Engi ne Systems The Gas Turbine Engine All gas turbi ne engines consist of an air inlet section, a compressor section , the combustion section , the turbine section and the exhaust section . The terminology used to describe gas turbines is a little inconsistent at times. Sometimes the inlet and/or the exhaust sections are considered part of the airframe rather than the engine. At a minimum, a turbine engine has a compressor, combuster and a turbine section. Engines are sometimes divided into cold and hot sections. The cold section is forward of the combuster and the hot section is from the combuster aft. Air enters the inlet at roughly ambient temperature an d pressure. As it passes through the compressor, the pressure increases and so does the temperature due to the heat of compression. Bleed air is tapped off the compressor for such accessories as air conditioning and thermal anti-icing. Often this bleed air is taken from more than one point on the compressor or compressors. The section connecting the compressor and the combustion sections is called the diffuser . In the diffuser, the cross-sectional area of the engine increases. This allows the air stream from the compressor to slow and its pressure to increase. In fact, the highest pressure in the engine is attained at this point. Next, the air enters the combustion section were it is mixed with fuel and the mixture is ignited. There is no need for an ignition system that operates continuously (such as the spark plugs in a piston engine) because the uninterrupted flow of fuel and air will sustain combustion after an initial light-off. The combustion of the fuel-air mixture causes a great increase in volume, and because there is higher pressure at the diffuser, the gas exits through the turbine section. The temperature of the gas rises rapidly as it passes from the front to the rear of the combustion section. It reaches its highest point in the engine at the turbine inlet. The maximum turbine inlet temperature is a major limitation on turbojet performance and without cooling, it could easily reach up to 4,000 F, far beyond the limits of the materials used in the turbine section. To keep the temperature down to an acceptable 1,100 to 1,500 F, surplus cooling air from the compressor is mixed in aft of the burners. The purpose of the turbine(s) is to drive the compressor(s) which are connected by a drive shaft. Since the turbines take energy from the gas, both the temperature and pressure drop. The gases exit the turbine section at very high velocity into the tailpipe. The tailpipe is shaped so that the gas is accelerated even more reaching maximum velocity as it exits into the atmosphere. The four events in the energy release cycle of a gas turbine engine (intake, compression, combustion, and exhaust) are the same as in a piston engine. The difference in the cycle is that when energy is added, the pressure remains relatively constant while the volume increases dramatically, just the reverse of a piston engine. Types of Turb ojet Engines Turbojet engines can be divided into two types based on the type of compressor they employ. The centrifugal-flow engine uses a compressor that is very similar in design to the compressor used in piston engine turbochargers. This was the type of engine found on many early jet aircraft, but it has been almost entirely replaced by Engine Systems 11/10/2009 19:28 Page 1