AIR STANDARD ASSUMPTIONS

Transcription

1 AIR SANDARD ASSUMIONS In powe enges, enegy is poided by bung fuel with the system boundaies, i.e., tenal combustion enges. he followg assumptions ae commonly known as the aistandad assumptions: he wokg fluid is ai, which contuously ciculates a closed loop (cycle). Ai is consideed as ideal gas. All the pocesses (ideal) powe cycles ae tenally eesible. Combustion pocess is modeled by a heat-addition pocess fom an extenal souce. he exhaust pocess is modeled by a heat-ejection pocess that estoes the wokg fluid (ai) at its itial state. Assumg constant specific heats, (@5 C) fo ai, is called cold-ai-standad assumption. Some Defitions fo Recipocation Enges he ecipocation enge is one the most common maches that is beg used a wide aiety of applications fom automobiles to aicafts to ships, etc. Intake ale Exhaust ale Boe DC Stoke Fig. : Recipocation enge. op dead cente (DC): he position of the piston when it foms the smallest olume the cylde. Bottom dead cente (): he position of the piston when it foms the lagest olume the cylde. Stoke: he lagest distance that piston taels one diection. Boe: he diamete of the piston. Cleaance olume: he mimum olume fomed the cylde when the piston is at DC. Displacement olume: he olume displaced by the piston as it moes between the DC and. M. Bahami MECH0 (F07) ICE Cycles

2 Compession atio: he atio of maximum to mimum (cleaance) olumes the cylde: max m Mean effectie pessue (ME): A fictitious (constant though the cycle) pessue that if acted on the piston will poduce the wok. W net ME x A iston x Stoke ME x Displacement ol. ME W net max m DC ( ka) An enge with highe ME will poduce lage net put wok. Intenal Combustion Enges. spak ignition enges: A mixtue of fuel and ai is ignited by a spak plug Applications euig powe to ab 5 kw (00 H) Relatiely light and low cost. compession ignition enge: Ai is compessed to a high enough pessue and tempeatue that combustion occus when the fuel is jected Applications whee fuel economy and elatiely lage amounts of powe ae euied. Low pm. Spak-Ignition (Gasole) Enge Spak Ai +Fuel Combustion poducts Compession stoke (-) owe (expansion) stoke (-) Exhaust stoke (-) Intake stoke (-) Fig. : Actual cycle fo spak-ignition enges, fou-stoke. M. Bahami MECH0 (F07) ICE Cycles

3 Fig. : - diagam fo spak-ignition enges. Otto Cycle he Otto cycle is the ideal cycle fo spak-ignition ecipocatg enges. It sees as the theoetical model fo the gasole enge: Consists of fou tenally eesible pocesses Heat is tansfeed to the wokg fluid at constant olume he Otto cycle consists of fou tenally eesible pocesses seies: - compession - Constant-olume heat addition - expansion - Constant-olume heat ejection Q Const. Const. Q s DC Fig. : -s and - diagams fo Otto cycle. M. Bahami MECH0 (F07) ICE Cycles

4 he Otto cycle is executed a closed system. Neglectg the changes potential and ketic enegies, the st law, on a unit mass base, can be witten: ( ) + ( w w ) Δu ( kj / kg) whee u ( ) ( ) hemal efficiency can be witten : η th, Otto u u w u net c c ( / ) ( / ) ocesses - and - ae isentopic, and and. hus, k ηth, Otto k whee is called the compession atio: max m ypical compession atios fo spak-ignition enges ae between 7 and 0. he themal efficiency ceases as the compession atio is ceased. Howee, high compession atios can lead to auto ignition o enge knock. ypical themal efficiency fo Otto cycles is 0-5%. Diesel Enge he diesel cycle is the ideal cycle fo compession ignition enges. It is ey simila to spak-ignition, expect the method of ignition. In diesel enge, ai compessed to a tempeatue that is aboe the ignition tempeatue of the fuel. k Q Const. Const. Q s DC Fig. 5: -s and - diagam of Diesel enge. M. Bahami MECH0 (F07) ICE Cycles

5 he themal efficiency of the diesel enge unde the cold ai standad assumptions becomes: η th, Diesel whee max m w net k k c k( c ) fo Otto We also defe the cutoff atio c, as the atio of cylde olumes afte and befoe the combustion pocess (ignition peiod): c Compaison of the Otto and the Diesel Cycle η Otto > η Diesel fo the same compession atio. Howee, the diesel cycle wok on a much highe compession atio; thus its themal efficiency is highe. ypical themal efficiency of a diesel cycle is 0-5%. Diesel enges bun the fuel moe completely sce they usually opeate at lowe pm and ai-fuel atio is much highe than ignition-spak enges Diesel enges compession atios ae typically between and, wheeas spak-ignition (SI) enges ae between 7 and 0. hus a diesel enge can toleate a highe compession atio sce only ai is compessed a diesel cycle and spak knock is not an issue Dual Cycle (Limited essue Cycle) Combustion pocess tenal combustion enges eithe as constant-olume (Otto cycle) o constant-pessue (Diesel cycle) heat addition is oely simplified and it is not ealistic. Dual cycle is a bette epesentation of the combustion pocess both the gasole and the diesel enges Both the Otto and the Diesel cycles ae special cases of the dual cycle. M. Bahami MECH0 (F07) ICE Cycles 5

6 Const. Q H, Const. Q H, 5 5 Const. s Defg: c p he η Dual Compession atio Cutoff atio essue atio Fig. 6: -s and - diagams fo an ideal dual cycle. themal efficiency of the dual cycle becomes : k [( ) + k ( ) ] p k p c p c Note that when p, we get the diesel enge efficiency. M. Bahami MECH0 (F07) ICE Cycles 6