Ses.53-1 MX221 Sessie 53 DRYWINGS-KRINGLOPE (GAS-FASE) Session 53 POWER CYCLES (GAS PHASE) Dr. Jaco Dirker hese slides also appear on Click-UP Hierdie skyfies verskyn ook op Click-UP 8 th edition / 8e uitgawe 10.9 10.10
Ses.53-2 Otto cycle is used to model a spark ignition engine Vonk-aansteking Druk-aansteking Diesel cycle is used to model a compression ignition engine Air is compressed to high pressure (and temp.) Fuel is injected Due to high temperature the fuel automatically ignites here is thus no problem with engine knock. DIESEL CYCLE: 1 2 : Compression (BDC DC) 2 3 : Ignition at constant pressure (DC ) - Heat addition occurs at such a rate that the pressure remains constant 3 4: Expansion (Isentropic) ( BDC) 4 1: Cycle is closed by representing the exhaust and intake processes as heat transfer to the surroundings at a constant volume.
Ses.53-3 PLEASE NOE HA WE ARE WORKING HERE WIH A CONROL MASS (not a control Volume as in the previous sections) 1st Law: q u u w (no dke or dpe) For constant Volume (4 1) : C v q u u 0 q C v For constant Pressure (2 3) : C p q u q h q C p u h P v v
Ses.53-4 Efficiency can be expressed as: Rement th W net 1 Q H Q Q L H After some mathematical and thermodynamic manipulation: th 4 1 1 1 k 2 3 2 1 1 Can not be simplified further (as before) 4 3 In this case because the 1 isentropic compression ratio is greater than the isentropic expansion ratio. (See figure) 2
Ses.53-5 Comparison of the thermal efficiency of the Otto and Diesel Cycles: Having the same compression inlet and outlet states: Selfde kompressor in- en uitlaat toest. Otto cycle has higher efficiency than Diesel cycle (Compare the enclosed area on the S diagram in terms of heat inflow). = bigger Having the same compression ratio (ie max pressure): Selfde samedrukking-verhouding Diesel cycle has higher efficiency than Otto cycle (Compare the enclosed area on the S diagram in terms of heat inflow). = bigger he correct associated comparison is needed deping on what one is interested in. Diesel engines can normally run at compression ratios of up to around 20.
Ses.53-6 Diesel Cycle Example Based on EX10.8 (Ed 8) Values are changed. An air standard Diesel cycle has a compression ratio of 18. he heat transferred to the working fluid is 1600 kj/kg per cycle. At the of the compression process, the pressure is 100 kpa and the temperature is 30ºC. Determine: a) P and at the of each process b) hermal efficiency c) Mean Effective Pressure. n Lug-standaard het n samedrukking-verhouding van 18. Hitte-toevoeging tot die werksvloeier is 1600 kj/kg per siklus. Aan die begin van die same-drukkingsproses is die druk 100 kpa en die temperatuur 30ºC. Bepaal: a) P en na afloop van elke proses b) ermiese Rement c) Gemiddelde Effektiewe Druk
Ses.53-7 Stirling-kringloop 10.10 Stirling Cycle he Stirling cycle is similar to the Otto Cycle. Otto Cycle: 2x const vol processes (at DC and BDC) 2x adiabatic (isentropic) processes (Expansion and Compression) Stirling Cycle: 2x const vol processes 2x Isothermal processes
Ses.53-8 Stirling-kringloop 10.10 Stirling Cycle he ideal Stirling cycle (with regeneration) has the same efficiency as the Carnot Cycle It is used more for academic purposes Seldom implemented practically (various configurations does however exist) Selde prakties toepasbaar