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Saab Combustion Control


Saab Combustion Control system:
Lower emissions through extensive use of exhaust gases

Three main components of the SCC concept
The SCC system is based on a combination of direct injection of petrol (gasoline), variable valve timing and variable spark gap. Unlike the direct injection systems available on the market today, the SCC system puts to use the benefits of direct injection, but without disturbing the ideal air-to-fuel ratio (14.6:1 = lambda 1) necessary for a conventional three-way catalytic converter to perform satisfactorily.

The most important components of the SCC system are:

Catalyst still most important emission control element
The three-way catalytic converter is still the most important single exhaust emission control component. During normal operation, it will catalyse up to 99 percent of the harmful chemical compounds in the exhaust gases.

The inside of the catalytic converter consists of a perforated core, the walls of which are coated with a precious metal catalyst (platinum and rhodium). The total area of the catalyst is equivalent to the area of three football pitches. The precious metal coating traps carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) in the exhaust gases and enables these substances to react with one another so that the end product will be carbon dioxide (CO2), water (H2O) and nitrogen (N2).

Weaknesses of catalytic converter
Although it is highly effective in neutralizing the harmful substances in the exhaust gases, the catalytic converter suffers certain limitations. For the three-way catalyst to be fully effective, its temperature must be around 400°C. So the catalyst has no emission control effect immediately after the engine has been started from cold (the concept of 'starting from cold' is not related to the weather conditions or the ambient temperature, but in this context denotes all starting circumstances in which the engine coolant temperature is below 85°C).

Moreover, the proportion of free oxygen in the exhaust gases must be kept constant. The amount of oxygen, in turn, is decided by the air/fuel ratio in the cylinder during combustion. The ideal ratio is 1 part of fuel to 14.6 parts of air (i.e. lambda = 1). If the mixture is richer, i.e. if the proportion of fuel is higher, the emissions of carbon monoxide (CO) and hydrocarbons (HC) will increase. If the mixture is leaner, i.e. if the amount of fuel is lower, the nitrogen oxide (NOx) emissions will increase.

The catalytic converter has no influence on the carbon dioxide (CO2) emissions, which are directly proportional to the fuel consumption. The greater the amount of fuel used, the higher the carbon dioxide emissions.

Much of the work of designing less polluting petrol engines therefore has two objectives - to achieve the lowest possible fuel consumption, and to ensure that the catalyst is at optimum working conditions during most of the operating time. These are the guidelines that have been followed in the development of the SCC system.

Conventional direct injection for lower fuel consumption...
In an engine with a conventional injection system, the petrol is injected into the intake manifold, where it is mixed with the combustion air and is drawn into the cylinder. But part of the petrol is deposited on the sides of the intake manifold, and extra fuel must then be injected, particularly when the engine is started from cold, to ensure that the necessary amount of fuel will reach the cylinder.

Direct injection of petrol was launched a few years ago by some carmakers as a way of lowering the fuel consumption. Since petrol is injected directly into the cylinder, the fuel consumption can be controlled more accurately, and the amount of fuel injected is only that necessary for each individual combustion process. In such cases, the entire cylinder is not filled with an ignitable mixture of fuel and air, and it is sufficient for the fuel/air mixture nearest to the spark plug to be ignitable. The remainder of the cylinder is filled with air.

...but higher nitrogen oxide emissions
This leaner fuel/air mixture results in lower fuel consumption under certain operating conditions, but makes it impossible to use a conventional three-way catalytic converter to neutralize the nitrogen oxide emissions. A special catalytic converter with a 'nitrogen oxide trap' must be used instead.

Compared to conventional three-way catalytic converters, these special converters suffer a number of major disadvantages. In the first place, they are more expensive to produce, since they have higher contents of precious metals. Moreover, they are more temperature-sensitive and need cooling when under heavy load, which is usually done by injecting extra fuel into the engine. The nitrogen oxide trap must also be regenerated when full, i.e. the nitrogen oxide stored must be removed, which is done by the engine being run briefly on a richer fuel/air mixture. Both cooling and regeneration have a significant effect on the fuel consumption.

In addition, special catalytic converters of this type are sensitive to sulphur, and the engine must therefore be run on fuel with extremely low sulphur content. The petrol desulphurizing process causes higher carbon dioxide emissions from the refinery.

Direct injection and lambda 1 with SCC
In evolving the SCC system, Saab engineers have developed a way of putting to use the benefits of direct injection, while still maintaining lambda 1. Compressed air is used to inject the fuel directly into the cylinder through the spark plug injector. However, unlike other direct injection systems, the cylinder is still supplied with only a sufficient amount of air to achieve lambda 1. The remainder of the cylinder is filled with exhaust gases from the previous combustion process.

The benefit of using exhaust gases instead of air for making up the cylinder fill is that the exhaust gases are inert. They add no oxygen to the combustion process, and they therefore do not affect the lambda 1 ratio. So the SCC system does not need a special catalytic converter and performs well with a conventional three-way catalyst. Moreover, the exhaust gases are very hot, and they therefore occupy a large volume, while also providing a beneficial supply of heat to the combustion process.

Reduced pumping losses for lower fuel consumption
At the same time, the SCC system contributes towards minimizing the pumping losses. These normally occur when the engine is running at low load and the throttle is not fully open. The piston in the cylinder then operates under a partial vacuum during the suction stroke in order to draw in the air. The principle is roughly the same as when you pull out a cycle pump plunger while shutting off the air opening with your thumb. The extra energy needed for pulling down the piston causes increased fuel consumption.

In an SCC engine, the cylinder is supplied with only the amount of fuel and air needed for the operating conditions at any particular time. The remainder of the cylinder is filled with inert exhaust gases. The pumping losses are reduced since the engine need not draw in more air than that necessary for achieving lambda 1.

Different sparks for different operating conditions
The fuel/air mixture in the cylinders of a car with an SCC system consists mainly of exhaust gases and air. The exhaust gases account for 60 - 70 percent of the combustion chamber volume, while 29 - 39 percent is air, and less than 1 percent is occupied by the petrol. The exact relationships depend on the prevailing operating conditions. As a general rule, a higher proportion of exhaust gases is used when the engine is running at low load, and a lower proportion when it is running at high load.

An ignition system that provides good spark firing quality is needed to ignite a gas mixture consisting of such a high proportion of exhaust gases and to ensure that the mixture will burn sufficiently quickly. A large amount of energy must be applied locally in the combustion chamber. In the SCC system, this is achieved by employing a variable spark gap and a high spark firing energy (80 mJ).

The spark gap is variable between 1 and 3.5 mm. At low load, the spark is fired from the central electrode in the spark plug injector to a fixed earth electrode at a distance of 3.5 mm. At high load, the spark is fired somewhat later, and the gas density in the combustion chamber is then too high for the spark to bridge a gap of 3.5 mm. A pin on the piston is then used instead as the earth electrode. Following the laws of physics, the spark will be struck to the electrode on the piston as soon as the gap is less than 3.5 mm.

SCC developed by Saab
The Saab Combustion Control system has been developed at the Saab Engine Development Department, which is also the Center of Expertise for the development of turbocharged petrol engines in the GM Group. The variable spark gap in the SCC system is a further development of the spark-to-piston concept that Saab unveiled at the Frankfurt Motor Show in 1995. In the air-assisted direct injection system, Saab engineers are cooperating with the Australian company Orbital.

The SCC system is a 'global' engine system, since it meets the demands in the U.S.A., where greatest emphasis is placed on limiting the nitrogen oxide and hydrocarbon emissions, and also those in Europe, where greater emphasis is placed on the carbon dioxide emissions. The SCC system will be launched in the next generation of Saab cars.

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