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Four stroke cycle theory
Intake strokePiston moving downIntake valve openExhaust valve closed
Copyright 2003 Gary Lewis - Dave Capitolo
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 intervalOdd fire V-6 engine (90° block with 120° rod journals)
Four stroke diesel theory
Compression ignitionDiesel fuel low volatilityHigh compression ratios produce the heat necessaryPre-chamber for vaporization
Piston dwell time
Piston travel is at a minimum. . . TDC and BDCCrank moves horizontally
Piston velocityMaximum when rod is 90° to crank
AccelerationMaximum 30° earlier
Best VE is obtained by synchronizing valve openingwith piston speeds
Other engine cycles
OverlapBoth valves are openEnd of exhaust & start of intakeLow pressure in exhaust port
BlowdownExhaust valve opens before BDCTo help evacuate cylinder before piston reversesPumping losses at end of exhaust stroke
Valve events
Intake valve openingBTDCLow pressure in cylinder
Intake valve closingABDCCylinder pressure is effected by timing
Exhaust valve openingBBDCResidual pressure helps blowdown
Exhaust valve closingATDCLow pressure in exhaust port draws air in
Effects on valve timing
Intake valve openingLate – Reduced VEEarly – Dilution of intake with exhaust
Intake valve closingLate – Reduces cylinder pressureEarly – Increases cylinder pressure
Exhaust valve openingLate – Pumping lossesEarly – Power reduction
Exhaust valve closingLate – Reduces vacuumEarly – Reduces VE
Combustion
Spark ignitionMaximum cylinder pressure 15° ATDCTumble and swirlMotion reduces misfiresExcess motion inhibits flowAFR 14.7:1 at part throttle, 12.5:1 under load
Compression ignition18:1 direct injection23:1 pre-chambers for better startingCranking heats air to 600°FCompression heats are to 800-1200 °F
Diesel fuels
Cetane volatility numbers 50-55Higher cetane #1 fuel for cold weatherLower cetane #2 fuel for warm weatherParaffin separates from fuel at 20°F
Valve trains
OHV (overhead valve)Pushrod configurationMany reciprocating partsHigher valve spring pressure requiredCompact engine size compared to OHC
Valve trains
OHC (overhead cam)Fewer reciprocating partsReduced valve spring pressure requiredHigher RPM capabilityCylinder head assemblies are taller
Valve lash compensators
Hydraulic liftersTo maintain zero lashQuieterNo periodic adjustmentAnti-scuff additives are required in oils
Hydraulic lifter operation
Valve closed• Oil flows through lifter bore, &
past check valve• Plunger return spring maintains
zero lash
Hydraulic lifter operation
Valve open• Check valve seats and limits the slippage• Now operates as a solid lifter
Hydraulic lifter operation
Return to valve closed• New oil enters the lifter body• This oil replaces oil that has leaked between plunger and body (predetermined leakage)
Timing sets
Gear sets• Cam and crank rotate in opposite directions• Noisy if not free of burrs• Helical and spur cut gears
Camshaft terminology
Cam lift (A-B)Valve lift = Cam lift times rocker ratio
Valve lift.300” cam lift times1.5 rocker ratio = .450” valve opening
Engine oiling
Full flow oil filtering system• Oil pump output flows through filter first• Bypass circuit for restricted filters will allow oil to flow to engine
Engine oil additives
Viscosity index improvers• To reduce viscosity change with heat
Detergents• To dissolve varnish and sludge
Dispersants• To keep sludge, carbon and other materials from recombining and suspends them in oil to be drained
Scuff inhibitors• To reduce friction and wear
Antifoam and antioxidants• To prevent foaming and to slow oxidation in oil
Engine measurements
Displacement per cylinder• r² S
Displacement for the engine• Disp per cylinder times the Number of cylinders
Engine measurements
Deck clearance• Top of piston to top of block deck• Measured with dial indicator or depth mic
Fits and clearances
Running fit• Clearance between bearing and shaft• Clearance for oil• Listed as diametral
Fits and clearances
Interference (press) fit• OD is larger than ID• Example is piston pin pressed into rod
Fits and clearances of pistons
Full floating• .0003 - .0005 clearance in rod• .0001 - .0003 clearance in piston
Oscillating• .0008 - .0012 interference in rod• .0003 - .0005 clearance in piston
Rod offset• Beam offset to center of cylinder• Enlarged chamfers to clear fillets
Pin offset• Offset to major thrust side • Quieter engine, less cylinder wear
Cooling system operation
Heat energy• 1/3 usable power• 1/3 released through exhaust system• 1/3 released through cooling system
Engine temperature• Cool enough to prevent part failure• Warm enough to maximize engine efficiency
Cooling system operation
Engine heat is transfered . . .• through walls of the combustion chambers• through the walls of cylinders
Coolant flows . . .• to upper radiator hose• through radiator• to water pump• through engine water jackets• through thermostat• back to radiator
Cooling system operation
Fans increase air flow through radiator• Hydraulic fan clutches• Hydraulic fans consume 6 to 8 HP• Electric fans
Coolant (ethylene glycol)• 50/50 mixture increases boiling point to 227°F• pressurizing system to 15 PSI increases to 265°F
Coolant (propylene glycol)• Less protection at the same temperatures• Less toxic
Combustion efficiency
Under perfect conditions . . .• Only byproducts would be carbon dioxide and water• Iso-octane fuel is laboratory fuel
Because conditions are not perfect . . .• Carbon monoxide and hydrocarbons are produced• Oxides of nitrogen are produced from pressure & temp
Emission controls• Catalytic converters – Convert CO & HC to carbon dioxide & water• O2 sensors – To monitor oxygen content in exhaust• EGR – To reduce peak cylinder temperatures