Advantages Of Internal Combustion Engines Engineering Essay

Advantages Of Internal Combustion Engines Engineering EssayThe most widely used heating locomotive is the internal blaze locomotive engine. The advantages that it has over gas turbines moderate seen its widespread usage in passenger car applications. 1All the components of internal burn engines knead at an average temperature which is be secondary the supreme temperature of the working fluid in the working cycle.This is because the high temperature of the working fluid in the cycle persists only for a really minute fraction of the cycle time.As a result, fluids with high working temperatures can be used to increase thermal faculty at mode step level best working presss.Weight to authority ratio is less than that of steam turbine and gas turbines.It is therefore possible to develop reciprocating IC engines of very meek power output with reasonable thermal dexterity and cost.Higher brake thermal efficiency can be obtained as only a small fraction of heat energy of the go off is dissipated to the cooling system.Initial cost is low.Materials used in the manufacture of gas turbines must be dep removeable and heat resistant in order to sustain the heat generated. Machining operations required for gas turbines construction ar excessively more complex.Reciprocating IC engines are more efficient at idle rushs than gas turbines in terms of enkindle consumption at idling.Gas turbines have delayed responses to different power requirements changes.Gas turbines must be removed for overhaul and servicing, which is usually non the case in internal combustion engines.Gas turbines require more ambiance than IC engines for its normal operation. It also consumes more go off whenever the load fluctuates, which is common in the domestic usage.All these explain why passenger cars do not use gas turbine engines, but use internal combustion engines instead. distrust 2Define the following parameters and give typical value for spark- discharge and compression ign ition IC enginesSpecific go off consumption,Specific elicit consumption (SFC) is the open fire die hard rate per unit power output . It measures how efficiency of an engine in using the evoke to produce useful work.The equation for the specific fuel consumption isWhereKe= specific fuel consumptionK Fuel Consumption, kg/sPe=Useful work per cycle, i = 0.5 for 4, 1 for 2ne=real efficiencyH =Heat of Combustion = 42.000 KJ/KgLow values of SFC are obviously desirable.For SI engines typical values of brake specific fuel consumption are about 270 g/kWh. Range (345 285 g/kWh)For CI engines, values are lower and in large engines can go below 200 g/kWh.Range (285 190 g/kWh) 2Mean telling storm,Relative engine performance measure is obtained by dividing the work per cycle by the cylinder intensiveness displaced per cycle. The parameter so obtained has units of force per unit scope and is called the mean rough-and-ready pressure (mep).WhereW=Indicated WorkVh=Piston Displacement (cyli nder) Volume (cc, cm3, lt)H=Length TDC Length BDCFor,Naturally aspirated spark ignition engines, maximum values are in the melt d bear 850 to 1050 kPa at the engine speed where maximum torque is obtained (about 3000 rev/min).Turbocharged automotive spark-ignition engines the maximum bmep is in the 1250 to 1700 kPa range.Naturally aspirated four-stroke diesels, the maximum bmep is in the 700 to 900 kPa rangeTurbocharged four-stroke diesel maximum bmep values are typically in the range 1000 to 1200 kPaTurbocharged aftercooled engines this can rise to 1400 kPaTwo-stroke cycle diesels have alike(p) performance to four-stroke cycle engines. Large low-speed two-stroke cycle engines can achieve bmep values of about 1600 kPa.2Power-torque relation as function of engine rpm,Engine torque is thrifty using a dynamometer. The engine is clamped and the output shaft is connected to the dynamometer rotor. The rotor is coupled electromagnetically, hydraulically, or by mechanical friction to a s tator, which is support in low friction bearings. The stator is balanced keeping the rotor stationary. The torque exerted on the stator with the rotor turning is measured by balance the stator with weights, springs, or pneumatic means.Fig.1 Brake dynamometer- engine torque test 2Torque is a measure of an engines ability to do work and power is the rate at which work is done. The value of engine power measured as described above is called brake power Pb. This power is the usable power delivered by the engine to the load-in this case, a brake.Fig.2 Engine power, torque vs. speed plot 3Correlation between measured force and engine torqueMeasured power (1 PS = 0.736 kW)Conversion between different units may be necessary for power, torque, or angular speed. For example, if rotational speed (revolutions per time) is used in place of angular speed (radians per time), a factor of 2 radians per revolution have to be multiplied.Dividing on the left by 60 seconds per minute and by 1000 watts per kilowatt gives us the following.mboxpower (kW) = frac mboxtorque (Ncdotmboxm) times 2 pi times mboxrotational speed (rpm) 60,000Volumetric efficiencyVolumetric efficiency is the ratio of the mass internal the engine cylinder to the mass of job of the displacement volume at atmospheric conditions. It measures the effectiveness of an engines induction process. Volumetric efficiency is used for four-stroke cycle engines which have a distinct induction process and not for two stroke engines.Where pai is the inlet air densityAlternatively volumetric efficiency can also be defined as,Indicative values 4-Otto 0.7 0.92-Otto 0.5 0.7Typical maximum values of v for naturally aspirated engines are in the range 80 to 90 percent. The volumetric efficiency for diesels is somewhat higher than for SI engines.2Engine real efficiency as function of engine power, fuel consumption and fuel calorific valueThe real engine efficiency of an engine can be found out using the relationWhere,ne=real eff iciencynth= theoretical thermodynamic efficiencyng=quality coefficient (0.4-0.7 Otto 0.6-0.8 diesel engine)nm=mechanical efficiency (0.8)ni=actual efficiency (nth.ng=Pi/Q)K= fuel consumption Kg/sH =Heat of Combustion = 42.000 KJ/KgQuestion 3Describe with simple terms the main air fall down path developing inside the cylinder of IC engines relative to the diver motion make a simple schematic to indicate them.Laser Doppler Velocimetry (LDV) helps us to visualise the charge motion indoors the cylinder with the help of optically transparent research engines. Computational Fluid Dynamics (CFD) can help in validating the average flow field in the cylinder but the process is expensive. One such CFD software is KIVA-4v, which helps to predict the air charge motion. go around flowSwirl is defined as the micro mass rotational motion of charge indoors the cylinder. It is generated by constructing the intake system to give a tangential component to the intake flow as it enters the cylinder . This is done by shaping and contouring the intake manifold, valve ports, and sometimes even the piston face. Swirl enhances the mixing of air and fuel to give a solid compartmentalisation in a short time in modern high-speed engines. It is also responsible for very rapid spreading of flame front during the combustion.Fig.3 Swirl flow in the engine cylinder 3Swirl flow can be generated by changing the geometry of the inlet portFig.4 Geometry of inlet port affecting crack flow 3(a) Deflector wall (b) directed (c) shallow ramp coiled (d) steep ramp helicalSimilarly inlet valve approach geometry can also generate swirl flow by producing net in-cylinder angular momentum of the charge.Fig.5 Inlet valve geometry affecting swirl flow 2Squish flowWhen the piston approaches TDC at the end of the compression stroke, the volume around the outer edges of the combustion bedchamber reduces drastically. New combustion chamber designs have the clearance volume near the centerline of the cyl inder. As the piston approaches TDC, the gas mixture occupying the volume at the outer radius of the cylinder is agonistic radially inward as this outer volume is reduced to near zero. This radial inward motion of the gas mixture is called squish. It adds to other mass motions within the cylinder to mix the air and fuel, and quickly spreads the flame front. Maximum squish velocity usually occurs at about 10bTDC.During combustion, the expansion stroke begins and the volume of the combustion chamber increases. As the piston moves away from TDC, the burning gases are propelled radially outward to fill the now-increasing outer volume along the cylinder walls. This knock over squish helps to spread the flame front during the latter part of combustionPiston motion influences squish as in the case of wedge do and bowl-in combustion chambers.Fig.6 Piston motion generating squish 2(a) Wedge shaped SI combustion chamber (b) bowl-in-piston DI Diesel combustion chamberTumbleAs the piston nea rs TDC, squish motion generates a secondary rotational flow called tumble. This rotation occurs about a circumferential axis near the outer edge of the piston bowlFig.7Tumble- result of piston motion and squish 3TurbulenceDue to the high velocities involved, all flows into, out of, and within engine cylinders are turbulent flows. The riddance to this is those flows in the corners and small crevices of the combustion chamber where the close proximity of the walls dampens out turbulence. As a result of turbulence, thermodynamic transfer rates within an engine are increased by an order of magnitude. Heat transfer, evaporation, mixing, and combustion rates all increase. As engine speed increases, flow rates increase, with a jibe increase in swirl, squish, and turbulence. This increases the real-time rate of fuel evaporation, mixing of the fuel vapor and air, and combustion.Intake turbulent mixture flow Turbulence superimposed on mixture swirlFig.8 Turbulence of the charge within cylin der 4Question 4The Figure below shows a conceptual model of a quasi-steady Diesel combustion plume, as presented by Dec et al in 1997. Indicate the following areas shown on this schematicliquid fuel , productive vapour fuel-air mixture ,fuel-rich premixed flame,initial soot formation ,diffusion flame boundary ,thermal NO production zone ,soot oxidation zone ,25398f1.jpgFig.9 Quasi-steady Diesel combustion plume 5The above figure describes the formation and features of a quasi-steady diesel fuel jet. This model is applicable to large bore, quiescent chamber combustion or a free fuel jet without wall interactions. At the fate of fuel nip, fuel penetrates into the combustion chamber and air which is at a high temperature collectible to end of compression stroke begins to mix with the nebuliser. Fuel absorbs energy from the hot air and evaporates. This process continues until a point where no liquid fuel is present. The point at which this occurs is called the liquid length. This li quid length reduces after the start of combustion but thereafter remains constant until the end of injection. Beyond the liquid length, the rich premixed fuel and air are still heated by the surroundings until they start to react in the rich premixed zone. The products of rich combustion continue downstream and diffuse and mix radially outward until reaching the surrounding cylinder gases. At a location where the rich products and cylinder gases mix to produce a stoichiometric mixture, a diffusion flame is produced. The diffusion flame surrounds the jet in a thin turbulent sheet, which extends upstream towards the beak. The axial distance from the nozzle exit to the diffusion flame is the lift-off length. The lift-off length controls the amount of oxygen mixed into the fuel jet and therefore the stoichiometry. Soot is burned out and NOX is produced on the outside of the diffusion flame, where temperatures are high and oxygen and nitrogen are abundant.Question 5What are the main req uirements of the fuel injection system for a direct injection engine?In recent years, significant progress has been made in the development of advanced computer-controlled fuel injection systems, which has had much to do with the research and development activities related to Direct Injected engines being expanded.6The main requirements of the fuel injection system for a direct injection engine are strong atomised fuel spray independent of chamber pressure conditionsInjection during the compression stroke against pressures up to 20barInjection during the intake stroke against atmospheric pressures with stoichiometric homogeneous mixtureTo have uniform distribution of fuel in a multi cylinder engineTo improve breathing capacity of an engine i.e. volumetric efficiencyTo reduce or eliminate detonationTo prevent fuel loss in the form of scavenging in the case of two stroke engines.For an efficient combustion of a stratified mixture, a stable and compact spray geometry is necessaryInject ion pressure has been determined to be very important for obtaining both effective spray atomization and the required level of spray penetration.Accurate fuel metering (generally a +2% band over the linear flow range)Desirable fuel mass distribution pattern for the applicationMinimal spray skew for both sac and main spraysGood spray axisymmetry over the operating rangeMinimal drippage and zero fuel leakage, particularly for cold operationSmall sac volumeGood low-end linearity between the dynamic flow and the fuel pulse widthSmall pulse-to-pulse variation in fuel quantity and spray characteristicsMinimal variation in the above parameters from unit to unit.Question 6Describe the injection process requirements for direct injection Diesel engines and the evolution of the fuel injection equipment over the last few decades.The functional requirements of the fuel injection system are as followsAccurate fuel metering per engine working cycleInjection time to ensure maximum power, good fuel economy and low emissionsObtain the desirable heat release pattern by control of injection rateAtomisation of the fuelProper spray pattern to ensure better mixing of fuel and airUniform distribution of fuel droplets in the combustion chamberSupply equal quantities of fuel to all cylinders, in the case of multi cylinder enginesEliminate dripping of fuel droplets into the combustion chamber by eliminating injection lag between start and end of injectionEvolution of fuel injection equipmentIn-line pumpFig.10 Layout of In-line fuel injection pump 7Though in-line pumps are primitive injection systems, they are still in use among heavy duty marine engines.Individual fuel pumps fuel each of the injectorsEngine operational speed has a major influence on the fuel injection pressuresAs a result, there is a hydraulic delay between the pressure increase and the start of injectionFuel flows by dint of high pressure connecting pipesFuel injection pressures range from 600 1200 barInjector with discharging in the combustion chamber (the nozzle with one or more holes)Distributor pillow slip pumpsThese are still used in a number of enginesThough it started as mechanically operated, now electronic control modifications have been madeIt has a mechanism which controls the spill valve responsible for cutting off the high pressure generated inside the pumping chamber, and thus, responsible for the termination of injectionOne pumping chamber delivers high pressure to all the injectors of the enginePressure depends on engine speed, so a hydraulic delay exists between the pressure generation and start of injection relatively low injection pressures (up to 1200bar)Fig.11 Distributor type pump (Lucas CAV) 7Unit injectorsConsists of the pump and the injector integrated into one body, which does not require a high pressure liaison pipeHigh fuel pressure is generated close to the nozzle exit, which can be upto 2500 bar.These gave accurate control over injectionEach cylinder has its own individual systemHigh pressure developed depends of the engine rpm and the load.Fig.12 General Layout of Unit injector 76Delphi Diesel Systems electronic unit injectors (EUI fig13.) control the quantity and the timing of injection electronically through a solenoid actuator. The solenoid can respond very quickly (injection periods are of the order 1 ms), to control very high injection pressures (up to 1600 bar or so). The solenoid controls a spill valve, which in turn controls the injection process. The pumping element is operated directly from a camshaft (or indirectly via a rocker), and the whole assembly is contained within the cylinder head.Fig.13 Electronic Unit injector (Lucas EUI system) 76An alternative approach to the EUI is the Caterpillar Hydraulic Electronic Unit Injector (HEUI, also supplied by other manufacturers). HEUI uses a hydraulic pressure intensifier system with a 7 1 pressure ratio to generate the injection pressures. The hydraulic pressure is generated by pump ing engine lubricant to a controllable high pressure. Similar to CR injection systems, there is control of the injection pressure. The HEUI uses a two-stage valve to control the oil pressure, and this is able to control the rate at which the fuel pressure rises, thereby controlling the rate of injection, because a lower injection rate can help control NOx emissions. putting green rails fuel injection systemsOne of the last improvements to the fuel injection system is the Common Rail System that was implemented first by the Fiat Company.Fig.14 Common rail fuel injection system 8Common rail (CR) fuel injection systems decouple the pressure generation from the injection process and have beseem popular because of the possibilities offered by electronic control.The key elements of a CR fuel injection system are as followsA (controllable) high-pressure pumpThe fuel rail with a pressure sensorElectronically controlled injectorsAn engine management system (EMS)The injector is an electro-hy draulic device, in which a control valve determines whether or not the injector needle lifts from its seat. The engine management system can divide the injection process into four phases two pilot injections, main injection, and post-injection (for supplying a controlled quantity of hydrocarbons as a reducing agent for NOx catalysts). Common rail injection also enables a high output to be achieved at a comparatively low engine speedFuel injectorsFig.15 Types of nozzles used in Diesel fuel injectors 1