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

21

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

221

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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