Variable Compression Ratio - Saab SVC
Saab stunned the world by showing its variable compression ratio engine in
the 2000 Geneva motor show. I’ve heard such engine for some 2 years, but
this is the first time Saab disclose the details to the press. In my
opinion, this is perhaps the largest single breakthrough in engine
technology since turbocharging and electronic engine
management.
Why is variable compression
ratio so fascinating? As everybody knows, fixed compression ratio is always
a constraint for supercharging or turbocharging engines. To prevent
excessive pressure in combustion chamber, hence pre-ignite ("knocking") and
overheating to the cylinder head, turbo/supercharger engines always employ a much
lower compression ratio than normally aspirated engines so that the total
pressure won’t exceed the limits of the engine. when the boost pressure is added.
The
problem is, when the charger (especially turbocharger) is not yet getting
into full boost, that is, at low and mid revs, the combustion runs at
lower compression
ratio than normally aspirated engines. Therefore power efficiency at low
speed is even lower than normally aspirated engines.
Saab have finally realized the variable
compression ratio engine.
Named SVC (Saab Variable
Compression), the engine implement VC by an innovative and interesting
method - a slidable cylinder head and cylinder. look at the following
pictures for illustration.
As seen, the SVC engine have a
cylinder head with integrated cylinders - which is known as monohead. The
monohead is pivoted at the crankcase and its slope can be adjusted slightly
(up to 4 degrees) in relation to the engine block, pistons, crankcase etc.
by means of a hydraulic actuator, therefore the volume of the combustion
chamber (when piston is in compressed position) can be varied. In other
words, compression ratio is also variable.
SVC is smarter than any
previous patents for variable compression ratio engines in that it involves
no additional moving parts at the critical combustion chamber or any
reciprocating components, so it is simple, durable and free from leakage.
The monohead is self-contained,
that means it has its own cooling system. Cooling passages across the head
and the cylinder wall. There is a rubber sealing between the monohead and
engine block.
The VC allows the Saab engine to run on
very high supercharging pressure - 2.8 bar, compared with the latest 911
turbo’s 1.94 bar, or about twice the boost pressure used by 9-3 Viggen. So
high that today’s turbochargers cannot provide the boost. Therefore it employs
a supercharger instead. The VC is adjustable continuously
according to needs - depends on revs, load, temperature, fuel used etc., all
decided by engine management system. Therefore power and fuel consumption
(and emissions) can be optimized at any conditions.
The SVC engine shown in Geneva
is the third generation prototype, although production is still far away. It
is an inline 5-cylinder with 4-valve head. The displacement is just 1598
c.c. to take advantage of the outstanding efficiency. Compression ratio can
be varied between 8:1 and 14:1. With the supercharger, it output a maximum
225 hp and 224 lbft, something similar to a Honda 3.2-litre V6. However, its
fuel consumption is very low. Saab claims it saves 30% compared with equally
powerful conventional engines.
In terms of specific output, it
achieves a healthy 150 hp per litre, At the same time, it is expected to fulfill all foreseeable emission
regulations, including the tightest EU4. Another advantage is the
suitability to different grades of fuel, especially in America where low
Octane petrol is common. The engine management system detects the fuel grade and
decides the most appropriate compression ratio to be used.
Saab started developing SVC in
the late 80s and acquired the first patent in 1990. The first prototype was
a 2-litre unit but was considered as being more powerful than needed ??? The second
prototype was a 1.4-litre inline-6 but it had problems with packaging, so
the inline-5 configuration was eventually chosen.
More work has to be done to get an SVC into production. The production unit might not be the same as this
one, but it is believed that General Motors has green lighted the full
development, which requires big investment from parent
company.
| Advantage: |
Enhance
efficiency a lot for turbo/supercharged engines across the whole rev
range, thus enable the engine to be smaller and lighter; highly
adaptable to different grade of fuel; cleaner emission
possible. |
| Disadvantage: |
Engine head and block
more complicated |
| Used by ? |
Only Saab is
developing. GM funding! Vauxhall to benefit? |
