propulsion mod 1

8/13/2019 Propulsion Mod 1

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PROPULSION


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Propulsion is a means of creating force leading tomovement.

A propulsion system has a source of mechanical

power (some type of engine or motor, muscles), andsome means of using this power to generate force,such as wheel and axles, propellers, a propulsivenozzle, wings, fins or legs.

The term propulsion is derived from two Latin words:pro meaning before or forwards and pellere meaningto drive.


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Air propulsion

An aircraft propulsion system generally consists of an aircraft

engine and some means to generate thrust, such as a

propeller or a propulsive nozzle.

An aircraft propulsion system must achieve two things. First,

the thrust from the propulsion system must balance the drag

of the airplane when the airplane is cruising. And second, the

thrust from the propulsion system must exceed the drag of

the airplane for the airplane to accelerate. In fact, the greater

the difference between the thrust and the drag, called theexcess thrust, the faster the airplane will accelerate.


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Some aircraft, like airliners and cargo planes, spend most of

their life in a cruise condition. For these airplanes, excess

thrust is not as important as high engine efficiency and low

fuel usage. Since thrust depends on both the amount of gas

moved and the velocity, we can generate high thrust byaccelerating a large mass of gas by a small amount, or by

accelerating a small mass of gas by a large amount. Because of

the aerodynamic efficiency of propellers and fans, it is more

fuel efficient to accelerate a large mass by a small amount.That is why we find high bypass fans and turboprops on cargo

planes and airliners.


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Some aircraft, like fighter planes or experimental high speed

aircraft, require very high excess thrust to accelerate quickly

and to overcome the high drag associated with high speeds.

For these airplanes, engine efficiency is not as important as

very high thrust. Modern military aircraft typically employafterburners on a low bypass turbofan core. Future

hypersonic aircraft will employ some type of ramjet or rocket

propulsion.


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Spacecraft propulsion

Spacecraft propulsion is any method used to accelerate

spacecraft and artificial satellites. most spacecraft today are

propelled by forcing a gas from the back/rear of the vehicle at

very high speed through a supersonic de Laval nozzle. This

sort of engine is called as rocket engine.


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AIRCRAFT ENGINE

An aircraft engine is the component of the propulsion system

for an aircraft that generates mechanical power. Aircraft

engines are almost always either lightweight piston engines or

gas turbines.

Reciprocating (piston) engines

Jet engines


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Reciprocating (piston) engines

V-type engine


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In-line engine


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horizontally opposed air-cooled aero

engine


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Rotary engine


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Jet engine

A jet engine is a reaction engine discharging a fast

moving jet that generates thrust by jet propulsion in

accordance with Newton's laws of motion. This

broad definition of jet engines includes turbojets,turbofans, rockets, ramjets, and pulse jets. In

general, jet engines are combustion engines but non-

combusting forms also exist.


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The term jet engine loosely refers to an internal combustionair breathing jet engine (a duct engine). These typically consist

of an engine with a rotary (rotating) air compressor powered

by a turbine ("Brayton cycle"), with the leftover power

providing thrust via a propelling nozzle. Jet aircraft use these

types of engines for long-distance travel. Early jet aircraft used

turbojet engines which were relatively inefficient for subsonic

flight. Modern subsonic jet aircraft usually use high-bypass

turbofan engines. These engines offer high speed and greaterfuel efficiency than piston and propeller aero engines over

long distances.


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Jet engine types

Air breathing engine

Non Air breathing engine


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Commonly aircraft are propelled by air breathing jet

engines. Most air breathing jet engines that are in

use are turbofan jet engines which give good

efficiency at speeds just below the speed of sound.


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Turbine powered

Gas turbines are rotary engines that extract energy from a

flow of combustion gas. They have an upstream compressor

coupled to a downstream turbine with a combustion

chamber in-between. In aircraft engines, those three core

components are often called the "gas generator.

Turbojet

Turbofan

Turboprop and turboshaft

Propfan


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Ram powered (non-Turbine powered)

Ram powered jet engines are air breathing enginessimilar to gas turbine engines and they both followthe Brayton cycle. Gas turbine and ram poweredengines differ, however, in how they compress the

incoming airflow. Whereas gas turbine engines useaxial or centrifugal compressors to compressincoming air, ram engines rely only on aircompressed through the inlet or diffuser. Ram

powered engines are considered the most simpletype of air breathing jet engine because they cancontain no moving parts.


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Types

Pulsejet

Ramjet

Scramjet


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Turbojet engine

A turbojet engine is a gas turbine engine that works by

compressing air with an inlet and a compressor (axial,

centrifugal, or both), mixing fuel with the compressed air,

burning the mixture in the combustor, and then passing the

hot, high pressure air through a turbine and a nozzle. Thecompressor is powered by the turbine, which extracts

energy from the expanding gas passing through it. The

engine converts internal energy in the fuel to kinetic energy

in the exhaust, producing thrust. All the air ingested by theinlet is passed through the compressor, combustor, and

turbine, unlike the turbofan engine


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Turbofan

A turbofan engine is a gas turbine engine that is very similar

to a turbojet. Like a turbojet, it uses the gas generator core

(compressor, combustor, turbine) to convert internal energy in

fuel to kinetic energy in the exhaust. Turbofans differ from

turbojets in that they have an additional component, a fan.Like the compressor, the fan is powered by the turbine section

of the engine. Unlike the turbojet, some of the flow

accelerated by the fan bypasses the gas generator core of the

engine and is exhausted through a nozzle. The bypassed flowis at lower velocities, but a higher mass, making thrust

produced by the fan more efficient than thrust produced by

the core. Turbofans are generally more efficient than

turbojets at subsonic speeds, but they have a larger frontal

area which generates more drag.


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There are two general types of turbofan engines, lowbypass and high bypass. Low bypass turbofans have abypass ratio of around 2:1 or less, meaning that for eachkilogram of air that passes through the core of the engine,two kilograms or less of air bypass the core. Low bypass

turbofans often used a mixed exhaust nozzle meaning thatthe bypassed flow and the core flow exit from the samenozzle. High bypass turbofans have larger bypass ratios,sometimes on the order of 5:1 or 6:1. These turbofans canproduce much more thrust than low bypass turbofans or

turbojets because of the large mass of air that the fan canaccelerate, and are often more fuel efficient than lowbypass turbofans or turbojets.


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Turboprop engines Turboprop engines are jet engine derivatives, still gas

turbines, that extract work from the hot-exhaust jet to turna rotating shaft, which is then used to produce thrust bysome other means. While not strictly jet engines in thatthey rely on an auxiliary mechanism to produce thrust,turboprops are very similar to other turbine-based jet

engines, and are often described as such. In turboprop engines, a portion of the engines' thrust is

produced by spinning a propeller, rather than relying solelyon high-speed jet exhaust. As their jet thrust is augmentedby a propeller, turboprops are occasionally referred to as a

type of hybrid jet engine. While many turboprops generatethe majority of their thrust with the propeller. Turbopropsgenerally have better performance than turbojets orturbofans at low speeds where propeller efficiency is high,but become increasingly noisy and inefficient at high

speeds.


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Turboshaft

Turbo shaft engines are very similar to turboprops, differingin that nearly all energy in the exhaust is extracted to spinthe rotating shaft, which is used to power machinery ratherthan a propeller, they therefore generate little to no jetthrust and are often used to power helicopters. Turboshaft

engines are used primarily for helicopters and auxiliarypower units. A turboshaft engine is very similar to aturboprop, with a key difference: In a turboprop thepropeller is supported by the engine and the engine isbolted to the airframe. In a turboshaft, the engine does not

provide any direct physical support to the helicopter'srotors. The rotor is connected to a transmission, whichitself is bolted to the airframe, and the turboshaft enginesimply feeds the transmission via a rotating shaft.


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Propfan

A propfan engine (also called "unducted fan", "open rotor",or "ultra-high bypass") is a jet engine that uses its gasgenerator to power an exposed fan, similar to turbopropengines. Like turboprop engines, propfans generate most oftheir thrust from the propeller and not the exhaust jet. The

primary difference between turboprop and propfan designis that the propeller blades on a propfan are highly swept toallow them to operate at speeds around Mach 0.8, which iscompetitive with modern commercial turbofans. Theseengines have the fuel efficiency advantages of turboprops

with the performance capability of commercial turbofans.While significant research and testing (including flighttesting) has been conducted on propfans, no propfanengines have entered production.


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Pulse jets

Pulse jets are mechanically simple devices thatin arepeating cycledraw air through a no-return valve atthe front of the engine into a combustion chamber andignited it. The combustion forces the exhaust gases outthe back of the engine. It produces power as a series ofpulses rather than as a steady output, hence the name.The only application of this type of engine was theGerman unmanned V1 flying bomb of World War II.Though the same engines were also used

experimentally for ersatz fighter aircraft, the extremelyloud noise generated by the engines causedmechanical damage to the airframe that was sufficientto make the idea unworkable.


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Ramjet

Ramjets are the most basic type of ram powered jetengines. They consist of three sections; an inlet tocompress incoming air, a combustor to inject and combustfuel, and a nozzle to expel the hot gases and producethrust. Ramjets require a relatively high speed to efficiently

compress the oncoming air, so ramjets cannot operate at astandstill and they are most efficient at supersonic speeds.A key trait of ramjet engines is that combustion is done atsubsonic speeds. The supersonic incoming air isdramatically slowed through the inlet, where it is then

combusted at the much slower, subsonic, speeds. Thefaster the incoming air is, however, the less efficient itbecomes to slow it to subsonic speeds. Therefore ramjetengines are limited to approximately Mach 5.


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S j


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Scramjet Scramjets are mechanically very similar to ramjets. Like a

ramjet, they consist of an inlet, a combustor, and a nozzle.

The primary difference between ramjets and scramjets isthat scramjets do not slow the oncoming airflow tosubsonic speeds for combustion, they use supersoniccombustion instead. The name "scramjet" comes from"supersonic combusting ramjet." Since scramjets use

supersonic combustion they can operate at speeds aboveMach 6 where traditional ramjets are too inefficient.Another difference between ramjets and scramjets comesfrom how each type of engine compresses the oncomingair flow: while the inlet provides most of the compressionfor ramjets, the high speeds at which scramjets operateallow them to take advantage of the compressiongenerated by shock waves, primarily oblique shocks. Veryfew scramjet engines have ever been built and flown. InMay 2010 the Boeing X-51 set the endurance record for thelongest scramjet burn at over 200 seconds


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