advanced aircraft engines -open rotor engines

7/22/2019 Advanced Aircraft Engines -Open Rotor Engines

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Advanced Aircraft Engines-Open Rotor Engines

M. Sathyanarayanan11Aeronautical Department,

Park College of Engineering and [email protected]

Abstract

This paper discusses on an efficient way ofpowering the Aircrafts by using Open Rotor Engines(ORE) other two modifications to Turbo Fan Engines.The existing Turbo Fan Engines produce Thrust morethan any other engine because of the By-Pass Ratiothrust. But there is a limit to how much the by-passratio can be increased. So open rotor engines i.e.

without Engine Nacelle is used for powering the

aircrafts. Rolls-Royce predicts that Open Rotor Engineswill offer a 25-30% reduction in Specific Fuel

Consumption. Alternative for Open Rotor Engines:Geared Turbo-fan Engine (GTF) & Rolls-Royceturbo-fan engine with novel 3 Shaft Architecture can

also be used.

KEY WORDS: ORE (Open Rotor Engines).

I. INTRODUCTION

This paper discusses on Open RotorEngines, a technology that may be used on the nextgeneration of Aircrafts for Efficient Propulsion andhas the Potential significantly to reduce the CO2

emissions.

II. TURBO-FAN ENGINES

Fig. 1 Turbofan engine

Since the invention of the jet engine byFrank Whittle in 1930, the greatest advance inaviation has arguably been the development of theturbofan engine, the engine most commonly in usein civil aviation today. The turbofan engine isessentially a jet engine with a large ducted fan atthe front. This fan allows the engine to suck in amassive volume of air, some of which goes through

the engine core (jet engine), the rest of whichtravels relatively slowly through the enginesbypass duct.

Turbofan engines that have a high bypassratio (ratio of air entering the bypass to air enteringthe engine core) can produce large amounts ofthrust for significantly less fuel than traditional jetengines. As the CO2 emissions produced by theengine are directly proportional to the amount offuel burnt, these engines also have much lower

emissions than pure jet engines, where most or allof the air passes through the core. These twodesirable characteristics, low fuel burn and CO2emissions, make turbofan engines the currentuniversal choice for high speed civil aircraft.

III. HOW DO THE TURBO FAN ENGINES

PRODUCE LOW EMISSIONS??

To reduce the CO2 emissions, the amountof fuel that is being burnt to produce the enginethrust must be reduced.

The engine thrust equation is given by

F = Mv (Vj Va)Where, F - is the thrust that is produced.

MV - is the Mass Flow Rate of Air throughthe Engine core.

Vj - is the velocity of the Engine Jet.Va - is the velocity of the Aircraft.

It is clear from the above equation that,the thrust produced by the engine can be eitherincreased by Increasing the Mv or the (Vj-Va). But

the Fuel Burn varies with the so a Small

increase in Vj will lead to a Larger Increase in theCO2 Emissions. Therefore it is better to increasethe the thrust by increasing the Mass Flow Rate of

Air through the engine. This can be achieved byusing a turbofan engine with a high bypass ratio asthis passes a large amount of air with a low meanjet velocity.


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Fig. 2 A Turbojet and a High Bypass ratio Turbofan

engine and their jet velocities

Engine manufacturers have exploited thischaracteristic by progressively increasing thebypass ratios of modern turbofan engines. Howeverthere is a limit to how much the bypass ratio of anengine can be increased, as eventually the fuel burnbenefit associated with the high bypass engine willbe offset by the weight and drag and hence fuelburn penalties associated with the largenacelle(engine casing). At this point, a betterapproach may be to utilize an open rotor engine, anengine where the propeller/fan is not containedwithin the nacelle, offering a significant furtherincrease in bypass ratio.

IV. AN OPPORTUNITY FOR THE OPEN

ROTORS (ORE)

Ageing fleets requiring replacement

coupled with high growth in low cost carriers inboth traditional markets (US, Europe, etc.) andemerging markets (China, India, Latin America,Africa, etc.) mean that the forecast demand for newsingle aisle aircraft up to 2025 is in excess of15,000. This represents a market opportunity ofaround US$1 trillion.

However, this significant demand for newsingle aisle aircraft offers more than just economicbenefit. It provides the perfect opportunity for newengine and aircraft designs with radically improvedenvironmental performance to be implemented on agrand scale.

Open rotor engines are one technologythat has the potential to deliver significantenvironmental improvements for new single aisleaircraft. The increase in propeller diameter andremoval of the heavy, drag-inducing, nacelle meansthat the fuel burn and hence CO2 emissions of anopen rotor engine could be significantly less thanthat of an equivalent high bypass ratio turbofanengine. Rolls-Royce predicts that open rotor

engines could offer a reduction in specific fuelconsumption (amount of fuel consumed to produceeach pound of engine thrust) of 25-30% relative tocurrent single aisle turbofans.

Fig. 3 Single Aisle Aircraft Market between 2005 and2025

V. WHAT WILL OPEN ROTORS LOOK

LIKE??

Two common open rotor configurationsexist:

1. A Puller and2. A Pusher.

In a Puller configuration, the propellersare mounted at the front of the engine. In a pusherconfiguration, the propellers are mounted directlybehind the turbine stage.

Fig.4 Puller Configuration

Engine configurations can either have asingle propeller or a pair of contra-rotating


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propellers, however historically, pullerconfigurations have generally had only onepropeller and pusher configurations have usuallyhad two contra-rotating propellers. The advantageof using the second propeller is that the rotationalcomponent of velocity (swirl) of the air leaving thefirst set of propeller blades will be corrected by the

second set of blades and so will increase theengines effective thrust.

Fig. 5 Pusher Configuration

This means that to achieve a given thrust,an open rotor engine with contra-rotating bladescan have a reduced propeller diameter and speedcompared to a single propeller engine. Reducedpropeller diameter offers the benefit of easierintegration with the aircraft design. As the noisegenerated by the propeller increases with therotational speed of the blades, reduced propellerspeed offers the benefit of a quieter propeller;

however, the interaction between the blade rows ofa contra-rotating propeller creates additional noise.Using a pair of contra-rotating propellers alsoenables the aircraft to fly at significantly higherspeeds, almost as fast as todays turbofan poweredaircraft. The drawbacks associated with a pair ofcontra-rotating propellers are increased weight,cost and complexity.

There are two ways to drive the large, slowmoving propeller(s):

Using a reduction gearbox driven by ahigh speed, low stage count, low pressure

turbine. Directly using a low speed, multi-stage,low pressure turbine.

Both geared and direct drive options haveadvantages and disadvantages. Reductiongearboxes can be heavy and complex which canlead to unreliability and increased maintenanceeffort. On the other hand, direct drive engines willendure a performance penalty. A low pressureturbine operates most efficiently at high speeds

whilst the propellers optimal efficiency occurs atlow speeds. In direct drive engines as the propelleris driven directly by the low pressure turbine, thespeed at which the propeller/turbine system rotateswill be a trade-off between the optimal speeds ofthe two. So there has to be compromise in order toachieve the perfection.

VI. BUT, ARE OPEN ROTORS ACTUALLY

ANYTHING NEW??

Open rotor concepts have been around forsome time. The Turboprop engine, a basic jetengine that drives a single propeller through a gearbox, has been around since 1939 and is still widelyused to power small, low speed aircraft. In the1980s when fuel prices rose considerably, aturboprop engine that used a pair of contra-rotatingpropellers was developed. However, these engines,termed propfans or UnDucted Fans (UDFs) werenever produced commercially due to theunresolved technical issues (complexity, noisiness,etc.).

However, the open rotor engine designsbeing considered for the new single aisle aircraftwill not simply replicate past turboprop or propfanConfigurations. Designs will incorporate new andnovel technologies currently under development inresearch and validation programmes. Although it isnot yet known exactly which technologies will beincorporated into open rotor engines, it is likelythat at least some of the following advances willfeature:

Advanced turbine and compressorsystems new materials and coatings,improved aerodynamics and reduced useof cooling air will lead to improvedcomponent efficiency and/or fuel burn.

Low emissions combustors novel leanburn combustor designs will enable NOxto be reduced.

Improved control systems moreadvanced electronics, sensors, actuatorsand software will help to optimise engineperformance and thus emissionsthroughout the flight envelope.

Light weight structures new materials,innovative design and manufacturingtechniques will be used to produce lightweight structures and hence reduce fuelburn.

Advanced propeller blades improvedaerodynamics and pitch control systems will allowoptimized propeller performance in all flightconditions.


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VII. CHALLENGES OF OPEN ROTOR

ENGINES

There are some substantial challengesassociated with the use of open rotors. Theseinclude noise (cabin and community), bladecontainment and other certification issues,

integration with the airframe, accessibility andmaintenance issues.

i. NOISE:Open rotor engines are likely to be noisier

than turbofan engines as there is no nacelle toabsorb and attenuate the noise generated by theengine. As a result, it is very unlikely that newopen rotor engines will be able to achieve noiselevels comparable to or less than future equivalentturbofan engines. However, with the latesttechnology they will be quieter than todaysturbofan engines. Some have suggested that

stringent noise requirements should be relaxed sothat the significant fuel burn reductions offered byopen rotors can be realized.

ii. ACCESSIBILTY&MAINTENNANCEISSUES:

The most popular mounting configuration forcurrent aircraft engines is under the wing. Thisoffers easy access for on-wing checks andmaintenance. However, under wing mounting isunlikely to be suitable for the new open rotorengines. New engine mounting positions couldhave a significant impact on maintenance times andcost .In addition, reliability concerns and thepotential maintenance burden of certain open rotorengine components such as the gearbox andpropeller pitch control systems may make openrotor engines a less desirable choice for operators.

VIII. WORK BEING DONE TO OVERCOME

THE CHALLENGES

Easy Jet eco Jet:

In 2007, easy Jet released a concept designfor the eco Jet, which it asserts could be anenvironmentally friendly single aisle aircraftreplacement. Easy Jet predicts that the eco Jet,which is to be powered by open rotor engines,could be in operation by 2015 and has the

potential to deliver CO2 and NO emission

reductions of 50% and 75% respectively, and a

25% reduction in noise, compared to currentBoeing 737 and Airbus A320 variants.

Fig. 6 Model of easyJets eco design

The open rotor engines proposed for theecoJet design have a geared drive system and arerear-mounted for improved efficiency and reducednoise. The proposed airframe is lightweight, witha significant proportion of the structure likely to bemade from composite materials. The proposeddesign has wings that are slightly swept forward

for reduced drag at cruise. The ecoJet is designedto have a lower cruise speed and a shorter designrange than a traditional single aisle aircraft, toreduce drag and weight respectively.

Producing the ecoJet design conceptdemonstrates the eagerness of airlines like easyJetto adopt new technology offering improvedenvironmental performance. However it isimportant to remember that the ecoJet is a conceptdesign only, significant work is required bymanufacturers to develop and validate thetechnologies proposed by the design.And also RearMounted Aircraft engines have a high PropulsiveEfficiency.

IX. FIRST OPEN ROTOR ENGINE USAGE

Engine manufacturers such as Rolls-Royce have begun work on open rotor enginedesigns for the next generation single aisle aircraft.It predicts that by 2013 the open-rotor concept willbe sufficiently mature for the anticipatedperformance benefits to be confirmed and thatengines will be available between 2015 and 2020.

The open rotor engine design thatwould use the above technology, would be of a

Pusher Configuration.


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Fig.7 The six Integration technologies

X. ALTERNATIVES

Open rotor engines are not the onlytechnology being developed for the next generationof single aisle aircraft. Some engine manufacturersbelieve that the installation impact and extra weightof open rotor engines will offset their fuel burnbenefit and so are taking a different route,developing a geared turbofan (GTF) engine. AGTF is a very high bypass ratio turbofan engine

whose fan is driven through a reduction gearbox

rather than being directly connected to the

engine. This allows the fan and the compressor-turbine system that drives the fan to both run at

their optimal speeds resulting in an engine that isquieter and more fuel efficient than a conventionaltwo-shaft aero-engine.

Pratt & Whitney claim their GTF engineoffers a 12% reduction in fuel burn over currentengines with a further potential improvement of5-7% available by 2020.13 Although this fuelburn reduction is less than that predicted for anopen rotor engine, GTF technology offers thebenefit of enhanced maturity and hence reducedtime for entry into service, is likely to be slightlyquieter than an open rotor engine and is exemptfrom some of the challenges associated with open

rotor technology (e.g. fan blade off and someintegration issues).

A third option for powering the new singleaisle aircraft is a turbofan engine with novel three-shaft architecture. The three-shaft architecture hassome inherent benefits over the two-shaftarchitecture of the GTF; it is shorter and lighter andhas fewer stages of compression. Furthermore, as

Fig 8. GTF design

there is no compressor(booster)running on the fanshaft, the fan can be allowed to run at its optimumslow speed, producing less noise, without the addedcomplexity of the gearbox. Rolls-Royce predictsthat novel three shaft engines will have specificfuel consumption comparable to the GTF,

around 15% less than current two- and three-shaft engines. The three-shaft engines also haveenhanced maturity and are exempt from many ofthe challenges facing open rotors. Rolls-Royce isleading the ENVIRONMENT FRIENDLYENGINE (EFE) programme and will utilize thistechnology if open rotors are not the chosenarchitectural style for future single aisle aircraft.

XI. FUTURE

Whether open rotor, GTF or novelthree-shaft engines are selected for the newsingle aisle aircraft, it is clear that the

opportunity to implement radical environmentalimprovements on a grand scale must not be

missed. The next generation single aisle aircraftmarket represents a rare and exciting opportunitythat the aviation industry is determined to seize tomake a significant difference to the future of theenvironment.

XII. CONCLUSION

Open rotors proposed for new single aisleaircraft (Boeing 737/Airbus A320replacement) will not be conventionalturboprops/propfans: they will incorporate

new, advanced technologies. Open rotors are predicted to provide a 25

30% reduction in specific fuelconsumption & CO2 emissions relativeto current, equivalent turbofan engines,and reduce the NOx emissions by 70%..

Challenges associated with Open Rotorsinclude Noise, Aircraft Integration,accessibility & maintenance can all beovercome.


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Increase in Journey Time for OpenRotor Powered Aircrafts will be small(for eg:

advanced aircraft engines -open rotor engines

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