mixture formation in spark ignition engines · mixture formation in si engine engine induction and...

MIXTURE FORMATION IN SPARK IGNITION ENGINES

Chapter 5

Mixture formation in SI engine

Engine induction and fuel system must prepare a fuel-air mixture thatsatisfies the requirements of the engine over its entire operating regime

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

Relative proportions of fuel and air that give the above requirementsdepend on engine speed and load.

Optimum air-fuel ratio for SI engine is that which gives

1. Required power output

2. Lowest fuel consumption

3. Consistent with smooth and reliable operation

Air induction systems

SI engine fuel supply system

Functional requirement of fuel supply system

The functional objectives for fuel injection systems can vary, beside the central task of supplyingfuel to the combustion process

There are several competing objectives such as:

Power output Fuel efficiency Emissions performance Ability to accommodate alternative fuels Reliability Driveability and smooth operation Initial cost Maintenance cost Diagnostic capability Range of environmental operation Engine tuning


There are three mixture formation techniques exist for gasoline engines

I. Carburetor

II. Port fuel injection - inject the fuel into the intake manifold

Depending the position of the injector

a. Single point injectionb. Multipoint injection

III. Direct injection - inject the fuel directly into the cylinder

The primary difference between carburetors and fuel injection is that fuel injectionatomizes the fuel by forcibly pumping it through a small nozzle under highpressure

Mixture formation in SI engine - Carburetor

The process of formation of a combustible fuel-air mixture by mixing theproper amount of fuel with air before admission to engine cylinder is calledcarburetion and the device which does this job is called a carburetor.

Mixture formation in SI engine - Carburetor

The process of carburetion is influenced by

The velocity of incoming air

The velocity of the air stream at the point where the fuel is injected has to beincreased. This is achieved by introducing a venturi section in the path of the air.

The vaporization characteristics of the fuel

Will require a volatile fuel for quick evaporation and mixing with air

The temperature of the incoming air and

Higher atmospheric air temperature increases the vaporization of fuel andproduces a more homogeneous mixture.

The design of the carburetor

Proper design of carburetor elements alone ensures the supply of desiredcomposition of the mixture under different operating conditions of the engine.

Mixture formation in SI engine - PFI

The point or location of fuel injection is one way to classify a gasolineinjection system.

A single-point injection system, also called throttle body injection (TBI),has the injector nozzles in a throttle body assembly on top of the engine.Fuel is sprayed into the top center of the intake manifold .


A multi-point injection system, also called port injection, has an injector in theport (air-fuel passage) going to each cylinder.

Gasoline is sprayed into each intake port and toward each intake valve.


Electronic control unit (ECU) is a generic term for anyembedded system that controls one or more of the electricalsystems or subsystems in a motor vehicle.

An engine control unit (ECU) is a type of electronic controlunit that determines the amount of fuel, ignition timing andother parameters an internal combustion engine needs to keeprunning.

It does this by reading values from multidimensional mapswhich contain values calculated by sensor devices monitoringthe engine.

Mixture formation in SI engine-PFI

Most port injection today occurs with the intake valve closed

Evaporation and mixture formation outside the cylinderprevents large droplets from directly entering the combustionchamber

large drops enter the cylinder, may be deposited on the walls, due to the low pressure and density of the gas, and increase HC emissions.


The classical gasoline injector today is the solenoid injector

The majority of injectors used these days are low-and medium-pressure injectors with injection pressures in the range of 0.5 to 1.5MPa


The injectors can survive the excessive temperature and pressure ofcombustion by using the fuel that passes through it as a coolant

The electronic fuel injector is normally closed, and opens to injectpressurized fuel as long as electricity is applied to the injector'ssolenoid coil.

When the injector is turned on, it opens, spraying atomized fuel atthe combustion chamber .

Depending on engine operating condition ,injection quantity willvary .


Injectors for port fuel injection differ mainly in the type of nozzle used togenerate the desired spray shape

The pintle-type injector produces a cone-shaped spray, the single-holeinjector a pencil-shaped spray, and the multi hole plate injector producesmultiple sprays according to the number of holes

Mixture formation in SI engine - DFI

Compared to the conventional diesel injection near top dead center, thedirect injection of gasoline may occur already during the induction stroke inthe case of homogeneous mixture formation (full load), or very late duringthe compression stroke in the case of stratified charge operation (part load)

Mixture formation in SI engine -DFI


Among other techniques like cylinder cutoff or the application of variablevalve trains the direct injection of gasoline is the measure with the highestpotential to reduce fuel consumption and thus also CO2-emissions

Compared to a similar PFI engine, about 15–25% reduction of fuelconsumption at part load are theoretically possible

Depending on the operating point of the engine, the direct injection ofgasoline offers different advantages compared to the PFI technique.

Mixture formation in SI engine-DFI

Mixture formation in SI engine-DIF

Mixture formation in SI engine-DISC

Mixture formation in SI engine-MM

mixture formation in spark ignition engines · mixture formation in si engine engine induction and...

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