Compression ratios find freedom to express

The importance of compression ratios to engine efficiency has long been known, and there are many imaginative patents for designs of variable compression (VC) engines. But Saab is bringing a VC engine closer to production than ever before

Saab’s novel new combustion engine claims to cut fuel consumption while increasing engine performance per litre of engine displacement.

The unique feature of the SVC engine – and the key to its high efficiency – is that the engine has a variable compression ratio.

The fixed compression ratio of a conventional engine is a compromise between a wide variety of operating conditions – stop and go city traffic, urban motoring at constant speed, or high-speed motorway driving. But the compression ratio of the SVC engine is continually adjusted to provide the optimum value for varying driving conditions.

The compression ratio of an engine is the amount by which the fuel/air mixture is compressed in the cylinder before it is ignited. The energy in the fuel will be better utilised if the compression ratio is as high as possible. But if the compression ratio is too high, the fuel will pre-ignite, causing `knocking’, which could damage the engine. In a conventional engine, the maximum compression ratio that the engine can withstand is therefore set by the conditions in the cylinder at high load, when the fuel and air consumptions are at maximum levels. The compression ratio remains the same when the engine is running at low load, such as when the car is travelling at constant speed.

Due to its variable compression ratio, the SVC engine can be run at the optimum compression ratio of 14:1 at low load in order to maximise the use of the energy in the fuel, and the compression ratio can then be lowered to 8:1 at high load to enable the engine performance to be enhanced by supercharging without inducing `knocking’.

The SVC engine comprises a cylinder head with integrated cylinders, which is known as the monohead, and a lower portion consisting of the engine block, crankshaft and pistons. The monohead is pivoted at the crankcase and sealed by a rubber bellows.

The compression ratio is varied by adjusting the slope of the monohead (with a hydraulic actuator) in relation to the engine block and internal reciprocating components. This alters the volume of the combustion chamber with the piston at top dead centre, which, in turn, changes the compression ratio. The monohead can be sloped by up to 4 degrees. The optimum compression ratio is calculated by the Saab Trionic engine management system based on the engine’s speed, engine load and fuel quality.

But, as Saab stresses, it is only the combination of small engine displacement, a high pressure supercharger and a system for varying the compression ratio that enables the SVC engine to use fuel energy more efficiently than conventional automotive engines.

Saab engine designers began thinking about developing a variable compression engine in the early 1980s, but it was not until the end of the 1980s that more concrete development work was started, albeit on a modest scale. The first patent application was lodged in 1990.

However, even in its latest version, the Saab five-cylinder, 1.6-litre SVC engine is still at the prototype stage and further development work is needed before the engine can be used in regular production. The final design and size, and also the performance and fuel consumption properties of the ultimate production engine are dependent on many factors, including meeting the future demands of customers.

Saab Tel: 00 1 800 438 7222

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