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Saab's Variable Compression Engine

Supercharged, intercooled - and with a comp ratio of up to 14:1!

By Saab Automobile AB

Click on pics to view larger images


This article was first published in 2000.

Saab Automobile has unveiled a revolutionary 1.6-litre petrol engine that reduces fuel consumption by up to 30 per cent without impairing engine performance, but can also deliver the power of a 3.0-litre V6 engine when needed.

The Saab Variable Compression (SVC) engine concept was unveiled at the Geneva Motor Show on February 28, 2000. The prototype 1.6 SVC engine - developed entirely by Saab - is a supercharged five-cylinder engine producing a maximum 168kW of power and a massive 305Nm of torque. However, the SVC engine is as fuel-efficient under normal conditions as a conventional 1.6-litre engine. Its breakthrough is that it can produce tremendous power on request, to give the best of both worlds.

There are three fundamentals to the SVC concept - reduced engine displacement, high supercharging pressure and a unique system for varying the compression ratio. The SVC concept can be used to develop engines with specific torque of 200Nm per litre and specific power of 112kW per litre of engine displacement. Harmful carbon dioxide (CO) emissions are also reduced proportionately to the fuel consumption, while the CO, HC and NOx emissions will enable the SVC engine to meet all current and proposed future legal requirements.

The SVC engine consists of an upper part comprising a cylinder head with integrated cylinders (which is known as the monohead) and a lower part consisting of the engine block, crankshaft, connecting rods and pistons. Compression ratio is varied by adjusting the slope of the upper part of the engine (maximum 4 degrees) in relation to the lower part. This alters the volume of the combustion chamber, in so the compression ratio.

It is this variable compression, in combination with considerable overboost and a scaled-down cylinder displacement, that makes the SVC design so strong and at the same time so fuel-efficient. Generous overboost means it is possible to supply more fuel to the engine as and when needed. This in turn promotes both greater torque and higher power output. A smaller cylinder displacement also means that the engine is lighter and operates with lower friction, so it uses fuel more efficiently compared to a conventional engine. Fuel consumption can be reduced by up to 30 percent - while still retaining existing performance levels

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Due to its variable compression ratio, the SVC engine can be run at the optimum compression ratio of 14:1 at low load (ie steady highway speed) in order to put the energy in the fuel to best possible use. The compression ratio can then be reduced to 8:1 at high load (ie under hard acceleration) to enable the engine performance to be raised by supercharging without the problem of engine knocking occurring. The variable compression ratio also gives the engine great fuel flexibility. Since the compression ratio can be varied and adjusted to suit the properties of the fuel, the engine will always run at the compression ratio best suited to the fuel. The various functions of the SVC engine are controlled by a special version of Saab's Trionic engine management computer.

Saab's Variable Compression concept opens the door to the development of both small, extremely fuel-efficient engines with good performance, and bigger engines delivering genuine sports car performance at reasonable fuel consumption. Saab's pictured prototype shows the use of a positive displacement supercharger blowing through a plenum chamber housing a liquid/air intercooler.

Coming from another manufacturer, such an engine development could be taken less than seriously, but downsizing and forced induction have long been well-known concepts at Saab. Saab launched the turbo concept for passenger cars back in 1976 as one way of boosting engine performance without making the engine bigger, heavier and thirstier.

The SVC concept is the brainchild of long-time Saab drivetrain engineer, Mr. Per Gillbrand, who sketched the variable compression concept more than 20 years ago. Saab is still at the prototype stage with the SVC engine, with more development work needed before the engine can be taken into regular production.

Saab Variable Compression (SVC) engine technical data:
Engine displacement: 1 .598 litres
Number of cylinders: Five
Maximum power: 168 kW
Maximum torque: 305 Nm
Cylinder bore: 68 mm
Piston stroke: 88 mm
Compression ratio: 8:1 to 14:1, depending on engine load
Max supercharger boost: 2.8 bar
Max. monohead tilt angle: 4 degrees

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