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The question being answered is: What are typical types of aircraft engines used?
An aircraft engine is the propulsion system for an aircraft. Different propulsion systems develop thrust in different ways, but the principle is based on Newton's third law of motion. In any propulsion system a working fluid is accelerated by the system and a reaction to this acceleration produces force( in this case thrust) on the system. Thrust is the force which moves an aircraft forward through the air.
Types of Aircraft EnginesEdit
All aircraft engines operate by compressing outside air, mixing it with fuel, burning the mixture, and extracting energy from the resulting high pressure hot gases.
Different types of aircraft engines are:
- Reciprocating Engine/Propeller/Piston-Prop
The above engines are also known as Air-breathing engines.
The piston-prop was the first form of aircraft propulsion. This type of engine consists of a piston-cylinder arrangement, and typically operate on four basic strokes viz. intake, compression, combustion and exhaust. During Intake, air-fuel mixture is taken into the cylinder, then it is compressed during compression stroke to raise the pressure. The compressed air-fuel mixture is then ignited which produces tremendous power due to expansion of gases. This is called power stroke. The hot gases are then pushed out of the cylinder during exhaust stroke and then the cycle repeats. Piston engines drive a propeller via a spinning shaft which are used for producing the required force/thrust. This type of engine is shown in the figure Reciprocating Engine
Piston Engine Configurations: The pistons can be arranged in four ways: radial, in-line, oppositional and "V."
In its simplest form, a turboprop consists of an intake, compressor,combustor, turbine and a propelling nozzle as shown in figure TurboProp Engine. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts. The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor. The rest is transmitted through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust, as compared to the propeller, generated by a turboprop.
A turbojet engine contains diffuser, compressor, burner, turbine and nozzle as shown in figure Turbojet Engine. Flow enters the compressor through a diffuser. Rotating compressor blades in the compressor increase the pressure to many times the atmospheric pressure. Compressed air then enters the burner, where it is mixed with fuel and the resulting mixture is ignited. The burned fuel air mixture then expands through a turbine which extracts work from the gas. The turbine is connected to the compressor by a shaft, and the work extracted from the turbine is transmitted via the shaft to operate the compressor. Finally, the gas expands through a nozzle and is exhausted into the air with the exit jet velocity producing the forward thrust. The key part of a jet engine is the exhaust nozzle. This is the part which produces thrust for the jet.
A turbofan engine is shown in the figure Turbofan Engine. The turbojet engine forms the core of a turbofan. However in a turbofan engine, the turbine drives not only the compressor, but also a large fan external to the core. The flow itself is contained in a shroud that is wrapped around the core. The flow through turbofan engine is split into two paths. One passes through the fan and flows externally over the core and the second passes through the core itself. The engine produces thrust both from hot jet leaving the main nozzle and from acceleration of the cold bypass flow outside the engine core. This ratio of the air passing externally over the core to that of the air passing through the core is called the bypass ratio. Bypass ratio ranges from as high as 6 to as low as 0.25.
Selecting Propulsion System Edit
Piston-prop and turboprop engine produces comparably low thrust with great efficiency, a turbojet produces considerably higher thrust with less efficiency, and rocket engine produces tremendous thrust with poor efficiency. The choice of proper propulsion system depends on the mission of the aircraft i.e. what the aircraft must do. In most of the cases the speed of the aircraft limits the choice on the type of the engine.
Various types of engines and their Mach regimes are shown in the figure Mach Values For Engine Type
Piston-prop engines are cheap and have lowest fuel consumption, but they are heavy and produce lot of noise and vibration when operating at Mach number greater than 0.5 or 0.6. Also propellers produce less and less thrust as the velocity increases because of loss in efficiency at high speeds. Hence piston-props are limited to light airplanes and some agricultural aircraft where high speed is not a criterion.
Turbo props use more fuel than piston-props of the same horsepower and have higher initial cost than piston-props. However, because of their light weight and reliability they are prefered for business twins and short-range commuter aircrafts. Piston props and turboprops can operate efficiently below the Mach 0.5 or 0.6 as shown in the figure. As the propeller speed increases, the tips of the blades may approach supersonic speeds. If this happens, the flow may separate and shocks may form, decreasing the air flow into the engine. For these reasons this type of engine is still restricted to lower speeds.
For the flight speeds in the transonic regime—Mach numbers from 0.75 to 0.9—the most common engine configurations are turbofans. The engines with higher bypass ratios (of the order 5) are the class of turbofans that power the civil transports. The performance of these seems to be closer to the propeller than that of a turbojet in terms of high engine efficiency and low fuel consumption.
For low bypass ratio turbofans (between 0 and 1) the performance is somewhat closer to that of a turbojet than that of propeller. They can produce high thrusts at the expense of engine efficiency i.e. at low engine efficiency and high fuel consumption. For fighter planes and high speed aircrafts engine efficiency and fuel consumption are not as important as very high thrust, as they are required to accelerate quickly during their flight.
The turbojet is the most inefficient engine when compared to the turboprop and the turbofan but produces high thrusts. It consumes more fuel than its counterparts but the range of velocities at which is operated cannot be achieved by the turboprop or the turbofan. The top operational velocity is the vicinity of Mach 3.0. Military fighters and fast business jets use turbojet engines.
Engine Operational EnvelopesEdit
Any given engine type will operate only within a certain range of altitudes and Mach numbers (velocities), this is called engine operational envelope.Figure Flight Limits shows the approximate velocity and altitude limits, or corridor of flight, within which airlift vehicles can operate. The corridor is bounded by a lift limit, a temperature limit, and an aerodynamic force limit. The lift limit is determined by the maximum level-flight altitude at a given velocity. The temperature limit is set by the structural thermal limits of the material used in construction of the aircraft. At any given altitude, the maximum velocity attained is temperature-limited by aerodynamic heating effects. At lower altitudes, velocity is limited by aerodynamic force loads rather than by temperature.The operating regions of all aircraft should lie within the flight corridor.
1.Aircraft Performance and design by John D.Anderson,Jr.
2.Aircraft Design: A Conceptual Approach, Fourth Edition by Daniel P. Raymer.
3.Mechanics and Thermodynamics of Propulsion, Second Edition by Philip Hill and Carl Peterson.