1 1 Internal combustion Engines: Gas exchange processes and in- cylinder charge motion-fuel injection, combustion in IC engines, thermochemistry of fuel-air mixtures Dr. Primal Fernando [email protected]Ph: (081) 2393608 Fuel injection • Fuel injection is a system for mixing fuel with air in an internal combustion engine. It has become the primary fuel delivery system used in gasoline automotive engines, having almost completely replaced carburetors in the late 1980s. • The carburetor was invented by Karl Benz (founder of Mercedes‐Benz) in 1885 and patented in 1886. • Carburetors were the usual fuel delivery method for almost all gasoline (petrol)‐ 2 fuelled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery. In the U.S. market, the last carbureted cars were the 1990 Oldsmobile Custom Cruiser, Buick Estate Wagon, and Subaru Justy, and the last carbureted light truck was the 1994 Isuzu. Elsewhere, Lada cars used carburetors until 1996. A majority of motorcycles still use carburetors due to lower cost and throttle response problems with early injection set ups, but as of 2005, many new models are now being introduced with fuel injection. Carburetors are still found in small engines and in older or specialized automobiles, such as those designed for stock car racing. • A fuel injection system is designed and calibrated specifically for the type(s) of fuel it will handle. Most fuel injection systems are for gasoline or diesel applications. Venturi‐type Carburetor P+1/2 V 2 = Constant Bernoulli Effect: Valve Stem Fuel Inlet Throttle Plate Air/Fuel Mixture To Engine Atomized Fuel 3 Ref. Obert Constant level is maintained in bowl -as float moves down, valve stem moves down, allowing more fuel into bowl, float moves up and closes valve Float Metering Orifice Choke Plate Fuel Nozzle Inlet Air Bowl Venturi Gasoline Fuel Injection • In SI engines the air and fuel are usually mixed together in the intake system prior to entry to the engine cylinder. • Ratio of air to fuel ≈ 15 : 1 4 • Fuel is injected to trough individual injectors from a low‐pressure fuel supply system into the intake port.
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Internal combustion Engines: Gas exchange processes and in-cylinder charge motion-fuel injection, combustion in IC engines,
thermochemistry of fuel-air mixturesDr. Primal [email protected]: (081) 2393608
Fuel injection• Fuel injection is a system for mixing fuel with air in an internal combustion
engine. It has become the primary fuel delivery system used in gasoline automotive engines, having almost completely replaced carburetors in the late 1980s.
• The carburetor was invented by Karl Benz (founder of Mercedes‐Benz) in 1885 and patented in 1886.
• Carburetors were the usual fuel delivery method for almost all gasoline (petrol)‐
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y g (p )fuelled engines up until the late 1980s, when fuel injection became the preferred method of automotive fuel delivery. In the U.S. market, the last carbureted cars were the 1990 Oldsmobile Custom Cruiser, Buick Estate Wagon, and Subaru Justy, and the last carbureted light truck was the 1994 Isuzu. Elsewhere, Lada cars used carburetors until 1996. A majority of motorcycles still use carburetors due to lower cost and throttle response problems with early injection set ups, but as of 2005, many new models are now being introduced with fuel injection. Carburetors are still found in small engines and in older or specialized automobiles, such as those designed for stock car racing.
• A fuel injection system is designed and calibrated specifically for the type(s) of fuel it will handle. Most fuel injection systems are for gasoline or diesel applications.
Venturi‐type Carburetor
P+1/2 V2 = Constant
Bernoulli Effect:
Valve StemFuel Inlet
Throttle Plate
Air/Fuel Mixture To Engine
Atomized Fuel
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Ref. Obert
Constant level is maintained in bowl -as float moves down, valve stem moves down, allowing more fuel into bowl, float moves up and closes valve
Float
Metering Orifice
Choke Plate
Fuel Nozzle
Inlet Air
Bowl
Venturi
Gasoline Fuel Injection
• In SI engines the air and fuel are usually mixed together in the intake system prior to entry to the engine cylinder.
• Ratio of air to fuel ≈ 15 : 1
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• Fuel is injected to trough individual injectors from a low‐pressure fuel supply system into the intake port.
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Gasoline Fuel Injection‐Injector types
• Mechanical injection using an injection pump driven by the engine.
• Mechanical, driveless, continuous injection.
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j
• Electronically controlled driveless injection.
Fuel Injection (electronic, multi‐port)
Monitored Engine Operating Conditions:
Manifold PressureEngine Speed
Air TemperatureCoolant Temperature
Acceleration
COMPUTERTRIGGER
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50 psi typical
INJECTOR DRIVE UNIT
Pressure Regulator FuelFilter
Fuel Pump
FUEL TANK
Injectors
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Engine Information‐valve timing and volume relationship
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Engine Information‐valve timing and volume relationship• Between 10‐40 crank angles before TDC electrical discharge
across the spark plug starts the combustion process. Typically duration of burning 40 ‐ 60 crank degrees.
• About 2/3 of the way through the expansion stroke, the exhaust valve starts to opens. It open before the end of the expansion
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p p pstroke. It remains open until just after TDC.
• The intake opens just before TDC.
• Valves open and closes slowly to reduce noise and excessive cam wear. Valve open period often overlaps.
Valve Timing Profile
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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
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
in CI engines (CI:12 to 24), compressed to pressure about 4 Mpa d t t b t 800 K
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and temperature about 800 K.• 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.
Comparison of SI and CI Engines
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Typical Brake Thermal Efficiencies of CI and SI Engines
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15 16
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Four Stroke & Two stroke SIEngines
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Four Stroke & Two Stroke CI Engines
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Diesel engine (CI)
• 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 auto ignition starts.
• At full load air fuel ratio is ≈ 20: 1
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• At full load air fuel ratio is ≈ 20: 1
Diesel fuel‐injection system consists of
1. Injection pump
2. Delivery pipes
3. Fuel injector nozzles
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THE DIESEL FUEL SYSTEM
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• Injection Pump usually mechanical drive
–Belts and rollers not good, use gears and chains• Note spill line from injector, pump, separator
Timing sets
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Gear sets• Cam and crank rotate in opposite directions• Noisy if not free of burrs• Helical and spur cut gears
Timing sets
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Timing chains• Single and double roller• Tensioners
Fuel Injection Systems
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General Characteristics
• Pump runs at ½ engine speed
–Controls Quantity AND timing of injection
–Max fuel limited by smoke limit
• How does timing vary with load?
–Ignition delay is SHORTER (higher density) BUT:
–Although ignition delay is shorted, still need more advance to ensure all fuel is burnt during stroke
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–Timing varies with load and speed
–Timing accurate to 1o crank angle
• At max load fuel variance among cylinders should be less than 3% otherwise power limited by smoky exhaust of richest cyl.
A pump ain’t so simple!
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Layout of conventional fuel system
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In‐Line Pumps (most common)‐a set of cam driven plungers (one for each cylinder)
• Driven from crank ½ speed
• Multi‐lobe cam
• This example uses rack, not lever
• Rack rotates plunger assy and
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p g ycontrols flow
• Governor and advance coupling driven by rotating weights acting against a spring (like mechanical advance on distributor)
• Fuel trapped in the plunger is forced through a check valve into the injection line. The injection nozzle has one or more holes through which the fuel is sprayed to cylinder.
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Plunger Design – Traditional Injection Pump
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• Plunger forces fuel through fitting
• Rotating Lever controls how much spills back – lever controls fuel flow (no throttle)
• All run by cam driven by crank
Plungers
• Operation:
–Plunger moves up and blocks inlet
–Fuel is allowed to escape through spill port (notice helical grove)
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grove)
–Reminder of fuel forced out outlet port
–Stroke is constant by delivery varied by rotation
Rotary Pump
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• Much less complicated but lower pressures
• Few moving parts
• Fed by transfer pump
• Metering through governor mechanism – rotor slides
• Pressurization via sliding pistons
Typical Rotary Pump
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Fuel Injectors
• Nozzle type dictates performance
• Single Hole
–Good for ID–1mm hard to clog
• Multi hole
–Better misting
–Easy clog as size ‐> 0.1mm
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• Clogs caused by decomp of leaked fuel
• Differential pressures cause opening
• Note needle design – pressure OPENS nozzle
• Differential pressures
–f(needle diameter vs. seat diameter)
–Spring closing–Harder to open than to keep open
• Smaller seat contact area and strong spring enhance sealing, eliminate dribble
• Dribble leads to emissions and deposits
Operation of needle
• This is why it’s easier to keep a needle open than to open it initially
• Good idea to provide pressure release mechanism to fast and accurate closing