Download AN electronic turbosupercharger control system, newly developed

A
N electronic turbosupercharger control system, newly
developed by the Minneapolis-Honeywell Regulator
Company, solves a major problem confronting military
pilots by providing accurate, dependable control of engine
manifold pressures under widely divergent, rapidly varying
conditions. It enables the combat pilot to devote his entire
time to maneuvering his plane, relieves him of the
necessity for continual manipulation of engine controls to
maintain desired manifold pressures and power output.
This completely automatic mechanism controls
induction system manifold pressures during take-off,
climb, cruise, and glide. A single convenient dial installed
in the cockpit or cabin enables the pilot quickly and easily
to select desired manifold pressures on either single or
multi-engine planes. Use of this electronic equipment
eliminates the usual control system inaccuracies caused by
extreme temperature changes.
Whether the turbo speed is increased by a gain in
altitude, duct failure from gun-fire, or by sudden throttle
manipulation, the overspeed control prevents excessive
turbo rpm. On a four-engine plane, four simple calibrating
adjustments coordinate the manifold pressures on all
engines with the throttle full open and the rpm
synchronized at maximum. Once this calibration has been
made, it need not be changed for subsequent flights. When
the pilot wishes to reduce the manifold pressure on one or
more engines, he may do so by retarding the corresponding
throttle or throttles without lowering the selector dial
setting. This gives him instant power in reserve for landing,
as well as unequal engine powers for maneuvering while in
close formation, and power "cut-off" on any given engine
should trouble develop.
The electronic turbosupercharger control system consists
of an induction system pressuretrol, turbo waste gate motor,
manifold pressure selector, turbosupercharger governor,
turbo control amplifier, and main and nacelle junction
boxes. The power to operate the system is supplied by a 115volt, 400 cps alternator or inverter.
Since the turbo speed is regulated by opening and closing
the turbo waste gate, the control system includes a motor for
operating the waste gate. Inasmuch as the induction system
pressure is the factor to be controlled, the primary sensing
discriminator stage. The tubes used are as follows: a 7Y4 as
a rectifier, a 7F7 duo-triode as a voltage amplifier and two
7C5 beam power amplifiers as discriminators. Each
amplifier is provided with its own plate voltage power
supply so that in the event of failure of one of these power
supplies, only one amplifier and one control system is
affected.
The incoming voltage signal from the bridge is amplified
in the two stages of the 7F7 tube and is then applied to the
grids of the two 7C5 tubes. Connected together, the grids of
these two tubes both go positive at the same instant. When
the plate of the upper 7C5 tube goes positive, the plate of
the lower one goes negative, and vice versa. Thus, if the
plate of the upper tube goes positive at the same instant that
device of the system is an induction system pressuretrol,
which is actuated by pressure variations at the carburetor
intake. A manifold pressure selector is provided to enable
the pilot to select any desired safe manifold pressure.
It is also necessary to include devices to prevent the turbo
speed from exceeding a safe limit and to prevent too rapid
acceleration of the turbo in response to abrupt changes of
throttle or manifold pressure selector settings. These
functions are performed by an overspeed governor and an
accelerometer, which are combined in a unit called the turbogovernor.
The induction system pressuretrol is essentially a voltagedividing potentiometer, mechanically powered by a pressure
bellows which is piped to the induction system at the
carburetor inlet. As the pressure in the operating bellows is
increased, a wiper is caused to move across the pressuretrol
potentiometer in a downward direction, thereby introducing
a voltage signal to the electrical bridge which will initiate
action in other controls and equipment to bring the pressure
back again to the desired value. The turbo control amplifier
consists of two voltage amplifier stages and one
the grids go positive, it will pass current. At this same
instant, however, the plate of the lower tube is negative and
will therefore not pass current. At this same instant, (half a
cycle later), when the plate of the lower tube is positive,
both grids are negative and therefore current does not flow
in either tube. As long as this phase relationship between
incoming signal voltage and plate voltage exists, the upper
tube will pass current and the lower tube will not.
If the phase relationship between incoming signal voltage
is such that the plate of the lower tube goes positive when
the grids go positive, the reverse condition is true and the
lower tube passes current while the upper one does not.
With amplifier and bridge powered by the same 115-volt,
400 cps inverter, the phase relationship between incoming
signal voltage from the bridge and the plate voltage of the
two 7C5 discriminator tubes determines which of the two
will operate and pass current, thus resolving the direction in
which the waste gate motor rotates. Motor rotation is
transmitted through a gear train and linkage to position the
waste gate. Closing the gate increases manifold pressure and
opening the waste gate decreases it.
The waste gate motor is a two phase unit electrically
connected to the inverter line and to the discriminator stage
of the amplifier. One field winding is continually excited
from the inverter line, and the other field winding is excited
from the amplifier. If no signal is applied to the grids of the
discriminator tubes, the current flowing through the
amplifier-excited field winding of the motor is negligible
and no motor rotation occurs.
The turbo-governor is driven from the turbosupercharger
tachometer connection by means of a short flexible shaft. It
has the dual purpose of providing a definite overspeed
control function as well as preventing the turbine from
accelerating at too high a rate, and thereby producing
momentary surges of pressure in the induction system.
First, the engine rpm is selected and set by adjusting
propeller pitch. This remains constant throughout the climb
because of the action of the automatic propeller governors.
The throttle is then moved forward to full-open position and
the desired manifold pressure is selected.
The pressure "boost" supplied by the internal engine
blower, as represented by the difference between the
induction system pressure and manifold pressure, is a fixed
value as long as rpm remains constant. Therefore, to
maintain a constant manifold pressure, it is necessary to
maintain a constant induction system pressure.
Now, the weight and volume of air which must be
compressed for use in the aircraft engine induction system is
extremely great. An engine producing 1,500 hp consumes
over five tons of air for each hour of operation. But with an
increase in altitude, atmospheric pressure decreases and the
weight of oxygen in a unit volume of air also decreases.
Therefore, a greater volume of air is required to furnish
sufficient oxygen to support complete combustion of the
normal fuel charge. But the amount of fuel-air mixture
drawn into the engine cylinder is limited volumetrically.
Consequently, at a higher altitude the weight of oxygen in
the fuel charge is not sufficient to support complete
combustion, and the horsepower output of the engine drops
off.
To maintain a constant air-scoop and manifold pressure
during a climb, it is, therefore, necessary that the turbo unit
increase in speed. If the induction system pressure should
start to decrease due to an increase in altitude, the
pressuretrol wiper will move toward the low pressure end of
the potentiometer and will cause the waste gate motor to
close the gate. As this happens, the turbine starts to speed up
and increases the induction system pressure; and again a
new position of balance occurs in which the waste gate is
slightly more closed.
As altitude is increased above 30,000 feet, the turbo
discharge pressure, drops off quite rapidly. Then the overspeed portion of the turbosupercharger functions to prevent
a further increase in speed, which might cause structural
failure of the turbine wheel, From this, it can readily be seen
that, for a given plane, safe turbo speed is really the
determining factor which limits the altitude at which the
plane may be flown and still obtain the full rated hp of its
engines. Although the plane can actually fly to higher
altitudes, it will eventually reach a ceiling above which it
cannot rise, because the delivered hp will decrease in direct
proportion to the density of the atmosphere. This ceiling
condition, of course, will be obtained when the engine can
no longer generate climbing power without exceeding the
top safe turbine speed.
This article was originally published in the April, 1944,
issue of Air Tech magazine, vol 4, no 3, pp 30-32, 56.