Mixture formation for spark ignition engine.pdf

MIXTURE FORMATION IN

SPARK IGNITION ENGINES

Mixture formation in SI engine

 Engine induction and fuel system must prepare a fuel-air mixture that

satisfies the requirements of the engine over its entire operating regime

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

 Relative proportions of fuel and air that give the above requirements depend on engine speed and load.

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

1. Required power output 2. Lowest fuel consumption

Functional requirement of fuel supply system

fuel 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

Mixture formation in SI engine

I. Carburetor

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

 Depending the position of the injector

a. Single point injection

b. Multipoint injection

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

 The primary difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure

Mixture formation in SI engine -

Carburetor

 The process of formation of a combustible fuel-air mixture by mixing the

proper amount of fuel with air before admission to engine cylinder is called

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 be

increased. 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 and

produces a more homogeneous mixture.

 The design of the carburetor

 Proper design of carburetor elements alone ensures the supply of desired

Mixture formation in SI engine -

PFI

 The point or location of fuel injection is one way to classify a gasoline

injection 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 .

Mixture formation in SI engine -

PFI

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

Mixture formation in SI engine -

PFI



Electronic control unit (ECU)

is a

generic term

for any

embedded system that controls one or more of the

electrical

systems or subsystems in a motor vehicle.



An engine control unit (ECU) is a type of electronic control

unit that determines the amount of

fuel, ignition timing

and

other parameters an internal combustion engine needs to keep

running.



It does this by reading values from

multidimensional maps

which contain values calculated by sensor devices monitoring

the engine.

Mixture formation in SI engine-PFI

Most port injection today

occurs with

the intake valve closed

Evaporation

and

mixture formation outside the cylinder

prevents

large droplets

from

directly entering the combustion

chamber



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.

Mixture formation in SI engine -

PFI



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 1.5

MPa

Mixture formation in SI engine -

PFI



The injectors can survive the excessive temperature and pressure of

combustion by using the fuel that passes through it as a coolant



The electronic fuel injector is normally closed, and opens to inject

pressurized fuel as long as electricity is applied to the injector's

solenoid coil.



When the injector is turned on, it opens, spraying atomized fuel at

the combustion chamber .



Depending on engine operating condition ,injection quantity will

Mixture formation in SI engine -

PFI

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

 The pintle-type injector produces a cone-shaped spray, the single-hole

injector a pencil-shaped spray, and the multi hole plate injector produces

Mixture formation in SI engine -

DFI

 Compared to the conventional diesel injection near top dead center, the

direct injection of gasoline may occur already during the induction stroke in the case of homogeneous mixture formation (full load), or very late during the 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 variable

valve trains the direct injection of gasoline is the measure with the highest potential to reduce fuel consumption and thus also CO2-emissions

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

 Depending on the operating point of the engine, the direct injection of