At the Geneva Motor Show, Saab Automobile AB unveils the 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 enables the SVC engine to use energy in fuel far more efficiently than today's conventional automotive engines. SVC offers an entirely new concept for combining high performance with low fuel consumption and low exhaust emissions.
Fuel consumption 30 percent lower
The SVC concept reduces the fuel consumption of a conventional naturally aspirated engine by up to 30 percent while at the same time providing impressive performance. The five-cylinder SVC engine developed by Saab has a displacement of 1.6 liters and is as fuel-efficient under normal conditions as a conventional 1 .6-liter engine, but can deliver power comparable to a highly tuned 3.0-liter engine when needed. The carbon dioxide (CO2) emissions are reduced proportionately to the fuel consumption, while the CO, HC and NOx emissions will enable the SVC englne to meet all current and proposed future emissions regulations. 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, highway motoring at constant speed, or high-speed freeway driving. The compression ratio of the SVC engine is continually adjusted to provide the optimum value for varying driving conditions.
Variable combustion chamber volume for variable compression ratio
The SVC engine is comprised of 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 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 (highest position of the piston in the cylinder), which, in turn, changes the compression ratio.
The combination of reduced engine displacement, high supercharging pressure and a variable compression ratio enables the SVC concept to provide engines with tremendous power output capabilities. The 1.6 liter, 5-cylinder engine produces 147 Ib.-ft. of torque and 150 horsepower per liter of engine displacement! The SVC concept opens the door to the development of both small, extremely fuel-efficient engines with good performance, and larger engines delivering sports car performance with high fuel-efficiency. Alternative fuels
The variable compression ratio also gives the engine excellent fuel flexibility. Since the compression ratio can be varied and adjusted to suit the properties of fuel, the engine will always run at the compression ratio that is best suited to the fuel being used.
Three cornerstones of the SVC concept
Although a vanable compression ratio is what makes the SVC engine unique, the fuel efficiency of a conventional naturally aspirated engine would only improve 4 - 5 percent if it were equipped with a variable compression system. The full potential of variable compression can only be realized when it's used in combination with reduced engine displacement and high supercharging pressure.
1. Reducing the engine dispIacement - size does matter
A conventional four-stroke gasoline engine is most efficient (maximizing the energy in the fuel) when it is running at a high load. 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 capacity. The small engine often extracts more energy from every drop of fuel.
One reason for this is because the pumping losses are lower in a small engine. Pumping losses arise when the engine is running at low load and when its 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. The effect is roughly the same when you cover the air hole of a bicycle tire pump with your thumb while trying to pull out the pump handle. The extra energy needed for pulling the piston down is known as the pumping loss. 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 lighter internal reciprocating mass and has lower frictional losses. Therefore, a small engine is generally more efficient than a big engine.
2. Supercharging - Power on tap
Although a small engine is efficient, it is not powerful enough to be used for anything other than powering small, lightweight cars. 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 piston stroke, which results in 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.
Small displacement engines and supercharging have long been well-known concepts at Saab. Saab launched the turbo concept back in 1976 as a way of boosting the performance of an engine by raising the intake air pressure, but without making the engine bigger and heavier, and therefore less fuel efficient.
Over the last 25 years, Saab has developed a number of innovative turbo-enhanced engine systems, all of which have resulted in boosting performance, lowering fuel consumption and reducing exhaust emissions. However, engine development has now reached the stage at which a new parameter of the combustion process must be optimized to meet future demands for reducing the carbon dioxide emissions and enabling alternative fuels to be used. Varying the compression ratio is the ideal parameter to optimize.
3.Variable compression - pearl of wisdom
The compression ratio of an engine is the piston displacement volume plus the volume of the combustion chamber divided by the volume of the combustion chamber—in other words, the amount by which the fuel/air mixture is compressed in the cylinder before it is ignited. The compression ratio is one of the most important factors that determine how efficiently the engine can utilize the 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 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 on the highway 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 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."
New ways of using known engine components
An objective in the development of the SVC concept was to retain as many of the basic components of a conventional engine as possible. The crankshaft, connecting rods, pistons and valves are all of the same type as those of today's engines. 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 centimeters (almost eight inches) lower. The upper part, known as the monohead, consists of the cylinder head with integrated cylinders, whereas the lower part — the crankcase — consists of the engine block, crankshaft, connecting rods and pistons.
The monohead is pivoted at the crankcase. The compression ratio is altered by tilting the monohead in relation to the crankcase by means of a hydraulic actuator. The volume of the combustion chamber will then increase and therefore lower the compression.
To increase the compression, the slope of the monohead is reduced. The volume of the combustion chamber will then decrease and the compression will be higher. The monohead is sealed at the crankcase by a rubber bellows.
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. The compression ratio is continuously variable.
Efficient four-valve combustion chambers
An important benefit of the SVC concept is that the variable compression can be achieved without modifying the design of the efficient four-valve combustion chamber. The combustion chamber design is of vital importance to the combustion process, and therefore directly affects the exhaust emissions, fuel consumption and engine performance. One of the essential conditions in the work of developing the SVC concept was that the new technique should not impair the existing design.
Since the monohead is made as one unit, it is also possible to enhance the design of the coolant passages. This is essential for being able to supercharge the engine sufficiently to achieve high performance.
Mechanical compressor for maximum boost pressure and fast response
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 the boost pressure delivered by today's Saab 9-3 Viggen and 9-5 Aero high output turbo engines. Saab engine designers 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 needed by the SVC engine.
A plafform for continued development
The SVC concept and the 1.6-liter, five-cylinder engine represent a leap forward in engine technology and provide a completely new plafform for further engine development. The fact that the compression ratio parameter can now be controlled enables more accurate engine operation, and therefore, higher efficiency. SVC can be combined with other engine technologies to further improve performance, lower fuel consumption and reduce exhaust emissions.
The SVC engine represents a decisive step in the long-term development work aimed at combining the benefits of the Otto engine and the diesel engine. This trend is already visible in engine development. Direct injection will be used on the Otto engine just as it is on the diesel engine, while the diesel engine will have much more electronics. Variable compression has thus far been the missing link between the two.
The importance of the compression ratio to the efficiency of an engine has long been known, and there are many imaginative patents for different designs of variable compression engines. What Saab engine designers were first to achieve—just as they were with turbocharging in the 1970s—was to combine innovative new thinking with a known technique and proven theories in order to develop a system that is usable.
Saab engine designers began thinking about developing a variable compression engine in the early 1 980s, 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. The first usable experimental engine had a displacement of 2.0 liters, and delivered higher torque and power output than was necessary. But the engine did prove that the theory performed well.
Actual testing began when the second generation prototype engine—a 1.4 liter in-line six— was ready in the mid-1990s. The objective was that an SVC engine of that design would have the performance and power output of a naturally aspirated 3.0-liter V6 engine, but with 30 percent lower fuel consumption. In order to have the potential of the SVC engine assessed by
independent experts, Saab approached the renowned German engine development company FEV Motorentechnik in Aachen. They submitted a thorough evaluation to confirm that the engine met the desired targets, and that it was also possible to make further advances by continued development work.
However, the six-cylinder, 1.4-liter in-line engine was not appropriate to the performance level needed by the projected Saab range of cars. The engine also entailed packaging disadvantages. So it was dropped in favor of the current five-cylinder, 1.6-liter engine concept.
The SVC concept would have been impossible to develop without an advanced engine management system. The addition of variable compression as another control parameter in the already complex control system of today's automotive engines creates high demands on the engine management system. The engine management system for the SVC engine is a special version of the Saab Trionic system—developed in-house by Saab and in use on Saab turbocharged engines since 1991. Further development of the Saab Trionic system and the in- depth knowledge of the system accumulated by Saab engineers have been key elements in the development of the SVC concept.
However, even in its latest version, the Saab five-cylinder, 1 .6-liter 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.
The figures below relate to the 1 .6-liter test engines currently used in Saab's ongoing technical development work. The exact technical specifications of future regular production engines will be dependent on this development work, future customer demands for performance and fuel- efficiency and Saab's overall product plans.
Engine displacement 1.598 liter Number of cylinders 5 Cylinder bore 68 mm Piston stroke 88 mm Compression ratio 8:1 to 14:1, depending on engine load Max. compressor boost pressure 2.8 bar (40 psi) Max. monohead tilt angle 4 degrees Maximum torque 224 Ib.-ft. Maximum horsepower 225 hp