An electronic device, manufactured by the Minneapolis-Honeywell Regulator Co, which regulates the "breathing" of four-engined planes, is making high-altitude flying and bombing easier and safer for AAF pilots.
This completely automatic mechanism controls induction system manifold pressures during takeoff, climb, cruise and glide. Operating from one convenient dial installed in the cabin or mounted on a cockpit throttle column, it enables a pilot to quickly and easily control manifold pressures on either single- or multi-engined planes simultaneously.
Pursuit pilots in dogfights and bomber pilots flying in close formation are provided with accurate, dependable control of engine manifold pressures under widely divergent and rapidly varying conditions so that they can devote all their time to maneuvering. The need for continually manipulating engine controls to maintain manifold pressures and power output within desired limits is removed.
If a pilot desires to alter manifold pressures he simply readjusts his control knob. The control system operates continuously, making minute adjustments in waste-gate position as the airplane gains or loses altitude, moves into hot or cold fronts where air pressures vary, or changes speed through altered throttle positions.
Fig 1, showing a curve of power fluctuations taken from photographic records during a formation flight at 25,000 ft, illustrates the magnitude and frequency of power changes required. Overspeeding can take place in a fraction of a second and pilots have long sought a control to relieve them of the duty of watching turbo speeds. At extreme combat altitudes, previous regulators were "sluggish" and unreliable, frequently causing engine failure or serious accidents when a pilot's attention was occupied with other duties.
Supercharger speeds, depending upon compression requirements, range up to 28,000 rpm. But any speed over this will blow up the turbine, causing a drop in manifold pressure which renders the engine practically useless at high altitudes. Prior to the development of the electronic regulator, there was no fixed top limit on turbo speed or manifold pressure.
By an especially designed turbo-governor, this electronic device prevents superchargers from blowing up by automatically providing a safe maximum turbine speed and anticipating pressure increases from turbo accelerations to prevent overshooting of manifold pressures. Being electrical, it is unaffected by low temperatures. There are no make-and-break contacts to cause radio interference or service malfunctions.
Many variables enter into the operation of the supercharger. The available power output of the turbine varies with operating conditions of the engine. For a given engine power, turbine speeds will vary as a function of either exhaust back pressure or atmospheric pressure, or both. The amount of "boost" necessary for a given power will vary with atmospheric pressure.
The turbosupercharger is a turbine-driven centrifugal-type air compressor. Turbosuperchargers boost carburetor inlet pressures. By compressing rarefied air of the upper atmosphere and "feeding" it to the carburetor under pressure, high power output is obtained.
Fig 2 shows a schematic diagram of a typical engine with turbosupercharger. Hot exhaust gases from the engine pass through the inlet and enter the nozzle box surrounding the periphery of the turbine wheel. The speed of the turbine wheel and its direct-connected compressor impeller is controlled precisely by opening or closing the waste-gate. With the waste-gate closed, all of the gases impinge on the turbine buckets. As the gate is opened, an increased proportion of the exhaust gas is permitted to pass to the atmosphere without doing work on the turbine buckets, and the speed of the turbine decreases.
Air at the prevailing atmospheric pressure is brought into the supercharger air intake and its pressure is increased by the impeller. It passes from there to the intake of the carburetor after losing some of its heat of compression to the intercooler. The internal blower further increases this pressure to the manifold, except as modified by the throttle.
The electronic turbosupercharger control consists of a number of individual control units electrically interconnected to operate as a system. A single—engine installation consists of an induction system Pressuretrol, an amplifier, turbo waste—gate motor, a turbo-boost selector, turbosupercharger governor, a nacelle junction box and a main junction box. In a multi-engined airplane, the installation includes one turbo-boost selector and one main junction box, plus one of each of the other units for each engine.
The complete system is operated on the 150V 400Hz AC supplied by the airplane's inverters. Power required is 67VA per engine. Since turbo speed is regulated by opening and closing the turbo waste-gate motor, the control system includes a motor for operating the waste-gate. This motor is automatically controlled by electrical signals from the turbo-boost selector, the Pressuretrol, and the turbo-governor. These signals, amplified and analyzed by the amplifier, control the power delivered to the waste-gate motor. Inasmuch as the induction system pressure is the factor to be controlled, the primary sensing device of the system is an induction-system Pressuretrol, which is actuated by pressure variations at the carburetor intake. A turbo-boost selector is provided to enable the pilot to select any desired safe manifold pressure.
The induction-system Pressuretrol measures electrically the pressure of the air supplied by the turbo to the carburetor. It is essentially a voltage-dividing 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 in a downward direction across the Pressuretrol potentiometer by means of a sector arm and pinion gear, 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 is an intermediate unit between the control units and the waste-gate motor. It receives two kinds of signals from the other control units. One kind calls for rotation to open the waste- gate. After amplifying the signal, the amplifier determines the direction of waste-gate movement called for and controls the power delivered to the waste-gate motor accordingly.
The turbo-control amplifier consists of two voltage amplifier stages and one 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 the grids go positive, it will pass current. At this same instant, however, the plate of the lower tube is negative and, therefore, will 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 up- per tube will pass current and the lower tube will not.
If, however, 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 the amplifier and bridge both powered by the same 115V, 400Hz 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. In addition, the waste-gate motor operates a balancing potentiometer within its own case which serves to neutralize signals from the rest of the system.
When the amplifier field winding of the turbo waste-gate motor is energized, magnetic action on the rotor moves the shaft lengthwise and disengages the braking surfaces, thus allowing the motor to rotate. Therefore, in case of power failure, the waste-gate will be held in whatever position it is in when power failure occurs.
The waste-gate motor is a two-phase reversible 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.
If, however, a voltage signal is applied to the grid which is in phase with the plate voltage of either tube "A" or tube "B" (Fig 3), the voltage thus developed is impressed across the amplifier field winding. The current in the field winding when tube "A" is operating will be 180 electrical degrees out of phase with that when tube "B" is operating. The condenser in the lead to the line-excited field winding has been so chosen that the current in the amplifier—excited field winding either leads or lags the current in the line-excited field winding by approximately 90 electrical degrees, causing motor rotation. Direction of motor rotation is determined by whether current in the amplifier field winding leads or lags the current in the fixed field winding. If tube "A" is operating, motor rotation in one direction is obtained and if tube "B" is operating, the opposite rotation is obtained. Therefore, the phase relationship between the incoming signal voltage from the bridge and the plate voltage of the discriminator tubes in the amplifier determines the direction in which the motor rotates.
The turbo-boost selector, conveniently installed in the cockpit or cabin, is the pilot's control device by which he regulates the operation of the turbo control system. With a given dial setting, the calibration potentiometer can be adjusted to regulate the manifold pressure to the desired value. Once this calibration has been made, it need not be changed for subsequent flights. Adjustments are required only to compensate for small differences in engine and turbo performance.
Equal manifold pressures on a four-engine installation are assured by four small calibrating potentiometers which coordinate the manifold pressures of all engines with the rpms synchronized. The pilot can control the turbo boosts on all four engines simultaneously by turning the large central control potentiometer. This single-knob control serves as a vernier control of power. A vernier knob and dial is utilized to rotate the dial shaft, to which is attached the wiper for the manifold pressure selector potentiometer.
Control of power at high altitudes is most economically affected by change of boost pressures, rather than by throttling. It in no way interferes with the control of power through throttles alone when the pilot desires to obtain more rapid power changes because the electronic control maintains almost constant carburetor inlet pressure.
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 turbo—governor is a dual safety device driven by means of a short flexible drive shaft from the gear-type oil pump of the turbosupercharger tachometer connection. One part of the mechanism, called the overspeed control, prevents the turbo from exceeding its safe operating speed through a gear—driven linkage—operated potentiometer. The other part, the accelerometer, prevents the turbine from accelerating at too high a rate and producing momentary surges of pressure in the induction system in response to abrupt changes of throttle or manifold pressure selector settings. The accelerometer anticipates the pressure increase from turbo acceleration and provides a potentiometer movement to introduce a signal to start opening the waste-gate in time to prevent overshooting of the manifold pressure.
The potentiometers in the control units turbo-boost selector, turbo-governor, and Pressuretrol are interconnected with the balancing potentiometer in the waste-gate motor to form an electrical circuit.
Fig 4 shows a schematic diagram of the turbo control bridge circuit consisting of three distinct sections, "X," "Y," and "Z." Voltage signals produced by any of these three sections either add to or subtract from one another. To obtain a condition of balance, these voltage signals must cancel out so that no voltage appears between the waste-gate-motor wiper (grid) and the turbo-boost- selector wiper (ground).
Fig 5 is a schematic of a balanced control system bridge. Starting with ground as a reference point and working up through the bridge, a 6V potential exists between wipers A and B. B is negative to A. In section "Y," D is six additional volts negative to C (or B), or D is 12V negative to A. F is 12V positive to E (or D), so that by algebraic addition, F and A are at the same potential.
When this condition of "balance" exists, there is no signal to the grid of the amplifier. If wiper F were three volts to the left of its present position, F would be three volts negative to A. But a negative signal causes the waste-gate motor to rotate in such a direction as to close the waste gate, bringing F back to its original position and rebalancing the bridge. Similarly, were wiper F three volts to the right of its present position, the potential of F would be three volts positive to A, causing the waste gate to run toward the open position, rebalancing the bridge.
Waste-gate action results from the movement of any one wiper. If the selector wiper is moved to the right (increased dial setting), a negative signal appears which tends to close the waste-gate. If the calibration screw is moved to the right (calibration screw turned counter-clockwise), a positive voltage signal appears between grid and ground, which tends to open the waste gate.
If the accelerometer wiper is moved to the right (accelerometer action due to turbine acceleration), the resultant positive signal tends to open the waste gate. If the overspeed wiper is moved to the left (overspeed action), the resultant positive signal tends to open the waste-gate. In actual operation, two or more wipers are caused to move simultaneously.
If, with the bridge balanced, the selector dial is turned to a higher setting, the waste gate moves toward a closed position. As this happens, the turbine starts to speed up and increase induction-system pressure. Increasing this pressure moves the Pressuretrol wiper to the left, which in turn calls for the waste-gate to move toward the open position. A new condition of balance results.
The set of curves in Fig 6 outlines typical operating conditions for the turbosupercharger system during a climb from an altitude of 5000 to 32,000 ft.
First, the engine rpm is selected (2300 for this illustration) by setting the propeller governor control. The rpm remains constant throughout the climb because of the action of the automatic propeller governors.
Then the throttle is moved forward to full-open position, and a manifold pressure of 37" is selected by means of the turbo-boost selector. The manifold pressure selected for any given flight depends upon speed of flight desired, rate of climb desired, and airplane load.
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 engine rpm remains constant. Therefore, to maintain a constant manifold pressure, it is necessary to maintain a constant induction-system pres- sure.
The internal combustion engine converts into mechanical power the energy liberated by the combustion of fuel and oxygen. The theoretical or indicated horsepower output of a given engine is represented by:
P = Average pressure applied to piston (psi)
L = Length of piston stroke (ft)
A = Area of piston head (sq in)
N = Number of power strokes per min
K = Number of cylinders in engine
For a given engine operating at any small rpm, the only variable which can affect horsepower output is combustion pressure.
To increase horsepower output, the weight of fuel and air admitted to the cylinder during each cycle must be increased. This can only be accomplished by supercharging.
To maintain a constant induction-system pressure and consequent manifold pressure during a climb, the turbine unit must increase in speed. This increase in speed is represented by the turbo rpm line in Fig 6. The maximum turbine rpm was reached at approximately 30,000 ft and then the overspeed portion of the turbo governor became effective to prevent further increase in speed.
Turbo rpm is a function of the pressure differential across the turbine and compressor wheels, and is controlled by regulating the differential across the turbine wheel by positioning the waste-gate. If induction-system pressure should 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 pressure differential across the turbine wheel; and again a new position of balance occurs in which the waste—gate is slightly more closed. A
s altitude is increased above 30,000 ft, and the turbo reaches rated speed, the turbo discharge pressure, and hence the manifold pressure, drops off quite rapidly. From this it can be seen that for a given airplane, safe turbo speed is really the determining factor which limits the altitude at which the plane may be flown and still obtain full rated horsepower 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 will be obtained when the engine can no longer generate climbing power without exceeding the top safe turbine speed.
The curves shown in Fig 7 are reproduced from photographic records of a test flight made at an altitude of 25,000 ft, and illustrate the operation of the regulator under extreme conditions. The procedure was to retard one throttle fully and wait until the system restabilized or "folded up." The throttle was then rammed to full-open position in less than a second. This is a severe test for any regulator.
As the throttle was retarded, manifold pressure dropped quickly. The propeller governor reestablished prop speed after a period of several seconds. The waste gate went fully closed and the turbo speed dropped.
When the throttle was rammed full open, manifold pressure increased, turbo rpm increased. It was the regulator's job to see that original conditions were reestablished as soon as possible without exceeding safe turbo speed or manifold pressure, and without setting up a "hunt" or other unstable condition.
This article was originally published in the January, 1945, issue of Industrial Aviation magazine, vol 2, no 1, pp 7-11, 100.
In addition to the figures included above, the original article included a drawing showing the schematic layout of the control system.
Figures are not credited.