Power Boosting Technology
engines have one spark plug per cylinder. However, since decades ago,
Romeo insisted to put 2 spark plugs in each cylinder. As ignition takes
place in two locations rather than one, this enable more efficient
and cleaner emission. However, besides Alfa, in the past 15 years only
Mercedes and Porsche have ever applied Twin Spark design to their
This is mainly because of the complexity of cylinder head - it would be
too difficult to put 4 valves and 2 plugs into the small cylinder head
area. ( Mercedes' and Porsche's engines are 3 valves and 2 valves per
respectively, so they have no such problem.) Only Alfa Romeo applied it
to 4-valve engines.
famous 2.0 TS
efficiency, hence more power and cleaner emission.
enough for most car makers
|Who use it ?
||Alfa 1.6 to
Mercedes V6 and V8
ratio; Right: low compression ratio
Variable Compression Ratio - Saab SVC
has 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
opinion, this is perhaps the largest single breakthrough in engine
since turbocharging and electronic engine management.
Why is variable
ratio so fascinating? As everybody knows, fixed compression ratio is
a constraint for supercharging or turbocharging engines. To prevent
pressure in combustion chamber, hence pre-ignite ("knocking") and
to cylinder head, turbo/supercharger engines always employ a much lower
compression ratio than normally aspirated engines so that the total
won’t exceed the limit when the boost pressure is added. The problem
when the charger (especially is turbocharger) is not yet getting into
boost, that is, at low and mid rev, the combustion runs at lower
compression ratio than
aspirated engines. Therefore power efficiency at low speed is even
than normally aspirated engines.
I remember when I was
13 or 14 years old, I realized that problem and "designed" a variable
ratio engine on paper. It involved variable length connecting rods to
the position of piston’s top dead center, hence compression ratio. When
the turbo is not in full boost, compression ratio is as high as
aspirated engine (10:1 by then). This lower to 7:1 for full boost. Of
that concept is completely out of imagination and is no way to be
Today - a dozen years later - Saab finally realized the variable
Named SVC (Saab
Compression), the engine implement VC by an innovative and interesting
method - slidable cylinder head and cylinder. Let’s look at the
pictures for illustration.
As seen, the SVC engine
have a cylinder head with integrated cylinders - which is known as
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,
etc. by means of a hydraulic actuator, therefore the volume of the
chamber (when piston is in compressed position) can be varied. In other
words, compression ratio is also variable.
SVC is cleverer than
previous patents for variable compression ratio engines is that it
no additional moving parts at the critical combustion chamber or any
components, so it is simple, durable and free of leakage.
The monohead is
that means it has its own cooling system. Cooling passages across the
and the cylinder wall. There is a rubber sealing between the monohead
VC allows the Saab engine to run on very high supercharging pressure -
2.8 bar, compare with the latest 911 turbo’s 1.94 bar, or about twice
boost pressure using by 9-3 Viggen. So high that today’s turbochargers
cannot provide. Therefore it employs supercharger instead. At other
the VC is adjustable continuously according to needs - depends on rev,
load, temperature, fuel used etc., all decided by engine management
Therefore power and fuel consumption (hence emission) can be optimized
at any conditions.
The SVC engine shown
is the third generation prototype, although production is still far
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
a maximum 225 hp and 224 lbft, something similar to a Honda 3.2-litre
However, its fuel consumption is very low. Saab claims it saves 30%
with equally powerful conventional engines.
In terms of specific
it achieve 150 hp per litre, which must be a world record for
car. At the same time, it is expected to fulfill all foreseeable
regulations, including the tightest EU4. Another advantage is the
to different grade of fuel, especially in America where lower Octane
is common. The engine management system detect the fuel grade and
the most appropriate compression ratio to be used.
in the late 80s and acquired the first patent in 1990. The first
was a 2-litre unit but was considered as more powerful than needed. The
second prototype was a 1.4-litre inline-6 but it had problems about
so the inline-5 configuration was eventually chosen.
More work has to be
to make a SVC into production. The production unit might not be the
as this one, but it is believed that General Motors has green lighted
full development, which requires big investment from parent company.
a lot for turbo/supercharged engines across the whole rev range, thus
the engine to be smaller and lighter; highly adaptable to different
of fuel; cleaner emission possible.
and block more
|Who use it ?
||Only Saab is
High compression engine - Mazda
Higher compression ratio brings higher combustion
efficiency hence power. That's why automotive engineers want to raise
compression as high as possible. However, a compression too high will
lead to early explosion of fuel-air mixture, or what we call
"knocking". Knocking is bad to engines, not only because it causes NVH
but also it reduces output. When I started reading about cars, most
engines in the world ran at lower than 10:1 compression. As engine
management and valve-timing technology improves, nowadays the figure
can be higher than 11:1. Direct injection engine may even lift that
figure to 12:1 or so thanks to its cooling effect, but anything higher
than that remains a dream. However, Mazda made a breakthrough with its
Skyactiv-G engine in 2010. It works at an incredible 14:1 compression !
How can Mazda avoid knocking ? A crucial factor causing knocking
is the high temperature of combustion chambers. Temperature in the
chamber rises during compression stroke. It peaks when the piston
reaches the top dead center (TDC, i.e. the highest position). At this
point, knocking is most likely to occur. Obviously, if we want to
reduce the risk of knocking, we had better to lower the combustion
Then why is the combustion chamber so hot ? One of the reasons is the
existence of residual exhaust gas, i.e. the exhaust gas that flows back
into the combustion chamber during the intake stroke just before the
exhaust valves close. No one can completely get rid of residual exhaust
gas, because for high breathing efficiency engines always need to run
with a certain level of valve overlapping (overlapping between the
opening period of intake and exhaust valves). Suppose exhaust gas is
750degC and the fresh intake air is 25degC, and their mixture ratio is
1 to 10, you can see the residual exhaust gas can raise the combustion
chamber temperature a lot. The more the amount of residual exhaust gas,
the higher the combustion chamber temperature is. In other words, if we
want to reduce temperature, we can reduce the amount of residual
exhaust gas in the combustion chamber.
The graphs above show that a 14:1 compression engine always has higher
comnbustion chamber temperature than a 10:1 engine on a given residual
exhaust gas level. However, if the amount of residual exhaust gas is
reduced to 4 percent, combustion chamber temperature will be about the
same as a 10:1 engine running with 8 percent of residual exhaust gas.
Now the question is: how to lower the percentage of residual exhaust
Surprisingly, Mazda uses a very conventional method to do that: a long,
4-to-2-to-1 exhaust manifold. On a typical inline-4 cylinder engine
with short, 4-to-1 exhaust manifolds (the first picture below), once
the exhaust valve of Cylinder 3 opens, its exhaust pressure waves (grey
area) flows through the short manifolds to the exhaust valve of
Cylinder 1, which is at the end of its exhaust phase. This pumps some
exhaust gas back into Cylinder 1 and becomes residual exhaust gas. When
the engine is running at low speed (2000 rpm in the picture below), the
exhaust pressure wave arrives Cylinder 1 early enough to cause high
percentage of residual exhaust gas. As engine rev rises, the opening
and closing of valves speeds up as well, thus the exhaust pressure
waves of Cylinder 3 reaches Cylinder 1 at later stage, causing lower
percentage of residual exhaust gas. In short, from low to mid-range
engine speed the level of residual exhaust gas is pretty high for this
In the case of Skyactiv-G's 4-2-1 exhaust manifolds (the second picture
above), exhaust pressure waves from Cylinder 3 has to travel a long way
to reach Cylinder 1, by the time Cylinder 1 has already, or nearly
completed its exhaust phase. Therefore the level of residual exhaust
gas is much lower than the previous case, especially for low to
mid-range rpm. As a result, the Skyactiv-G engine attains lower
temperature in its combustion chambers and allows a higher compression
ratio to be used.
Well, if the principle is so simple, why not others discovered already
? It's not that simple, of course. One critical drawback of the long
4-2-1 exhaust manifold is that it takes relatively long time to heat up
the NOx catalyst during cold start. In fact, this is exactly the reason
why most modern production engines have abandoned this exhaust
configuration - with the exception of high-performance engines which
may use thin-wall fabricated stainless steel exhaust manifolds to
compensate for its extra length. On cost-conscious mass production
engines, cheap cast-iron exhaust manifolds are still the norm. Its
extra mass and surface area absorb a great deal of heat and delay the
proper functioning of catalyst. This causes difficulty to comply with
Mazda overcomes the cold-start problem by retarding ignition. This
leads to a higher exhaust gas temperature to compensate for the long
manifolds. The late ignition may result in unstable combustion. This is
dealt with a specially shaped piston (pictured above) which
concentrates the stratified air-fuel mixture around the spark plug.
Other supporting features like high-pressure direct injection and
six-hole injectors also contribute to the optimized combustion.
higher fuel efficiency, 15% better torque output at low to mid-range
|Who use it ?
1998-2011 by Mark Wan
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