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INTRODUCTIONThe 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.

Fig.1 Saab Combustion Control Engine

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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:

1. 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.

2. 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.

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3. Variable spark plug gap with high spark energyThe 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.

CATALYTIC CONVERTOR

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).

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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.

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.

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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.

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.

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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

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REDUCED PUMPING LOSSES

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 stoichiometric mixtures.

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DIFFERENT SPARKS

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.

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WORKING OF THE SCC ENGINE

EXPANSION STROKE

1. The air/fuel mixture burns. The combustion heat causes the pressure of the

gas mixture to rise, which presses the piston downwards.

EXHAUST STROKE

2. The exhaust valves open when the piston has reached the bottom of its

stroke. Most of the exhaust gases are discharged through the exhaust ports due

to the pressure difference between the interior of the cylinder and the outside of

the gas ports. This takes place during a short period when the piston is at the

Bottom Dead Centre.

The remainder of the exhaust gases is discharged through the exhaust ports as

the piston moves up.

Fig 2 Expansion Stroke

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Fig 3 Exhaust Stroke

3. Just before the piston reaches Top Dead Centre, petrol is injected into the

cylinder through the spark plug injector. The inlet valves open at the same time.

Exhaust gases mixed with petrol are discharged through both the exhaust and

the inlet ports. The prevailing operating conditions determine the exact length of

time during which the opening of the exhaust and inlet valves overlaps (and thus

the proportion of exhaust gases that will remain in the combustion chamber

during combustion).

Fig 4 Exhaust Stroke

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INTAKE STROKE

4. The piston moves downwards. The exhaust and inlet valves are open. The

mixture of exhaust gases and petrol is drawn back from the exhaust ports into

the cylinder. A large proportion of the exhaust gas/petrol mixture flows up into

the inlet ports.

Fig 5 Intake Stroke

5. The piston continues on its downward travel. The exhaust valves close but

the inlet valves remain open, and part of the exhaust gas/petrol mixture that

flowed up into the inlet manifold is drawn back into the cylinder.

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Fig 6 Intake Stroke

6. The piston approaches Bottom Dead Centre. All of the exhaust gas/petrol

mixture has now been drawn back into the cylinder, and during the final phase

of the inlet stroke, the air needed for combustion is drawn in (14.6 parts of air

for every part of fuel).

Fig 7 Intake Stroke

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Compression stroke

7. The inlet valves close. The piston moves upward, and the mixture of exhaust

gases, air and petrol is compressed. About half-way up the compression stroke

(about 45 degrees of crankshaft rotation), and before the spark has ignited the

air/petrol mixture, the spark plug injector delivers a blast of air into the cylinder.

The air blast creates the turbulence needed to facilitate combustion and shorten

the combustion time.

Fig 8 Compression Stroke

8. Just before the piston has reached Top Dead Centre, a spark from the

electrode of the spark plug injector ignites the air/petrol mixture, and the next

expansion stroke begins. The exact instant of ignition is determined by the

prevailing operating conditions.

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Fig 9 Compression Stroke

Depending on when the ignition instant occurs, the spark is fired either to the

fixed electrode across a gap of 3.5mm or to the electrode in the piston. The

spark follows the laws of physics and is fired to the piston as soon as the piston

electrode is closer than 3.5mm to the centre electrode. As a general rule, the

spark is fired to the fixed electrode at low load and to the piston electrode at

high load.

Fig 10 Compression Stroke

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CONCLUSION

The Saab Combustion Control system has been developed at the Saab

Engine Development Department, which is also the Centre 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 USA, 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.

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REFERENCES

1. Internal Combustion Engine - M.L.Mathur &R.P.Sharma

2. Advanced Engine Technology - Heinz Heisler

3. www.autospeed.com