Carburetors and Fuel injection - University of Peradeniyaeng.pdn.ac.lk/.../class/downloads/notes/04-ME207-new.ppt.pdf · 2020-01-22 · 1 1 Internal combustion Engines: Carburetor,

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Internal combustion Engines: Carburetor, Fuel injection, valve timing

Dr. Primal [email protected]: (081) 2393608

Carburetors and 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

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, Ladacars 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.

3

Gas Review November 1913

Used on tractors, boats, and stationaryengines, including the Waterloo Boy and Model D tractors

Gas Review September 1917

4

Well, lets see if we can figure it out……

Carburetor Theory

• It’s all due to Air Pressure (or lack thereof)

• 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 psi

– Often measured in inches of mercury

5

y14.7 psi ~ 30 in Hg

• As engine runs, intake strokes create

“vacuum” or lower air pressure in manifold

– Normal ~10 psi (~20 in Hg)

• With throttle plate open, carburetor throat

exposed to manifold pressure

Carburetor Theory

• Venturi

– What is it?

• Wind blowing in downtown Chicago

– always stronger in the smaller areas between 2 b ld

6

buildings

• River currents

– always faster in a narrower, shallower place than deep, wide pools

2

Carburetor Theory

7

• Carburetors operate on the venturi effect• The venturi is a narrowing of the bore

Carburetor Theory

• What causes air flow through carburetor?

– Intake stroke of piston creates vacuum

Intake valve open, transmits vacuum to throttle plate

– Position of throttle plate determines air flow

Closed no flow high manifold vacuum

8

Closed – no flow – high manifold vacuum

Open – full flow – low manifold vacuum

– Air (at ~ atmospheric pressure) flows from air cleaner side, through venturi, past throttle plate, through manifold and intake valve, into cylinder

• Model A running at 975 rpm flows about 70 cfm (cubic feet per minute)

• As air flows through venturi, pressure decreases in venturi– Bernoulli’s Law tells us as Area decreases, velocityincreases and– As velocity increases, pressure decreases• Ai f l i b l i l t h i

Carburetor Theory

9

• Air pressure on fuel in bowl is always ~atmospheric• As pressure difference between 1) fuel in bowl and2) at tip of nozzle (located in venturi) increases, fuelflow increases from nozzle– Throttle opens, more air flow, greater ΔP, more fuel flow– Throttle closes, less air flow, less ΔP, less fuel flow

• Important factors– Amount of vacuum created by intake stroke• Less vacuum if– Intake valve guides leak air– Exhaust valve leaks airPi i l k i

Carburetor Theory

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– Piston rings leak air– Manifold gasket leaks air– Position of throttle plate• Determines air flow through carburetor– Determines difference in pressure on fuel in bowl and attip of nozzle in venturi» Greater difference – more fuel flow

Carburetor Theory

• To further regulate the mixture, two “air regulators” or butterfly valves are also added:

– These restrict the amount of air flow through the carburetor‐‐either manually or automatically.

This action decreases the power and speed and

11

» This action decreases the power and speed and the richness of the mixture within the engine.

– Throttle valves restrict air movement at all speeds and are generally manually controlled.

– Choke valves restrict air movement at start‐up to allow for a richer mixture and can be manually or automatically engaged.

Carburetor types

Venturi‐type Carburetor

P+1/2 V2 = Constant

Bernoulli Effect:

Valve StemFuel Inlet

Throttle Plate

Air/Fuel Mixture To Engine

Atomized Fuel

12

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

3

θ % Bore Open0 0.010 1.514 3.017 4.424 8.630 13.4

θ

13

33 16.141 25.045 29.360 50.075 75.084 90.090 100

% Bore open = πb²(1 ‐ cosθ)x100“Bore Open” is difference between boresize and area of throttle plateb = radius of bore size

The Throttle

• The throttle is a round disc mounted on a shaft beyond the main fuel nozzle in the

14

carburetor.

• It regulates the amount of air‐fuel mixture entering the cylinder.

The Choke

• The choke is a round disc mounted on a shaft located at the intake end of the carburetor.

• Since cold fuel is hard to vaporize, the choke is used during cold engine starts to provide a rich mixture to the

b d h d

15

carburetor in order to get the engine started.

Natural Draft Carburetor

• This carburetor is used where there is little space on top of the

16

engine. The air horizontally into the manifold.

Updraft Carburetors

• This type is placed low on the engine and use a gravity fed‐fuel supply. In other

17

words, the tank is above the carburetor and the fuel falls to it.

Down‐draft Carburetors

• This carburetor operates with lower air velocities and larger passages. This is because gravity assists the air‐fuel mixture flow to the cylinder.

18

• The downdraft carburetor can provide large volumes of fuel when needed for high speed and high power output.

4

Diaphragm Carburetors

• This type does not have a float, rather the difference between atmospheric pressure and the vacuum created in the engine pulsates a flexible diaphragm.

19

pu a e a e i e iap ag

• The pulsation of the diaphragm takes place on every intake and compression stroke.

Mixture Requirements

Engine induction and fuel system must prepare a fuel‐air mixture that satisfies the requirements of the engine over its entire operating regime.

Optimum air‐fuel ratio for an SI engine is that which

20

Optimum air‐fuel ratio for an SI engine is that which gives

1. required power output

2. with lowest fuel consumption

3. consistent with smooth and reliable operation

21 22

Calculation of Air‐fuel Ratio

ot

2out

outoutoutoutin

2in

inininin gZ2

hmWQgZ2

hmWQ

in

2in

ininot

2out

outoutinoutoutin gZ2

hmgZ2

hmWWQQ

Energy balance for a steady flow system

23

22

General form

in

2in

ininot

2out

outout gZ2

hmgZ2

hmWQ

Note: In the above equation, heat input to the system and work output from the system is positive (+) and heat output from the system and work input to the system is negative (‐).


in

2in

ininot

2out

outout gZ2

hmgZ2

hmWQ

in

ininot

outout gZhgZhwq

22

22

Applying the steady flow energy equation to i A A d B B i fl f i

24

1

21

12

22

2 22gZhgZhwq

Here, q and w are the heat and work transfers from the entrance to the throat and h and v stand for enthalpy and velocity respectively.If we assume reversible adiabatic conditions, and there is no work transfer, q=0, w=0, and if approach velocity v1≈0 we get

sections A‐A and B‐B per unit mass flow of air:

5


20

22

12

hh

212 2 hhv

writecanwehenceTch

getwegasperfectabetoassumedisairIf

p

25

212 2 TTcv p

p

k

k

k

k

p

pTTT

p

p

T

Tthen

isentropicbetothroattoinletfromflowAssume

1

1

2121

1

1

2

1

2

1


k

k

p

pTTT

1

1

2121 1

212 2 TTcv p

k

k

p1

26

k

p p

pTcv

1

212 12

By the continuity equation we can write down the theoretical mass flow rate of air

222111

.

vAvAm a where A1 and A2 are the cross‐sectional areas at the air inlet (point 1)and venturi throat (point 2).


k

k

p p

pTcv

1

1

212 12

222111

.

vAvAm a

(velocity)

27

To calculate the mass flow rate of air at the throat, we have assumed theflow to be isentropic till the throat so the equation relating p and v (orρ) can be used.

kk vpvp 2211 kk

pp

2

2

1

1

k

p

p1

1

212

(specific volume)


k

k

p p

pTcv

1

1

212 12

222111

.

vAvAm a 1

28

k

p

p

1

212

k

k

p

k

ap

pTcA

p

pm

1

1

212

1

1

21

.

12


k

k

p

k

ap

pTcA

p

pm

1

1

212

1

1

21

.

12

For a perfectgas we have 1

11 RT

p

29

k

k

p

k

a p

pTcA

RT

p

p

pm

1

1

212

1

1

1

1

2.

12

rearranging the above equation we have

k

k

k

pa p

p

p

pc

TR

pAm

1

1

2

2

1

2

1

12.

2


k

k

k

pa p

p

p

pc

TR

pAm

1

1

2

2

1

2

1

12.

2

Since the fluid flowing in the intake is air, we can put in theapproximate values of R = 287 J/kgK, cp = 1005 J/kgK and k = 1.4 at 300K.

711431

30

1

12

71.1

1

2

43.1

1

2

1

12.

1562.0

1562.0

T

pA

p

p

p

p

T

pAma

71.1

1

2

43.1

1

2

p

p

p

pwhere

6


1

12

71.1

1

2

43.1

1

2

1

12.

1562.0

1562.0

T

pA

p

p

p

p

T

pAma

71.1

1

2

43.1

1

2

p

p

p

p

31

Here, pressure p is in N/m2, area A is in m2,and temperature T is in K.If we take the ambient temperature T1 = 300Kand ambient pressurep1 = 10

5 N/m2, then2

.

8.901 Ama

Above equation gives the theoretical mass flow rate of air. The actualmass flow rate, can be obtained by multiplying the equation by thecoefficient of discharge for the venturi, Cd,a.

.

.

,

a

aad

m

mC

1

12,

.

1562.0T

pACm ada

The coefficient of discharge and area are both constant for a givenventuri, thus


71.1

1

2

43.1

1

2

p

p

p

p

32

venturi, thus

1

1.

T

pma

Since we have to determine the air‐fuel ratio, we now calculate the fuel flow rate.

1

1.

T

pma

The fuel is a liquid before mixing with the air, it can be taken to be incompressible.

We can apply Bernoulli’s equation between the atmospheric conditions prevailing at the top of the fuel surface in the float bowl,


33

e ue su ace i e oa bo ,which corresponds to point 1 and the 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.

(Constant) C gz2

VP 2

2

22

2

21

21

1

1

22gz

VPgz

VP

or

1

1.

T

pma Fuel flow will take place because of

the drop in pressure at point 1 dueto the venturi effect.

2

22

2

21

21

1

1

22gz

VPgz

VP

2

22

2

2

1

1

2gz

VPP


34

(1)

(2)gzVPP f

ff

2

2

21

where ρf is the density of the fuel in kg/m3, Vf is the velocity of the fuel

at 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 valueof z is usually of the order of 10 mm.

gzVpp f

ff

2

2

21

From above equation we can obtain an expression for the fuel velocity atthe jet exit as

gz

ppVf

212


35

f

Applying the continuity equation for the fuel, we can obtain the theoretical mass flow rate,

gzppA

VAm

fff

ffff

21

.

2

where Af is the exit area of the fuel jet in m2. If Cd,f is the

coefficient of discharge of the fuel nozzle (jet) given by

.

.

,

f

ffd

m

mC

.

21, 2 gzppACm ffffdf


36

f

Since .

.

f

a

m

m

F

A

Fuel

Air

gzppT

p

A

A

C

C

F

A

ffffd

ad

211

12

,

,

21562.0

1

12,

.

1562.0T

pACm ada

71.1

1

2

43.1

1

2

p

p

p

p

7


k

k

k

pad

a p

p

p

pc

TR

pACm

1

1

2

2

1

2

1

12,.

2 .


1

11 RT

p

11

1 1

Tp

R

11

1 1

Tp

R

37

k

k

k

pada p

p

p

pc

R

p

p

RACm

1

1

2

2

1

21

1

12,

.

2

k

k

k

pada p

p

p

pc

R

pACm

1

1

2

2

1

2112,

.

2


.


k

k

k

pada p

p

p

pc

R

pACm

1

1

2

2

1

2112,

.

2

k

k

k ppc12

. 1

1

.

R

c

R

c

c

Rcc

pp

v

vp

38

p

ada p

p

p

p

R

cpACm

1

2

1

2112, 2

1

11

k

k

R

c

c

R

k

p

p

k

k

k

ada p

p

p

p

k

kpACm

1

1

2

2

1

2112,

.

1

2


.


k

k

k

ada p

p

p

p

k

kpACm

1

1

2

2

1

2112,

.

1

2

39

gzppAC

pp

pp

kk

pAC

m

m

ffffd

k

k

k

ad

f

a

21,

1

1

2

2

1

2112,

.

2

12


gzppAC

pp

pp

kk

pAC

m

m

ffffd

k

k

k

ad

f

a

21,

1

1

2

2

1

2112,

.

2

12

40

k

k

k

ffffd

ad

f

a

p

p

p

p

k

k

gzpp

p

A

A

C

C

m

m1

1

2

2

1

2

21

12

1

12

,

,

.

1

21 pppa If we put 1

1

21p

pp

pa

and


k

k

k

ffffd

ad

f

a

p

p

p

p

k

k

gzpp

p

A

A

C

C

m

m1

1

2

2

1

2

21

12

1

12

,

,

.

1

21 pppa 1

1

21p

pp

pa

41

1

2

1

1

2

2

1

2

2

1

12

,

,

.

1

1

pp

pp

pp

kk

gzp

p

A

A

C

C

m

m

k

k

k

fa

a

fffd

ad

f

a

C


1

2

1

1

2

2

1

2

2

1

12

,

,

.

1

1

pp

pp

pp

kk

gzp

p

A

A

C

C

m

m

k

k

k

fa

a

fffd

ad

f

a

42

gzp

p

A

A

C

C

F

A

fa

a

f

a

ffd

ad

2

,

,

2

1

1

2

1

1

2

2

1

2

11

pp

pp

pp

k

k

k

k

k

8

if we take T1 = 300K and p1 = 105 N/m2 then

gzppA

A

C

C

F

A

ffffd

ad

21

2

,

,

28.901

The coefficient of discharge represents the effect of all deviations fromthe ideal one‐dimensional isentropic flow. It is influenced by many


43

factors of which the most important are:

1.Fluid mass flow rate,2.Orifice length‐to‐diameter ratio,3.Orifice area‐to‐approach area ratio,4.Orifice surface area,5.Orifice surface roughness,6.Orifice inlet and exit chamfers,7.Fluid specific gravity,8.Fluid viscosity, and9.Fluid surface tension.

Air‐fuel ratio neglecting compressibility of air

• 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

2221 vpp

Thus

44

2221

aa

Thus

a

ppv

21

2 2

Applying the continuity equation for the fuel, we can obtain the theoretical mass flow rate,

21222

.

2 ppACAm aaa

where A2 is the venturi in m2. If Cd,a is the coefficient of discharge of the

venturi given by.

m

45

.,

a

aad

m

mC

then .

212,

.

2 ppACm aada

Since .

.

f

a

m

m

F

A

Fuel

Air

gzpp

pp

A

A

C

C

F

A

ff

a

ffd

ad

21

212

,

,

ppACA aad 212,

46

gzppACF ffffd 21,

If we assume z = 0, then

f

a

ffd

ad

A

A

C

C

F

A

2

,

,

The equivalence ratio, (ratio between stoichiometric airfuel ratio to actual air fuel ratio)

AAFA

s 6.14

2

112

k

gzp

p

A

A

C

C

F

A

fa

a

f

a

ffd

ad

2

,

,

Typical value for a gasoline engine

47

2

1

2,

, 1

a

f

a

ff

ad

fds

p

gz

A

A

C

CFA

FF2

1

2

1

2

1

2

11

pp

pp

pp

k

k

kk

2

1

2,

, 1

a

f

a

ff

ad

fds

p

gz

A

A

C

CFA

The effects of equivalence ratio variations

• Mixture requirement at full load: Complete utilization of air to obtain maximum power, wide operation of throttle, rich‐of‐stoichiometric mixtures, 1.1.

• Mixture requirement at part loads: Part throttle, dilute air i i h i h d l d (EGR)

48

mixture with excess air or exhausted gas recycled (EGR) (improves the fuel conversion efficiency).

• The equivalent ratio of the mixture delivered by an elementary carburetor is not constant.

9

pACA ad 2


1

2

1

1

2

2

1

2

2

1

12

,

,

.

1

1

pp

pp

pp

kk

gzp

p

A

A

C

C

m

m

k

k

k

fa

a

fffd

ad

f

a

49

gzp

p

A

A

CF

A

fa

a

f

a

ffd

ad

2

,

,

2

1

1

2

1

1

2

2

1

2

11

pp

pp

pp

k

k

k

k

k

50

Carburetor Performance

• Figure shows the performance of an elementary carburetor. The top graph shows the variation of Cd,a and Cd,f and Φ with the venturi pressure drop (typically vary with pressure drop). For Δpa ≤ ρfgz, there is no fuel flow. Once fuel starts to flow, the fuel flow rate increases more

idl th th i fl t Th b t d li

51

rapidly than the air flow rate. The carburetor delivers a mixture of increasing equivalence ratio as the flow rate increases. z is typically order of 10 mm. Usually fuel level in the float chamber is held below the fuel discharge nozzle to prevent the fuel spillage when the engine is inclined to horizontal.

The deficiencies of a elementary carburetor

1. At low loads the mixture becomes leaner; the engine requires the mixture to be enriched at low loads.

2. At intermediate loads, the mixture equivalence ratio increases slightly as the air flow increases. The engine requires an almost constant equivalence ratio.

3. As the air flow approaches the maximum wide open‐throttle

52

value, the equivalence ratio remains essentially constant. However, the mixture equivalence ratio should increase to 1.1 or greater to provide maximum engine power.

4. The elementary carburetor cannot compensate for transient phenomena in the intake manifold. Nor can enrich the mixture during engine starting and warm‐up.

5. The elementary carburetor can not adjust to changes in ambient air density (due primarily to changes in altitude).

Modern Carburetor Design

The changes required in the elementary carburetor so that it provides the equivalence ratio required at various air flow rates are as follows.

1. The main metering systemmust be compensated to provide a constant lean or stoichiometric mixture over 20 to 80% of the air flow range.

2. An idle system must be added to meter the fuel flow at idle and light loads to provide a rich mixture.

3. An enrichment systemmust be provided so that the engine can get a rich mixture as wide open throttle conditions is approached and maximum power can be obtained.

53

p4. An accelerator pumpmust be provided so that additional fuel can be

introduced into the engine only when the throttle is suddenly opened.5. A chokemust be added to enrich the mixture during cold starting and

warm‐up to ensure that a combustible mixture is provided to each cylinder at the time of ignition.

6. Altitude compensation is necessary to adjust the fuel flow which makes the mixture rich when air density is lowered.

7. Increase in the magnitude of the pressure drop available for controlling the fuel flow is provided by introducing boost venturis (Venturis in series) or Multiple‐barrel carburetors (Venturis in parallel).

Two common methods used to achieve above are

• Boost venturis

Double venturi system, multiple venturis.

54

• Multiple barrel carburetors

Two barrel carburetors usually consists of two single barrel carburetors mounted in parallel.

10

Fuel injection systems

• 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‐‐‐‐‐

• Diesel fuel injection

55

j– Fuel sprayed in cylinder near TDC

– Atomization, vaporization & mixing delay ignition

– Ignition occurs wherever conditions right

– 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

– Power output controlled only by amount of fuel injected

Merits of Fuel Injection in the SI Engine

• Absence of Venturi – No Restriction in Air Flow/Higher Vol. Eff./Torque/Power

• Hot Spots for Preheating cold air eliminated/Denser air enters

• Manifold Branch Pipes Not concerned with Mixture Preparation (MPI)

• Better Acceleration Response (MPI)

56

• Fuel Atomization Generally Improved

• Use of Greater Valve Overlap

• Use of Sensors to Monitor Operating Parameters/Gives AccurateMatching of Air/fuel Requirements: Improves Power, Reducesfuel consumption and Emissions

• Precise in Metering Fuel in Ports

• Precise Fuel Distribution Between Cylinders (MPI)

Limitations of Petrol Injection• High Initial Cost/High Replacement Cost

• Increased Care and Attention/More Servicing Problems

• Requires 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 to Service by Roadside Service Units

57

• Contain More Mechanical and Electrical Components Which May Go Wrong

• Increased Hydraulic and Mechanical Noise Due to Pumping and Metering of Fuel

• Very Careful Filtration Needed Due to Fine Tolerances of Metering andDischarging Components

• More Electrical/Mechanical Power Needed to Drive Fuel Pump and/orInjection Devices

• More Fuel Pumping/Injection Equipment and Pipe Plumbing Required‐May be Awkwardly Placed and Bulky

Gasoline Fuel Injection System Components

1. Electric Fuel Pump

2. Fuel Accumulator – Maintains Fuel Line Pressure When Engine is Shut Off and Quietness the Noise Created by the Roller Cell Pump

3. Fuel Filter ‐ A Pleated Paper or Lint‐of‐fluff Type Plus Strainer

4. Primary Pressure Regulator – Maintains Output Delivery Pressure to be About 5 Bar

58

5 Push Up Valve – Prevents Control Pressure Circuit Leakage.

It is a Non‐return Valve Placed at Opposite End of Pressure Regulator

6. Fuel Injection Valve – Valves are Insulated in Holders to Prevent FuelVapor Bubbles Forming in the Fuel Lines Due to Engine Heat.

Valves Open at about 3.3 Bar and Spray Fuel.

Valve Oscillates About 1500 cycles per second and so Helps in Atomization

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.

Indirect Injection

• Also Called Manifold Injection or Single Point Injection (SPI) or Throttle Body Injection (TBI)

• Injector Usually Upstream From Throttle (Air Intake Side) or In Some Cases Placed on the Opposite Side

• Pressures are Low – 2 to 6 Bar. Maybe Injected Irrespective of Intake Process

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• Cost Would be Low

• Has Same Air and Fuel Mixing and Distribution Problems asCarburetor but Without Venturi Restriction so Gives HigherEngine Volumetric Efficiency

• Higher Injection Pressures Compared to Carburetion – Speeds upAtomization of Liquid Fuel

11

Semi‐direct Injection• Also Called Port Injection or Indirect Multipoint Injection (IMPI) or

Simply Multi‐point Injection (MPI)

• Injectors Positioned in Each Induction Manifold Branch Just in Front of Inlet Port

• Injection at Low Pressure (2‐6 Bar)

• Need Not Be Synchronized With Engine Induction Cycle

• Fuel Can Be Discharged Simultaneously to Each Induction Pipe Where

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g y pit is Mixed and Stored Until IVO

• Need Not Be Timed – Requires Low Discharge Pressures – Injectors Not Exposed to Combustion Products so Complexity Reduced – Less Cost

• No Fuel Distribution Difficulties Since Each Injector Discharges DirectlyInto Its Own Port and Mixture Moves a Short Distance Before EnteringCylinder

• Induction Manifold Deals Mainly With Only Inducted Air – So BranchPipes Can Be Enlarged and Extended to Maximize Ram Effect

Direct Cylinder Injection• Also Called Direct Multi‐point Injection (DMPI) or Gasoline Direct

Injection (GDI)

• Injection May be During Intake or Compression Process

• Increased Turbulence Required

• To Compensate For Shorter Permitted Time For Injection/Atomization/Mixing Injection Pressure Must Be Higher

• More Valve Overlap Possible So Fresh Air Can Be Utilized For Scavenging

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Scavenging

• Injector Nozzle Must Be Designed For Higher Pressure and Temperature So Must Be More Robust and Will Be Costlier Than Other Types

• Position and Direction of Injection Are Important – No One Position Will Be Ideal For All Operating Conditions

• Air and Fuel Mixing Is More Thorough in Large Cylinders Than In Small Cylinders Because Droplet Size is the Same

• Condensation and Wall Wetting in Intake Manifold Eliminated But Condensation On Piston Crown and Cylinder Walls

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

ELECTRONIC FUEL INJECTION

• Strict emission standards require precise fuel delivery

• Computers used to calculate fuel needs

• EFI very precise, reliable & cost effective

• EFI provide correct A/F ratio for all loads, speeds, & temp

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ranges

The Fuel Injector

• Electromechanical device

• Engine rpm determines when injector opens

• How long it stays open determined by:

– Engine temp

– Engine load

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– Engine load

– Throttle pos.

– O2 sensor voltage

12

Throttle Body Injection (TBI)

• First injection unit used

• Housing similar to Carb

• One or two

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One or two injector

• One or two of these units mounted to intake manifold

FIG 6-40 CLASS

LOW PRESSURE FUEL INJECTOR

• 13‐16 psi operating pressure

B ll l

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• Ball style pintle

• Easily replaceable

Multi‐Port Fuel Injection

• One injector per cylinder

• Mounts in intake manifold, sprays directly at intake valve

• Fired in groups or

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• Fired in groups or individually (SFI)

• Ram Tuning for denser air charge

• Lower A/F temps

• Leaner mixture during warm‐up

Fuel Pressure Regulator

• Located at end of fuel rail

• Maintains constant pressure at injectors

• Internal chamber contains a diaphragm – Pressurized fuel on one side

– Manifold vacuum & spring tension on other

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• Manifold vacuum pulls up on diaphragm, metering fuel that is returned to tank

• Excess fuel pressure can overcome spring tension, allowing fuel to return to tank

• Increases in manifold pressure causes spring tension to push diaphragm down, blocking return line, increasing pressure in rail.


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Vacuum hose connection Fuel rail


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Fuel return

13

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

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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14

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 controls flow

• Governor and advance coupling

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• 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.

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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15

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

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

Pintle Nozzle

• Excellent disbursement, provides conical spray pattern

• Looks Similar to that used in CIS systems

• Opens UPWARD

E ll t l i t

88

• Excellent clog resistance

More Injector Considerations• Aux hole to bleed excess fuel and prevent deposits

• 4V Heads:

–Upside• Vf Up• Central injector position

–Downside

• Less swirl• More nozzle holes for good disbursion/combustion, as

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g / ,small as 0.1 mm

• Nozzles cooled by fuel

–Cooling important to maintain tolerances and sealing

• Spray Pattern Critical!

–Aspect Ratio of 2‐8–Larger Aspect Ratio – more penetration

–Larger Aspect ratio – Smaller cone

–Atomization up –w‐ velocity, but restricts penetration as well

Pilot Injection

• Small Amount of fuel early to initiate flame front

• Allows for large advance

• Eliminates knock and corresponding problems associated with high peak pressures and wave impingement

• 2 Spring Special injector needed for 2 mode operation

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16

Electronic Unit Injection

• Electronic Unit Injection

–Solenoid Controlled–So fast pilot injection can be used–Expensive to produce–Widely used in heavy truck

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where emissions and economy are critical

–Controlled just like SI EFI

• Variation is HEUI

Moving Components

• Valves

– Intake: open to admit air to cylinder (with fuel in Otto cycle)

– Exhaust: open to allow gases to be rejected

• Camshaft & Cams

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– Used to time the addition of intake and exhaust valves

– Operates valves via pushrods & rocker arms

Valve trains

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OHV (overhead valve)Pushrod configurationMany reciprocating partsHigher valve spring pressure requiredCompact engine size compared to OHC

Valve trains

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OHC (overhead cam)Fewer reciprocating partsReduced valve spring pressure requiredHigher RPM capabilityCylinder head assemblies are taller

Valve trains

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Cam-in-headNo pushrodsUse rocker arms

Valve Locations

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17

Combustion process: stratified chargeCombustion process: stratified charge

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jet guided wall guided inlet air guided

Charge Stratification

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Combustion Chamber Designs

99

Combustion Chamber Design

100


101


102

18


103


104


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CLASSIFICATION OF INTERNAL COMBUSTION ENGINES

Cooling

1. Direct Air‐cooling

2 Indirect Air cooling (Liquid Cooling)

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2. Indirect Air‐cooling (Liquid Cooling)

3. Low Heat Rejection (Semi‐adiabatic) engine.

Cooling system operation

Engine heat is transfered . . .• through walls of the combustion chambers• through the walls of cylinders

Coolant flows . . .t di t h

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• 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

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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

19

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.

Diesel: Gasoline’s Dirty Cousin?

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y

How is Diesel Different from Gasoline?

• Diesel is a petroleum‐based fuel with a higher energy content than gasoline.

– contains about 30% more energy per gallon as compared to gasoline.

• Diesel is a safer fuel than gasoline or other alternatives.

– less flammable and explosive than gasoline due to lower

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p gcombustibility.

• Diesel is Cheaper than Gasoline

– Current Cost of a Gallon of Gasoline and Diesel

• Gasoline = $1.78• Diesel = $1.65

Misconceptions About Diesel

• It’s Dirty

• It Causes a lot of Pollution

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• It has Limited Uses

Benefits of Diesel

• A well maintained diesel engine usually emits lower levels of carbon monoxide, hydrocarbons and carbon dioxide than gasoline engines.

• Better fuel economy,

• Increased durability for longer engine life

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• Increased durability for longer engine life.

Problems with “Old” Diesel Technologies

• High Sulfur Content of Fuel

• High NOx Emissions

• High Particulate Matter Emissions

– The “Black Smoke” everyone sees

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• Noisy Engines

20

Sulfur Content

• Diesel fuel available in the U.S. currently contains from 340 ppm of sulfur to 140 ppm in California.

• European Standards are much lower

– As low as 10 ppm in Germany and Sweden

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NOx Emissions

• High cylinder pressure and temperature with excessive air is the recipe for making NOx

• Because of excess air in diesel engines, current catalytic can’t scrub out NOx

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Particulate Matter

• Unburned fuel in the compression ignition process becomes soot, a pervasive form of particulate matter.

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Clean Diesel

• Clean diesel is an evolutionary systems‐based process that combines advancements in diesel engines, cleaner burning fuels and emissions control system, all working and optimized together.

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What Makes Diesel Clean?

• The Three Pillars of Clean Diesel Technology:

– cleaner‐burning fuels

– state‐of‐the‐art engines

– effective emissions‐control systems

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Cleaner Burning Fuels

• The newest in diesel fuels is called Ultra‐low Sulfur Diesel (ULSD)

– Ultra‐low sulfur diesel fuel is a specially refined diesel fuel that has dramatically lower sulfur content than

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yregular diesel and can be used in any diesel engine just like regular diesel fuel.

• Today, the sulfur content of ULSD ranges from 15 to 30 parts per million. Regular diesel has a maximum of 500 parts per million of sulfur.

21

How Does ULSD Help?

• Reduces sulfate emissions

• Allows the use of particulate traps and catalytic converters

• Lowers engine maintenance costs

• Easy to convert to

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– No retrofitting required

• Only costs a few cents more

State of the Art Engines

• New Engine Technologies

– Electronic Controls

– Common‐rail Fuel Injection

– Variable Injection Timing

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– Improved Combustion Chamber Configuration

– Turbocharging

Comparison of SI and CI Engines

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Typical Brake Thermal Efficiencies of CI and SI Engines

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125 126

22

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(port fuel injection)

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Roger Krieger, GM R&D Center

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Summary Diesel Engines

Advantages:

• Efficiency (most efficient prime mover)• Emissions (low CO, CO2, good durability)• Very high torque and performance

Di d t

130


Disadvantages:

• Emissions (more challenging to control NOx, particulates)

• Higher cost• Heavier• Noise (more challenging to make quiet)

Documents

Carburetors and Fuel injection - University of Peradeniyaeng.pdn.ac.lk/.../class/downloads/notes/04-ME207-new.ppt.pdf · 2020-01-22 · 1 1 Internal combustion Engines: Carburetor,