2.3. FUEL CONTROLS

The principles and operation of fuel controls used on current engines are discussed in this paragraph. Depending upon the type of engine and the performance expected of it, fuel controls may range from simple valves to automatic computing controls containing hundreds of intricate parts.

Strictly speaking, a pilot of a gas-turbine-powered aircraft does not directly control his engine. His command over the engine corresponds to that of the captain of a ship who obtains engine response by relaying orders to an engineer below deck who, in turn, moves the throttle of the engine. But before he moves the throttle, he monitors certain operating pressures, temperatures, and rpm that are not apparent to the captain. The engineering officer then refers to a chart and computes a fuel flow or throttle change which will not allow the engine to exceed its operating limitations. If you think of the pilot as the captain of the ship, then think of the automatic controls as the engineer. They, too, monitor operating pressures, temperatures, and rpm, and make the necessary fuel and throttle adjustments.

Fuel controls can be divided into two basic groups: hydromechanical, and electronic. There are as many variations in controls as there are engines. Although each type of fuel control has its particular advantage, most controls in use today are hydromechanical. Some fuel controls are extremely complex devices composed of speed governors, servo systems, valves, metering systems, and sensing pickups.

This section limits discussion mainly to fuel control theory of the hydromechanical type. A schematic of one is shown in figure 2.2. A fuel control in the simplest form consists of a plain metering valve to regulate fuel flow to the engine. A hydromechanical fuel control consists of the following main components, but it is not limited to only these.

1. Pump to pressurize fuel.
2. Governors to control rpm.
3. Relief valves to protect the control.
4. Manual control systems (emergency control system).
5. Fuel shutoff valve.

Most modern fuel control units meter the flow of fuel by keeping the pressure drop or difference across the metering valve a constant value, while varying the orifice of the metering valve. Another way to control fuel is to keep the valve orifice a constant size and vary the pressure acting upon the fluid. The operation of a gas turbine requires that a number of variable conditions be given careful thought to provide for safe, efficient operation. Among these are engine rpm, acceleration, exhaust gas temperature (egt), compressor inlet temperature, compressor discharge pressure, and throttle or power control setting. All these conditions affect or are affected by fuel flow, which is increased only to the point where the limiting temperature is reached. As the engine accelerates and airflow through the engine increases, more fuel is added. If turbine inlet temperature were the only engine limitation, a temperature pickup sensing this temperature could be used. However, it is also necessary to avoid the operating range that would cause a compressor surge and stall. Because more than one factor limits engine operation, it is necessary to schedule the accelerating fuel in accordance with a combination of these factors. Because turbine engine compressors are susceptible to surges and stalls, a control with a longer acceleration time is used than is needed for a reciprocating engine. This acceleration time is known as a "lag," and the pilot must be aware of the time it takes the engine to accelerate and give him the power change he requires. Compressor discharge pressure or burner pressure is commonly used as the variable for these controls, since they vary both with engine speed and inlet air temperature. By evaluating these variable conditions, a fair indication of the amount of fuel which can be burned without exceeding engine limitations is obtained.

Two fuel control systems are discussed in the following subparagraphs.

• Automatic control system. The amount of fuel required to run the engine at rated rpm varies with the inlet air temperature and pressure. For example, it requires less fuel to run the engine on a hot day than on a cold day. To relieve the pilot of the necessity of resetting the power lever to compensate for changes in outside air temperature and pressure, a speed governor is used. A simple speed governor consists of flyweights balanced by a spring. When the engine is running unloaded, at rated speed, the metering valve is open only far enough to supply the small amount of fuel required. If a load is applied to the engine, the speed decreases. This decrease in rpm causes the flyweights to move in under the force of the spring tension and the fuel valve to open wider and admit more fuel. With the additional fuel, the engine picks up speed again, and, as the rated speed is reached, the flyweights move the fuel valve in the closing direction until the proper steady-state fuel flow is reached.
• Manual (emergency) control system. When the governor control switch in the cockpit is moved from the automatic position to the manual (emergency), a valve is actuated in the fuel control, and fuel is redirected to the manual system metering valve. The throttle in a helicopter is of the motorcycle twist-grip type. When the governor is in the automatic position the throttle is rolled full open and left there, with the fuel control making all fuel-flow changes automatically. If the automatic fuel control fails, the pilot switches to the emergency mode and takes manual control of the throttle, which is mechanically linked to the manual metering valve. The manual throttle control has no compensation for altitude or temperature, and it has no protection against an engine overspeed.

Keep in mind that so far the discussion has been on principles of operation, and any specific fuel control may differ.

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