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

The Saab Combustion Control (SCC) system is a new engine control system developed to lower fuel consumption while radically reducing the exhaust emissions, but without impairing engine performance. By mixing a large proportion of exhaust gases into the combustion process, the fuel consumption can be reduced by up to 10 percent, at the same time lowering the exhaust emissions to a value below the American Ultra Low Emission Vehicle 2 (ULEV2) requirements that will come into force in the year 2005. Compared to today's Saab engines with equivalent performance, this will almost halve the carbon monoxide and hydrocarbon emissions, and will cut the nitrogen oxide emissions by 75 percent.  

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:  

Air-assisted fuel injection with turbulence generator The injector unit and spark plug are integrated into one unit known as the spark plug injector (SPI). The fuel is injected directly into the cylinder by means of compressed air. Immediately before the fuel is ignited, a brief blast of air creates turbulence in the cylinder, which assists combustion and shortens the combustion time.  

Variable valve timing

The SCC system uses camshafts with variable cams to enable the opening and closing of the inlet and exhaust valves to be steplessly varied. This allows exhaust gases to be mixed into the combustion air in the cylinder, which puts to use the benefits of direct injection while maintaining the value of lambda at 1 under almost all operating conditions. Up to 70 percent of the cylinder contents during combustion consist of exhaust gases. The exact proportion depends on the prevailing operating conditions.  Variable spark plug gap with high spark energy The spark plug gap is variable between 1 and 3.5 mm. The spark is struck from a central electrode in the spark plug injector either to a fixed earth electrode at a distance of 3.5 mm or to an earth electrode on the piston. The variable spark gap together with a high spark firing energy (80 mJ) is essential for igniting an air/fuel mixture that is so highly diluted with exhaust gases. 

(September 29, 2000)