Superchargers
pletely filled, and while the speed may mount, the actual power
developed falls off, because the charge volume becomes less.
The supercharger, being nothing more than an air compressor, puts
a pressure behind the air going through the carburetor, and, no
matter how fast the pistons and valves operate, this increased
pressure of the mixture tends to fill the cylinders to the fullest
extent possible.
Definition of Supercharger
The following definition of supercharger is given by Mr. David
Gregg:'
"A supercharger is a device that can supply to the engine
cylinders a greater weight of charge than the pistons would of
their own accord suck in. In other words, the final pressure in
the cylinder can be greater than when the pistons alone suck in
the charge.
Superchargers are divided into two classes, positive and centrifugal.
In the positive type, such as the Roots, the pressure delivered
by the supercharge is to a large extent independent of the speed
at which the supercharger is driven. In the centrifugal type the
pressure increases roughly with the square of the speed.
Each of these types is subdivided into suction and blower superchargers.
A suction type being one in which the carburetor is attached to
the intake or suction side of the super-charger, a blower type
being one in which the supercharger forces air under pressure through
the carburetor."
Advantages of Supercharging
Supercharging makes possible an increase in engine power output
without making the engine any larger. Thus, we might say, an
eight-cylinder engine of 122 cu. in. piston displacement will
develop considerably over 100 h.p. with a supercharger. The same
power might he derived from a much larger and heavier four-cylinder
engine of some 600 cu. in. piston displacement and operating
without a super-charger.
Fig. 1. Here is shown a cross-sectional view of two engines, the
one at the left without, and the one on the right with a supercharger.
In the illustration at the left the mixture under atmospheric pressure
is shown by dots. This mixture enters the cylinders owing to the
depression or vacuum created in the cylinder by the descending
piston. When the piston moves very fast this depression becomes
less and the engine cannot take in a full charge.
That is where the supercharger comes in. As shown at the right,
it is nothing more than a blower driven by the engine, usually
by gears, which puts a pressure behind the charge and fills the
cylinders more completely. The impeller of the super-charger usually
is made with "entrance buckets" where the air comes in
and the vanes usually are referred to as "diffusing vanes." The
above illustration is purely hypothetical.
But the advantages lie with the small, multicylindered, high-speed
super-charged engine. In the first place, the small engine weighs
less, has lighter reciprocating parts, less vibration, presents
less
i Not from Motor Age; additions made by the author.difficult cooling
problems, has better fuel economy and is more flexible at all speeds.
The small displacement engine is' necessarily a, high-speed engine
and, as conventionally designed,, means usually the use of inlet
and exhaust cams which insure rapid opening and closing of the
valves. This abrupt action of the valves is naturally hard on the
mechanism, and if ordinary cam layouts are used the engine performance
is impaired.
That is where the supercharger comes in. With a supercharger ordinary
cams can be used to open and close the valves. Since the mixture
is under pressure it is forced into the cylinders through the valves
in greater quantity and far easier than by suction, or, rather,
manifold depression, and severe cam action.
Furthermore, the valve timing does not have to be set to a hair
to get maximum efficiency. It may vary several degrees one way
or another. Just as long as the valves open and close, the supercharger
takes care of getting the mixture into the engine.
The supercharger has shown it is a most useful device in mechanically
breaking up the fuel and more completely atomizing it. This was
amply illustrated in the last year's Indianapolis 500-mile race
in the supercharged Duesenberg car driven in the first part of
the race by the late Joe Boyer.
Part of the mechanism of the supercharging device on the car gave
way, leaving the impeller, however, free to rotate, but not driven
at the required speed. Nevertheless, with but a single carburetor
and with the impeller revolving by virtue of the engine suction,
the car was almost as fast as with the super-charging device intact.
The impeller in this case churned the mixture. Naturally this atomization
of the fuel took place because the supercharger was located between
the engine and carburetor.
Value with Small Engines
In ordinary passenger car operation the value of supercharging
would lie in the increased engine flexibility and maneuverability
of the car. It means smoother and quicker acceleration and on the
approach of a hill calling for a shift into second, the hill could
be taken in high gear with the supercharger in action. There would
be little or no danger of stalling the engine at low speeds, and
certainly the abundance of reserve power for hill climbing and
hard pulling is desirable.
It must be remembered that all the foregoing is possible with a
very small engine, on which the super-charger, if desired, would
come into action only when this reserve power was needed. Such
a small engine, then, would be very economical on fuel.
The German Mercedes is the first stock passenger car to fit a supercharger,
and this car already has demonstrated its remarkable flexibility
and all-around performance. The car is rated at 23.8 h.p., but
with the supercharger develops 100 h. p.
Although a little early to make predictions regarding the application
of superchargers, it would seem the future holds possibilities
for the two-cycle engine corning into its own by virtue of supercharging.
Here the usual crankcase compression could be dispensed with, since
the charge from the blower would be under pressure. Crankcase compression
and its attending evils has been one of the stumbling blocks in
the development of the two-cycle engine, so far as motor vehicles
are concerned, where a flexible engine is required. The entire
absence of mechanically operated valves, of course, favors the
two-cycle engine.
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