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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
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.