hpter 4 he second lw of thermodynmics Directions of thermodynmic processes et engines Internl-combustion engines Refrigertors he second lw of thermodynmics he rnotcycle Entropy
Directions of thermodynmic processes hermodynmic processes re nturlly irreversible processes. he second lw of thermodynmics cn determine the preferred direction for the processes. In reversible process, the thermodynmic process cn be reversed nd re thus equilibrium processes. In qusi-equilibrium process, the system cn keep close to equilibrium stte nd nerly reversible.
et engine et engine Device tht trnsforms het prtly into work or mechnicl energy. Mtter inside the engine undergoes inflow nd outflow of het, expnsion nd compression, nd sometimes chnges of phse. he simplest process is cyclic process. het engine bsorbs het from source t higher temperture het engine rejects some het t lower temperture. U 2 -U 0 -W or W in cycle
Energy flow digrm of het engine het is supplied to the engine by the hot reservoir ( the mount of het show by the width of pipelines). het is rejected from the engine into the cold reservoir in the exhust. he portion of the het supplied by the engine converts to mechnicl work (W). W + - herml efficiency e W
Internl-combustion engines here re generlly four strokes in combustion engine. Intke stroke intke vlve opens, piston descends, volume increses from minimum to mximum r ( r is compression rtio). ompression stroke intke vlve closes, piston compresses dibticlly to volume. Power stroke sprk plug ignites, het gs expnds dibticlly bck to volume r. Exhust stroke exhust vlve opens, the combusted gs re pushed out.
he Otto cycle he Otto cycle n idelized model of the thermodynmic process in gsoline engine. from to b, the system compresses dibticlly. from b to c, het is dded by burning gsoline by isochoric process. DU n ( b ) > 0 from c to d, the system expnds dibticlly. from d to, the gs is cooled to the temperture of the outside ir, het is rejected by isochoric process. DU n ( d ) < 0
he Otto cycle b d b e + + dibtic process ( ) b r γ γ nd ( ) c d r γ γ ( )( ) ( ) d d d d d r r r r r r e γ γ γ γ γ γ + + finlly, r e γ herml efficiency in Otto cycle.
he Diesel cycle Diesel engine is similr in opertion to the gsoline engine. he difference from gsoline engine is tht no fuel t the beginning of the compression stroke. he fuel is injected to the engine t little before the beginning of the power stroke. he high temperture occur when the system is compressed nd is enough for ignition without sprk plug. r for diesel engine is greter thn the gsoline engine (r 5-20). his engine hs more efficiency thn gsoline engine, hevier, need no ignition system, nd hrder to strt.
he diesel cycle
Refrigertors Refrigertor et engine operting in reverse. Refrigertor tkes het from cold plce nd give it off to wrmer plce. Refrigertor requires net input of mechnicl work. > 0, < 0 nd W< 0 First lw + W + W coefficient of performnce (K) K W
Refrigertors compressor compresses the refrigernt fluid dibticlly. the fluid with high temperture is delivered to the condenser coil nd het is give off to surrounding. the fluid expnds dibticlly into the evportor controlled by the expnsion vlve. while expnding, the fluid temperture decrese until lower thn the surrounding t et from surrounding cn be bsorbed, vporizes, nd then sent into the compressor. K W t Pt P
he second lw of thermodynmics Sttement of impossibility It is impossible for ny system to undergo process in which it bsorbs het from reservoir t single temperture nd converts het completely into mechnicl work, with the system ending in the sme stte in which it begn. One sttement of second lw of thermodynmics he conversion of work to het is irreversible process. he het flow from hot to cold cross finite temperture grdient is irreversible process.
he rnotcycle he rnot cycle he het engine tht hs the mximum possible efficiency consistent with the second lw et flow should be t sme temperture. isotherml process work should converse from internl energy without het trnsfer dibtic process
he rnotcycle he gs expnds isothermlly t, bsorbing (b). b Wb nr ln It expnds dibticlly until its temperture drops to (bc). It is compressed isothermlly t, rejecting het (cd). d Wcd nr ln < 0 c It is compressed dibticlly bck to its initil stte t (d). ln ln ( d ) ( ) b
he rnotcycle ( ) ( ) b d n n l l dibtic process: ( ) b γ γ ( ) d γ γ d c b ( ) ( ) b d n n l l het trnsfer in rnot engine rnot e het trnsfer in rnot engine
he rnotrefrigertor he rnot cycle is reversible therefore ll process, if revesed, give the rnot refrigertor. K K rnot
Problem 4.:rnot engine ท างานท อ ณหภ ม 850 K และ 300 K และให งานออกมา 200 J จงหา. ค า e 2. ค าก าล งของเคร องยนต 3. ค า และ
Entropy he second lw cn be stted s quntittive reltion with the concept of Entropy. Severl processes tht proceed nturlly in the direction of incresing disorder. Entropy (S) provides quntittive mesure of disorder. idel gs in isotherml process nr d dw d sign of disorder d d nr S S S 2 d S 2 infinitesiml reversible process d reversible process
Entropy in cyclic process rnot cycle 0 + 0 S S + 0 d S cyclic he totl entropy chnge during ny reversible cycle is zero
Problem 4.2: จาก Problem 4. จงหาการเปล ยนแปลงเอนโทรป ของว ฏจ กร คาร โนร ด งกล าว
Problem 4.3: จงหาค าการเปล ยนแปลงเอนโทรป ของระบบแกสอ ดมคต จ านวน โมล ท ม การขยายต วแบบ Isotherml จนกระท งม ปร มาตรเป น 2 เท าของปร มาตรเด ม
Entropy nd the second lw When ll systems tking prt in process re included, the entropy either remins constnt or increses. No process is possible in which the totl entropy decreses, when ll systems tking prt in the process re included. S universe S 0 is incresing he end. good luck in your test.