up to four degrees via an hydraulic actuator. This tilting effectively changes the volume at the top of the combustion chamber, thereby changing the compression ratio. Because the cylinder liners are pressed into the head on the SVC engine, the cylinders tilt along with the "monohead," eliminating the need for a head gasket or even head bolts.

"I think you want to make it as a monohead because there are many advantages," Drangel asserts. "On a normal block you machine your block and head, put the head on and when you tighten the bolts you distort the liners. Therefore, you have to have higher ring forces to counteract the distortion. With this design there is no distortion, so you can have a closer fit in the rings with less tension. Plus you can put cooling channels where they are needed because the bolts don't run through the hottest part of the head and block."

Although the tilting head appears to create opportunity for serious NVH concerns, Drangel says the added piston angle caused by the articulation of the head is really no dofferent than Volkswagen's narrow-angle VR engine. However, VW engineers claim block stiffness, not piston angle, is their concern.

Proper sealing where the head pivots on the block is also a challenge. The concept engine is sealed with an elastomeric gasket, but Drangel admits it has not yet been optimized for durability and may change in production. Similarly, the exhaust manifold has to flex where it meets the exhaust pipe, creating potentially dangerous sealing concerns.

Many questions remain, but Saab engineers claim the SVC design is sound, and could be production ready in three years. "From a technical standpoint, yes, there is still work to be done," Drangel acknowledges. "But there is no problem we see that we cannot resolve."

Automotive Industry

Saab Unveils Radical Variable-Compression Engine - Brief Article - Statistical Data Included

Automotive Industries, April, 2000 by Gerry Kobe

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