Powerplant Flashcards
Describe the major components of a gas turbine engine.
An air inlet, compressor section, combustion section, turbine section, exhaust section, accessory section, the systems necessary for starting, lubricating, fuel supply, and auxiliary purposes, such as anti icing, cooling, and pressurization
Turbine engines are classified according to the type of compressors they use. What are the three types if compressors found in turbine engines?
Centrifugal flow, axial flow, and centrifugal-axial flow
Describe a centrifugal-flow compressor
The compressor has an impeller surrounded by a ring of diffuser vanes. The impeller is driven at high speed by a turbine. Air is drawn into the air inlet and directed to the center of the impeller. The air is then forced centrifugally outward into a diffuser, where the pressure of the air is increased. The pressurized air is then supplied to the combustion section
What is the main function of the diffuser section of a turbine engine?
The diffuser is the divergent section of the engine after the compressor and before the combustion section. It has the all important function of reducing high velocity compressor discharge air to a slower velocity at increased pressure. This prepares the air for entry into the flame burning area of the combustion so that the flame combustion can burn continuously
Describe an axial-flow compressor
The axial flow compressor consists of two main elements, a rotor and a stator. The rotor, turning at high speeds, has blades fixed on a spindle that takes in air at the compressor inlet and impels it rearward through a series of stages, paralleling the longitudinal axis of the engine. The action of the rotor increases the compression of the air at each stage, accelerating it rearward through several stages. With this increased velocity, energy is transferred from the compressor to the air in the form of velocity energy. The stator blades act as diffusers at each stage, partially converting high velocity to pressure. Each consecutive pair of rotor and stator blades constitutes a pressure stage; the greater the number of stages, the higher the compression ratio. Most present day engines utilize of to 16 stages.
Explain the function of stator vanes.
The function of the stator vanes is to receive air from the air inlet duct or from each preceding stage and increase the pressure of the air and deliver it to the next stage at the correct velocity and pressure. They also control the direction of air to each rotor stage to obtain the maximum possible compressor blade efficiency.
Explain the operation of a centrifugal-axial flow compressor.
Utilizes both kinds of compressors to achieve the desired compression. A typical free power turbine engine has two independent counter-rotating turbines. One turbine drives the compressor, while the other drives the propeller through a reduction gearbox. The compressor stages combined with a single centrifugal compressor stage. The axial and centrifugal stages are assembled on the same shaft and operate as a single unit.
What are the four types of gas turbine engines?
- Turbojet
- Turbofan
- Turboprop
- Turboshaft
Explain a Turbojet.
Consists of a compressor, combustion chamber, turbine section, and exhaust section. The compressor section passes inlet air at a high rate of speed to the combustion chamber, which contains the fuel inlet and igniter for combustion. The expanding air drives a turbine, which is connected by a shaft to the compressor, sustaining engine operation. The accelerated exhaust gases from the engine provide thrust. Turbojet engines are limited in range and endurance. They are slow to respond to throttle applications at slow compressor speeds.
Explain a Turbofan.
Developed to combine the best features of the turbojet and turboprop. Turbofan engines create additional thrust by diverting secondary airflow around the combustion chamber. The turbofan bypass air generates increased thrust, cooks the engine, and aids in exhaust noise suppression and provides, turbojet type cruse speed and lower fuel consumption. The inlet air that passes through a turbofan engine is usually divided into two separate streams of air. One stream passes through the engine core, while the second bypasses the engine core. A turbofans bypass ratio refers to the ratio of the mass airflow that passes through the fan divided by the mass airflow that passes through the engine core.
Explain a Turboprop.
A turbine engine that drives a propeller through a reduction gear. Exhaust gases drive a power turbine connected by a shaft that drives the reduction gear assembly. Reduction gearing is necessary in turboprops because optimum propeller performance is achieved at much slower speeds than the engines operating RPM. Turboprop engines are most efficient at speeds between 250 and 400 mph and altitudes between 18,000 and 30,000 feet. They also perform well at the slow airspeeds required for takeoff and landing are fuel efficient. The minimum specific fuel consumption of the turboprop engine is normally available in the altitude range of 25,000 feet to the tropopause.
Explain a Turboshaft.
Delivers power to a shaft that drives something other than a propeller. The biggest difference between a turbojet and turboshaft engine is that on a turboshaft engine, most of the energy produced by the expanding gases is used to a drive a turbine rather than produce thrust. Many helicopters use a turboshaft gas turbine engine. In addition, turboshaft engines are widely used as auxiliary power units on large aircraft.
Explain the term engine pressure ratio (EPR).
EPR is the ratio of turbine discharge to compressor inlet pressure. Pressure measurements are recorded by probes installed in the engine inlet and at the exhaust. Once collected, the date is seen to a differential pressure transducer, which is indicated on a flight deck EPR gauge. An EPR gauge is used to indicate the power output of a turbojet/turbofan engine.
Define EGT.
Exhaust gas temperature - The temperature of the exhaust gases as they enter the tail pipe, after passing through the turbine.
Define TIT.
Turbine inlet temperature - The temperature of the gases from the combustion section of the engine as they enter the first stage of the turbine.
Define ITT.
Interstate turbine temperature - The temperature of the gases between the high pressure and low pressure turbine wheels
Define TOT.
Turbine outlet temperature - like EGT, temperature is taken aft of the turbine wheel(s)
What is the N1 indicator providing?
Represents the rotational speed of the low pressure compressor and is presented on the indicator as a percentage of design RPM. After start, the speed of the low pressure compressor is governed by the N1 turbine wheel. The N1 turbine wheel is connected to the low pressure compressor through a concentric shaft.
What is the N2 indicator providing?
Represents the rotational speed of the high pressure compressor and is presented on the indicator as a percentage of design RPM.
Why is there sometimes an N3 indicator in turbofan engines?
A triple spool engine would have an N3 indicator. This is a turbofan engine design where the fan is the N1 compressor followed but the N2 intermediate compressor and then the N3 high pressure compressor, all of which rotate on separate shafts at different speeds.
Where are the highest temperature located in a turbine engine?
Occurs at the turbine inlet. TIT is therefore usually the limiting factor in turbine engine operation.
Explain the factors that would affect the thrust output of a turbine engine.
- Air density
- Increasing temperature
- Humidity
How does an air turbine starter accomplish turning the high pressure compressor?
An axial flow turbine that turns a drive coupling through a reduction gear train and a starter clutch mechanism. The air to operate the air turbine starter is supplied from either a ground operated air cart, the APU, or a cross bleed start from an engine already operating.
What are igniters?
A typical gas turbine engine is equipped with igniters that provide a high heat intensity spark used to ignite the fuel air mixture. A typical ignition system includes two exciter units, two transformers, two intermediate ignition leads, and two high tension leads. As a safety factor, the ignition system is actually a dual system, designed to fire to igniter plugs. This type of ignition system provides a high degree of reliability under widely varying conditions of altitude, atmospheric pressure, temperature, fuel vaporization, and input voltage.
