Saab Combustion Control System
29 September 2000
Saab Combustion Control System: Lower Emissions Through Extensive Use of Exhaust Gases
PARIS, Sept. 29 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.
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.