[Diy_efi] Graham Anal Optical Confusion

Robert Harris bob at bobthecomputerguy.com
Fri Dec 27 16:47:11 GMT 2002


Automotive Engines by Kuns-Plumridge - copyright 1945 by American Technical
Society - reprinted 1948.  Just another antique book - doesn't matter since it
was pre-computer and formal dweebdom. You can see how dumb Graham, Allison,
Rolls Royce, Pratt-Whitney, Curtis Wright, Corvair, Buick, Oldsmobile and all
the rest were about having the fuel introduced ahead of the supercharger.
Dumasses probably right? Page 395 of useless text.

"Graham Supercharger

The Graham supercharger, fig 19, is of centrifugal type which supplies power
to the feeding of the fuel mixture into the engine cylinders.  Instead of
having the engine dependent upon its natural pumping or sucking action to draw
the mixture through the carburetor and into the combustion chambers, the
supercharger forces the mixture into them under pressure.  The suction of the
engine is replaced by pressures ranging from one to several pounds.  Under
full throttle, the manifold pressures are positive at all engine speeds.
Since with a non-supercharged engine a negative pressure (vacuum) ranging from
one to several pounds is required to draw the mixture into the cylinders, the
effect of the supercharger is actually much greater than any gauge pressure
reading would indicate.  Several pounds of vacuum is replace by several pounds
of pressure, approximate twice the amount, in fact.

The supercharger is of the centrifugal or "blower" type and is mounted between
the carburetor and intake manifold of the engine.  It consist primarily of a
casing within which rotor, similar in form to that of a vacuum sweeper,
revolves at speeds of up to approximately 30,000 rpm.  The centrifugal action
of the rotor draws mixture of fuel and air from the carburetor into center of
casing and expels it under pressure at the rim or edge, forcing it through
intake manifold to combustion chambers of the engine.

The carburetor is mounted directly upon the supercharger rotor cover and is of
the down-ddraft type.  After passing through the supercharger, the fuel
mixture is carried over the top to the engine through a tube connected to the
intake manifold, which is similar in type to that used on Graham standard
engines.  Thence, it is forced under pressure into the combustion chambers.

The lower part of the unit consists of a gear cased enclosing worm gearing
expecially developed for driving centrifugal superchargers. The worm wheel is
driven from the accessory shaft which has a ratio of .2 to 1 with the
crankshaft.  The worm gearing itself has a ratio of 4.8 to 1.  Thus the rotor
revolves 5.75 times engine speed, or 23,000 revolutions per minute at the
horsepower peak speed of 4000 revolutions per minute.  The rotor is 7.5 inches
in diameter, and with its shaft, is accurately balanced and tested to run
smoothly.

Simplicity of Graham Supercharger.  Both the rotor and the worm wheel shafts
are mounted on plain bearings.  Complicated bearing mountings and adjustments
have bee completely eliminated.  The unit has been so designed as to make it
just  as much a part of the engine as the lubrication or cooling systems.  No
adjustment of either the bearings or the worm gearing is required.

Full Pressure Lubrication.  All the bearings and the worm gearing of the
supercharger are full-pressure lubricated by the same oil used to lubricate
the engine.  A reserve supply of oil is automatically retained within the worm
gears and bearings during the starting and warming-up period. The cover which
forms the top half of the rotor casing is water-jacketed.  No exhaust heat is
required to warm the mixture.  There are no hot spots on the intake manifold.
The cooling water controls the mixture temperature at desirable levels under
partial and full-throttle operating conditions.

Uses Leaner Mixture.  By placing the supercharger between the down-draft
carburetor and intake manifold, the rotor scrubs and thoroughly mixes the fuel
and air after it is taken in a relatively raw condition from the carburetor.
Obviously, the high speed rotor is a most effective means of putting the
mixture in a most explosive condition before it reaches the cylinders.  The
result is that since the supercharger begins to operate the instant the engine
begins to be rotated by the starter, the mixture is in better condition for
quick starting in cold weather.  No slugs of fuel can pass the rapidly
whirling rotor.  It is a most effective mixing device.

A more important result from the standpoint of operating economy, is that
since the mixture is continuously churned by the rotor at all engine speeds,
it is never necessary for the engine to require more that the most efficient
and power-giving amount of fuel.  A rich mixture is never needed.  The
supercharger, by its thorough vaporization of the fuel, makes it possible for
the engine to operate on a leaner mixture than ordinarily.  Even with
exceptionally lean mixtures the supercharger so completely prepares it for the
engine that backfiring ( which usually results from excessively lean mixtures
) is almost and impossibility.  The supercharger consumes less than two
horsepower.  By consuming this small amount, it in turn enables the power of
the engine to be tremendously increased.  This small consumption of power is
due to the fact that the supercharger, by forcing mixture into the cylinders,
offsets the necessity for the engine to pump or suck it into them.

All automobile engines are relatively inefficient in the operation of pumping
the mixture into the cylinders.  This is due to the large clearance volume (
in this case the combustion chamber ) and other technical reasons.  The fact
that the supercharger is efficient as a means of offsetting this pumping loss,
is, that although it requires from 5 to 6 horsepower to drive the supercharger
at 3,000 revolutions per minute engine speed when disconnected, the amount
required when it is supplying it with mixture is 1.8 horsepower at the same
engine speed.  Indicative also of the gain in efficiency of the the engine
when equipped with the supercharger, is the fact that the thermal efficiency
is about 21 per cent from 2,000 to 3,00 revolutions per minute engine speed,
decreasing above and below this range;  the efficiency of the supercharged
engine remains above 19.5 per cent until a speed of 4,000 revolutions per
minute is attained.  At this speed the thermal efficacy of the
non-supercharged engine has dropped to 16 per cent.

Acceleration.  While the supercharged engine does develop much more power,
this alone would not necessarily make the engine accelerate faster at from 50
to 60 miles per hour than at 25 to 30 miles per hour.  Herein lies one of the
most unusual facts relating to the action of the supercharger.  The factor in
an engine which gives the car greater acceleration is the torque or twisting
force on the crank shaft.  The greater this is, the faster the acceleration.
Also, if it is greater at one speed than another, the acceleration in turn
will also be faster.  This is exactly why the supercharged engine gives the
car such fast pick-up at twice the car speed at which it is ordinarily the
greatest.

The supercharged engine delivers not only more torque at car speeds around 25
miles per hour thus making it have a faster get away, but it also delivers its
maximum torque at twice the speed of the ordinary engine.  Instead of
delivering its greatest torque at 1200 revolutions per minute, as is usually
the case, it delivers it at 2400 revolutions per minute and, for this reason,
the supercharged engine can pull harder and faster than the non-supercharged
engine which begins to level off and gradually increase its speed.  The
supercharged engine seems to have no top speed when the throttle is pushed to
the floor at from 50 to 60 or even more miles per hour."

Like ah said - a bunch of pre-historic myths.


Habaneros - Not Just for breakfast anymore!

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