Master minding the TURBO

An 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 115V, 400Hz 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 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 turbo-governor.

The induction system pressuretrol 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 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 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 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 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.

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 over-speed 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.
The PDF of this article includes schematics of the wiring of the control system, and a cutaway drawing and a flow diagram of the operation of a turbosupercharger.