Saab Reveals Saab Combustion Control System Which Reduces Fuel Consumption And Exhaust Emissions

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 exhaust emissions, without impairing engine performance.
 By mixing a large volume of exhaust gases into the combustion process, Saab's
 fuel consumption can be reduced by up to 10 percent, and exhaust emissions
 lowered enough to comply with the California Ultra Low Emission Vehicle 2
 (ULEV2) requirements, set to take effect in 2005.  Compared to today's Saab
 engines with equivalent performance, this will reduce the carbon monoxide and
 hydrocarbon emissions by almost half, 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 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 in low torque conditions, a second brief blast of air creates
        turbulence in the cylinder, which facilitates combustion and shortens
        the combustion time.
     *  Variable valve timing
        The SCC system uses camshafts with variable cam timing, which enables
        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 1 under almost
        all operating conditions. In low torque conditions, the exhaust gases
        compose up to 70 percent of the cylinder contents during combustion.
        The exact proportion depends on the prevailing operating conditions --
        with the proportion of exhaust gas to ambient air decreasing as the
        torque demand increases.
     *  Variable spark plug gap with high spark energy
        The spark plug gap is variable between 1 and 3.5 mm.  A central
        electrode in the spark plug injector strikes a spark to either a fixed
        earth electrode 3.5 mm away (in low torque) or to an earth electrode on
        the piston (under high torque).  The variable spark gap together with a
        high spark firing energy (80 mJ) is essential for igniting an air/fuel
        mixture that may have a high percentage of 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 catalyze
 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 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 C (752 degrees F). 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 80 degrees C (185 degrees F).
     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 fuel to 14.6 parts
 air (i.e. lambda = 1) by mass.  As a general rule, 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.
     Other than converting CO to CO2 for emission control, the catalytic
 converter does not significantly affect the amount of carbon dioxide (CO2)
 produced, which is directly related to the amount of gasoline consumed.  The
 lower the fuel economy, the greater the carbon dioxide emissions.
     Much of the work of designing less polluting gas 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 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.
     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 gasoline.  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 quickly enough.  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. (A normal spark plug gap is less than 1
 mm.)  At high load, the spark is fired 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 as the earth electrode.  The spark will be
 struck to the electrode on the piston (1 mm gap).
     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 gasoline 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., 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.