Saab Combustion Control System: Lower Emissions Through Extensive Use of Exhaust Gases

Sep 29, 2000, 01:00 ET from Saab

    PARIS, Sept. 29 /PRNewswire/ -- 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
     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
     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.
     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 degrees Celsius.  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 degrees Celsius).
     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.