Jet Engine 4p4055

JET ENGINE PROPULSION IN AIRCRAFT

MAYANK BHARDWAJ 8MA1

HISTORY OF JET ENGINES • The first jet engine was built by Egyptian scientists during 100 B.C.

• The device was known as Aeolipile and used steam power directed through two nozzles to cause a sphere to spin rapidly on its axis

• It was not used for supplying mechanical power and was simply considered a curiosity

• Dr. Hans von Ohain and Sir Frank Whittle were the pioneers behind today’s jet engines.

INTRODUCTION • Jet engine is also called Gas Turbine Engine. • It works under the principle of Newton’s third law which states that “For every acting force there is an equal and opposite force.’’

MAIN COMPONENTS OF JET ENGINE

FAN COMPRESSOR COMBUSTOR TURBINE

MIXER NOZZLE

WHAT IS A JET ENGINE ? • A jet engine is a machine designed for the purpose of creating large volumes of high velocity exhaust gases. • This is done in order to overcome the aerodynamic drag of an airplane. • In the process of producing high velocity exhaust, the engine also produces:  Electrical Power  Hydraulic Power  Pneumatic power for air-conditioning & pressurization  Hot Air for anti-icing protection

BASIC OPERATION OF A JET ENGINE The basic operations of Jet Engine are as follows: • • • • •

Air enters the compressor where it is compressed. Fuel is then added and ignited. The resulting gas spins the gas turbine. The turbine powers the compressor. The gas then exists the engine at tailpipe.

The way a jet engine operates is similar To the way an automobile engine operates i.e. • • • •

Intake Compression Ignition exhaust

STRUCTURE OF JET ENGINE

STRUCTURE OF JET ENGINE • The engine shown below is known as Whittle Type Engine, since it follows the original design features developed by Sir Frank Whittle in the 1930’s. • The first flight of a jet engine of his design was in 1941.

• All engines in use on today's commercial jet aeroplanes have been developed based on this original design.

TYPES OF JET ENGINES • • • •

Turbojet Turbofan Turboprop Turbo shaft

Pratt Whitney Turbojet Engine

Pratt-Whitney Turbofan Engine

TURBOJET ENGINE • The turbojet engine is a reaction engine. • A turbojet engine works by compressing air in compressor, mixing fuel with the compressed air, burning the mixture in the combustor, and then ing the hot, high pressure air through a turbine and a nozzle. • Substantial increases in thrust can be further obtained by employing an afterburner

• Used in fighter planes, and were used in the Concorde.

TURBOPROP ENGINE • A turboprop engine is a jet engine with a propeller attached in front. • Majority of their thrust as in turbofan is obtained from propeller. However, it is efficient only up to certain height and speed.

TURBOFAN ENGINE • A turbofan engine is a gas turbine engine which is similar to a turbojet. Turbofans differ from turbojets as they have an additional component, a fan. • Fan in the engine produces as high as it 70-80 % of the total engine thrust without increasing fuel consumption because the fan air can exit separately from the core engine in other words 70-80% of the air is byed from core engine. • It achieves this by increasing the total air-mass flow and reducing the velocity within the same total energy supply.

TURBO SHAFT ENGINE Turboshaft engines are very similar to turboprops, with a difference that nearly all energy in the exhaust is extracted to spin the shaft. They therefore generate little to NO jet thrust.

This engine is used to drive shaft which in turn provides power to rotate helicopter rotor. The engine also has provision through Gear Box mechanism to maintain rotor speed constant even when the speed of the generator is varied .

STRUCTURE OF JET ENGINE – SINGLE SPOOL The earliest commercial jet engine airplanes used a Single Spool turbojet engine like shown below.

STRUCTURE OF JET ENGINE – SINGLE SPOOL  The term Single Spool refers to the fact that there is only one Shaft.  This shaft connects the Turbine section to one compressor section.  All Jet engines in current use are Axial Flow Engines meaning that the compression phases is done axially (parallel to the axis of the engine) as the airflows through the compressor.

Thermodynamic Cycles Through a Jet Engine (Similar to a 4 Stroke Engine)

(suck)

(squeeze) (bang)

(whoosh!)

STRUCTURE OF JET ENGINE – TWIN SPOOL • The first significant development after the

introduction of the early axial-flow, single-spool turbojets was the introduction of a second shaft.

• This second shaft allowed the engine to have two independent stages of compression powered by two independent turbines.

STRUCTURE OF JET ENGINE – TWIN SPOOL • The first stage of compression is the low-speed rotor and the second stage is the high-speed rotor. These refer to the fact that the first stage of the turbine, which rotates the second stage compressor, turns at a faster rate than the second stage turbine/ first stage compressor.

• These are often referred to as the N1 rotor (low-speed) and the N2 rotor (high-speed).

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Turbofan Engine • The by ratio is the ratio of the air which exits the engine

without going through the rest of the engine core compared to the amount of air which goes through the engine core (the primary flow). Each of these produces thrust. • Turbofan engines produce lower noise levels than earlier engines, and have considerably improved fuel economy.

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High By Ratio Turbofan Engine • Early turbofan engines were “Low-By ratio” engines. Approximately ½ of the thrust was produced by the fan stage, and the other half by the primary flow. • Engines currently in production for most commercial airplanes are all high-by ratio turbofans. The difference is that these engines have a much higher ratio of by air compared to the primary air. • In a typical turbofan engine with by ratios around 5:1 and higher, the fan stage provides about 75 to 80 percent of the total thrust produced by the engine. • All bigger aero planes operating in India with M/s Air India. Jet Airways, Indigo, Spice jet are powered by Turbofan Engines.

High By Ratio Turbo Fan Engine High by ratio engines take in a large amount of air and accelerate it only a small amount (relative to low by ratio engines).

High By Ratio Turbofan Engine

High By Ratio Turbo Fan Engines - few examples P & W PW 4084 engine: By ratio - 6.8:1

Rolls Royce Trent 800 engine by ratio - 6.5 :1 P1, Page 16

High By Ratio Turbo Fan Engines –

Another example

GE-90B engine By ratio: 9:1

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High By Ratio Turbo Fan Engines

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Triple Spool Turbofan Engines Designed to achieve better fuel economy due to the ability of the triplespool design to better match the design of the compressors and turbines to the airflow. RR engines now in production use a triple-spool design incorporating three independent rotors.

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Propulsive Efficiency The propulsive efficiency of an engine can be expressed in of the inlet velocity of the air and the exhaust velocity.

ῃp =

2XVinlet (Vinlet + Vexit)

Vinlet

Vexit Jet Engine

Propulsive Efficiency • An efficiency of 100% would be attained if the exhaust velocity was equal to the inlet velocity. However, for this to occur, the mass flow through the engine would need to be infinite. • Infinite mass flow is obviously not achievable in the real world, but this does indicate that greater efficiency is obtained when a large mass of air is accelerated by a small amount rather than a small mass of air being accelerated by a large amount.

Propulsive Efficiency

The Thrust Equation

• From Newton's third law: – For every action there is an equal and opposite reaction • The jet engine's action is accelerating a mass of gas and sending it out tailpipe. • The equal and opposite reaction is thrust.

The Thrust Equation • From Newton’s second law: F = d(mv)/dt (= ma for a constant mass) • In jet engine , we can re-write this as:

F= • Where:

w g * (V2 – V1)

F

is force in pounds

w

is the gas flow in pounds per second

g

is the gravitational constant

V1

is the initial velocity of the gas, in ft/sec

V2

is the final velocity of the gas, in ft/sec P1, Page 24

The Thrust Equation We can re-write the thrust equation to make it more meaningful in the context of a jet engine:

Fnet =

thrust of engine

wair + wfuel g

total mass flow out tailpipe

X Vjetexhaust

exhaust velocity

-

wair g

X Vinlet

incoming velocity mass flow of incoming of air mass flow

The Thrust Equation • This is called the “Net” Thrust, because it s for the momentum of the incoming air; “Gross” Thrust is given by the first term in the equation – which is the force created at the exhaust of the engine. • To compute usable thrust, the gross thrust has to be reduced by the amount of the second term, which is the momentum already existing because of the airplane’s speed. • From the equation, it can be seen that net thrust is a function of the mass flow rate of the air and fuel ing through the engine, and of the exhaust velocity minus the incoming velocity.

Additional Thrust Due to Internal Pressure • The thrust equation as written is somewhat simplified in that it ignores one more possible component of thrust i.e. Thrust due to internal pressure. • Most of the internal pressure within the engine is converted to velocity of the exhaust gasses, which in turn produces thrust. • At the exhaust, if the total pressure of the gasses is greater than the total pressure at the intake, this surplus of pressure will produce some additional thrust. F = Aexhaust X

pexhaust- pambient

• This component of thrust is small compared to the thrust due to exhaust velocity, but should not be ignored.

Factors Affecting Thrust

Thrust

Thrust

Air Temperature

tropopause

Air density, a function of temperature and pressure altitude, is a very significant component affecting thrust.

Altitude

Factors Affecting Thrust • Velocity affects both the momentum and the pressure of the air entering the engine intake. • Increasing aircraft speed increases the momentum of the incoming air, lowering thrust, while at the same time compressing the air at the intake (ram effect) increasing thrust by increasing density. The combined effect is show below. 100 %

with ram % Thrust

with out

0% Airspeed

ram

Other Factors Affecting Thrust

•

Bleed air extraction affect thrust

•

Power extraction for hydraulic pumps, electric generators, fuel pumps, etc., affects thrust.

•

Humidity has a negligible effect on thrust.

Commonly used Jet Engine • EPR - Engine Pressure Ratio: – Ratio of total pressure at the exhaust to total pressure at the front of the fan/compressor. – This is commonly used as a measure of engine thrust, and is the primary thrust setting parameter. • N1 or %N1: – N1 is the rotation rate, in RPM, of the low-speed rotor of a two or three-spool engine. – N1 is usually expressed as %N1, a percentage of some nominal value. – General Electric and CFMI engines use %N1 as the primary thrust setting parameter.

Commonly used Jet Engine • Engine Stall (compressor stall): – A condition characterized by stalled airflow over the compressor blades. • Surge: – Refers to a condition of unsteady airflow through an engine as the result of abnormal flow conditions. – Surge can result from strong crosswinds at low airspeeds (e.g., during takeoff) or other conditions such as very rapid acceleration or deceleration of the engine. • Flameout: – A condition in which the combustion chambers lose their ignition. This could be the result of unsteady airflow (e.g., strong turbulence) or other conditions. P1, Page 39

Other Commonly Encountered Jet Engine • Bleed: – Extraction of compressed air from the engine. – Bleed air is used for air conditioning and pressurization, as well as for providing icing protection. – Engine bleeds are also used in some cases to prevent surging. • EGT – Exhaust Gas Temperature: – This is the temperature at the engine exhaust

ADVANTAGES OF JET ENGINES • Gas turbine engines have a great power-to-weight ratio compared to reciprocating engines. That is, the amount of power you get out of the engine compared to the weight of the engine itself is very good.

• Gas turbine engines are smaller than their reciprocating counterparts of the same power. • Jet engine designs are frequently modified for non aircraft applications, as industrial gas turbines. These are used in electrical power generation, for powering water, natural gas, or oil pumps, and providing propulsion for ships and locomotives. • Industrial gas turbines can create up to 50,000 shaft horsepower

DISADVANTAGES OF JET ENGINES • Compared to a reciprocating engine of the same size, they are expensive. • Because of high speeds and high operating temperatures, deg and manufacturing gas turbines is a challenge from both the engineering and materials point of view. • Gas turbines also tend to use more fuel when they are idling.

THANK YOU…