: 1

Conversion of Thermal EnergyAlmost all of the mechanical energy produced today is produced from the conversion of thermal energy in some sort of heat engine. The operation of all heat-engine cycles can usually be approximated by an ideal thermodynamic power cycle of some kind.A basic understanding of these cycles can often show the power engineer how to improve the operation and performance of the system. 2

Conversion of Thermal EnergyAlmost all of the mechanical energy produced today is produced from the conversion of thermal energy in some sort of heat engine. The operation of all heat-engine cycles can usually be approximated by an ideal thermodynamic power cycle of some kind.A basic understanding of these cycles can often show the power engineer how to improve the operation and performance of the system. 3

P- and T-s Diagrams of Power CyclesThe area under the heat addition process on a T-sdiagram is a geometric measure of the total heatsupplied during the cycle qin, and the area under theheat rejection process is a measure of the total heatrejected qout. The difference between these two (thearea enclosed by the cyclic curve) is the net heattransfer, which is also the net work produced duringthe cycle. 4

Reversible Heat-Engine CyclesThe second law of thermodynamics states that it is impossible to construct a heat engine or to develop a power cycle that has a thermal efficiency of 100%. This means that at least part of the thermal energy transferred to a power cycle must be transferred to a low-temperature sink.There are four phenomena that render any thermodynamic process irreversible. They are: Friction Unrestrained expansion Mixing of different substances Transfer of heat across a finite temperature difference 5

Categorize Cycles Thermodynamic cycles can be divided into twogeneral categories: Power cycles and refrigerationcycles. Thermodynamic cycles can also be categorized asgas cycles or vapor cycles, depending upon the phaseof the working fluid. Thermodynamic cycles can be categorized yetanother way: closed and open cycles. Heat engines are categorized as internal or externalcombustion engines. 6

Air-Standard AssumptionsTo reduce the analysis of an actual gas power cycle to a manageable level, we utilize the following approximations, commonly know as the air- standard assumptions:1. The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.2. All the processes that make up the cycle are internally reversible.3. The combustion process is replaced by a heat- addition process from an external source.4. The exhaust process is replaced by a heat rejection process that restores the working fluid to its initial state. 7

Air-Standard CycleAnother assumption that is often utilized to simplifythe analysis even more is that the air has constantspecific heats whose values are determined at roomtemperature (25oC, or 77oF). When this assumption isutilized, the air-standard assumptions are called thecold-air-standard assumptions. A cycle for which theair-standard assumptions are applicable is frequentlyreferred to as an air-standard cycle.The air-standard assumptions stated above provideconsiderable simplification in the analysis withoutsignificantly deviating from the actual cycles.The simplified model enables us to study qualitativelythe influence of major parameters on the performanceof the actual engines. 8

Bore andstroke of a cylinder 9

Mean Effective PressureThe ratio of the maximum volume formed in the cylinder tothe minimum (clearance) volume is called the compressionratio of the engine. V V r max BDC Vmin VTDCNotice that the compression ratio is avolume ratio and should not beconfused with the pressure ratio.Mean effective pressure (MEP) is afictitious pressure that, if it actedon the piston during the entirepower stroke, would produce thesame amount of net work as thatproduced during the actual cycle. MEP Wnet Vmax Vmin 10

Three Ideal Power CyclesThree ideal power cycles are completely reversible power cycles, called externally reversible power cycles. These three ideal cycles are the Carnot cycle, the Ericsson cycle, and the Stirling Cycle. 11

Three Ideal Power CyclesThe Carnot cycle is an externally reversible power cycle and is sometimes referred to as the optimum power cycle in thermodynamic textbooks. It is composed of two reversible isothermal processes and two reversible adiabatic (isentropic) processes.The Ericsson power cycle is another heat-engine cycle that is completely reversible or externally reversible. It is composed of two reversible isothermal processes and two reversible isobaric processes (with regenerator).The Stirling cycle is also an externally reversible heat- engine cycle and is the only one of the three ideal power cycles that has seen considerable practical application. It is composed of two reversible isothermal processes and two reversible isometric (constant volume) processes. 12

Carnot Cycle and Its Value in EngineeringThe Carnot cycle is composedof four totally reversibleprocesses: isothermal heataddition, isentropic expansion,isothermal heat rejection, andisentropic compression (asshown in the P- diagram atright). The Carnot cycle can beexecuted in a closed system (apiston-cylinder device) or asteady-flow system (utilizingtwo turbines and two TL th ,Carnot 1compressors), and either a gas THor vapor can be used as theworking fluid. 13

Internal-Combustion Engine Cycles Internal-combustion (IC) engines cannot operate onan ideal reversible heat-engine cycle but they can beapproximated by internally reversible cycles in whichall the processes are reversible except the heat-addition and heat-rejection processes.In general, IC engines are more polluting than external-combustion (EC) engines because of the formation of nitrogen oxides, carbon dioxide, and unburned hydrocarbons.The Otto cycle is the basic thermodynamic power cycle for the spark-ignition (SI), internal- combustion engine. 14

The Ideal Air Standard Otto Cycle 15

Otto Cycle: The ideal Cycle for Spark-Ignition EnginesFigures below show the actual and ideal cycles in spark-ignition (SI) engines and their P- diagrams. 16

Ideal Otto Cycle The thermodynamic analysis of the actual four-stroke or two- stroke cycles can be simplified significantly if the air-standard assumptions are utilized. The T- s diagram of the Otto cycle is given in the figure at left.The ideal Otto cycle consists of four internallyreversible processes:12 Isentropic compression23 Constant volume heat addition34 Isentropic expansion41 Constant volume heat rejection 17

Thermal Efficiency of an Otto CycleThe Otto cycle is executed in a closed system, anddisregarding the changes in kinetic and potentialenergies, we haveqin qout win wout u qin u3 u 2 Cv T3 T2 qout u 4 u1 Cv T4 T1 wnet qout T4 T1th ,Otto 1 1 qin qin T3 T2 T1 T4 / T1 1 T 1 1 1 1 1 k 1 T2 T3 / T2 1 T2 r k 1 k 1 T1 2 3 T4 Vmax V1 1 Where, ;and r T2 1 4 T3 Vmin V2 2 18

Example IV-4.1: The Ideal Otto CycleAn ideal Otto cycle has acompression ratio of 8. At thebeginning of the compressionprocess, the air is at 100 kPa and17oC, and 800 kJ/kg of heat istransferred to air during theconstant-volume heat-additionprocess. Accounting for the variationof specific heats of air withtemperature,determine a) the maximum temperature and pressurethat occur during the cycle, b) the net work output, c)the thermal efficiency, and d) the mean effectivepressure for the cycle. Solution: 19

a Maximum temperatur e and pressure in an Otto cycle:T1 290K u1 206.91kJ / kg, vr1 676.1Process 1- 2 (isentropic compressio n of an ideal gas) :vr 2 v2 1 vr1 676.1 vr 2 84.51 T2 652.4 K , u 2 475.11kJ / kgvr1 v1 r r 8P2 v2 P v1 T2 v1 652.4 1 P2 P 100 1 v 8 1799.7 kPa T2 T1 T1 2 290Process 2 - 3 (constant volume heat addition) :qin u3 u 2 u3 qin u 2 800 475.11 1275.11kJ / kg T3 1575.1 KP3v3 P2 v2 T3 v2 1575.1 1.797 MPa P3 P2 1 4.345MPa T3 T2 T2 v3 652.4Note : The property vr (relative specific volume) is a dimensionl essquantity used in the analysis of isentropic processes,and should notbe confused w ith the property specific volume. 20

b The net w ork output :Process 3 - 4 (isentropic expansion of an ideal gas) :vr 4 v4 r vr 4 rv r 3 8 6.108 48.864vr 3 v3 T4 795.6 K , u 4 588.74 kJ / kgProcess 4 - 1 (constant volume heat rejection) : qout u1 u 4 qout u 4 u1 588.74 206.91 381.83 kJ / kgThus, wnet qnet qin qout 800 381.83 418.17 kJ / kgc The thermal efficiency: 0.523 or 52.3% wnet 418.17th qin 800Under the cold - air - standard assumption s :th 1 k 1 1 r1 k 1 811.4 0.565 or 56.5% 1 rCare should be exercised in utlizing this assumption s. 21

d The mean effectivepressure is determined from its definition : kPa.m 3 0.287 290K RT1 kg .K m3v1 0.832 P1 100kPa kg wnet wnet 418.17 1kPa.m 3 574.4 kPaThus, mep v1 v2 v v1 0.832 0.832 1 kJ 1 r 8Therefore, a constant pressure of 574.4 kPa during the pow er strokew ould produce the same net w ork output as the entire cycle.Note that this problem could be solved by using equations show nonSlide #17 w ith given constant specific heats c p , cv (at room temperatur e). 22

Diesel Cycle: The Ideal Cycle for Compression-Ignition EnginesThe diesel cycle is the ideal cycle for CI (Compression-Ignition) reciprocating engines. The CI engine firstproposed by Rudolph Diesel in the 1890s, is verysimilar to the SI engine, differing mainly in the methodof initiating combustion. In SI engines (also known asgasoline engines), the air-fuel mixture is compressedto a temperature that is below the autoignitiontemperature of the fuel, and the combustion process isinitiated by firing a spark plug. In CI engines (alsoknown as diesel engines), the air is compressed to atemperature that is above the autoignition temperatureof the fuel, and combustion starts on contact as thefuel is injected into this hot air. Therefore, the sparkplug and carburetor are replaced by a fuel injector indiesel engines. 23

The Ideal Air Standard Diesel Cycle 24

Ideal Cycle for CI Engines (continued)In diesel engines, ONLY air is compressed during thecompression stroke, eliminating the possibility ofautoignition. Therefore, diesel engines can be designedto operate at much higher compression ratios, typicallybetween 12 and 24.The fuel injection process in diesel engines starts whenthe piston approaches TDC and continues during thefirst part of the power stroke. Therefore, thecombustion process in these engines takes place over alonger interval. Because of this longer duration, thecombustion process in the ideal Diesel cycle isapproximated as a constant-pressure heat-additionprocess. In fact, this is the ONLY process where theOtto and the Diesel cycles differ. 25

Ideal Cycle for CI Engines (continued)qin wb ,out u3 u2 qin h3 h2 C p T3 T2 qout u4 u1 Cv T4 T1 wnet qout T4 T1 1 rck 1 th ,Diesel 1 1 1 k 1 qin qin k T3 T2 r k rc 1 Where, 1 r 2 and rc 3 2 26

Thermal efficiency of Ideal Diesel CycleUnder the cold-air-standard assumptions, theefficiency of a Diesel cycle differs from the efficiency ofOtto cycle by the quantity in the brackets. (See Slide#26)The quantity in thebrackets is always greaterthan 1. Therefore, th,Otto> th, Diesel when bothcycles operate on thesame compression ratio.Also the cuttoff ratio, rcdecreases, the efficiencyof the Diesel cycleincreases. (See figure atright) 27

Internal-Combustion EnginesThe two basic types of ignition or firing systems arethe four-stroke-cycle engines, commonly called four-cycle engines, and the two-stroke-cycle engines,commonly called two-cycle engines.The four-cycle engines has a number of advantagesover the usual two-cycle engine, including better fueleconomy, better lubrication, and easier cooling.The two-cycle engine has a number of advantages,including fewer moving parts, lighter weight, andsmoother operation. Some two-cycle engines havevalves and separate lubrication systems. 28

Cylinder Arrangements for Reciprocating EnginesFigure below shows schematic diagrams of some of thedifferent cylinder arrangements for reciprocatingengines. 29

Vertical in-line engine is commonly used today infour- and six-cylinder automobile engines. The V-engine is commonly employed in eight-cylinder (V-8) and some six-cylinder (V-6) automobileengines. The horizontal engine is essentially a V-engine with180o between the opposed cylinders. This system wasused as the four-cylinder, air-cooled engine thatpowered the Volkswagon bug. The opposed-piston engine consists of two pistons,two crankshafts, and one cylinder. The two crankshaftsare geared together to assure synchronization. Theseopposed-piston systems are often employed in largediesel engines. 30

The delta engine is composed of three opposed-piston cylinders connected in a delta arrangement.These systems have found application in the petroleumindustry. The radial engine is composed of a ring of cylindersin one plane. One piston rod, the master rod, isconnected to the single crank on the crankshaft and allthe other piston rods are connected to the master rod.Radial engines have a high power-to-weight ratio andwere commonly employed in large aircraft before theadvent of the turbojet engine. When the term rotary engine is used today, itimplies something other than a radial engine with astationary crank. 31

Engine PerformanceThere are several performance factors that are commonto all engines and prime movers. One of the mainoperating parameters of interest is the actual output ofthe engine. The brake horsepower (Bhp) is the powerdelivered to the driveshaft dynamometer.The brake horsepower is usually measured bydetermining the reaction force on the dynamometerand using the following equation: 2FRNd Bhp 33,000Where F is the net reaction force of the dynamometer,in lbf, R is the radius arm, in ft, and Nd is the angularvelocity of the dynamometer, in rpm. 32

HorsepowerFor a particular engine, the relationship between themean effective pressure (mep) and the power is: mepVdis N p Bhp 33,000 Wnet w here mep Vmax Vmin bore2 stroke Vdis 4 CN e and N p is the number of pow er strokes per minute. Where C is the number of cylinders in the engine, Ne isthe rpm of the engine, and is equal to 1 for a two-stroke-cycle engine and 2 for a four-stroke-cycleengine. 33

Brake Thermal EfficiencyThe brake thermal efficiency of an engine, th, unlikepower plants, is usually based on the lower heatingvalue (LHV) of the fuel. The relationship betweenefficiency and the brake specific fuel consumption(Bsfc) is: 2545 th Bsfc LHV w here Bsfc fuel rate, lbm/h BhpNote that the brake specific fuel consumption (Bsfc) ofan engine is a measure of the fuel economy and isnormally expressed in units of mass of fuel consumedper unit energy output. 34

External-Combustion SystemsExternal-combustion power systems have severaladvantages over internal-combustion systems. Ingeneral, they are less polluting. The primary pollutantsfrom internal-combustion engines are unburnedhydrocarbons, carbon monoxide, and oxides ofnitrogen.In external-combustion engines, the CHx and CO canbe drastically reduced by carrying out the combustionwith excess air and the NOx production can bemarkedly reduced by lowering the combustiontemperature. By burning the fuel with excess air, moreenergy is released per pound of fuel.There are three general ideal external-combustionengine cycles, the Stirling and Brayton are ideal gas-power, and vapor power cycles. 35

Brayton Cycle: The Ideal Cycle for Gas-Turbine EnginesThe Brayton cycle was first proposed by George Braytonfor use in the reciprocating oil-burning engine that hedeveloped around 1870.Fresh air at ambient conditions is drawn into the compressor,where its temperature and pressure are raised. The high-pressure air proceeds into thecombustion chamber, wherethe fuel is burned at constantpressure. The resulting high-temperature gases then enterthe turbine, where theyexpand to the atmosphericpressure, thus producingpower. (An open cycle.) 36

Brayton Cycle (continued)The open gas-turbine cycle can be modeled as a closedcycle, as shown in the figure below, by utilizing the air-standard assumptions.The ideal cycle that the workingfluid undergoes in this closedloop is the Brayton cycle, whichis made up of four internallyreversible processes:12 Isentropic compression (in a compressor)23 Constant pressure heat addition34 Isentropic expansion (in a turbine)41 Constant pressure heat rejection 37

T-s Diagram of Ideal Brayton CycleNotice that all four processesof the Brayton cycle areexecuted in steady-flowdevices (as shown in thefigure on the previous slide,T-s diagram at the right), andthe energy balance for theideal Brayton cycle can beexpressed, on a unit-massbasis, asqin qout win wout hexit hinletw here qin h3 h2 C p T3 T2 and qout h4 h1 C p T4 T1 38

P- Diagram and th of Ideal Brayton CycleThen the thermal efficiency ofthe ideal Brayton cycle underthe cold-air-standardassumptions becomes wnet qoutth ,Brayton 1 qin qin C p T4 T1 T1 T4 / T1 11 1 C p T3 T2 T2 T3 / T2 1 11 rp k 1 / k k 1 / k k 1 / k T P P T3 P w here 2 2 3 , and rp 2 is the pressure ratio. T1 P 1 P 4 T4 P1 39

Thermal Efficiency of the Ideal Brayton CycleUnder the cold-air-standardassumptions, the thermalefficiency of an ideal Braytoncycle increases with both thespecific heat ratio of theworking fluid (if different fromair) and its pressure ratio (asshown in the figure at right) ofthe isentropic compressionprocess.The highest temperature in the cycle occurs at the endof the combustion process, and it is limited by themaximum temperature that the turbine blades canwithstand. This also limits the pressure ratios that canbe used in the cycle. 40

With the demise of the steam powered tractor in thelate 1800s, most modern tractors are equipped withinternal combustion engines. Internal combustion engines are identified by thenumber of strokes in the cycle and by the fuel that is used to run them. Common Tractor Classifications: 4 stroke cycle- gasoline- diesel- LP 41

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IntakeExhaustLubricatingElectricalCoolingFuelHydraulicDrive Train 44

Parts:1. Pre-Cleaner2. Air Cleaner3. Intake Manifold4. Intake Valve5. Turbocharger (if used)6. Intercooler (if used) 45

Parts:1. Exhaust Valve2. Exhaust Manifold3. Muffler4. Cap 46

Parts:1. Crankcase Oil Reservoir (Oil Pan)2. Oil Pump3. Oil Filter4. Oil Passages5. Pressure Regulating Valve Oil goes to:1. Camshaft Bearings2. Crankshaft Main Bearings3. Piston Pin Bearing4. Valve Tappet Shaft 47

Parts:1. Battery2. Ground Cable3. Key Switch4. Ammeter5. Voltage Regulator6. Starter Solenoid7. Starter8. Distributor * Gasoline Only9. Coil10. Alternator11. Spark Plug12. Power Cable 48

Cooling System Liquid & AirParts:1. Radiator2. Pressure Cap3. Fan4. Fan Belt5. Water Pump6. Engine Water Jacket7. Thermostat8. Connecting Hoses9. Liquid or Coolant 49

Cooling SystemAir cooledFins are used to dissipate heatLiquid cooledCoolant is used to dissipate heat. 50

Gasoline Diesel Liquid Propane (LP) Alternate Fuels 51

Parts: Fuel Tank Fuel Pump Carburetor Fuel Filter Fuel Lines 52

Diesel Fuel SystemParts: 1. Fuel Tank 2. Fuel Pump 3. Fuel Filters 4. Injection Pump 5. Injection Nozzles 53

Power Transmission Mechanical & HydraulicParts:1. Clutch Pedal2. Clutch3. Shift Controls4. Transmission5. Differential6. Differential Lock Pedal7. Final Drives8. Power Take Off (PTO) 54

CONVENTIONAL INTERNAL COMBUSTION ENGINESTWO STROKE ENGINES Migrating Combustion Chamber Engine (MCC)FOUR CYCLE ENGINES Conventional Four Cycle (OTTO ENGINE) Rotary Engine (WANKEL) Rotating Cylinder Valve Engine (RCV) 55

TWO STROKE ENGINESTwo-stroke engines do not have valves,which simplifies their construction andlowers their weight.Two-stroke engines fire once everyrevolution, while four-stroke engines fireonce every other revolution. This givestwo-stroke engines a significant powerboost. 56

TWO STROKE ENGINESThese advantages make two-stroke engines lighter, simpler and lessexpensive to manufacture.Two-stroke engines also have the potential to pack about twice thepower into the same space because there are twice as many powerstrokes per revolution.The combination of light weight and twice the power gives two-strokeengines a great power-to-weight ratio compared to many four-strokeengine designs. 57

TWO STROKE ENGINESTwo-stroke engines dont last nearly aslong as four-stroke engines. The lack ofa dedicated lubrication system meansthat the parts of a two-stroke enginewear a lot faster.Two-stroke oil is expensive, and youneed about 4 ounces of it per gallon ofgas. You would burn about a gallon ofoil every 1,000 miles if you used a two-stroke engine in a car. 58

TWO STROKE ENGINESTwo-stroke engines do not use fuelefficiently, so you would get fewer milesper gallon.Two-stroke engines produce a lot ofpollutionso much, in fact, that it is likely that you wontsee them around too much longer. 59

FUELINTAKE 60

COMPRESSION 61

COMBUSTION & EXHAUST 62

TWO STROKE OPERATION TWO STROKE OPERATI ON 63

FOUR CYCLE ENGINESconventional Otto engines 64

FOUR CYCLE ENGINE OPERATION 65

FOUR CYCLE ENGINE CHARACTERISTICSFOUR STROKE ENGINES LASTS LONGER THAN TWO STROKE ENGINES. Thelack of a dedicated lubrication system means that the parts of a two-stroke enginewear a lot faster.FOUR STROKE ENGINES DONT BURN OIL IN COMBUSTION CHAMBER. Two-stroke oil is expensive, and you need about 4 ounces of it per gallon of gas. Youwould burn about a gallon of oil every 1,000 miles if you used a two-stroke engine ina car.FOUR STROKE ENGINES ARE MORE FUEL EFFICIENT. Two-stroke engines donot use fuel efficiently, so you would get fewer miles per gallon.FOUR STROKE ENGINES ARE CLEANER. Two-stroke engines produce a lot ofpollutionINVERTED FLIGHTS MAY NOT BE EASY IN FOUR STROKE ENGINES. Two-stroke engines can work in any orientation, which can be important in acrobaticflights. A standard four-stroke engine may have problems with oil flow unless it isupright, and solving this problem can add complexity to the engine. 66

Unusual Four stroke engines applications ROTARY ENGINES WANKEL ENGINEROTARY CYLINDER VALVE ENGINE RCV ENGINE 67

ROTARY ENGINES Wankel EngineRotary engines use the four-stroke combustioncycle, which is the same cycle that four-strokepiston engines use. But in a rotary engine, this isaccomplished in a completely different way. 68

The heart of a rotary engine is the rotor. This is roughly theequivalent of the pistons in a piston engine. The rotor ismounted on a large circular lobe on the output shaft. Thislobe is offset from the centerline of the shaft and acts like thecrank handle on a winch, giving the rotor the leverage itneeds to turn the output shaft. As the rotor orbits inside thehousing, it pushes the lobe around in tight circles, turningthree times for every one revolution of the rotor. 69

How Rotary Engines Work For every three rotations of the engine shaft corresponds to one complete piston rotation (360 degrees)WANKEL ENGINE OPERATION 70

How Rotary Engines WorkIf you watch carefully, youll see the offsetlobe on the output shaft spinning three timesfor every complete revolution of the rotor. As the rotor moves through the housing, the three chambers created by the rotor change size. This size change produces a pumping action. Lets go through each of the four stokes of the engine looking at one face of the rotor. 71

Four Stroke Gas EnginesThe four strokes of a internal combustion engine are: Intake Compression Power ExhaustEach stroke = 180 ofcrankshaft revolution.Each cycle requires two revolutionsof the crankshaft (720 rotation), andone revolution of the camshaft to complete(360 rotation). 72

Intake StrokeFirst StrokeThe piston moves down the cylinderfrom TDC (Top Dead Center) to BDC(Bottom Dead Center).This movement of piston causes low air pressure in the cylinder (vacuum) Mixture of Air and Fuel in the ratio of 14.7 : 1 (air : fuel) is drawn into the cylinder. Intake valve stays open and the Exhaust valve stays closed during this stroke. 73

This starts at the highest point known as top dead center and ends at bottom dead center The intake stroke allows the piston to suck fuel and air into the combustion chamber through the intake valve 74

Compression strokeSecond strokeThe piston moves from BDC to TDCIntake and exhaust valves stay closedAir and fuel mixture is compressed8:1 to 12:1The pressure in the cylinder is raised 75

Compression starts at bottom dead center and ends at top dead center. The second motion of the stroke takes all the fuel and air that was stored and compresses it into one tenth its original sizes. Making the air/fuel mixture increase in temperature preparing it for the next stage in its combustion cycle. 76

Power strokeThird strokeAt the end of compression strokethe sparkplug fires, igniting the air/fuelmixture. Both the valves stay closed in this stroke.The expanding gases from thecombustion in the cylinder(with no escape) push the pistondown.The piston travels from TDC to BDC. 77

Force acting from pressure Pr e s s u r e In engines theamount of forceexerted on the top ofa piston is A re adetermined by thecylinder pressureduring thecombustion process. 78

The power stroke starts as soon as the piston reaches top dead center allowing the spark plug to ignite. This electric current created by the spark plug ignites the fuel and air mixture sending the piston back down the cylinder with a pressure reaching high as 600 PSI. 79

Exhaust strokeFourth and last strokeThe momentum created by theCounter-weights on the crankshaft,move the piston from BDC to TDC.The exhaust valve opens andthe burned gases escape into theexhaust system.Intake valve remains closed. 80

The final stage of the stroke releases all the burned fuel through the exhaust valve. As the piston moves from bottom dead center to top dead center it takes all the burned fuel and pushes it out of the cylinder, preparing it for the next cycle of strokes. 81

Indicator Diagrams and Internal Combustion Engine Performance Parameters Much can be learned from a record of the cylinder pressure and volume. The results can be analyzed to reveal the rate at which work is being done by the gas on the piston, and the rate at which combustion is occurring. In its simplest form, the cylinder pressure is plotted against volume to give an indicator diagram. 82

Pressure-Volume Graph 4-stroke SI engineOne power stroke for every two crank shaft revolutionsPressure Spark Exhaust valve Exhaust opens valve closes 1 atm Intake valve closes Intake valve opens TC BC Cylinder volume 83

Exhaust Valve : Valve Timing Diagram Pcyl Patm 84

Inlet Valve : Valve Timing DiagramPcyl Patm 85

Valve Timing for Better Flow 86

Efficiency In general, energy conversion efficiency is the ratio between the useful output of a device and the input. For thermal efficiency, the input, to the device is heat, or the heat-content of a fuel that is consumed. The desired output is mechanical work, or heat, or possibly both. Because the input heat normally has a real financial cost, a memorable, generic definition of thermal efficiency is; 87

When expressed as a percentage, the thermal efficiency must be between 0% and 100%. Due to inefficiencies such as friction, heat loss, and other factors, thermal engines efficiencies are typically much less than 100%. For example, a typical gasoline automobile engine operates at around 25% efficiency. The largest diesel engine in the world peaks at 51.7%. 88

Work done on the piston due to pressure 89

The term indicated work is used to define the net work done on the piston per cycle the indicated mean effective pressure (imep),can be defined by; 90

The imep is a hypothetical pressure that would produce the same indicated work if it were to act on the piston throughout the expansion stroke. The concept of imep is useful because it describes the thermodynamic performance of an engine, in a way that is independent of engine size and speed and frictional losses. Unfortunately, not all the work done by the gas on the piston is available as shaft work because there are frictional losses in the engine. These losses can be quantified by the brake mean effective pressure (bmep,), a hypothetical pressure that acts on the piston during the expansion stroke and would lead to the same brake work output in a frictionless engine. 91

Mechanical EfficiencySome of the power generated in the cylinder is used to overcome enginefriction and to pump gas into and out of the engine. The term friction power, W f , is used to describe collectively these powerlosses, such that: W f Wi , g WbFriction power can be measured by motoring the engine.The mechanical efficiency is defined as: Wb Wi , g W f Wf m 1 Wi , g Wi , g Wi , g 92

Mechanical Efficiency, contd Mechanical efficiency depends on pumping losses(throttle position) andfrictional losses (engine design and engine speed). Typical values for automobile engines at WOT are: 90% @2000 RPM and 75% @ maxspeed. Throttling increases pumping power and thus themechanical efficiencydecreases, at idle the mechanical efficiencyapproaches zero. 93

Brake Specific Fuel Consumption (BSFC) is a measure of fuel efficiency within a shaft reciprocating engine. It is the rate of fuel consumption divided by the power produced. Specific fuel consumption is based on the torque delivered by the engine in respect to the fuel mass flow delivered to the engine. Measured after all parasitic engine losses is brake specific fuel consumption [BSFC] and measuring specific fuel consumption based on the in-cylinder pressures (ability of the pressure to do work) is indicated specific fuel consumption [ISFC]. 94

The final parameter to be defined is the volumetric efficiency of the engine; the ratio of actual air flow to that of a perfect engine is In general, it is quite easy to provide an engine with extra fuel; therefore, the power output of an engine will be limited by the amount of air that is admitted to an engine. 95

Volumetric Efficiency Volumetric efficiency a measure of overall effectiveness of engine and its intake and exhaust system as a natural breathing system. It is defined as: 2 ma v r a , 0Vd N If the air density ra,0 is evaluated at inlet manifold conditions, the volumetric efficiency is a measure of breathing performance of the cylinder, inlet port and valve. If the air density ra,0 is evaluated at ambient conditions, the volumetric efficiency is a measure of overall intake and exhaust system and other engine features. The full load value of volumetric efficiency is a design feature of entire engine system. 96

Systems which are thermally insulated from their surroundings undergo processes without any heat transfer; such processes are called adiabatic. Thus during an isentropic process there are no dissipative effects and the system neither absorbs nor gives off heat. A reversible process, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy. Isentropic process is a process which is a process is both adiabatic and reversible . 97

A closed cylinder with a locked piston contains air. The pressure inside is equal to the outside air pressure. This cylinder is heated to a certain target temperature. Since the piston cannot move, the volume is constant, while temperature and pressure rise. When the target temperature is reached, the heating is stopped. The piston is now freed and moves outwards, expanding without exchange of heat (adiabatic expansion). Doing this work cools the air inside the cylinder to below the target temperature. To return to the target temperature (still with a free piston), the air must be heated. This extra heat amounts to about 40% more than the previous amount added. In this example, the amount of heat added with a locked piston is proportional to CV, whereas the total amount of heat added is proportional to CP. Therefore, the heat capacity ratio in this example is 1.4 98

99

Efficiencies of Real Engines The efficiencies of real engines are below those predicted by the ideal air standard cycles for several reasons. Most significantly, the gases in internal combustion engines do not behave perfectly with a ratio of heat capacities. 10 0

Ignition and Combustion in Spark Ignition and Diesel Engines Spark ignition (SI) engines usually have pre-mixed combustion, in which a flame front initiated by a spark propagates across the combustion chamber through the unburned mixture. Compression ignition (CI) engines normally inject their fuel toward the end of the compression stroke, and the combustion is controlled primarily by diffusion. Whether combustion is pre-mixed (as in SI engines) or diffusion controlled (as in CI engines) has a major influence on the range of air-fuel ratios (AFRs) that will burn. In pre-mixed combustion, the AFR must be close to stoichiometric-the AFR value that is chemically correct for complete combustion. In practice, dissociation and the limited time available for combustion will mean that even with the stoichiometric AFR, complete combustion will not occur. In diffusion combustion, much weaker AFRs can be used (i.e., an excess of air) because around each fuel droplet will be a range of flammable AFRs. Typical ranges for the (gravimetric) air-fuel ratio are as follows: 10 1

Diesel engines have a higher maximum efficiency than spark ignition engines for three reasons: The compression ratio is higher. During the initial part of compression, only air is present. The air-fuel mixture is always weak of stoichiometric. 10 2

Simple Combustion Equilibrium For a given combustion device, say a piston engine, how much fuel and air should be injected in order to completely burn both? This question can be answered by balancing the combustion reaction equation for a particular fuel. A stoichiometric mixture contains the exact amount of fuel and oxidizer such that after combustion is completed, all the fuel and oxidizer are consumed to form products. 10 3

Combustion stoichiometry for a general hydrocarbon fuel, with air can be expressed as; The amount of air required for combusting a stoichiometric mixture is called stoichiometric or theoretical air. 10 4

Methods of Quantifying Fuel and Air Content of Combustible Mixtures In practice, fuels are often combusted with an amount of air different from the stoichiometric ratio. If less air than the stoichiometric amount is used, the mixture is described as fuel rich. If excess air is used, the mixture is described as fuel lean. For this reason, it is convenient to quantify the combustible mixture using one of the following commonly used methods: Fuel-Air Ratio (FAR): The fuel-air ratio, f, is given by 10 5

Equivalence Ratio: Normalizing the actual fuel-air ratio by the stoichiometric fuel air ratio gives the equivalence ratio, The subscript s indicates a value at the stoichiometric condition. f 1 is a rich mixture Lambda is the ratio of the actual air-fuel ratio to the stoichiometric air-fuel ratio defined as 10 6

Fuel Requirements Gasoline is a mixture of hydrocarbons (with 4 to approximately 12 carbon atoms) and a boiling point range of approximately 30-200C. Diesel fuel is a mixture of higher molarmass hydrocarbons (typically 12 to 22 carbon atoms), with a boiling point range of approximately180-380C. Fuels for spark ignition engines should vaporize readily and be resistant to self-ignition, as indicated by a high octane rating. In contrast, fuels for compression ignition engines should self- ignite readily, as indicated by a high cetane number. 10 7

Octane number is a standard measure of the anti-knock properties (i.e. the performance) of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating. In broad terms, fuels with a higher octane rating are used in high-compression engines that generally have higher performance. Knocking (also called knock, detonation, spark knock, pinging or pinking) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, Effects of engine knocking range from inconsequential to completely destructive.. 10 8

Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality. 10 9

The octane or cetane rating of a fuel is established by comparing its ignition quality with respect to reference fuels in CFR (Co-operative Fuel Research) engines, according to internationally agreed standards. The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso- octane and n-heptane. 11 0

Engine Knock and thermal Efficiency of an Engine The thermal efficiency of the ideal Otto cycle increases with both the compression ratio and the specific heat ratio. When high compression ratios are used, the temperature of the air-fuel mixture rises above the autoignition temperature produces an audible noise, which is called engine knock. (antiknock, tetraethyl lead? unleaded gas) For a given compression ratio, an ideal Otto cycle using a monatomic gas (such as argon or helium, k = 1.667) as the working fluid will have the highest thermal efficiency. 11 1

Charge Stratification 11 2

Combustion Chamber Designs 11 3

Combustion Chamber Design 11 4

Combustion Chamber Design 11 5

Combustion Chamber Design 11 6

Combustion Chamber Design 11 7

Combustion Chamber Design 11 8

Turbocharging A turbocharger, or turbo, is a centrifugal compressor powered by a turbine that is driven by an engines exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine (forced induction), thereby resulting in greater performance (for either, or both, power and efficiency). They are popularly used with internal combustion engines (e.g., four-stroke engines like Otto cycles and Diesel cycles). 11 9

Engine Artificial Respiratory System: An Inclusion ofCV Turbo-Charged Engine 12 0

Turbo -Charger 12 1