cng engine.pdf

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American J. of Engineering and Applied Sciences 2 (2): 268-278, 2009ISSN 1941-7020 2009 Science Publications

Corresponding Author: Semin, Deaprtment of Marine Engineering, Institut Teknologi Sepuluh Nopember (ITS) SurabayaKampus ITS Keputih, Sukolilo, Surabaya 60111, Indonesia

268

Diesel Engine Convert to Port Injection CNG Engine Using Gaseous Injector

Nozzle Multi Holes Geometries Improvement: A Review

1Semin, 2Abdul Rahim Ismail and 2Rosli Abu Bakar1Department of Marine Engineering, Institute of Technology Sepuluh November (ITS),

Surabaya Kampus ITS Keputih, Sukolilo, Surabaya 60111, Indonesia2Faculty of Mechanical Engineering, University Malaysia Pahang, Malaysia

Locked Bag 12, 25000 Kuantan, Pahang, Malaysia, Malaysia

Abstract:The objective of this study was to review the previous research in the development of gaseousfuel injector for port injection CNG engine converted from diesel engine. Problem statement: Theregular development of internal combustion engines change direction to answer the two most importantproblems determining the development trends of engines technology and in particular, theircombustion systems. They were environmental protection against emission and noise, shortage ofhydrocarbon fuels, specific fuel consumption and other technical and economic parameters.

Approach: Several alternative fuels has been recognized as having a significant potential forproducing lower overall pollutant emissions compared to diesel fuel. Natural gas, which composedpredominately by identified as a leading candidate for transportation applications among these fuels foravailability, environmental compatibility and natural gas is that it can be used in conventional dieselengines. Results:Some advantages of CNG as a fuel are octane number is very good for SI enginefuel, engines can be operate with a high compression ratio, less engine emissions and less aldehydes.In the diesel engines converted or designed to run on natural gas with the port injection (sequential) ortrans-intake valve-injection system, a high-speed gas jet was pulsed from the intake port through theopen intake valve into the combustion chamber, where it caused effects of turbulence and chargestratification particularly at engine parts load operations. The system was able to diminish the cyclicvariations and to expand the limit of lean operation of the engine. The flexibility of gas pulse timingoffers the potential advantage of lower emissions and fuel consumption. There are several advantagesof port injection. The better possibility CNG engine is to equalize the air-fuel ratio of the cylinders,

optimization of the gas injection timing and of the gas pressure for different operating conditions.Conclusion: The fuel nozzle injector multi holes geometries development was to produce optimumfuel air mixing and increasing the volumetric efficiency of the engine that will promote a comparableengine performance and efficiency.

Key words: CNG engines, diesel engines, injector improvement, port injection

INTRODUCTION

In 2009, a hundred and thirty three years hadpassed since Nicolas August Otto constructed a four-stroke gas-fuelled internal combustion engine andpatented its cycle in 1876 with Patent DRP No.532,

year 1876. In spite of the fact that the power output ofthe engine was only approximately 2.2 kW or 3 hp andignition was started by hot gases, it was a precursor ofmodern Internal Combustion Engine (ICE) because themixture was compressed and burned in a singlecylinder, the machine also had other interesting designfeatures. It can be said without exaggeration that this

invention revolutionized the world. After ten years, in1886 an ICE was installed in a motorcar and aftertwenty seven years, in 1903 was installed in an aircraft.In 1974 some 35 million motor vehicles driven byengines based on the Otto cycle were produced[1].

In 1893, Rudolf Diesel constructed an experimental

compression-ignition engine and patented the relevantcycle in 1892 with Patent DRP N0, 67207, year 1892.Four years later, a working version of the engine had anefficiency of 26% at a power output of approximately14.7 kW or 20 hp. This although the engine was notoriginally applied for driving a vehicle, was the secondfundamental invention in the history of the automobile.


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Am. J. Engg. & Applied Sci., 2 (2): 268-278, 2009

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In 1976, the total output by the West Europeancountries only was 5,200,000 compression-ignitionengines. Although the gas turbine was invented by JohnBarber in 1791, it was not until 1939 that is became afully efficient driving engine. Its subsequent rapiddevelopment was due to its being applied in air-craftengineering[1].

The regular development of ICE changes directionin answer to changing requirement. In the 1970, the twomost important problems determining the developmenttrends of engines technology and in particular, theircombustion systems. They were environmentalprotection against emission and noise and shortage ofhydrocarbon fuels. The brief comparison of a variety ofengine undertaken in what follows principally concernsspecific fuel consumption, emissions and othertechnical and economic parameters[1].

Direct injection diesel engine: The diesel engines is atype of internal combustion engines, more specifically,it is a compression ignition engine, in which the fuel isignited solely by the high temperature created bycompression of the air-fuel mixture, rather than by aseparate source of ignition, such as a spark plug, as isthe case in the gasoline engine. The engine operatesusing the diesel cycle.

The direct injection diesel engines or directinjection compression ignition engines, where fuel isinjected by the fuel injection system into the enginecylinder toward the end of the compression stroke, justbefore the desired start of combustion[1-5]. The liquid

fuel, usually injected at high velocity as one or morejets through small orifices or nozzles in injector tip,atomizes into small drops and penetrates into thecombustion chamber. The fuel vaporizes and mixeswith high temperature and high pressure cylinder air.The air is supplied from intake port of engine. Since theair temperature and pressure are above the fuelsignition point, spontaneous ignition of portions of thealready-mixed fuel and after air a delay period of a fewcrank angle degrees. The cylinder pressure increases ascombustion of the fuel-air mixture occurs[6-10]. Themajor problem in compression ignition enginecombustion chamber design is achieving sufficiently

rapid mixing between the injected fuel and the air fromintake port in the cylinder to complete combustion inthe appropriate crank angle interval close to top-center[2-4,8,11-13]. Horsepower output of an engine can bedramatically improved through good intake port designand manufacture[14].

Compressed natural gas: Natural gas is producedfrom gas wells or tied in with crude oil production.

Table 1: Natural gas composition[15]

Volume fraction (%)-----------------------------------------

Composition Formula Ref. 1 Ref. 2 Ref. 3 Ref. 4

Methane CH4 94.00 92.070 94.390 91.820

Ethane C2H6 3.30 4.660 3.290 2.910Propane C3H8 1.00 1.130 0.570 -Iso-butane i-C4H10 0.15 0.210 0.110 -N-butane n-C4H10 0.20 0.290 0.150 -Iso-pentane i-C5H12 0.02 0.100 0.050 -N-pentane n-C5H12 0.02 0.080 0.060 -Nitrogen N2 1.00 1.020 0.960 4.460Carbon dioxide CO2 0.30 0.260 0.280 0.810Hexane C6+C6H14 0.01 0.170 0.130 -Oxygen O2 - 0.010


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worldwide. The disadvantages of compressed naturalgas as an engine fuel are low energy density resulting inlow engine performance, low engine volumetricefficiency because it is a gaseous fuel, need for largepressurized fuel storage, so there is some safety concernwith a pressurized fuel tank, inconsistent fuel propertiesand refueling of the compressed natural gas is a slowprocess. Natural gas can be used as a fuel essentially inthe form in which it is extracted. Some processing iscarried out prior to the gas being distributed. Methanecan also be produced from coal and from biomass orbiogas and a whole variety of biomass wastes such asfrom landfill sites and sewage treatment plants.

CNG as an alternative fuel for internal combustion

engines: Compressed Natural Gas (CNG) has longbeen used in stationary engines, but the application ofCNG as a transport engines fuel has been considerably

advanced over the last decade by the development oflightweight high-pressure storage cylinders[3]. Someresearchers[15,16,20-22] have been researched about thecompressed natural gas as alternative fuel motivated bythe economic, emissions and strategic advantages ofalternative fuels.

Several alternative fuels have been recognized ashaving a significant potential for producing loweroverall pollutant emissions compared to gasoline anddiesel fuel. Natural gas, which is composedpredominately by has been identified as a leadingcandidate for transportation applications among thesefuels for several reasons[15,17]. Shasby[15] has identified

tree reason, the first reason is availability, the secondattraction reason of natural gas is its environmentalcompatibility and the third attraction reason of naturalgas is that it can be used in conventional diesel andgasoline engines. According to Shasby[15], operatingcosts are another reasons, where natural gas poweredvehicles theoretically have a significant advantage overpetroleum-powered vehicles, the basis for this argumentis the lower cost per energy unit of natural gas ascompared to petroleum. Compressed natural gasvehicles exhibit significant potential for the reductionof gas emissions and particulates. There are problemsfor compressed natural gas applications such as

onboard storage due to low energy volume ratio, knockat high loads and high emission of methane and carbonmonoxide at light loads[17]. According to Sera[23], theCNG as fuel properties are shown in Table 2.

However, these can be overcome by the properdesign, fuel management and exhaust treatmenttechniques. Most existing compressed natural gasvehicles use petrol engines, modified by after-marketretrofit conversions and retain bi-fuel capability.

Table 2: CNG as a fuel properties at 25C and 1 atm[23]

CNG Properties Value

Density (kg m3) 0.72Flammability limits (volume % in air) 4.3-15Flammability limits () 0.4-1.6

Autoignition temperature in air (C) 723Minimum ignition energy (mJ) 0.28Flame velocity (m sec1) 0.38Adiabatic flame temperature (K) 2214Quenching distance (mm) 2.1Stoichiometric fuel/air mass ratio 0.069Stoichiometric volume fraction (%) 9.48

Lower heating value (MJ kg1) 45.8

Heat of combustion (MJ kgair1) 2.9

Bi-fuelled vehicle conversions generally suffer from apower loss and can encounter driveability problems,due to the design and/or installation of the retrofitpackages Shasby[15]. In bi-fuel for diesel engine, naturalgas as a fuel for diesel engines offers the advantage of

reduced emissions of nitrogen oxides, particulate matterand carbon dioxide while retaining the high efficiencyof the conventional diesel engine[22]. Single-fuelvehicles optimized for compressed natural gas arelikely to be considerably more attractive in terms ofperformance and somewhat more attractive in terms ofcost.

According to Poulton[17] that a natural gas-powered, single-fuel vehicle should be capable ofsimilar power, similar or higher efficiency and mostlylower emissions than an equivalent petrol-fuelledvehicle. Such a vehicle would have a much shorterdriving range unless the fuel tanks are made very large,

which would then entail a further penalty in weight,space, performance and cost.

The safety aspects of converting vehicles to run on

CNG are of concern to many people. However, the low

density of methane coupled with a high auto-ignition

temperature, CNG is 540C compared with 227-500C

for petrol and 257C for diesel fuel and higher

flammability limits gives the gas a high dispersal rate

and makes the likelihood of ignition in the event of a

gas leak much less than for petrol or diesel.Additionally, natural gas is neither the toxic,

carcinogenic nor caustic. According to Poulton[17] thelegal maximum operating pressure for a vehicle storage

cylinder will usually be in the range 20-25 MPa-commonly 20 MPa. Cylinders are tested beforeinstallation to a pressure of 30 MPa (300 bar or4,350 psi) or to a level to meet local requirements.Safety regulations specify a periodic re-inspection,typically at five-year intervals, including a pressure testand internal inspection for corrosion.

The usage of natural gas as an alternative fuel hasthe advantage of a comprehensive supply and


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distribution system already in place, therebysubstantially reducing the cost of adopting it as analternative fuel. A gas supply network has been inexistence, distribution and transmission mains.However, the refueling infrastructure would need to beestablished.

CNG as alternative fuel characteristic: The octanerating of natural gas is about 130, meaning that enginescould operate at compression ration of up to 16:1without knock or detonation[17]. In the CNG applicationpolicy, many of the automotive makers already builttransportation with a natural gas fuelling system andconsumer does not have to pay for the cost ofconversion kits and required accessories. Mostimportantly, natural gas significantly reduces CO2emissions by 20-25% compare to gasoline becausesimple chemical structures of natural gas (primarily

methane-CH4) contain one Carbon compare to diesel(C15H32) and gasoline (C8H18)

[16,17]. CNG as alternativefuel characteristics are shown in Table 3.

Like methane and hydrogen is a lighter than airtype of gas and can be blended to reduce vehicleemission by an extra 50%. Natural gas compositionvaries considerably over time and from location tolocation[17]. Methane content is typically 70-90% withthe reminder primarily ethane, propane and carbondioxide[15, 24]. At atmospheric pressure and temperature,natural gas exists as a gas and has low density. Sincethe volumetric energy density is so low, natural gas isoften stored in a compressed state at high pressure

stored in pressure vessels.According to Poulton[17]that natural gas has a high

octane rating, for pure methane the RON = 130 andenabling a dedicated engine to use a higher compressionratio to improve thermal efficiency by about 10% abovethat for a petrol engine, although it has been suggestedthat optimized CNG engine should be up to 20% moreefficient, although this has yet to be demonstrated.Compressed natural gas therefore can be easilyemployed in spark-ignited internal combustion engines.

Table 3: CNG characteristics[16]

CNG Characteristics Value

Vapor density 0.68Auto Ignition 700COctane rating 130Boiling point (Atm. Press) -162CAir-fuel ratio (Weight) 17.24Chemical reaction with rubber NoStorage Pressure 20.6 MpaFuel air mixture quality GoodPollution CO-HC-NOX Very low

Flame speed m sec1 0.63Combust. ability with air 4-14%

It has also a wider flammability range than gasoline anddiesel oil[19]. Optimum efficiency from natural gas isobtained when burnt in a lean mixture in the rangeA = 1.3-1.5, although this leads to a loss in power,which is maximized slightly rich of the stoichiometricair/gas mixture. Additionally, the use of natural gasimproves engine warm-up efficiency and together withimproved engine thermal efficiency more thancompensate for the fuel penalty caused by heavierstorage tanks. Natural gas must be in a concentration of5-15% in order to ignite, making ignition in the openenvironment unlikely. The last and most often citedadvantages have to do with pollution. The percentagesvary depending upon the source, but vehicles burningnatural gas emit substantially lesser amounts ofpollutants than petroleum powered vehicles. Non-methane hydrocarbons are reduced by approximately50%, NOxby 50-87, CO2by 20-30, CO by 70-95% and

the combustion of natural gas produces almost noparticulate matter[17]. Natural gas powered vehicles emitno benzene and 1,3-butadiene which are toxins emittedby diesel powered vehicles. The use of natural gas as avehicle fuel is claimed to provide several benefits toengine components and effectively reduce maintenancerequirements. It does not mix with or dilute thelubricating oil and will not cause deposits incombustion chambers and on spark plugs to the extentthat the use of petrol does, thereby generally extendingthe piston ring and spark plug life. In diesel dual-fueloperation evidence of reduced engine wear is reported,leading to expected longer engine life[5]. The use of

natural gas in a diesel Spark-Ignition (SI) conversion isexpected to allow engine life at least as good as that ofthe original diesel engine. Because of its very lowenergy density at atmospheric pressure and roomtemperature, natural gas must be compressed and storedon the vehicle at high pressure-typically 20 MPa,200 bar or 2,900 psi. The alternative storage method isin liquid form at a temperature of-162C. Because ofthe limited capacity of most on-board CNG storagesystems a typical gas-fuelled vehicle will need refuelingtwo to three times as often as a similar petrol or diesel-fuelled vehicle-a typical CNG-fuelled car engine willprovide a range of 150-200 km and a truck or bus some

300-400 km. It is possible that the space required andweight of CNG fuel storage systems will fall in thefuture as a result of improved engine efficiencies aswith dedicated designs and lightweight storage tanks[17].

When a vehicle is operating on CNG about 10% ofthe induced airflow is replaced by gas which causes acorresponding fall in engine power output. Inperformance terms the converted bi-fuel engine willgenerally have a 15-20% maximum power reduction


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than that for the petrol version. When a diesel engineconversion is fuelled on gas moreengine power can beobtained due to the excess air available which, due tosmoke limitations, is not fully consumed. Becausenatural gas has a low cetane rating, a spark ignitionconversion for diesel engines is required, adding to theconversion cost[17]. Even though more power may beavailable, experience has shown that SI diesel engineconversions are usually down-rated to preventexcessive combustion temperatures leading tocomponent durability problems. A diesel/gas dual-fuelconversion may experience a loss of efficiency, relativeto diesel-fuelling alone. A 15-20% loss in thermalefficiency was reported in a dual-fuel heavy-duty truckdemonstration in Canada, where natural gas provided60% of the total fuel requirement during dual-fueloperation. A further disadvantage of methane is that itis a greenhouse gas with a warming forcing factor many

times that of the principal greenhouse gas, CO2, Gasleakage or vehicular emission, therefore and the size ofrelease, will have an impact on the overall greenhousegas (GHG) emissions performance relative to the petrolor diesel fuel it substitutes[15,16,20-22].

CNG Fuel Emissions: The last and most often citedadvantages have to do with pollution. The percentagesvary depending upon the source, but vehicles burningnatural gas emit substantially lesser amounts ofpollutants than petroleum powered vehicles. Non-methane hydrocarbons are reduced by approximately50, NOx by 50-87, CO2 by 20-30, CO by 70-95% and

the combustion of natural gas produces almost noparticulate matter[17]. Natural gas powered vehicles emitno benzene and 1,3-butadiene which are toxins emittedby diesel powered vehicles. The natural gas is a muchsimpler hydrocarbon than those in petrol and diesel andmixes un-uniformly with air, combustion is likely to bemore complete in the time available fluids to inherentlylower CO and non-methane HC emissions. Because agas-fuelled does not require cold-start enrichment,emissions from "cold" engine operation are higher thanwith liquid fuels and because gas systems are designedto be air-tight, so relative emissions should benegligible. Generally low sulphur content of natural gas

is 5-10 ppm, mainly from the odorant

[5]

. One of theimportant sources of catalyst poisoning and, forexample, should allow placement of platinum by themuch more abundant palladium. Palladium also hasadvantage of being very effective for methaneoxidation. The magnitude and character of emissionsfrom CNG engine vehicles, like emissions from alcoholand other vehicles, will vary depending on the trade-offs made between performance, fuel economy,

emissions and other factors. However, the simplephysical composition of natural gas (predominantlymethane-CH4with no carbon-to-carbon bonds) tends touse it a basically lower emission fuel, since thecombustion process is less complex than liquidhydrocarbon fuels and there is less of the otherhydrocarbons.

Diesel engine converted to port injection dedicated

CNG engine: In the diesel engines converted ordesigned to run on natural gas, there are two mainoptions discussed. The first is dual-fuel engines. Theserefer to diesel engines operating on a mixture of naturalgas and diesel fuel. Natural gas has a low cetane ratingand is not therefore suited to compression ignition, butif a pilot injection of diesel occurs within the gas/airmixture, normal ignition can be initiated. Between 50and 75% of usual diesel consumption can be replaced

by gas when operating in this mode. The engine canalso revert to 100% diesel operation. The second isdedicated natural gas engines. Dedicated natural gasengines are optimized for the natural gas fuel. They canbe derived from petrol engines or may be designed forthe purpose. Until manufacturer Original Equipment(OE) engines are more readily available, however, thepractice of converting diesel engines to spark ignitionwill continue, which involves the replacement of dieselfuelling equipment by a gas carburetor and the additionof an ignition system and spark plugs[17].

Buses and trucks larger and greater numbers ofcylinders are used than for light-duty engines. For

compression ignition engines conversions to sparkignition, the pistons must be modified to reduce theoriginal compression ratio and a high-energy ignitionsystem must be fitted. The system is suitable for CNGand is ideally suited to timed (sequential) port injectionsystem but can also be used for single point and lowpressure in-cylinder injection. Gas production providesgreater precision to the timing and quantity of fuelprovided and to be further developed and becomeincreasingly used to provide better fuel emissions[17].

The port-inducted or port injection CNG producesnegligible levels of CO, CO2 and NOx. In order togreatly reduce exhaust gas emissions, a port injection

system was chosen by Czerwinski

[25,26]

, Hollnagel

[27]

and Kawabata[28]and the injector and pressure regulatorhave been newly developed. At the same time, preciseAir-Fuel (A/F) ratio control and special catalysts CNGexhaust gas have been utilized. The resulting CNGengines output power has been restored to near that ofthe gasoline base engine.

With the port injection (sequential) or trans-intakevalve-injection system, a high-speed gas jet is pulsed


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from the intake port through the open intake valve intothe combustion chamber, where it causes effects ofturbulence and charge stratification particularly atengine part load operations. The system is able todiminish the cyclic variations and to expand the limit oflean operation of the engine. The flexibility of gas pulsetiming offers the potential advantage of loweremissions and fuel consumption. With three types ofport injectors available on the market, Czerwinski[26]were compared for stationary and transient engineoperation. There are several advantages of portinjection, e.g., better possibility to equalize the air-fuelratio of the cylinders, optimization of the gas injectiontiming and of the gas pressure for different operatingconditions. The port injection has an injector for eachcylinder, so the injectors can be placed in closeproximity to the cylinder's intake port. It also enablesfuel to be delivered precisely as required to each

individual cylinder (called sequential) and enables moresophisticated technologies such as skip-firing to beused. Skip-firing is when only some of the cylinders areoperating (the other cylinders are being skipped). Thisenables even more efficient use of the fuel at low loads,further lowering fuel consumption and unburnedhydrocarbon.

Gaseous fuel injection improvement: Improvement ofCNG injector nozzle holes geometries and understandof the processes in the engine combustion is a challengebecause the compression-ignition combustion process isunsteady, heterogeneous, turbulent and three

dimensional, very complex and the nozzle fuel injectorhole is can be variation with any hole geometry[29]. Inport injection CNG engines, natural gas fuel is injectedby fuel nozzle injector via intake port into combustionchamber and mixing with air must occur before ignitionof the gas fuel. To improve the perfect of mixingprocess of CNG fuel and air in combustion chamber isarranging of nozzle holes geometry, nozzle spraypressure, modified of piston head, arranging of pistontop clearance, letting the air intake in the form ofturbulent and changing the CNG fuel angle of spray[30].The CNG fuel spraying nozzle is the level of earningvariation so that can be done by research experiment

and computational of engine power, cylinder pressure,specific fuel consumption and exhaust gas emissionswhich also the variation of them. Czerwinski[25,26]havebeen researched the sequential injection of CNG offersseveral advantages to increase the CNG engineperformance.

The fuel nozzle injector multi holes geometriesdevelopment is to produce optimum fuel air mixing andincreasing the volumetric efficiency of the engine that

will promote a comparable engine performance.According to Czerwinski[26], the CNG port injectionsystem has advantages to develop for the moreefficiency.

Gaseous fuel injection spray structure: Gaseous fuel

injection implications and its behavior on combustion

chamber design has received considerable attention in

recent years. Most present day direct injection engineshave been optimized for use with liquid fuels. The

optimal configuration may be different than what isrequired for gaseous fuels. Experiments by Abraham[31]

examined the relative effects of combustion on liquidand gaseous fuel direct injection jets. Results showed

that fuel-air mixing and burning rates were initially

slower for gas injection jets than for liquid sprays.

However, burning and mixing rates of the gaseous

direct injection in the subsequent stages of combustionincreased in comparison with liquid fuel sprays.

Initially, the liquid jet was more effective at entraining

air and hence was better at producing flammablemixture regions within the combustion chamber. In the

end, however, the liquid spray did not burn ascompletely as the gas jet. Beyond the initial stage, the

gas direct injection jet exhibited a higher combustionrate than that of the liquid fuel[24]. The relative decrease

in burning rate for the liquid spray results from

vaporization of the remainder of the liquid fuel in the

mixture. This causes an increase in the richness of

mixture behind the flame, leading to a smaller energy

release than exhibited by the gas jet[24]

.According to Brombacher[24], the significant

contributions to the understanding of transient gaseousjet behavior have been reviewed, made and researched.The research a jet model, based on experimental results,consisting of four main regions. Figure 1 shows atransient gas jet showing the four main regions; thepotential core region, the main jet region, the mixingflow region and the dilution region[32,33]. The gaseousjet plume is characterized by a high velocity, lowtemperature core of rich unmixed fuel confined to thejet axis[34]. This core region is referred to as the main jetregion and contains the bulk of the unmixed fuel.

Turbulent vortices are generated on the periphery of thejet core as a result of shear forces exerted by theambient air in the chamber[32,34]. In the region close tothe nozzle exit, however, very little turbulence isgenerated. This region, the potential core region, is verystable and is characterized by very low mixing rates. Itextends from the nozzle exit to a distancecorresponding to a z/d (penetration distance/nozzleorifice diameter) ratio of approximately 12.5[33,35,36].


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Fig. 1: Transient gaseous jet structure[33]

Beyond this region, large scale vortices aregenerated by shear forces as air is entrained into the jet,resulting in vigorous mixing along the jet periphery.This region of enhanced mixing surrounds the main jetand is referred to as the mixing flow region. As the fuelloses its momentum, it is pushed aside by fuel flowingfrom upstream. The tip of the jet expands radially,forming the dilution region of the jet. This regioncorresponds to the jet tip and is characterized by lowvelocities and high fuel concentration.

The experimental study by Tanabe[37]were shown

that the flammable region exists only within the thinlayer around the periphery of the jet. In the vicinity ofthe nozzle, the temperature is low and the jet velocity ishigh; thus resulting in poor mixing conditions. Suchconditions do not provide a favorable environment forchemical reactions to take place. Downstream of thenozzle, in the mixing flow region, the temperature inthe periphery of the jet is relatively high and themixture is roughly stoichiometric. The flow hasstabilized in this area as result of momentum changebetween the jet and the entrained air, which providessufficient residence time for a chemical reaction tooccur. In summary, the most likely location for ignitionto take place and correspondingly the ideal place toinstall a glow plug in the periphery of the jetdownstream of the potential core region[38]. Resultsfrom Aesoy[38]further support the finding.

The core of the jet is fuel rich and very cold. Thetemperature of the core is, in fact, lower than the fueltemperature before injection. This is due to rapidexpansion of the jet at the nozzle exit and the highvelocity of the jet[24]. The outer edge of the jet is higher

in temperature because of heat transfer that occurs asthe hot air is entrained into the jet[24]. It would be verydifficult to achieve ignition if an ignition assist devicewere to be placed the core of the jet because a greatdeal of thermal energy input would be required toelevate the core to the autoignition temperature.Furthermore, the high temperature gradient between theglow plug and the jet core would result in short plugservice life. These conclusions provide someexplanation for the experimental findings of Aesoy[38]

and Thring[39].

Injector nozzle geometries improvement effect onfuel-air mixing: Numerous studies have suggested thatdecreasing the injector nozzle orifice diameter is aneffective method of increasing fuel air mixing duringinjection[24]. The smaller nozzle holes were found to bethe most efficient at fuel-air mixing primarily because

the fuel rich core of the jet is smaller[34]. In addition,decreasing the nozzle orifice diameter dl would reducethe length of the potential core region (defined asz/d = 12-51 and hence increase the size of the mixingflow region[33]. Unfortunately, decreasing nozzle holessize causes a reduction in the turbulent energygenerated by the jet. Since fuel-air mixing is controlledby turbulence generated at the jet boundary layer, thiswill offset the benefits of the reduced jet core size.Furthermore, jets emerging from smaller nozzle orificeswere shown not to penetrate as far as those emergingfrom larger orifices. This decrease in penetration meansthat the fuel will not be exposed to al1 of the available

air in the chamber. For excessively small nozzle size,the improvements in mixing related to decreased plumesize may be negated by a reduction in radialpenetration[34]. Another significant feature of theinjection flow that requires attention is the tendency ofthe fuel plume to attach to the cylinder head. Thisbehavior is undesirable because it restricts penetrationto the chamber extremities where a large portion of theair m as resides. Furthermore, it hampers airentrainment from the head side of the plume becausethe exposed surface area of the plume is reduced.

The phenomenon responsible for the jet deflectionis known as the Coanda effect[24]. The effect arises as a

consequence of the velocity and pressure fieldssurrounding the jet. Low pressure areas are formedabove and below the jet, due to the entrainment of airmass into the jet from the local surrounding volume.Below the jet, there is significant air mass in thevolume between the piston and the injector. Above thejet, space is limited and the air must be entrained fromprogressively farther downstream. As the jet develops,the air must eventually be entrained from the air into


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which the jet would penetrate. The entrainment flow isstrong enough to deflect the fuel jet upwards, causing inthe attach to the cylinder head. This phenomenon mustbe carefully avoided in the design of a natural gasengine combustion chamber. The mixing wasmaximized when the nozzle tip was placed equidistantfrom the piston and cylinder head[34].

The nozzle containing many small holes wouldprovide better mixing than a nozzle consisting of asingle large hole[34]. This hypothesis has been tested bystudying injectors with varying numbers of nozzleholes. The diameters of the holes were adjusted suchthat each nozzle delivered the same overall fuel massflow. Computational analysis examining eight hole,four hole, two hole and one holes nozzles, revealed thatthe mixing rate improved with the number of nozzleholes[24]. Jennings[34] carried out similar analysis for 8,12 and 16 holes nozzles. Contrary to the trend, the 16

holes injector performed poorly due to plume merging.Plume merging has an adverse effect on mixing becausethe total plume surface area available for mixing isdecreased[24].

Injector nozzle coefficient of discharge: TheCoefficient of discharge (Cd) for micro-nozzles forcompressible gas flow has measured by Snyder [40]. Thecoefficient of discharge was defined as Aeffective perAgeometry, where Aeffective was calculated from thestandard isentropic compressible mass flow relation.There is a possibility of further increase in Cd valueswith a further increase in the Reynold number. Thesmall orifice diameter and large l/d ratio had strongeffect on Cd values for compressible gas flow [35]. Thefollowing Eq. 1, 3 were proposed for Cd [32]:

2

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