AME 436

More documents

Recommendations

Info

! Otto vs. Diesel cycle comparison" Thermal efficiency (ideal cycle, no throttling or friction loss) " th,Diesel =1# 1 & % $ #1 ) r $ #1 ( + ; recall " ' $ (% #1) th,Otto =1# 1 * r ( 1) $ #1 % $ #1 $(% #1) >1 for % >1,$ >1; % $ #1 ,1 as % ,1 $ (% #1) ⇒ For same r, η th (Otto) > η th (Diesel) ⇒ η th (Otto) ≈ η th (Diesel) as β → 1 (small heat input) Lower η th is due to burning at increasing volume, thus decreasing T - thus less efficient Carnot-cycle strips; most efficient burning strategy is at minimum volume, thus maximum T Note that (unlike Otto cycle) η th is dependent on the heat input ! V 4 = T 4 =1+ T 4 "T 3 =1+ fQ R / C P fQ =1+ R V 3 T 3 T 2 r ""1 T 3 Higher heat input ⇒ higher f ⇒ larger β ⇒ lower η th AME 436 - Lecture 8 - Spring 2013 - Ideal cycle analysis 27 ! C P T 2 r ""1 Must have V 4 /V 3 ≤ V 5 /V 3 (otherwise burning is still occurring at bottom of piston travel) end of thus β ≤ r fQ " # r $ 1+ R C P T 2 r # r $ f # (r &1)C PT 2 r % &1 % &1 Q R For r = 20, Q R = 4.3 x 10 7 J/kg, C P = 1400 J/kgK, γ = 1.3, T 2 = 300K, requirement is f < 0.417, which is much greater than stoichiometric f (≈ 0.065) anyway so in practice this limit is never reached Typical f ≈ 0.04 (with other parameters as above): β ≈ 2.67, η th ≈ 0.515 (Diesel) vs. 0.593 (Otto), so difference not large for realistic conditions As with Otto, η th increases as r increases - why not use r → ∞ (η th → 1) Unlike Otto, knock is not an issue - Diesel compresses air, not fuel/air mixture; main reason: heat losses Since knock is not an issue, Diesels use much higher r As gas is compressed more, T increases and volume of gas decreases, increasing ΔT/ΔX for conduction loss As conduction loss increases, compression work lost increases At some point, lost work outweighs inherently higher η th of cycle having higher r Also - as r increases, peak pressure increases - larger mechanical stresses for little improvement in η th AME 436 - Lecture 8 - Spring 2013 - Ideal cycle analysis 28 Otto vs. Diesel cycle comparison" • 14
Otto vs. Diesel cycle comparison" Pressure (atm) 12.0 10.0 8.0 6.0 4.0 2.0 Compression Combustion Expansion Blowdown Intake Exhaust Intake start 1 2 3 4 5 6 7 Otto Diesel P-V diagram 0.0 0.E+00 1.E-04 2.E-04 3.E-04 4.E-04 5.E-04 6.E-04 Cylinder volume (m^3) Unthrottled Otto & Diesel with same compression ratio & heat input: Otto has higher peak P & T, more work output AME 436 - Lecture 8 - Spring 2013 - Ideal cycle analysis 29 Temperature (K) 1200 1000 800 600 400 200 Compression Combustion Expansion Blowdown Intake Exhaust Close T-s cycle 1 2 3 4 5 6 7 Otto vs. Diesel cycle (animation)" T-s diagram Equal areas Diesel Otto Otto work Diesel work Otto heat input Diesel heat input 0 -200 0 200 400 600 800 Entropy (J/kg-K) Otto clearly has higher η th - every Carnot strip has same T L for both cycles, but every Otto strip has higher T H Unlike Otto cycle, η th for Diesel cannot be determined by inspection of the T - s diagram since each Carnot cycle strip has a different 1 - T L /T H AME 436 - Lecture 8 - Spring 2013 - Ideal cycle analysis 30 • 15
Page 1 and 2: AME 436    Energy and Propulsio
Page 3 and 4: Ideal 4-stroke Otto cycle process"
Page 5 and 6: Effect of compression ratio (Otto)"
Page 7 and 8: Throttling losses" Animation: gros
Page 9 and 10: Throttling loss" Another way to re
Page 11 and 12: Turbocharging & supercharging" 35.0
Page 13: P-V & T-s diagrams for ideal Diesel
Page 17 and 18: Effect of heat input (Diesel)" Ani
Page 19 and 20: Example (continued)" f) Thermal Eff
Page 21 and 22: Complete expansion cycle" Highest
Page 23 and 24: Complete expansion cycle" Thermody
Page 25 and 26: Ronney’s catechism (3/4)" So wha
Page 27 and 28: Fuel-air cycle analysis using GASEQ
Page 29 and 30: Example" Repeat the previous exampl

AME 436

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?