Chapter5Theory

Chapter5Theory - Four stroke cycle theory Intake stroke...

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Four stroke cycle theory Intake stroke Piston moving down Intake valve open Exhaust valve closed Copyright 2003 Gary Lewis - Dave Capitolo
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Four stroke cycle theory Compression stroke Piston moving up Intake valve closed Exhaust valve closed
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Four stroke cycle theory Power stroke Piston moving down Intake valve closed Exhaust valve closed
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Four stroke cycle theory Exhaust stroke Piston moving up Intake valve closed Exhaust valve open
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Four stroke cycle theory Each stroke takes 180° of crankshaft rotation to complete All cylinders fire in 720° of crankshaft rotation 720 divided by number of cylinders = firing interval Odd fire V-6 engine (90° block with 120° rod journals)
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Piston dwell time Piston travel is at a minimum. . . TDC and BDC Crank moves horizontally Piston velocity Maximum when rod is 90° to crank Acceleration Maximum 30° earlier Best VE is obtained by synchronizing valve opening with piston speeds
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Other engine cycles Overlap Both valves are open End of exhaust & start of intake Low pressure in exhaust port Blowdown Exhaust valve opens before BDC To help evacuate cylinder before piston reverses Pumping losses at end of exhaust stroke
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Valve events Intake valve opening BTDC Low pressure in cylinder Intake valve closing ABDC Cylinder pressure is effected by timing Exhaust valve opening BBDC Residual pressure helps blowdown Exhaust valve closing ATDC Low pressure in exhaust port draws air in
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Effects on valve timing Intake valve opening Late – Reduced VE Early – Dilution of intake with exhaust Intake valve closing Late – Reduces cylinder pressure Early – Increases cylinder pressure Exhaust valve opening Late – Pumping losses Early – Power reduction Exhaust valve closing Late – Reduces vacuum Early – Reduces VE
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Combustion Spark ignition Maximum cylinder pressure 15° ATDC Tumble and swirl Motion reduces misfires Excess motion inhibits flow AFR 14.7:1 at part throttle, 12.5:1 under load Compression ignition 18:1 direct injection 23:1 pre-chambers for better starting Compression heats to 800-1200 °F
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Diesel fuels Cetane volatility numbers 50-55 Higher cetane #1 fuel for cold weather Lower cetane #2 fuel for warm weather Paraffin separates from fuel at 20°F
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Valve trains OHV (overhead valve) Pushrod configuration Many reciprocating parts Higher valve spring pressure required Compact engine size compared to OHC
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This note was uploaded on 01/06/2012 for the course AUTO 94 taught by Professor Davecapitolo during the Fall '03 term at DeAnza College.

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Chapter5Theory - Four stroke cycle theory Intake stroke...

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