Chapter 6

SI ENGINE FUEL METERING AND MANIFOLD PHENOMENA

1

SI ENGINE MIXTURE REQUIREMENTS

CARBURETORS

FUEL-INJECTION SYSTEMS

6.1 SI ENGINE MIXTURE REQUIREMENTS

Most gasolines have the stoichiometric air/fuel ratio (A/F)s

in the range 14.4 - 14.7. A typical value could be 14.6.

In the absence of strict engine NOx emission requirements, excess air is the obvious diluent, and engines have traditionally operated lean.

2

FIGURE 6.1 Typical mixture requirements for two common operating strategies.

Top diagram shows equivalence ratio variation with intake mass flow rate. Bottom diagram shows recycled exhaust (EGR) schedule as a function of intake flow rate. When tight control of NOx, HC, and CO emission is required, operation of the engine with a stoichiometric mixture and a three-way catalyst can be used. 3

In take mass flow rate

Low speed

High speed

Medium speed

Equiv

ale

nce r

atio

Low speed Medium speedHigh speed

(a)

(b)

Wide-open throttle Wide-open throttle

6.2 CARBURETORS

Cross Section of Basic Carburetor4

6.2 CARBURETORS

6.2.1 Carburetor Fundamentals

FIGURE 6.2 Schematic of elementary carburetor.5

1.Inlet section

2.Venturi throat

3.Float chamber

4.Pressure equalizing passage

5.Calibrated orifice

6.Fuel discharge tube

7.Throttle plate

Air

Fuel

FLOW THROUGH THE VENTURI

where CDT : discharge coefficientAT: area of the venturi throatpT: pressure at the throatpo: stagnation pressureTo: stagnation temperature

6

In terms of pressure drop from upstream condition to theventuri throat,

7

where the compressibility function,

This accounts for compressibility effects.

Choke Flow (critical flow)

For a given values of p0 and T0, the maximum mass flowoccurs when the velocity at the minimum area or throatequals the velocity of sound. This condition is calledchoked or critical flow.

Critical Pressure Ratio:

8

For air

Choked flow when

9

otherwise

FIGURE 6.3 Relative mass flow rate and compressible flowfunction as function of nozzle or restriction pressure ratio for idealgas with =1.4

10

Critical ratio

0.70

Crit

ical

flo

w

FLOW THROUGH THE FUEL ORIFICE

where: discharge coefficient

: area of the orifice

where h is typically of order 10 mm.

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Example 6.1 SI engine with 3.6 liter capacity is designed so that an air flow rate at the throat equals the speed of sound at the engine speed of 6000 rev/min. At this speed, the engine has volumetric efficiency = 0.8. The height h of carburetor is 10 mm. If this engine has two equal-size venturi throats, find diameters of each throat and of the orifice. Let the discharge coefficients of the venture throats and orifice are 0.9 and 0.7 respectively.(air density = 1.184 kg/m3, fuel density = 750 kg/m3,(A/F)s = 14.6)

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Vtot = 3.6x10-3 m3

N = 6000 rpm

(A/F)s = 14.6

CDT = 0.9, CDo = 0.7

h = 10 mm

Given: Asked:

dT, do

Analysis:

Mass flow rate of air

= 0.1705 kg/s

From (1)

= 0.7 and = 0.528, knowing

At the condition that the air flow speed is equal to the speed of sound (choked flow),

that po = patm = 101330 N/m2. We get

13

From

For two venture throats of equal size,

= 0.000804/2 = 4.02x10-4 m2

Ans

From (2)

and

14

dT = 0.0226 m or 2.26 cm

Then

Pa

kg/s

Replacing values into equation (2), we obtain

Ans

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CARBURATOR PERFORMANCE

The air/fuel ratio delivered by a carburetor is given by

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FIGURE 6.4 Performance of elementary carburetor that varies with venturi pressure drop

17

lean

Rich

6.2.2 Modern Carburetor Design

FIGURE 6.5 Schematic of modern carburetor.

1.Main venturi2.Boost venturi3.Main metering spray tube4.Air-bleed orifice5.Emulsion tube or well6.Main fuel-metering orifice7.Float chamber8.Throttle plate

9.Idle air-bleed orifice10. Idle fuel orifice11. Idle mixture orifice12. Transition orifice13. Idle mixture adjusting screw14. Idle throttle setting adjusting screw

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In modern carburetor, the changes required in the elementary carburetor are:

1.The main metering system2.An idle system3.An enrichment system4.An accelerator pump5.A choke6.Altitude compensation

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COMPENSATION OF MAIN METERING SYSTEM

FIGURE 6.5 Schematic of main metering system with air-bleed compensation.

21

Air bleed mass flow rate is given by

where CDb : Discharge coefficientAb : Area of air-bleed orifice

Mass flow rate through the fuel orifice is given by

22

23

Density of emulsion is approximated by

~ 730 kg/m3 ~ 1.14 kg/m3 (typical values)

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FIGURE 6.6 Metering characteristics of system with air-bleed compensation.

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FIGURE 6.7 An enrichment system 26

FIGURE 6.8 An accelerator pump27

FIGURE 6.9 Basic automobile carburetor showing (A) venturi, (B) throttle valve, (C) fuelcapillary tube, (D) fuel reservoir, (E) main metering valve, (F) idle speed adjustment, (G)idle valve, and (H) choke. 28

When starting a cold engine, the first step is to close the choke. This restricts air flow and creates a vacuum in the entire system downstream of the choke. close

FIGURE 6.9 Basic automobile carburetor showing (A) venturi, (B) throttle valve, (C) fuelcapillary tube, (D) fuel reservoir, (E) main metering valve, (F) idle speed adjustment, (G)idle valve, and (H) choke. 29

When the throttle is closed or almost closed, pressure in the intake system downstream of the throttle is very low. Therefore, a substantial pressure drop through the idle valve causes the fuel injection.

CHOKEChoke is the butterfly valve positioned upstream of theventuri throat. When starting a cold engine, the first stepis to close the choke. This restricts air flow and creates avacuum in the entire system downstream of the choke.

IDLEWhen the throttle is closed or almost closed, pressure in the intake system downstream of the throttle is very low. Therefore, a substantial pressure drop through the idle valve causes the fuel injection.

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6.3 FUEL-INJECTION SYSTEMS

6.3.1 Multipoint Port Injection

•Fuel is injected into the intake port of each cylinder.•Require one injector per cylinder (plus, in some systems, one or two more injectors for starting and warm-up).

32

FIGURE 6.8 Speed-density electronic multipoint port fuel-injection system: Bosh D-Jetronic System.

Speed – density system (speed – density electronic multipoint port fuel – injection)

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The mass of air per cylinder per cycle

Electronic multiport fuel – injection (EFI) system with air-flow meter

FIGURE 6.9 Electronic multipoint port fuel-injection system with air-flow meter: Bosh L-Jetronic System. 34

The mass of air inducted per cycle to each cylinder

Mechanical, air-flow based metering, injection system (Bosch K-Jetronic system)

FIGURE 6.10 Mechanical multipoint port fuel-injection system: Bosh DK-Jetronic System.

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6.3.2 Single-Point Throttle-Body Injection

FIGURE 6.11 Cutaway drawing of a two-injector throttle-body electronic fuel-injection system

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Pressure regulator

Fuel from tank

Injector

Air flow

Returned fuel to tank

FLOW PAST THROTTLE PLATE

FIGURE 6.12 Throttle plate geometry37

Throttle plate open area as a function of

d is throttle shaft diameter,

= the throttle plate angle when tightly closed

D is the throttle bore diameter.

When the throttle open area is

maximum 38

Mass flow rate through the throttle valve

For the pressure ratio across the throttle

greater than the critical value

For the pressure ratio across the throttle equal

when theto or less than the critical value flow is choked

39

FIGURE 6.13 The relation between air flow rate, throttle angle,

intake manifold pressure, and engine speed 40

0.528 × patm

= 53.5 kN/m2

chokedCritical pressure

ratio

Yes, it’s pT !

1 kN/m2

~ 0.75 cmHg