23-08-2012, 03:37 PM
SAAB COMBUSTION CONTROL SYSTEM
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INTRODUCTION
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 into force in the year 2005.
Compared to today's Saab engines with equivalent performance, this will almost have
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.
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 stoichiometric value under
almost all operating conditions. Up to 70 percent of the cylinder contents during
combustion consist of exhaust gases - the exact proportion depending on the prevailing
operating conditions.
IMPORTANCE OF CATALYTIC CONVERTOR
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 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).
WEAKNESS OF CATALYTIC CONVERTOR
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. 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 C).
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 (stoichiometric).
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.
DIRECT INJECTION
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 car makers 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.
HIGHER NOX
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.
DIRECT INJECTION AND STOICHIOMETRIC
In evolving the SCC system, Saab engineers have developed a way of putting to
use the benefits of direct injection, while still maintaining stoichiometric mixtures.
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 a stoichiometric air/fuel ratio. 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 stoichiometric 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.