Assumptions in thermodynamics Air cycles 1) Perfect gas ( It heats.) 'obey's the gas laws and has constant specific (Without internal friction 2) compression and Expansion process are reversible (isentiop )c adiabatic 3) Combustion process is replaced by heat addition process from an external source. 4) Exhaust process is replaced by heat rejection process from an external source. 5) It should be a closed cycle with the the same air' always remaining in the cylindes to repeat cycle. 6) No chemical reaction takes place in the cylinder 7) Molecular weight of cylinder gas is 29. Cp=1.005 N/kg.k, Cr-0.718 w/kg.k cycle A cycle is, a repeated series of operations occuring in a certain order The farmer imaginary perfect engine is called ideal cycle and the latter actual engine is called actual cycle. you Air standard Efficiency medium is known as an Air standard efficiency This The efficiency of engine " using air as the working efficiency is oftenly called ideal efficiency The actual efficiency of a cycle is always less than the air standard efficiency of that cycle under idial Conditions. This is taken into account by introducing a new term Relative efficiency" Actual thermal efficiency n relative Air Standard efficiency
cylinder Terminologies. D I Bore 5 chearanu volume TDC clearable Vehase Xe 2 stroke 6 Swept stroke /Displacement 3 TDC 1/4 D'XL) Volum 7 1 swept Total Volume stock Volume V (Tated 4 BDC V- (vi+vs). long Vs volunde) 1 s compression Ratio, r = Vct Vs BPC Vel - Total volume clearance volume. 4 Mean effective pressure Pm Work done Stroke volume Constant volume or Otto cycle P to tripious T Adiabatic VEC 2 to 4 4 1 Adiabatic 1 V= c 1 int V2 V2 V swept Volume Total Volume V The Volume point 1 represents that cylinder is full of air with V1, pressure P and absolute temperature TJ. Line which 1-2 represents the adiabatic Compression of air due to P, V1 and TI change to 12, V2 and I2 respectively. Line 2-3 shows the supply of heat to the air at constant Volume the bame So that P2 and I2 changes to P3 and Is (V3 being as V2) Line 3-4 represents the adiabatic expansion of the air During expansion P3, V3 and T3 change to a final value of it, V4 or V1 and T4 respectively. Line 4-1 shows the rejection of heat by air at constant volume till orginal State (point 1) reaches. maximum temperature is at point 3.
consider 1Kg of our working Substanu): Heat supplied at Constant Volume - Cv (T3-T2) Heat rejected at Constant Volume cv(T4-TI). But, work done L Heat Supplied - Heat rejected = ev(73-7))- Cv TT- II) Efficiency = Work done = Cv CT3 72) - Cv(Ty-II) Heat supplied Cr (T3-T1) = 1- (T4-T1) - I T3-Tv) vi Let Compression ratio, Mc = and 22 expansion ratio, re. 24 23 (These two ratios are same in this cycle) (HC as T2 (21) similarly, - 5-1 71 6-1 T2. . II VI) ve r) T3 24 it us TH r-1 T2 IT (r)r-1 5-1 2 - (in 13 T4 (r) 3 2 and 3 in , we get, giving W Notto- 1- (4-II) 1- T4-M T4(A) Ti(r) Nolt' 1- - . (r) 5-1 this expression is known as the air standard efficiency of the Otto cycle It is cliar from the above enpression that efficiency increases with the increase in the value of r, deu to practical difficulties value of r is limited to about 8.