The SCC engine appears to be a good product....
from: saabnet.com
Conventional direct injection for lower fuel consumption...
In an engine with a conventional injection system, the gasoline is injected into the intake manifold, where it is mixed with the combustion air and is drawn into the cylinder. But part of the gas 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 gasoline was launched a few years ago by carmakers as a way of lowering the fuel consumption. Since gas is injected directly into the cylinder, the fuel consumption can be controlled more accurately, and the amount of fuel injected is limited to that necessary for each individual combustion process. In such cases, it is not necessary for the entire cylinder to be filled with an ignitable mixture of fuel and air, and is sufficient if only the fuel/air mixture nearest to the spark plug is 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 require 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 stored nitrogen oxide must be removed periodically, 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 gasoline desulphurizing process causes higher carbon dioxide emissions from the refinery.
Direct injection and lambda 1 with SCC
In creating 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.
Therefore, the SCC system does not require 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 helps minimize 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 retract a tire pump plunger while covering the air opening with your thumb. The extra energy needed for pulling down the piston requires 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 there is little resistance on the piston intake stroke. With the exhaust valve held open and no throttle plate restriction, the engine can freely draw in the correct proportion of air and inert exhaust gas to achieve lambda 1..........
The SCC system is a 'global' engine system, since it meets the demands in the U.S., 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. |
