MODULE-2CARBURETION AND FUEL INJECTION

In the SI engine a combustible fuel-air mixture is

prepared outside the engine cylinder. The process of

preparing this mixture is called carburetion. The

carburetor, a device which atomises the fuel and mixes it

with air. The pipe that carries the prepared mixture to

the engine cylinders is called the intake manifold.

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A SIMPLE OR ELEMENTARY CARBURETTOR

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DRAWBACKS (LIMITATIONS) OF SIMPLE CARBURETTOR (SINGLE JET)

(1) A single jet carburetor can not provide a very rich mixture as required at the time of starting the engine

(2) It can not provide very rich mixture required for sudden acceleration of the engine.

(3) For gradually increasing pressure differences over the jet (at higher speed of the engine), the weight of the petrol discharged from a single jet increases at a greater rate than does the air supply. Hence, a single jet carburetor gives a progressively richer mixture as the air speed increases when set to give a correct mixture at low air speeds.

(4) It can not reduce the quantity of air flow during starting as required in cold weather conditions.

(5) The automation control of air and fuel according to the required conditions is not possible.

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PETROL INJECTIONS

• There are two petrol injection systems which are mechanically operated (1) Continuous port Injection (2) Electronic Fuel Injection system.• (1) Continuous port Injection system (Lucas Mechanical petrol

Injection System) In this system, fuel is injected continuously into the inner port as shown

in figure at a varied rate. The petrol is sucked from the tank by a pump and pressurized petrol at 3 bar is supplied through a distributor to the fuel injector to a particular cylinder. The relief valve shown in figure maintains the pressure and allows excess petrol to return to the tank.

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LUCAS MECHANICAL PETROL INJECTION SYSTEM

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ELECTRONIC FUEL INJECTION SYSTEM

• Now-a-days electronically controlled fuel injection system are commonly used as it functions rapidly and responds automatically to the change in manifold air pressure, engine speed, crankshaft angle an many other secondary factors. The electronic control unit assesses data (manifold pressure, engine speed, crank ange) received from a various sensing devices and then adjust the A:F supply for the best performance of the engine.

This system has to contain a means of supplying extra fuel for cold starting, during warming and enriching the mixture during acceleration.

Ice formation is virtually impossible with this system and the danger of vaporization is minimized because the petrol is under pressure right upto the injection point.

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FUEL INJECTION SYSTEM-L-JETRONIC WITH AIR-FLOW METERING

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Contd.

(1) Fuel delivery system(2) Air induction system(3) Sensors and air flow control system(4) Electronic control unit

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REQUIREMENTS OF DIESEL INJECTION

(1) To supply the correct quantity of fuel to be injected as per the load of the engine and increase in speed for automobile engines.

(2) To supply the fuel within a precisely defined period of the cycle(3) To control the rate of fuel injection, such that it should result in the

desired heat release pattern.(4) To atomize the fuel into very fine particles.(5) To distribute the fuel uniformly in the combustion chamber of the engine

and results in rapid mixing of fuel and air(6) Injection starts and stops sharply. There should not be any after-injection.

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TYPES OF INJECTION SYSTEMS

Injection system may be divided into two general types, as follows;(1) Air Injection(2) Air-less or Solid or Mechanical Injection (i) Common Rail system (ii) Distributor system (unit Injector) (iii) Individual pump and nozzle system

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AIR INJECTION

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Contd.

Advantages:(1) It provides better automization and distribution of fuel(2) As the combustion is more complicated, the BMEP is higher than with

other types of injection system.(3) It allows to use the inferior fuelsDisadvantages:(1) It requires complicated mechanism to run the compressor(2) The weights of the engine increases(3) Part of the power is used to drive the compressor and so the BHP of

the engine is reduced.

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AIR-LESS OR SOLID OR MECHANICAL INJECTION

Here, Injection of fuel directly into the combustion chamber without primaryatomisation is termed as solid injection. It is also termed as mechanicalinjection.Theory: In this system ,the fuel is supplied at a very high pressure (150bar)from the fuel pump to the fuel injector where it is injected to the combustionchamber with the help of an injector. The main parts of this system are fuelpump and fuel injector. Depending upon the location of the fuel pumps andinjectors, and upon the method used to meter the fuel, solid injection mayfurther be classified as follows: Air-less Injection may further be classified as follows: (i) Common Rail system (ii) Distributor system (unit Injector)(iii) Individual pump and nozzle system

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COMMON RAIL SYSTEM

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Contd.

A single pump supplies high-pressure fuel to header, a relief valve holdspressure constant. The control wedge adjusts the lift of mechanicaloperated valve to set amount and time of injection.

Advantages: (1) It fulfills the requirements of either, (a) the constant load with variable speed or (b) constant speed with variable load.

(2) Only one pump is sufficient for multi-cylinder engine.(3) Variation in pump supply pressure will effect all the cylinder uniformly(4) The arrangement of the system is very simple and maintenance cost is lessDisadvantages:(1) Very accurate design and workmanship are required(2) There is tendency to develop leaks in the injection valve.

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DISTRIBUTOR SYSTEM (UNIT INJECTOR)

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Contd.

In this system, the fuel is metered at a central point; a pump pressurises, meters

the fuel and times the injection. From here, the fuel is distributed to cylindersin correct firing order by cam operated poppet valves which open to admit fuelto the nozzles.

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INDIVIDUAL PUMP AND NOZZLE SYSTEM

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Contd.

In this system an individual pump or pump cylinder connects directly to eachfuel nozzle. Pump meters charge and control injection timing. Nozzles containa delivery valve actuated by the fuel-oil pressure

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FUEL PUMP

Fuel Pump: A large number of igneous fuel pump designs have beendeveloped by the manufacturers, Only one type fuel pump will bediscussed here.

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BOSCH FUEL INJECTION PUMP

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Contd. (EXPLANATION)

When the plunger is at its bottom stroke the ports SP and Y are uncovered andoil from low pressure pump (not shown) after being filtered is forced into thebarrel. When the plunger moves up due to cam and tappet mechanism, a stagereaches when both the ports SP and Y are closed and with the further upwardmovement of the plunger the fuel gets compressed. The high pressure thusdeveloped lifts the delivery valve off its seats and fuel flows to atomizerthrough the passage P. With further rise of the plunger, at a certain moment, theport SP is connected to the fuel in the upper part of the plunger through therectangular vertical groove by the helical groove, as a result of which a suddendrop in pressure occurs and the delivery valve falls back and occupies its seatagainst the spring force.

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Contd.

The plunger is rotated by the rack R which is moved in or out by the governor.By changing the angular position of the helical groove (by rotating theplunger) of the plunger to the supply port, the length of stroke during whichthe oil is delivered can be varied and thereby quantity of fuel delivered to theengine is also varied accordingly.

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FUEL INJECTOR (ATOMISER)

(1) It consists of a nozzle valve (NV) fitted in the nozzle body (NB). Thenozzle valve is held on its seat by a spring (S) which exerts pressure throughthe spindle (E). (AS) is the adjusting screw by which the nozzle valve lift canbe adjusted. Usually the nozzle valve is seat to lift at 135 to 170 barpressure.(FP) is the feeling pin which indicates whether valve is workingproperly or not.(2) The fuel under pressure from the fuel pump enters the injector through thepassages (B) and C and lifts the nozzle valve. The fuel travels down nozzle Nand injected into the engine cylinder in the form of fine spray. Then thepressure of the oil falls, the nozzle valve occupies its seat under the springforce and fuel supply is cut off. Any leakage of fuel accumulated above thevalve is led to the fuel tank through the passage A. The leakage occurs when the

nozzle valve is worn out.

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FUEL ATOMISER OR INJECTOR

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TYPES OF NOZZLES AND FUEL SPRAY PATTERNS

Main Requirements of an Injector Nozzle: (1) To inject fuel at a sufficiently high pressure so that the fuel enters the

cylinder with a high velocity. Higher the velocity of the fuel smaller will be the droplet size. The momentum of smaller droplets is less, hence, penetration is also.

(2) Penetration should not be high so as to impinge on cylinder walls; this may result in poor starting

(3) Fuel supply and cut-off should be rapid; there should be no dribbling.Types of Nozzles: The type of nozzle used is greatly dependent on the type of combustionchamber as open type or pre-combustion chamber. The nozzles areclassified as per the type of orifice nd its number used for injecting thefuel in the combustion chamber. The nozzles are classified as:

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Contd.

The nozzles are classified as :(1) Single hole nozzle(2) Multi-hole nozzle(3) Circumferential nozzle(4) Pintle nozzle(5) Pintaux nozzle

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SINGLE HOLE NOZZLE

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Contd.

(a) This is the simplest type of nozzle and is used in open combustion chambers

(b) It consists of a single hole bored centrally through the nozzle body and closed by the needle valve. The size of the hole is usually larger than 0.2 mm

(c) Its sprays cone angle varies from 5 to 15 0

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MULTI-HOLE NOZZLE

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Contd.

(1)This type of nozzle finds extensive use in automobile engines, particularly having open combustion chambers.

(2)It mixes the fuel with air properly even with slow air movement available with open combustion chambers.

(3) The number of holes varies from 4 to 18; the greater number provides better fuel distribution. The hole diameter lies between 0.25 to 0.35 mm and hole angle lies between 20 to 45 0

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CIRCUMFERENTIAL ORIFICE (NOZZLE)

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Contd.

(1) It sprays characteristics are similar to a plate type opening(2) The injected fuel particles tend to be projected in the form of plane,

with wide angle cone; the purpose of which is to obtain as large as possible to come into contact with the air in the combustion chamber.

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PINTLE NOZZLE

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Contd.

(1) The stream of the nozzle valve is extended to form a pin or pintle which protrudes through the mouth of the nozzle body. It may be either cylindrical or conical in shape

(2) The size and shape of the pintle can be varied according to requirement. The spray core angle is generally 60 0

(3) The spray obtained by the pintle nozzle is hollow conical spray.

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END

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