| Afterburner | is a section added between the turbine and the nozzle of an aircraft turbine engine where additional fuel is injected into the oxygen-rich combustion gases leaving the turbine. As a result of this added energy, the exhaust gases leave at a higher velocity, providing extra thrust for short takeoffs or combat conditions.
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| Air-standard assumptions | reduce the analysis of gas power cycles to a manageable level by utilizing the following approximations: - The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.
- All the processes that make up the cycle are internally reversible.
- The combustion process is replaced by a heat-addition process from an external source.
- The exhaust process is replaced by a heat rejection process that restores the working fluid to its initial state.
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| Air-standard cycle | is a cycle for which the air-standard assumptions are applicable.
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| Autoignition | is the premature ignition of the fuel that produces an audible noise, which is called engine knock.
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| Back work ratio | is the ratio of the compressor work to the turbine work in gas-turbine power plants.
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| Bore | is the diameter of a piston.
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| Bottom dead center (BDC) | is the position of the piston when it forms the largest volume in the cylinder.
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| Brayton cycle | was first proposed by George Brayton around 1870. It is used for gas turbines, which operate on an open cycle, where both the compression and expansion processes take place in rotating machinery. The open gas-turbine cycle can be modeled as a closed cycle by utilizing the air-standard assumptions. The combustion process is replaced by a constant-pressure heat-addition process from an external source, and the exhaust process is replaced by a constant-pressure heat-rejection process to the ambient air. The ideal Brayton cycle is made up of four internally reversible processes: - 1-2 Isentropic compression (in a compressor)
- 2-3 Constant pressure heat addition
- 3-4 Isentropic expansion (in a turbine)
- 4-1 Constant pressure heat rejection
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| Brayton cycle with regeneration | is the Brayton cycle modified with a regenerator (a counterflow heat exchanger) to allow the transfer of heat to the high pressure air leaving the compressor from the high-temperature exhaust gas leaving the turbine.
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| Clearance volume | is the minimum volume formed in the cylinder when the piston is at top dead center.
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| Cold-air-standard assumption | combines the air-standard assumptions with the assumption that the air has constant specific heats whose values are determined at room temperature (25°C, or 77°F).
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| Compression-ignition (CI) engines | are reciprocating engines in which the combustion of the air-fuel mixture is self-ignited as a result of compressing the mixture above its self-ignition temperature.
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| Compression ratio | r of an engine is the ratio of the maximum volume formed in the cylinder to the minimum (clearance) volume. Notice that the compression ratio is a volume ratio and should not be confused with the pressure ratio.
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| Cutoff ratio rc | is the ratio of the cylinder volumes after and before the combustion process in the Diesel cycle.
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| Diesel cycle | is the ideal cycle for compress-ignition reciprocating engines, and was first proposed by Rudolf Diesel in the 1890s. Using the air-standard assumptions, the cycle consists of four internally reversible processes: - 1-2 Isentropic compression,
- 2-3 Constant pressure heat addition,
- 3-4 Isentropic expansion,
- 4-1 Constant volume heat rejection.
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| Displacement volume | is the volume displaced by the piston as it moves between top dead center and bottom dead center.
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| Dual cycle | is the ideal cycle which models the combustion process in both gasoline and diesel engines as a combination of two heat-transfer processes, one at constant volume and the other at constant pressure.
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| Gas power cycles | are cycles where the working fluid remains a gas throughout the entire cycle. Spark-ignition automobile engines, diesel engines, and conventional gas turbines are familiar examples of devices that operate on gas cycles.
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| Ericsson cycle | is made up of four totally reversible processes: - 1-2 T = constant expansion (heat addition from the external source)
- 2-3 P = constant regeneration (internal heat transfer from the working fluid to the regenerator)
- 3-4 T = constant compression (heat rejection to the external sink)
- 4-1 P = constant regeneration (internal heat transfer from the regenerator back to the working fluid)
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| Exhaust valve | is the exit through which the combustion products are expelled from the cylinder.
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| External combustion engines | are engines in which the fuel is burned outside the system boundary.
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| Fan-jet engine | (see turbofan engine)
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| Four-stroke | internal combustion engines are engines in which the piston executes four complete strokes (two mechanical cycles) within the cylinder, and the crankshaft completes two revolutions for each thermodynamic cycle.
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| Heat engines | are devices designed for the purpose of converting other forms of energy (usually in the form of heat) to work.
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| Ideal cycle | is an actual cycle stripped of all the internal irreversibilities and complexities. The ideal cycle resembles the actual cycle closely but is made up totally of internally reversible processes.
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| Intake valve | is an inlet through which the air or air-fuel mixture is drawn into the cylinder.
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| Intercooling | is a technique used to reduce the compression work for the gas turbine cycle. The compression process is completed in stages while cooling the working fluid between stages. Since the steady-flow compression work is proportional to the specific volume of the flow, the specific volume of the working fluid should be as low as possible during a compression process.
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| Internal combustion engines | are engines where the energy is provided by burning a fuel within the system boundaries.
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| Jet-propulsion cycle | is the cycle used in aircraft gas turbines. The ideal jet-propulsion cycle differs from the simple ideal Brayton cycle in that the gases are not expanded to the ambient pressure in the turbine. Instead, they are expanded to a pressure such that the power produced by the turbine is just sufficient to drive the compressor and the auxiliary equipment. The gases that exit the turbine at a relatively high pressure are subsequently accelerated in a nozzle to provide the thrust to propel the aircraft.
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| Knock, or engine knock | is the audible noise occurring in the engine because of autoignition, the premature ignition of the fuel.
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| Mean effective pressure MEP | is a fictitious pressure that, if it acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle. The mean effective pressure can be used as a parameter to compare the performances of reciprocating engines of equal size. The engine with a larger value of MEP will deliver more net work per cycle and thus will perform better.
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| Multistage compression with intercooling | requires the compression process in a compressor to be carried out in stages and to cool the gas in between each stage such that the work required to compress a gas between two specified pressures can be decreased.
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| Multistage expansion with reheating | requires the expansion process in a turbine be carried out in stages and reheating the gas between the stages such that the work output of a turbine operating between two pressure levels can be increased.
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| Octane rating | of a fuel is a measure of the engine knock resistance of a fuel.
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| Otto cycle | is the ideal cycle for spark-ignition reciprocating engines. It is named after Nikolaus A. Otto, who built a successful four-stroke engine in 1876 in Germany using the cycle proposed by Frenchman Beau de Rochas in 1862. The ideal Otto cycle, which closely resembles the actual operating conditions, utilizes the air-standard assumptions. It consists of four internally reversible processes: - 1-2 Isentropic compression
- 2-3 Constant volume heat addition
- 3-4 Isentropic expansion,
- 4-1 Constant volume heat rejection.
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| Pressure ratio | is the ratio of final to initial pressures during a compression process.
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| Propjet engine | is a turbojet engine in which the shaft work is used to drive the propeller.
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| Propulsive efficiency | of an aircraft turbojet engine is the ratio of the power produced to propel the aircraft and the thermal energy of the fuel released during the combustion process.
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| Propulsive power | is the power developed from the thrust of the aircraft gas turbines and is the propulsive force (thrust) times the distance this force acts on the aircraft per unit time, that is, the thrust times the aircraft velocity.
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| Ramjet engine | is a properly shaped duct with no compressor or turbine, and is sometimes used for high-speed propulsion of missiles and aircraft. The pressure rise in the engine is provided by the ram effect of the incoming high-speed air being rammed against a barrier. Therefore, a ramjet engine needs to be brought to a sufficiently high speed by an external source before it can be fired.
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| Regeneration | is a process during which heat is transferred to a thermal energy storage device (called a regenerator) during one part of the cycle and is transferred back to the working fluid during another part of the cycle.
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| Regenerator effectiveness | is the extent to which a regenerator approaches an ideal regenerator and is defined as the ratio of the heat transfer to the compressor exit gas to the maximum possible heat transfer to the compressor exit gas.
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| Reheating | is a technique used to increase the expansion work for the gas turbine cycle. The expansion process is completed in stages while reheating the working fluid between stages. Since the steady-flow compression work is proportional to the specific volume of the flow, the specific volume of the working fluid should be as large as possible during a expansion process.
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| Rocket | is a device where a solid or liquid fuel and an oxidizer react in the combustion chamber. The high-pressure combustion gases are then expanded in a nozzle. The gases leave the rocket at very high velocities, producing the thrust to propel the rocket.
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| Scramjet engine | is essentially a ramjet in which air flows through at supersonic speeds (speeds above the speed of sound).
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| Spark-ignition (SI) engines | are reciprocating engines in which the combustion of the air-fuel mixture is initiated by a spark plug.
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| Stirling cycle | is made up of four totally reversible processes: - 1-2 T constant expansion (heat addition from the external source)
- 2-3 v constant regeneration (internal heat transfer from the working fluid to the regenerator)
- 3-4 T constant compression (heat rejection to the external sink)
- 4-1 v constant regeneration (internal heat transfer from the regenerator back to the working fluid)
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| Stroke | is the distance between the top dead center and the bottom dead center and is the largest distance that the piston can travel in one direction within a cylinder.
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| Thermal efficiency ηth | is the ratio of the net work produced by a heat engine to the total heat input, ηth = Wnet/Qin.
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| Thrust | is the unbalanced force developed in a turbojet engine that is caused by the difference in the momentum of the low-velocity air entering the engine and the high-velocity exhaust gases leaving the engine, and it is determined from Newton's second law.
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| Top dead center TDC | is the position of the piston when it forms the smallest volume in the cylinder.
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| Turbine firing temperature | (see turbine inlet temperature)
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| Turbine inlet temperature | (turbine firing temperature) is the temperature of the working fluid at the turbine inlet. Increasing the turbine inlet temperature has been the primary approach taken to improve gas-turbine efficiency. These increases have been made possible by the development of new materials and the innovative cooling techniques for the critical components such as coating the turbine blades with ceramic layers and cooling the blades with the discharge air from the compressor or injected steam.
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| Turbofan (or fan-jet) engine | is the most widely used engine in aircraft propulsion. In this engine a large fan driven by the turbine forces a considerable amount of air through a duct (cowl) surrounding the engine. The fan exhaust leaves the duct at a higher velocity, enhancing the total thrust of the engine significantly. A turbofan engine is based on the principle that for the same power, a large volume of slower-moving air will produce more thrust than a small volume of fast-moving air. The first commercial turbofan engine was successfully tested in 1955.
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| Turboprop engine | uses propellers powered by the aircraft turbine to produce the aircraft propulsive power.
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| Two-stroke engines | execute the entire cycle in just two strokes: the power stroke and the compression stroke.
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