2. Compressor inlet temperature 3. Compressor discharge pressure 4. Burner pressure 5. Compressor inlet pressure 6. RPM 7. Turbine temperature The more sophisticated fuel controls sense even more operating variables. The fuel control is the   heart   of the gas turbine engine fuel system. This complex device schedules fuel flow to the engine combustion chamber. It automatically provides fuel flow as dictated by the operating conditions of the engine (temperature, pressures, altitude, throttle position, etc.). The fuel control combines the inputs of throttle position, compressor discharge pressure, compressor inlet temperature, and engine speed to produce the fuel flow to operate the engine. The fuel control governs the engine speed by controlling fuel flow. Fuel flow variations are limited to ensure fast stall-free acceleration and deceleration. During throttle bursts, the fuel control also postpones the initiation of the afterburner operation (if installed) to achieve the fastest possible acceleration. LUBRICATION SYSTEM.—The oil lubrication systems of modern gas turbine engines vary in design and plumbing. However, most systems have units that perform similar functions. In a majority of cases, a pressure pump or system furnishes oil to lubricate and cool several parts of the engine. A scavenging system returns the oil to the tank for reuse. Overheating is a problem in gas turbine engines. Overheating is more severe after the engine stops than while it is running. Oil flow, which normally cools the bearings, stops. The heat stored in the turbine wheel now raises the temperature of the bearings much higher than when the engine was running. The oil moves heat away from these bearings to prevent overheating. Most systems include a heat exchanger to cool the oil. Many systems have pressurized sumps and a pressurized oil tank. This equipment ensures a constant head pressure to the pressure lubrication pump to prevent pump cavitation at high altitudes. Oil consumption is relatively low in a gas turbine engine compared to a piston-type engine. Oil consumption in the turbine engine primarily depends upon the efficiency of the seals. However, oil can be lost through internal leakage, and, in some engines, by malfunctioning of the pressurizing or venting system. Oil sealing is very important in a jet engine. Any wetting of the blades or vanes by oil vapor causes accumulation of dust or dirt. Since oil consumption is so low, oil tanks are made small to decrease weight and storage problems. The main parts of the turbine requiring lubrication and cooling are the main bearings and accessory drive gears. Therefore, lubrication of the gas turbine engine is simple. In some engines the oil operates the servomechanism of fuel controls and controls the position of the variable-area exhaust nozzle vanes. Because each engine bearing gets its oil from a metered or calibrated opening, the lubrication system is known as the calibrated type. With few exceptions, the lubricating system is of the dry sump design. This design carries the bulk of the oil in an airframe or engine-supplied separate tank. In the wet sump system, the oil is carried in the engine itself. All gas turbine engine lubrication systems normally use synthetic oil. Figure 6-18 shows components that usually make up the dry sump oil system of a gas turbine engine. IGNITION SYSTEM.—Modern gas turbine engines use high voltage and a spark of high heat intensity. The high-energy, capacitor-discharge type of ignition system provides both high voltage and an exceptionally hot spark. This system assures ignition of the fuel-air mixture at high altitudes. There are two types of capacitor discharge ignition systems. The high-voltage and the low-voltage systems with dc or ac input. The high-voltage system produces a double spark. The double spark is a high-voltage com- ponent. This component ionizes (makes conductive) the gap between the igniter plug electrodes so that the high- energy, low-voltage component may follow. In the low-voltage system, the spark is similar to the high- voltage system, but uses a self-ionizing igniter plug. WARNING Because of the high power in these ignition systems, you must be careful to prevent a lethal electrical shock from capacitors. Always avoid contact with leads, connections, and components until the capacitors have been grounded and are fully discharged. 6-13