1. Reducing the engine displacement - size does matter
   An Otto engine is most efficient and puts the energy in the fuel to maximum use when it is running at a high load. A small engine must work harder and must thus run closer to its full load if it is to perform the same work as a bigger engine which utilizes only part of its maximum capacity during normal operation. In simple terms, the small engine can be said to extract more energy from every drop of fuel.
   
   One of the reasons is that, under these conditions, 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 then also be restricted by reducing somewhat further the opening of the butterfly valve in the air intake.
   
   However, this in turn 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 shut off the air hole of a bicycle 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 more 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.
   
   Moreover, a small engine is lighter and has lower friction. So 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 in practice to be used for anything other than powering small, lightweight cars, if it is to give the car acceptable performance. By supercharging, which involves forcing in more air than the engine would be able to draw naturally, more fuel can be injected and be burned efficiently. The engine then delivers more power for every piston stroke, which results in a higher torque and higher engine output. Moreover, if the engine is supercharged only at large throttle openings when extra power is really needed, the fuel economy of the small engine can be combined with the performance of a big engine.
   
   Downsizing and supercharging have long been well-known concepts at Saab. Saab launched the turbo concept back in 1976 as one way of boosting the performance of an engine by raising the intake air pressure, but without making the engine bigger and heavier, and thus more thirsty.
   
   Within the framework of the turbo concept, Saab has developed a number of innovative engine systems, all of which have contributed towards boosting performance, lowering the fuel consumption and reducing the exhaust emissions. However, engine development has now reached the stage at which a new parameter is needed to meet future demands for reducing the carbon dioxide emissions and enabling alternative fuels to be used. Variable compression ratio is just such a parameter.
   
   
3. Variable compression - pearl of wisdom
   The compression ratio 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.
   
   As a general rule, 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, giving rise to 'knocking', which could damage the engine. In a conventional engine, the maximum compression ratio which the engine can withstand is therefore set by the conditions in the cylinder at high load, when the fuel and air consumptions are a maximum. The compression ratio remains the same when the engine is running at low load, i.e. when the engine output and boost pressure are low, 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 put the energy in the fuel to best possible use, and the compression ratio can then be lowered to 8:1 at high load to enable the engine performance to be raised by supercharging without the problem of knocking occurring.

New ways of using known engine components
An objective in the development work on 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 parts. Compared to a conventional engine, the joint face between the two is about 20 centimetres lower. The upper part, which is 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 the increased volume will 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 means of 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 on the basis of the engine 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 the need for modifications to the design of the efficient four-valve combustion chamber. The combustion chamber design is of vital importance to the combustion process, and thus 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 technique.

Since the monohead is made as one unit, it has also been possible to make the design of the cooling water passages much more appropriate. This is also essential for being able to supercharge the engine sufficiently to achieve the required performance.

Mechanical compressor for adequate 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, which is much higher than the boost pressure delivered by today's Saab turbocharging system. Saab engine designers chose to use a compressor instead of a turbocharger for the SVC engine because none of the turbochargers available on the market today would be able to deliver the high boost pressure and have the fast response needed by the SVC engine.

A platform for continued development
The SVC concept and the 1.6-litre, five-cylinder engine now unveiled represent a surge forward in the development of the Otto engine and provide a totally new platform for further engine development. The fact that the additional compression ratio parameter can now be controlled enables engine operation to be controlled more accurately, and the engine can thus be made more efficient. SVC can be combined with other engine technologies for improving the performance further, lowering the fuel consumption, and reducing the 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 discernible 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 so far been the missing link between the two.

Engine innovators
The importance of the compression ratio to the efficiency of an engine has been known ever since the infancy of the car, and there are many more or less 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 known theories in order to develop a system that is practically usable in ordinary cars.

Saab engine designers began thinking about developing a variable compression engine back 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 late in 1990. The first usable experimental engine had a displacement of 2.0 litres, and delivered a far higher torque and power output than would have been practically usable. But the engine did prove that theory performed well in practice.

Actual testing began when the second generation prototype engine - a 1.4 litre in-line six - was ready to be taken into service 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-litre V6 engine, but at 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, which submitted a thorough evaluation to confirm that the engine attained the targets set up, and that it was also possible to make further advances by continued development work.

However, the six-cylinder, 1.4-litre 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 favour of the five-cylinder, 1.6-litre engine now being unveiled.

The SVC concept would have been impossible to develop without an advanced engine management system. The addition of variable compression as a further control parameter in the already complex control system of today's modern car engines makes very strict demands on the engine management system. The engine management system for the SVC engine is a special version of the Saab Trionic system - the engine management 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 central elements in the development of the SVC concept.

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 taken into 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, not least the demands that customers make on their cars.

Technical data
The figures tabulated below relate to the 1.6-litre test engines currently used in the technical development work. The exact technical specifications of future regular production engines will be dependent on this development work and thus cannot be specified before a regular production engine has been presented.

 - Autoworld.com

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