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Saab has stunned the world by showing its variable compression ratio engine in the 2000 Geneva motor show. I?ve heard such engine for some 2 years, but this is the first time Saab disclose the details to the press. In my opinion, this is perhaps the largest single breakthrough in engine technology since turbocharging and electronic engine management.
Why is variable compression ratio so fascinating? As everybody knows, fixed compression ratio is always a constraint for supercharging or turbocharging engines. To prevent excessive pressure in combustion chamber, hence pre-ignite ("knocking") and overheat to cylinder head, turbo/supercharger engines always employ a much lower compression ratio than normally aspirated engines so that the total pressure won?t exceed the limit when the boost pressure is added. The problem is, when the charger (especially is turbocharger) is not yet getting into full boost, that is, at low and mid rev, the combustion runs at lower
compression ratio than normally aspirated engines. Therefore power efficiency at low speed is even lower than normally aspirated engines.
I remember when I was still 13 or 14 years old, I realized that problem and "designed" a variable compression ratio engine on paper. It involved variable length connecting rods to vary the position of piston?s top dead center, hence compression ratio. When the turbo is not in full boost, compression ratio is as high as normally aspirated engine (10:1 by then). This lower to 7:1 for full boost. Of course, that concept is completely out of imagination and is no way to be feasible. Today - a dozen years later - Saab finally realized the variable compression ratio engine.
Named SVC (Saab Variable Compression), the engine implement VC by an innovative and interesting method - slidable cylinder head and cylinder. Let?s look at the following pictures for illustration.
Left: high compression ratio; Right: low compression ratio
As seen, the SVC engine have a cylinder head with integrated cylinders - which is known as monohead. The monohead is pivoted at the crankcase and its slope can be adjusted slightly (up to 4 degrees) in relation to the engine block, pistons, crankcase etc. by means of a hydraulic actuator, therefore the volume of the combustion chamber (when piston is in compressed position) can be varied. In other words, compression ratio is also variable.
SVC is cleverer than any previous patents for variable compression ratio engines is that it involves no additional moving parts at the critical combustion chamber or any reciprocating components, so it is simple, durable and free of leakage.
The monohead is self-contained, that means it has its own cooling system. Cooling passages across the head and the cylinder wall. There is a rubber sealing between the monohead and engine block.
The VC allows the Saab engine to run on very high supercharging pressure - 2.8 bar, compare with the latest 911 turbo?s 1.94 bar, or about twice the boost pressure using by 9-3 Viggen. So high that today?s turbochargers cannot provide. Therefore it employs supercharger instead. At other speed, the VC is adjustable continuously according to needs - depends on rev, load, temperature, fuel used etc., all decided by engine management system. Therefore power and fuel consumption (hence emission) can be optimized at any conditions.
The SVC engine shown in Geneva is the third generation prototype, although production is still far away. It is an inline 5-cylinder with 4-valve head. The displacement is just 1598 c.c. to take advantage of the outstanding efficiency. Compression ratio can be varied between 8:1 and 14:1. With the supercharger, it output a maximum 225 hp and 224 lbft, something similar to a Honda 3.2-litre V6. However, its fuel consumption is very low. Saab claims it saves 30% compare with equally powerful conventional engines.
In terms of specific output, it achieve 150 hp per litre, which must be a world record for production car. At the same time, it is expected to fulfill all foreseeable emission regulations, including the tightest EU4. Another advantage is the suitability to different grade of fuel, especially in America where lower Octane gas is common. The engine management system detect the fuel grade and decide the most appropriate compression ratio to be used.
Saab started developing SVC in the late 80s and acquired the first patent in 1990. The first prototype was a 2-litre unit but was considered as more powerful than needed. The second prototype was a 1.4-litre inline-6 but it had problems about packaging, so the inline-5 configuration was eventually chosen.
More work has to be done to make a SVC into production. The production unit might not be the same as this one, but it is believed that General Motors has green lighted the full development, which requires big investment from parent company
Izvor: http://www.autozine.org/technical_schoo ... _4.htm#SVC
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