上游充通大学 SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lecture 45 Chapter 9 Gas Power Cycles Spring,2017 Prof.,Dr.Yonghua HUANG 强 目e http://cc.sjtu.edu.cn/G2S/site/thermo.html 1日G
Engineering Thermodynamics I Lecture 45 Spring, 2017 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 closes Compression Reciprocating LPiston Exhaust motion Intake valve closes Intake Crank mechanism Rotary Top dead 1 Bottom 1 motion center dead centr Displacement 上游充通大 Monday,May 08,2017 2 SHANGHAI JIAO TONG UNIVERSITY
Monday, May 08, 2017 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. 上游气通大学 Monday,May 08,2017 3 SHANGHAI JLAO TONG UNIVERSITY
Monday, May 08, 2017 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 上游气通大粤 Monday,May 08,2017 4 SHANGHAI JLAO TONG UNIVERSITY
Monday, May 08, 2017 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=TH△S23 T QL=TL AS4 P3 Wnet qH -qL P2 Wnet nth,Camnot Wne _au -qL QH P4 =1-l=1-A 4 nth.Camnot THAS23 QL S1=S2 S3=S4 Tth.Camot =1- Question: How to have isothermal heat transfer with air as the working fluid? 上游充通大 Monday,May 08,2017 5 SHANGHAI JLAO TONG UNIVERSITY
Monday, May 08, 2017 5 Continue Air Standard Cycles Air Standard Carnot Cycle: H H 23 L L 41 net H L q T s q T s w q q Question: How to have isothermal heat transfer with air as the working fluid? s s 1=s2 s3=s4 T TH TL 1 2 3 4 qL p3 p2 p4 p1 qH 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 T s 1 1 q T s 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 =const. V1 V3 V4 上游通大学 Monday,May 08,2017 6 SHANGHAI JIAO TONG UNIVERSITY
Monday, May 08, 2017 6 Continue Air Standard Carnot Cycle Answer: Must have work! v p 1 2 3 4 p3 p2 p4 p1 TL = const. v2 v1 v3 v4 TH = const. s = const. s = const. For an ideal gas, if T = constant, then pv = constant!
Continue Air Standard Carnot Cycle s=const Physical Devices: s=const. TH=const. TL=const. V2 V1 V3 V4 1 isentropic isothermal isentropic isothermal compression expansion expansion compression QL 2 2 QH 1 1 3 土4 上降文通大学 Monday,May 08,2017 7 SHANGHAI JIAO TONG UNIVERSITY
Monday, May 08, 2017 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 qL qL qH qH 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 上游通大学 Monday,May 08,2017 8 SHANGHAI JLAO TONG UNIVERSITY
Monday, May 08, 2017 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 上游充通大学 Monday,May 08,2017 9 SHANGHAI JIAO TONG UNIVERSITY
Monday, May 08, 2017 9 Spark ignition (SI):
Continue Internal Combustion Engines p Compression ratio,r MEP… r= max BDC min min Mean effective pressure,MEP Vmav W MEP net work for one cycle displacement volume max min Notes: o MEP would produce the same net work with constant pressure as for actual cycle (includes both expansion and compression) e want high MEP (high power density) 上游充通大 Monday,May 08,2017 10 SHANGHAI JLAO TONG UNIVERSITY
Monday, May 08, 2017 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
