SI Engine Mixture Preparation

8/3/2019 SI Engine Mixture Preparation

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CarburetionPerhaps soon to be obsolete?


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A carburetor is basically a device for mixing air

and fuel in the correct amounts for efficient

combustion. The carburetor bolts to the engine intakemanifold. The air cleaner fits over the top of thecarburetor to trap dust and dirt.


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The process of formation of a combustiblefuel-air mixture by mixing the properamount of fuel with air before it is admittedinto the engine cylinder.

Comes from the words car and burettebecause the carburetor meters theappropriate quantity of liquid fuel (like a

burette) and mixed it with air beforesending the mixture into the enginecylinder.


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Carburetor size is stated in CFM (cubic feet of air per

minute).

This is the amount of air that can flow through thecarburetor at wide, open throttle.

CFM is an indication of the maximum air flow

capacity. Usually, small CFM carburetors are more

fuel-efficient than larger carburetors. Air velocity, fuelmixing, and atomization are better with small throttle

bores. A larger CFM rating is desirable for high enginepower output


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1-Carburetor body2-Air horn3-Throttle valve4-Ventur i5-Main discharge tube6-Fuel bowl


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Carburetor

A device that mixes air and fuelin correct proportion forefficient combustion.

Stoichiometric Ratio14.7 : 1 (Air : Fuel)

CFM of air flow:Cubic feet of air perminute


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This disc-shaped valve

controls air flow through

the air horn.

When closed, it restricts

the flow of air and fuel into

the engine, and when

opened, air flow, fuel flow,and engine power increase.


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The venturi produces

sufficient suction to pull

fuel out of the main

discharge tube
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Venturi

Venturi works on high-low pressure.

As the air speeds up when passing through the air horn(venturi), itcreates vacuum, causing suction to pull fuel from the discharge tube.


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The main discharge

tube is also called themain fuel nozzle

It is a passage that

connects the fuel bowlto the center of theventuri.


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The fuel bowl holds a

supply of fuel that is

NOT under fuel pump

pressure


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Engine induction and fuel system mustprepare a fuel-air mixture that satisfiesthe requirements of the engine over itsentire operating regime.

Optimum air-fuel ratio for an SI engine isthat which gives

1. Required power output2. Lowers fuel consumption, and3. Consistent with smooth and reliable

operation


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The constraints of emissions maydictate a different air-fuel ratio andalso require recycling some exhaust gas.(EGR)

Relative proportions of fuel and air that givethe above requirements depend on enginespeed and load.

Mixture strength is given in terms of air-fuel or

fuel-air ratio or equivalence ratio.


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Mixture requirements are different for fullload (wide-open throttle or WOT) and forpart-load operation.

At full load, complete utilization ofinducted

air to obtain maximum power for a givendisplaced volume is the critical issue.At part-load at a given speed, efficient

utilization offuel is the critical issue.


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At part-load (or part-throttle) it is advantageous to dilutethe fuel-air mixture with excess air or with recycledexhaust gas. This dilution improves fuel conversionefficiency for three reasons:

1. The expansion stroke work is increased for a givenexpansion ratio due to the change in thermodynamicproperties,

2. For a given mean effective pressure, the intake pressureincreases with increasing dilution, so pumping workdecreases,

3. Heat losses to the walls are reduced because the burnedgas temperatures are lower.

In the absence of strict NOx emission control, excess airis the obvious diluent at part load and the engine runslean


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HIGH LOW

Fuel Metering Force


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Air bled into the main metering system decreases thefuel density and destroys surface tension. This results in better vaporization and control of fueldischarge, especially at lower engine speeds.


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Air Bleed


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1. Float system2. Idle system3. Off idle system4. Acceleration system5. High-speed system6. Full-power system


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The float system (fig. 4-20) maintains a steady working

supply of gasoline at a constant level in the carburetor.

This action is critical to the proper operation of the

carburetor. Since the carburetor uses differences in

pressure to force fuel into the air horn,

The float system keeps the fuel pump from forcing toomuch gasoline into the carburetor bowl


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An excessively high float levelwill cause fuel to flow

too freely from the discharge tube, causing an overly

rich mixture

whereas an excessively low float level will cause anoverly lean mixtureThe basic parts of the float system are thefuel bowl, the float, the needle valve, theneedle seat, the bowl vent


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Idle system

Feeds fuel into air horn when thethrottle is closed (low enginespeed).

High vacuum below the throttleplate pulls fuel from the idle port.

Idle mixture screw allowsadjustment of fuel at idle.

Air bleed helps premix air and fuel.


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Off idle system feeds fuel to the engine when the throttle is opened slightly.

It adds a little extra fuel to the extra air flowing around throttle valve


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The off idle, also known as the part throttle, feedsmore fuel into the air horn when the throttle plate ispartially open. It is an extension of the idle system. Itfunctions above approximately 800 rpm or 20 mph.

Without the off idle system, the fuel mixture wouldbecome too lean slightly above idle. The idle systemalone is not capable of supplying enough fuel to theair stream passing through the carburetor. The off idlesystem helps supply fuel during the change from idle

to high speed.


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The, high-speed system, also called the mainmetering system, supplies the engine air-fuelmixture at normal cruising speeds.

This system begins to function when the throttle

plate is opened wide enough for the venturi action.Air flow through the carburetor must be relativelyhigh for venturi vacuum to draw fuel out of themain discharge tube.

The high-speed system provides the leanest, mostfuel efficient air-fuel ratio. It functions from about20 to 55 mph or 2,000 to 3,000 rpm.


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The carburetor acceleration system, like the off idle system,provides extra fuel when changing from the idle system tothe high-speed system. The acceleration system squirts astream of fuel into the air horn when the fuel pedal ispressed and the throttle plates swing open.

Without the acceleration system, too much fuel would rushinto the engine, as the throttle quickly opened. The mixturewould become too lean for combustion and the enginewould stallor hesitate. The acceleration system prevents alean air-fuel mixture from upsetting a smooth increase inengine speed.


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Acceleration System

Accelerator pump squirts fuel into theair horn every time the throttle is opened.

This adds fuel to the rush of air enteringthe engine and prevents a temporary

lean mixture.

Pump check ball allows fuel to onlyenter the pump reservoir.

Pump check weight prevents the fuelbeing drawn into the air horn by theventuri vacuum.


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High speed system (cruising speed)

The main jet controls the fuel flow andmixture.

At higher engine speeds, there is enoughair flow through the venturi to producevacuum.

This pulls fuel through the maindischarge.


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Choke System

When the engine is cold thethermostatic springcloses the choke.

High vacuum below the choke pulls largeamounts of fuel out of the main discharge.

When the engine warms the hot air opensthe spring

Some chokes are electrically operated.


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When the engine is cold, the fuel tends to condenseinto large drops in the manifold, rather thanvaporizing. By supplying a richer mixture (8:1 to

9:1), there will be enough vapor to assure completecombustion.The carburetor is fitted with a choke system toprovide this richer mixture.The choke system provides a very rich mixture to startthe engine and to make the mixture less richgradually, as the engine reaches operatingtemperature. The two types of choke systems are themanual and automatic


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The full-power system provides a means of enriching the

fuel mixture for high-speed, high-power conditions.

This system operates, for example, when the driver

presses the fuel pedal to pass another vehicle or to

climb a steep hill.

The full-power system is an addition to the high-speed

system. Either a metering rod or a power valve (jet) can

be used to provide variable, high-speed air-fuel ratio.


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The evolution of gasoline-engine fuel deliverysystems has been dictated by the need to improvetransient and cold engine performance andemissions.With each evolutionary change in the fuel deliverysystem, air-fuel mixture preparedness, within thecylinder, had to be engineered and restored to thetraditionally acceptable homogeneous state.


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Carburetor

Port-Fuel-Injection

Direct-Injection


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Port Fuel Injection System


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Port Fuel Injection System


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1900 1985Carburetor

1980 1995

Single-Point,Throttle-BodyFuel Injection

1980 ????

Multi-Port-Fuel-Injection1995 ????

Advanced Multi-Port-Fuel-Injection

1996 ????

Direct (In-Cylinder)Fuel-Injection


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Fuel System

CarburetorSingle-Point,Throttle-BodyFuel InjectionMulti-Port-Fuel-InjectionAdvancedMulti-Port-Fuel-InjectionDirect-Fuel-Injection

Transient

Emissions &

Control

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Cold

Emissions &

Control

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Mixture

Preparation

Quality

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Cost &

Complexity

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Will direct-fuel-injection replace electronic

port-fuel-injection at a similar rate?


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Preliminary goal for a Direct-Fuel-Injectionsystem is therefore to be able to achieve thetraditionally acceptable homogeneous air-fuelmixture state at the time of ignition, by:Promoting maximum air-fuel mixingUsing a finely atomized sprayPrevent wall wettingInjecting early during intake strokeIntake-port designInjector location


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However, because of the big increase in cost andcomplexity of a DFI system, would like to getmore benefits to offset system costs than justimproved cold and transient engine performanceand emissions.Are there any additional benefits of a DFI system?Yes. Increased Fuel Economy !!But, this increase in thermal efficiency is currentlypossible only if the mixture-preparation state,within the cylinder, is stratified and not thetraditionally acceptable homogeneous state.


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Increased volumetric efficiency increased compression ratio Decreased throttling losses Lean combustion Decreased heat lossesIn trying to work the above levers, DFI is an

enabler with high potential. Note that advancedMPFI systems are also enablers, but with lesserpotential than DFI.


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Airflow Characterisitics

7

7.2

7.4

7.6

7.8

8

80 120 160 200 240 280 320 360 400

End-of-Injection (bTDC Firing)

Airflow(g

/s)

Direct-Fuel-Injection

Port-Fuel-Injection

Engine Speed = 2000 rev/min,

Wide-open-throttle,

Air-Fuel ratio=15:1

INTAKE STROKECOMPRESSIONSTROKE EXHAUSTSTROKE

2.5%


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Direct-Fuel-Injection can result in anincrease (up to 8% has been reported) inairflow due to spray-cooling of the intakeair, when injection occurs during theintake stroke. The resulting increasedperformance can be converted to 1-2%

increase in fuel economy.


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Effect of Compression Ratio on Otto-Cycle Thermal

Efficiency

30

35

40

45

50

55

60

65

70

2 4 6 8 10 12 14 16 18 20 22

Compression Ratio (r)

OttoCycleEfficiency,

E(%) Typical Compression Ratio

for Port-Fuel-Injection engine

Typical Compression Ratio

for Direct-Fuel-Injection engine

Note: Otto-Cycle efficiency is used as a gross approximation for


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Direct-Fuel-Injection permits an increase incompression ratio from 10.5 to about 12.0,resulting in about 2% increased efficiency.The increase in compression ratio resultsfrom a higher knock-tolerance (I.e., higherknock-limited spark advance) due to:1. Spray cooling of the intake air wheninjection occurs during the intake stroke2. Reduced end-gas temperature wheninjection occurs during compression stroke


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Knock-Limited Spark Advance

4

6

8

10

12

14

16

18

80 120 160 200 240 280 320 360 400

End-of-Injection (bTDC Firing)

Sp

arkAdvance(bTDC)

Direct-Fuel-Injection

Port-Fuel-InjectionEngine Speed = 2000 rev/min,Wide-open-throttle,

Air-Fuel ratio=15:1

INTAKE STROKECOMPRESSION

STROKEEXHAUST

STROKE


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Throttling losses are reduced by diluting themixture with EGR or with excess air. But in aconventional homogeneous-charge system,the extent of dilution is limited due to flameinitiation and propagation limits.By stratifying the fuel-air mixture within thecombustion chamber, the engine can beoperated with extended dilution, at air-fuelratios of 50:1 or greater.

Diluting the Air-Fuel Mixture Reduces


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Ln Volume

Diluting the Air Fuel Mixture ReducesPumping (or Throttling) Losses

Undiluted Combustion Partially Diluted

Combustion(Partially Unthrottled)

Fully Diluted Combustion

(Fully Unthrottled)

PMEP

Net MEP = IMEP - PMEP Reduced Pumping LossDue to Dilution

PMEP


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Effect of Specific-Heat Ratio on Otto-Cycle ThermalEfficiency

10

15

2025

30

35

40

45

50

55

60

65

70

75

80

1.1 1.2 1.3 1.4 1.5 1.6

Specific-Heat Ratio

OttoCycleEfficienc

y,

E(%) Specific-Heat Ratio for a

Stoichiometric mixture

of Fuel and Air

Specific-Heat Ratio for Pure Air

Compression Ratio = 12:1

Note: Otto-Cycle efficiency is used as a gross approximation for


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When the working fluid has a higher specific-heat ratio like that of lean air-fuel mixtures,less fuel energy is wasted in raising theinternal energy of the charge, so more isavailable for useful work.By stratifying the fuel-air mixture within thecombustion chamber, the engine can beoperated at very lean (up to 50:1) air fuelratios.


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By stratifying the fuel-air mixture inthe center of the combustion chamberand keeping the hot burnt productsaway from the walls, heat losses can bedecreased.


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Uses pressure (not Vacuum) from anelectrical pump to spray fuel into theintake manifold.

Provides the engine with properair-fuel ratio (14.7 : 1)


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Advantages

Improved Atomization

Better fuel flow

Smoother idle

Improved fuel economy

Lower emissions

Better cold weather drivability

Increased engine power

Simpler
http://www.pdm-racing.com/products/imag/Fuel_pump_large.jpg


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Electrical Fuel Pump draws fuel fromtank and forces it into the regulator.

Pressure Regulator controls the amountof pressure that enters the injector and anyextra fuel is returned to the fuel tank.

Fuel Injector is simply a coil or solenoidoperated valve.

Spring pressure holds the injector closed.

When engaged, the injector sprays fuelinto the engine.

Injector Pulse Width indicates the time each

Injector is energized (Kept Open).
http://www.pdm-racing.com/products/imag/Fuel_pump_large.jpg


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Air filter

Throttle valve

Sensors

Connecting ducts


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Monitors engine operating condition and reports this information

to ECM (computer).

Sensors are electrical devices that change resistance or voltagewith change in condition such as temperature, pressure and position.


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Uses electrical data from the sensors to control the operation of

the fuel injectors.

Engine Control Module (ECM)- Brain of the electronic fuelinjection.


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Oxygen Sensor measures the oxygen content in engine exhaust.

Mounted on the exhaust system before thecatalytic converter.

Voltage out-put of O2 sensor changes with

the change in oxygen content of exhaust.

Lean mixture decreases the voltage.Rich mixture increases the voltage.

Signal is sent to ECM and the ECM changes the time that an injectoris open or close.


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Open Loop

When the electronic injection system doesnt use the input fromthe engine exhaust.

System operates on information stored in the computer (PROM).

Computer ignores the sensors when the engine is cold.

Closed Loop

Ones engine reaches the operating temperature, computer usesinformation from oxygen sensor and the other sensors.


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Manifold Absolute Pressure Sensor (MAP)

Measures the pressure, or vacuum insidethe engine intake manifold.

Manifold pressure = Engine load

High pressure (low intake vacuum) =High load = Rich mixture

Low pressure (high intake vacuum) =

Little load = Lean mixture

Computer senses the change in resistanceand alters the fuel mixture.


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Throttle Position Sensor (TPS)

Variable resister connected to thethrottle plate.

Change in throttle angle =change in resistance.

Based on the resistance, ECM

richens or leans the mixture.


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Engine Temperature Sensor

Monitors the operating temperature of the engine.

Exposed to engine coolant.

Engine cold = Low Resistance = Rich Mixture

Engine Hot = High Resistance = Lean Mixture.


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Mass Air Flow Sensor (MAF)

Measures the amount of outside air entering the engine.

Contains an air flap or door that operates a variable resistor.

Helps computer to determine how much fuel is needed.


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Inlet Air Temperature Sensor

Measures the temperature of air entering the engine.

Cold air (more dense) = More fuel for proper AF ratio.


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Crankshaft Position Sensor

Detects engine speed.

Changes injector timing and duration.

Higher engine speed = More fuel
http://mykidz.net/GTP/Images/CrankPosSensor/images/RemoveCover_JPG.jpg


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Uses one or two injectors.

Injectors (pulse) spray fuel into thetop of throttle body air horn.

Atomized fuel mixes with air anddrawn into the engine.

Fuel pressure regulator is spring loadedand is part of the housing.


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EFI Multi port Injection System

Injector is pressed into the runner(Port)in the intake manifold.

Injector sprays towards an engineintake valve.

EFI Direct fuel Injection System

Injectors are pressed into thecombustion chamber and spray fueldirectly into the combustion chamber.


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THE INJECTION SYSTEM IN DIESELENGINES CAN BE OF TWO TYPES:1. AIR INJECTION.2. AIRLESS INJECTION.

IN AIR INJECTION SYSTEM THE DIESEL ISINJECTED ALONG WITH THE COMPRESSED AIRWHEREAS INAIRLESS INJECTION SYSTEM ONLYTHE LIQUID DIESEL IS INJECTED INTO

THE CYLINDER.


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In an indirect injection(abbreviated IDI) dieselengine, fuel is injectedinto a small prechamber,which is connected tothe cylinder by a narrow

opening. The initial combustion

takes place in thisprechamber.

This has the effect ofslowing the rate ofcombustion, which tendsto reduce noise.

FIGURE 4-3 An indirect injection diesel engineuses a prechamber and a glow plug.


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FIGURE 4-4 A direct injectiondiesel engine injects the fueldirectly into the combustionchamber. Many designs do not usea glow plug.


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High pressure circuit comprises: High pressure pump with pressure control valve The high pressure accumulator (Rail )with the rail pressure sensor Injectors, and The respective high pressure connection lines.


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EVAPORATIONis the changing of a liquid to a vapor.

*The rate of evaporation is dependent on the following

1- TEMPERATURE.2- ATMOSPHERIC PRESSURE

3- VOLATILITY.

4- ATOMIZATION


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The rate of movement of the molecules increase with

temperature. Because of this, the amount of molecules

leaving the liquid for a given time will increase, as thetemperature increases.


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As atmospheric pressure increases, the amount of airmolecules present over the liquid also increases. The

increased presence of air molecules will slow the rate

of evaporation. This is because the molecules of liquid

will have more air molecules to collide with. In many

cases, they will fall back into the liquid after thecollision


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The term volatility refers to how fast a liquidvaporizes. Some liquid vaporizes easily at room

temperature. Alcohol, for instance, vaporizes more

easily than water. A highly volatile liquid is one that

is considered to evaporate easily.


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Atomization is the process of

breaking up a liquid into tiny

particles or droplets.

When a liquid is atomized, the

droplets are all exposedindividually to the air.

For this reason, atomization

greatly increases evaporation by

increasing the exposed surface areaof the liquid.


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Carburetor Flooding: Occurs when fuel pours out the top of the carburetor.

Check float level (might be too high).

Float level too low: Will cause lean AFR. Will cause miss at high speed andaround cornering.

Clogged idle air bleed: Will effect at Idle, because it can enrich the mixture.

Engine Surge: Caused by extremely lean Air Fuel mixture.

Choke system: will make the engine perform poorly when the engine is cold.


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Carburetor Icing And Heat Use Carburetor ice means ice at any location in theinduction system.

Impact ice Fuel ice Throttle ice


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Impact ice Formed by the impingement of moisture-laded air attemperatures below freezing onto the elements of theinduction system which are at temperatures below

freezing. Air scoop, heat valve, carburetor air screen, throttlevalve and metering elements.


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Fuel Ice Forms when any air or fuel entrained moisture reachesa freezing temperature as a result of cooling of themixture by fuel vaporization.

Cooler air holds less water vapor and theexcess water is precipitated in the form ofcondensation. Condensate freezes. Can occur at ambient temperatures well abovefreezing.


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Throttle ice Formed at or near a partly closed throttle when watervapor in the induction air condenses and freezes dueto the expansion cooling and lower pressure at the

throttle. Temperature drop normally does not exceed

5 F.

Documents

SI Engine Mixture Preparation