CI vs. SI Engines - University of · PDF fileCI vs. SI Engines • SI engines ... 2...
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Internal combustion Engines: Carburetor Fuel injection Internal combustion Engines: Carburetor, Fuel injection Dr. Primal Fernando 1 [email protected] Ph: (081) 2393608 CI vs. SI Engines • SI engines draw fuel and air into the cylinder. • Fuel must be injected into the cylinder at the desired time of b ti i CI i combustion in CI engines. • Air intake is throttled to the SI engine ‐‐ no throttling in CI engines. • Compression ratios must be high enough to cause auto ignition • Compression ratios must be high enough to cause auto‐ignition in CI engines (CI:12 to 24), compressed to pressure about 4 Mpa and temperature about 800 K. • Upper compression ratio in SI engines is limited by the auto‐ Upper compression ratio in SI engines is limited by the auto ignition temperature (SI: 8 to 12). • Flame front in SI engines smooth and controlled. • CI combustion is rapid and uncontrolled at the beginning. • The valve timing in both CI and SI are similar. 2 Comparison of SI and CI Engines 3 Typical Brake Thermal Efficiencies of CI and SI Engines 4
Transcript of CI vs. SI Engines - University of · PDF fileCI vs. SI Engines • SI engines ... 2...
Internal combustion Engines: Carburetor Fuel injectionInternal combustion Engines: Carburetor, Fuel injection
Dr. Primal Fernando
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[email protected]: (081) 2393608
CI vs. SI Enginesg• SI engines draw fuel and air into the cylinder.• Fuel must be injected into the cylinder at the desired time of
b ti i CI icombustion in CI engines.• Air intake is throttled to the SI engine ‐‐ no throttling in CI
engines.• Compression ratios must be high enough to cause auto ignition• Compression ratios must be high enough to cause auto‐ignition
in CI engines (CI:12 to 24), compressed to pressure about 4 Mpa and temperature about 800 K.
• Upper compression ratio in SI engines is limited by the auto‐Upper compression ratio in SI engines is limited by the autoignition temperature (SI: 8 to 12).
• Flame front in SI engines smooth and controlled.• CI combustion is rapid and uncontrolled at the beginning.• The valve timing in both CI and SI are similar.
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Comparison of SI and CI Enginesp g
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Typical Brake Thermal Efficiencies of CI and SI Engines
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Four Stroke & Two stroke SIEnginesEngines
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Four Stroke & Two Stroke CI Enginesg
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Diesel engine (CI)g ( )
• The liquid fuel jet atomizes into drops and entrains air;• The liquid fuel jet atomizes into drops and entrains air; evaporates‐fuel vapor mixes with air‐air temperature and pressure are above the fuel’s ignition point. After a short delay
t i iti t tauto ignition starts.• At full load air fuel ratio is ≈ 20: 1
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Diesel fuel‐injection system consists ofj y
1. Injection pump
2. Delivery pipes
3. Fuel injector nozzles
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THE DIESEL FUEL SYSTEM
• Injection Pump usually mechanical driveB lt d ll t d d h i
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–Belts and rollers not good, use gears and chains• Note spill line from injector, pump, separator
Timing setsTiming sets
Gear setsGear sets• Cam and crank rotate in opposite directions• Noisy if not free of burrs
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• Helical and spur cut gears
Timing setsTiming sets
Timing chainsTiming chains• Single and double roller• Tensioners
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Fuel Injection Systemsj y
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General Characteristics
P t ½ i d • How does timing vary with load?• Pump runs at ½ engine speed–Controls Quantity AND timing of injectionMax fuel limited by smoke
• How does timing vary with load?–Ignition delay is SHORTER (higher density) BUT:–Although ignition delay is shorted, ll d d ll–Max fuel limited by smoke
limit–Timing varies with load and speed
still need more advance to ensure all fuel is burnt during stroke
• At max load fuel variance among cylinders should be less than 3% speed
–Timing accurate to 1o crank angle
yotherwise power limited by smoky exhaust of richest cyl.
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A pump ain’t so simple!p p p
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Valve trainsValve trains
OHV (overhead valve)Pushrod configurationgMany reciprocating partsHigher valve spring pressure required
i i d
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Compact engine size compared to OHC
Valve trainsValve trains
OHC (overhead cam)Fewer reciprocating partsReduced valve spring pressure requiredHigher RPM capabilit
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Higher RPM capabilityCylinder head assemblies are taller
Valve trainsValve trains
Cam in headCam-in-headNo pushrodsUse rocker arms
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Use rocker arms
Valve Locations
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Combustion process: stratified chargeCombustion process: stratified charge
jet guided wall guided inlet air guided
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Charge Stratificationg
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Combustion Chamber Designsg
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Combustion Chamber Designg
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Combustion Chamber Designg
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Combustion Chamber Designg
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Combustion Chamber Designg
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Combustion Chamber Designg
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CLASSIFICATION OF INTERNAL COMBUSTION ENGINES
CoolingCooling
1. Direct Air‐cooling
2. Indirect Air‐cooling (Liquid Cooling)
3. Low Heat Rejection (Semi‐adiabatic) engine.
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Cooling system operationCooling 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• through radiator• to water pump• through engine water jacketsthrough engine water jackets• through thermostat• back to radiator
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Cooling system operationCooling system operation
Fans increase air flow through radiator• Hydraulic fan clutches• Hydraulic fans consume 6 to 8 HP
El t i f• Electric fans
Coolant (ethylene glycol)Coolant (ethylene glycol)• 50/50 mixture increases boiling point to 227°F• pressurizing system to 15 PSI increases to 265°Fp g y
Coolant (propylene glycol)• Less protection at the same temperatures
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Less protection at the same temperatures• Less toxic
