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Saab’s Unique Engine Concept Provides Variable Compression
New concept enables more accurate engine operation, higher efficiency vehicles
Saab Automobile AB recently unveiled its Saab Variable Compression (SVC) engine, a new engine concept that enables fuel consumption to be radically cut while increasing engine performance per liter of engine displacement. The combination of reduced engine displacement, high supercharging pressure and a unique system for varying the compression ratio, allows the SVC engine to use energy from fuel far more efficiently than today’s conventional automotive engines. In fact, Saab reports the SVC concept reduces the fuel consumption of a conventional naturally aspirated engine by up to 30%.
The concept is so unique that the editors of Popular Science magazine have given it a "Best of What’s New’" award for automotive technology.
The five-cylinder SVC engine has a displacement of 1.6L, though it can deliver power comparable to a highly tuned 3.0L engine when needed. The unique feature of the SVC engine is that it has a variable compression ratio. Unlike a fixed compression ratio of a conventional engine used for a variety of operating conditions (stop-and-go traffic, highway driving, etc.), the compression ratio of the SVC engine is continually adjusted.
The SVC engine has a cylinder head with integrated cylinders (referred to as a monohead). The compression ratio is varied by adjusting the slope of the monohead in relation to the engine block and internal reciprocating components. This alters the volume of the combustion chamber with the piston at top-dead center, which in turn, changes the compression ratio.
The combination of reduced engine displacement, high supercharging pressure and a variable compression ratio — the cornerstones of the SVC concept — provides engines with tremendous power output capabilities. The 1.6L, five-cylinder engine produces 147 ft.lbs. of torque and 150 hp per liter of engine displacement. The SVC concept opens the door to development of both small, fuel-efficient engines with good performance, as well as larger engines delivering sports car performance with high fuel efficiency.
Saab engineers expressed that "size does matter," which is why they opted for reducing the engine displacement. It’s understood that a conventional four-stroke gasoline engine is most efficient (maximizing the energy in the fuel) when it is running at high load. Since a small engine must work harder and run closer to full load if it is to perform the same work as a bigger engine (which utilizes only part of its maximum load), the small engine often extracts more energy from every drop of fuel.
One reason is because the pumping losses are lower in a small engine. Pumping losses arise when the engine is running at low load and when fuel consumption is relatively low. In order to maintain the ideal air-to-fuel ratio (14.7:1), the air supply must be restricted by reducing the opening of the butterfly valve in the air intake. However, this means that the piston in the cylinder is under a slight vacuum during the suction stroke, when it is drawing air into the cylinder.
Since a small engine frequently runs at full load and the throttle is therefore more often fully open, the pumping losses in the small engine are usually lower than they are in a big engine.
Additionally, a small engine is lighter, has a lighter reciprocating mass and has a lower frictional loss, making it generally more efficient than a big engine.
By supercharging the intake air and forcing more air into the engine, more fuel can be injected and burned efficiently. The engine then delivers more power for every stroke, which results in a higher torque and horsepower output. By supercharging the engine only at greater throttle openings when extra power is really needed, the fuel economy of a small engine can be combined with the greater performance of a big engine.
The mechanical compressor used for supercharging is engaged and disengaged by the Saab Trionic engine management system. The compressor is equipped with an intercooler and delivers a maximum boost pressure of 2.8 bar (40 psi), which is double to boost power of today’s Saab 9-3 Viggen and 9-5 Aero high output turbo engines. Saab engineers said they chose to use a compressor instead of a turbocharger for the SVC engine because today’s turbochargers are not able to deliver the high boost pressure and fast response required by the SVC engine.
The compression ratio is one of the most important factors that determine how effectively the engine can utilize energy in the fuel. The energy in the fuel will be better utilized 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 consummations are at maximum levels.
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 maximize 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."
A Saab objective while developing the SVC concept was to retain as many of the basic components of a conventional engine as possible. What distinguishes the SVC engine is the way it is split into upper and lower portions. Compared to a conventional engine, the joint face between the two is about 20 cm (almost 8") lower. The monohead is pivoted at the crankcase, which consists of the engine block, crankshaft, connecting rods and pistons. Saab notes that all of these parts are all of the same type used on today’s engines.
The compression ratio is altered by tilting the monohead in relation to the crankcase using a hydraulic actuator. The volume of the combustion chamber will then increase and therefore lower compression. The monohead, which is sealed at the crankcase by a rubber bellows, can be sloped by up to 4°. The optimum compression ratio is calculated by the Saab Trionic engine management system based on the engine’s speed, engine load and fuel quality.
An important benefit of the SVC concept is that the variable compression can be achieved without modifying the design of the four-valve combustion chamber, which directly affects the exhaust emissions, fuel consumption and engine performance.
Since the monohead is made as one unit, it also is possible to enhance the design of the coolant passages, which is essential for being able to supercharge the engine sufficiently to achieve high performance.
The SVC concept and the 1.6L five-cylinder engine represent a leap forward in engine technology and provide a completely new platform for further engine development. The fact that the compression ratio parameter can now be controlled enables more accurate engine operation, and therefore higher efficiency. The SVC can be combined with other engine technologies to further improve performance, lower fuel consumption and reduce exhaust emissions.