上游充通大￥ SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lecture 45 Chapter 9 Gas Power Cycles Spring,5/15/2018 Prof.,Dr.Yonghua HUANG 强 T http://cc.sjtu.edu.cn/G2S/site/thermo.html SHANG 1日 ERSITY
Engineering Thermodynamics I Lecture 45 Spring, 5/15/2018 Prof., Dr. Yonghua HUANG Chapter 9 Gas Power Cycles http://cc.sjtu.edu.cn/G2S/site/thermo.html
Internal Combustion Engine -Spark plug or fuel injector Valve Power Top Clearance dead center volume Bore Ignition Cylinder Stroke wall Exhaust valve Exhaust opens Bottom valve dead center LPiston closes Compression Reciprocating Exhaust motion Intake valve closes Intake Crank mechanism 厂、 二一、 Rotary I Top dead 1 Bottom motion center dead centor Displacement 上游充通大率 May15,2018 2 SHANGHA BAO TONG LINIERSITY
May 15, 2018 2 Internal Combustion Engine Ignition
Air Standard Cycles Air standard cycles are idealized cycles based on the following approximations: >A fixed amount of air modeled as an ideal gas is the working fluid. >The combustion process is replaced by a heat transfer from an external source. >There are no exhaust and intake processes as in an actual engine. The cycle is completed by a constant-volume heat transfer process taking place while the piston is at the bottom dead center position. >All processes are internally reversible. Cold air-standard analysis >The specific heats are assumed constant at their ambient temperature values. 上游充通大率 May15,2018 3 SHANGHAI BAO TONG LINIERSITY
May 15, 2018 3 Air Standard Cycles Air standard cycles are idealized cycles based on the following approximations: A fixed amount of air modeled as an ideal gas is the working fluid. The combustion process is replaced by a heat transfer from an external source. There are no exhaust and intake processes as in an actual engine. The cycle is completed by a constant ‐volume heat transfer process taking place while the piston is at the bottom dead center position. All processes are internally reversible. Cold air ‐standard analysis The specific heats are assumed constant at their ambient temperature values
Continue Air Standard Cycles Cycles under consideration: Carnot cycle -maximum cycle efficiency 。 Otto cycle-Spark-ignition engine (SI engine) Diesel cycle-Compression-ignition engine (CI engine) Dual cycle-modern CI engine Brayton cycle-gas turbines Other cycles for interests: ·Stirling cycle ·Ericsson cycle 上游充通大 May15,2018 4 SHANGHAI BAO TONG LINIERSITY
May 15, 2018 4 Continue Air Standard Cycles Cycles under consideration: • Carnot cycle – maximum cycle efficiency • Otto cycle – Spark‐ignition engine (SI engine) • Diesel cycle – Compression‐ignition engine (CI engine) • Dual cycle – modern CI engine • Brayton cycle – gas turbines • Other cycles for interests: • Stirling cycle • Ericsson cycle
Continue Air Standard Cycles Air Standard Carnot Cycle: QH THAS23 T qL=TL ASal P3 Wnet qH -qL P2 Wnet Tth.Carnot Wnet an-qL QH QH P4 nth.Carnot =1-=1-IAs QH TH△S23 P1 S1=S2 S3=S4 Tth.Camot Question: How to have isothermal heat transfer with air as the working fluid? 上游充通大率 May15,2018 5 SHANGHA BAO TONG LINIVERSITY
May 15, 2018 5 Continue Air Standard Cycles Air Standard Carnot Cycle: H H 23 L L 41 net H L q Ts q Ts w qq Question: How to have isothermal heat transfer with air as the working fluid? s s 1=s 2 s 3=s 4 T T H T L 1 2 3 4 q L p 3 p 2 p 4 p 1 q H wnet net H L th,Carnot H H L L 41 th,Carnot H H 23 L th,Carnot H w q q q q q Ts 1 1 q Ts T 1 T
Continue Air Standard Carnot Cycle Answer:Must have work! For an ideal gas,if T=constant,then py=constant! p P2 s=const. S const. P3 3 TH=const. 4 P4 TL=const. V2 V1 VA V3 V 上游充通大率 May15,2018 6 SHANGHAI BAO TONG LINIERSITY
May 15, 2018 6 Continue Air Standard Carnot Cycle Answer: Must have work! v p 1 2 3 4 p 3 p 2 p 4 p 1 TL = const. v 2 v 1 v 3 v 4 TH = const. s = const. s = const. For an ideal gas, if T = constant, then pv = constant!
Continue Air Standard Carnot Cycle S=const ④Physical Devices:: s =const Tu=const. T=const V2 V1 V3 V4 net 1 isentropic isothermal isentropic isothermal compression expansion expansion compression AL 2 2 AH 人●2 1 上游充通大率 May15,2018 7 SHANGHA BAO TONG LINIERSITY
May 15, 2018 7 Continue Air Standard Carnot Cycle Physical Devices: 2 3 4 wnet isentropic compression isentropic expansion isothermal compression isothermal expansion 1 1 2 3 2 4 3 4 1 q L q L q H q H v p 1 2 3 4 p3 p2 p4 p1 TL = const. v2 v1 v3 v4 TH = const. s = const. s = const
Internal (ignition)Combustion Engines Spark ignition (SD: combustion initiated by spark air and fuel can be added together Light and lower in cost,used in automobiles Compression ignition (CD: combustion initiated by auto ignition requires fuel injection to control ignition large power,heavy trucks,locomotives,ships 上游充通大率 May15,2018 8 SHANGHAI BAO TONG LINIERSITY
May 15, 2018 8 Internal (ignition) Combustion Engines Spark ignition (SI): • combustion initiated by spark • air and fuel can be added together • Light and lower in cost, used in automobiles Compression ignition (CI): • combustion initiated by auto ignition • requires fuel injection to control ignition • large power, heavy trucks, locomotives, ships
Spark ignition (SI): Spark Plug Cutaway Connector (to plug wire】 CONVENTIONAL SPARK IGNITION Ceramic Insulator Spark- initiated flame Gasket Electrode 上游充通大率 May15,2018 9 SHANGHA BAO TONG LINIERSITY
May 15, 2018 9 Spark ignition (SI):
Continue Internal Combustion Engines Compression ratio,r MEP r- max BDC min 'min Mean effective pressure,MEP MEP= Wnet net work for one cycle V-V m min displacement volume Notes: 。 MEP would produce the same net work with constant pressure as for actual cycle (includes both expansion and compression) want high MEP (high power density) 上游充通大率 May15,2018 10 HANGHAI HAO TONG LINIVERSITY
May 15, 2018 10 Continue Internal Combustion Engines Compression ratio, r Mean effective pressure, MEP Notes: • MEP would produce the same net work with constant pressure as for actual cycle (includes both expansion and compression) • want high MEP (high power density) max BDC min TDC V V r V V net max min W net work for one cycle MEP V V displacement volume v p MEP Vmin Vmax
