fuel injection - faculty of...
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Internal combustion Engines: Carburetor Fuel injectionInternal combustion Engines: Carburetor, Fuel injection
Dr. Primal Fernando
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wpd@pdn.ac.lkPh: (081) 2393608
Fuel injectionj• 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 g g p y p1980s.
• The carburetor was invented by Karl Benz (founder of Mercedes‐Benz) in 1885 and patented in 1886.p
• Carburetors were the usual fuel delivery method for almost all gasoline (petrol)‐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
th 1990 Old bil C t C i B i k E t t W d S bwere 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 injectionas 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
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• 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.
Gas Review November 1913
Used on tractors, boats, and stationaryengines, including the Waterloo Boy d M d l D t t
Gas Review September 1917
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and Model D tractors
Well, lets see if we can figure it out……Well, lets see if we can figure it out……
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Venturi‐type CarburetorVenturi type Carburetor
Bernoulli Effect:
Air/Fuel Mixture To Engine
P+1/2 V2 = Constant Throttle Plate
Atomized Fuel
Valve StemFuel Inlet
Float Venturi
Constant level is maintained in bowl -as
Choke Plate
Fuel N l
Inlet Air
Bowl
float moves down, valve stem moves down, allowing more fuel into bowl, float moves up and
Metering Orifice
Nozzle
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Ref. Obert
closes valve
Carburetor Theoryy
• Venturi• Venturi– What is it?
• Wind blowing in downtown Chicagog g– always stronger in the smaller areas between 2 buildings
River currents• River currents– always faster in a narrower, shallower place than deep, wide pools
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Carburetor Theoryy• It’s all due to Air Pressure (or lack thereof)• Close to sea level pressure is 14 7 psi• Close to sea level pressure is 14.7 psi
– Air has weight – 88 lbs in a 12x12x8 ft room• “Vacuum” is a pressure less than 14.7 psiacuu is a p essu e ess a psi
– Often measured in inches of mercury14.7 psi ~ 30 in Hg
• As engine runs, intake strokes create“vacuum” or lower air pressure in manifoldN l 10 i ( 20 i H )– Normal ~10 psi (~20 in Hg)
• With throttle plate open, carburetor throatexposed to manifold pressure
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exposed to manifold pressure
Carburetor Theoryy
• Carburetors operate on the venturi effect• The venturi is a narrowing of the bore
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The venturi is a narrowing of the bore
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Calculation of Air‐fuel Ratio
The fuel is a liquid before mixing with the air, it can be taken to be incompressibleincompressible.
We can apply Bernoulli’s equation between the atmospheric
di i ili h fconditions prevailing at the top of the fuel surface in the float bowl, which corresponds to point 1 and the point where the fuel will flowthe point where the fuel will flow out, at the venturi, which corresponds to point 2.
Fuel flow will take place because of the drop in pressure at point 1due to the venturi effect.
VP 2 22 VPVP
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(Constant) C gz2
VP 2
2
2
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1
1
1
22gzVPgzVP
or
Fuel flow will take place because of
Calculation of Air‐fuel RatioFuel flow will take place because ofthe drop in pressure at point 1 dueto the venturi effect.
2
22
2
21
21
1
1
22gzVPgzVP
2VPP2
22
2
2
1
1
2gzVPP
2
(1)(2)gz
VPP f
ff
2
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h i h d i f h f l i k / 3 V i h l i f h f lwhere ρf is the density of the fuel in kg/m3, Vf is the velocity of the fuelat the exit of the fuel nozzle (fuel jet), and z is the depth of the jet exitbelow the level of fuel in the float bowl. This quantity must always beabove zero otherwise fuel will flow out of the jet at all times The value
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above zero otherwise fuel will flow out of the jet at all times. The valueof z is usually of the order of 10 mm.
Vpp f2
21Calculation of Air‐fuel Ratiogzpp f
ff
2
21
f fFrom above equation we can obtain an expression for the fuel velocity atthe jet exit as
pp
gzppV
ff
212
A l i th ti it ti f th f l bt i thApplying the continuity equation for the fuel, we can obtain the theoretical mass flow rate,
VA.
VAm ffff
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gzppA fff 212
Calculation of Air‐fuel Ratio
where Af is the exit area of the fuel jet in m2. If Cd,f is theffi i t f di h f th f l l (j t) i bcoefficient of discharge of the fuel nozzle (jet) given by
.m
.
.,
f
ffd
m
mC
21, 2 gzppACm ffffdf
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Air‐fuel ratio neglecting compressibility of airg g p y
• If we assume air to be incompressible, then we can apply Bernoulli’s equation to air flow also. Since initial velocity is assumed zero, we have
22 VPVP
2
2
22
2
21
21
1
1
22gzVPgzVP
2
2221 vpp
aa
Thus
Thus
ppv 21
2 2
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Thus
a
v2 2
Air‐fuel ratio neglecting compressibility of airg g p y
• If we assume air to be incompressible, then we can apply Bernoulli’s equation to air flow also. Since initial velocity is assumed zero, we have
Thus
2
2221 vpp
aa
aa
Thu ppThus
a
ppv
212 2
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Applying the continuity equation for the fuel, we can obtain the theoretical mass flow rate, ,
21222
.2 ppACAm aaa 21222 ppaaa
where A2 is the venturi in m2. If Cd,a is the coefficient of discharge of theenturi gi en byventuri given by
.
.
,a
admC
am
then .
212
.2 ppACm da then 212, 2 ppACm aada
.
amAAir
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Since .
fmFFuel
gzpp
ppAA
CC
FA
ff
a
ffd
ad
21
212
,
,
ffffd 21,
C gzpp
ppAA
CC
FA
ff
a
ffd
ad
21
212
,
,
If we assume z = 0, then
f
a
ffd
ad
AA
CC
FA
2,
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fffd ,
Fuel injection systems• Gasoline fuel injection• Gasoline fuel injection
– Inject the fuel into the engine intake system– Required one injector per cylinder– There are both mechanical and electronic injector systems– Increased power and torque, uniform fuel distribution, rapid engine
response to throttle position, precise control of equivalence ratio‐‐‐‐‐p p p q
• Diesel fuel injection– Fuel sprayed in cylinder near TDC– Atomization, vaporization & mixing delay ignition– Ignition occurs wherever conditions right– Combustion rate controlled by injection characteristics (injection rate,Combustion rate controlled by injection characteristics (injection rate,
spray angle, injection pressure, nozzle size and shape), chamber shape, mixture motion, & turbulence
– Glow plug may be used to aid cold starting
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Glow plug may be used to aid cold starting– Power output controlled only by amount of fuel injected
Limitations of Petrol Injectionj• High Initial Cost/High Replacement Cost• Increased Care and Attention/More Servicing Problems• Requires Special Servicing Equipment to Diagnose Faults and FailuresRequires Special Servicing Equipment to Diagnose Faults and Failures• Special Knowledge of Mechanical and Electrical Systems Needed to
Diagnose and Rectify Faults• Injection Equipment Complicated Delicate to Handle and Impossible toInjection Equipment Complicated, Delicate to Handle and Impossible to
Service by Roadside Service Units• Contain More Mechanical and Electrical Components Which May Go
WrongWrong• Increased Hydraulic and Mechanical Noise Due to Pumping and
Metering of Fuel• Very Careful Filtration Needed Due to Fine Tolerances of Metering and• Very Careful Filtration Needed Due to Fine Tolerances of Metering and
Discharging Components• More Electrical/Mechanical Power Needed to Drive Fuel Pump and/or
Injection Devices
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j• More Fuel Pumping/Injection Equipment and Pipe Plumbing Required‐
May be Awkwardly Placed and Bulky
Gasoline Fuel Injectionj• In SI engines the air and
fuel are usually mixedfuel are usually mixed together in the intake system prior to entry to the engine cylinderthe engine cylinder.
• Ratio of air to fuel ≈ 15 : 1• 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 System Componentsj y p
1. Electric Fuel Pump2 Fuel Accumulator – Maintains Fuel Line Pressure When Engine is2. Fuel Accumulator Maintains Fuel Line Pressure When Engine is
Shut Off and Quietness the Noise Created by the Roller Cell Pump3. Fuel Filter ‐ A Pleated Paper or Lint‐of‐fluff Type Plus Strainer4 Primary Pressure Regulator Maintains Output Delivery Pressure to4. Primary Pressure Regulator – Maintains Output Delivery Pressure to
be About 5 Bar5 Push Up Valve – Prevents Control Pressure Circuit Leakage.
It is a Non return Valve Placed at Opposite End of Pressure RegulatorIt is a Non‐return Valve Placed at Opposite End of Pressure Regulator6. Fuel Injection Valve – Valves are Insulated in Holders to Prevent Fuel
Vapor Bubbles Forming in the Fuel Lines Due to Engine Heat.Valves Open at about 3 3 Bar and Spray FuelValves Open at about 3.3 Bar and Spray Fuel. Valve Oscillates About 1500 cycles per second and so Helps in Atomization
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Gasoline Fuel Injection‐Injector typesj j yp
• Mechanical injection using an injection j g jpump driven by the engine.
• Me ha i al d i ele o ti uou• Mechanical, driveless, continuous injection.
• Electronically controlled driveless injection.
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Fuel Injection (electronic, multi‐port)Fuel Injection (electronic, multi port)
M it d E iMonitored Engine Operating Conditions:
Manifold PressureEngine Speed
Air Temperature
COMPUTERTRIGGER
pCoolant Temperature
Acceleration
INJECTOR DRIVE UNIT
50 psi typicalPressure Regulator Fuel
FilterFuel Pump
Injectors
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FUEL TANK
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Engine Information‐valve timing and volume relationshiprelationship
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