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11-Aug-2014Category

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internal combustion engine - second year machine & equipment dept. theoretical briefcase

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- 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...