fuel injection systems

INTERNAL COMBUSTION ENGINES-II Fuel Injection System

I C ENGINES – II : Fuel Injection System

Introduction

• Fuel-Injection System is vital to the working and

performance of CI engine

• This system serves the purpose of initiating and

controlling the combustion to meet the demand

requirements

• Fuel is injected into combustion chamber towards

the end of compression. It is atomized as it enters

under high velocity and the droplets get vaporized

to form a fuel-air mixture. Due to continued heat

transfer from hot air to fuel, the fuel reaches to its

self ignition temperature to ignite spontaneously


Introduction

initiating combustion. Depending upon the demand

requirements the fuel injection system continues to

deliver the fuel during initial part of combustion.

Functional Requirements of an Injection System

For proper engine operation and satisfactory performance,

the following requirements must be met by the Fuel

Injection (FI) System

Accurate metering of fuel injected per cycle to meet

changing demand of speed & load

Precise timing of fuel injection in the cycle to ensure


performance; power, fuel economy, emissions

Proper control of rate of injection to achieve desired

heat release during combustion without knocking.

Proper atomization of fuel into fine droplets

Proper spray pattern to ensure rapid mixing of fuel & air

Uniform distribution of fuel droplets throughout the

combustion chamber.

To supply equal quantities of metered fuel to all

cylinders in case of multi cylinder engines.

No lag during beginning and end of injection to eliminate

dribbling of fuel droplets into the cylinder


Classification of Injection System :

Air Injection system and Solid Injection system

Air Injection system : Fuel is forced by means of

compressed air. Good mixing with higher mep. It

requires compressor. Ability to use high viscosity fuel.

Not much in use

Solid Injection system : Liquid fuel is injected directly

into combustion chamber. Solid injection systems can

be classified into ;

i) Individual pump and nozzle system

ii) Unit Injector system

iii) Common rail system

iv) Distributor system


All the FI systems comprise of following components:

Fuel tank

Fuel feed pump to supply fuel from fuel tank to FI system

Injection pump to meter and pressurize the fuel for

injection

Governor to ensure that the amount of fuel injected is in

accordance with variation of load

Injector to take the fuel from the pump and distribute it in

the combustion chamber by atomizing it into fine

droplets

Fuel filters to prevent dust and abrasive particles from

entering the pump & injectors to reduce wear & tear of

components


A typical arrangement of various components for

solid injection system used in CI engine is shown


Types of solid injection systems used in CI engines

• Individual Pump and Nozzle System

- Each cylinder is provided with a pump& injector providing

separate metering and fuel compression pump

- The pump may be placed;

a)close to cylinder b) in culster



• Unit Injector System

- In this, pump and injector nozzle are combined in one unit

- Each cylinder is provided with a unit injector

- Fuel is brought up by a low pressure pump and injected

by rocker arm actuating the plunger at a given instant

- Amount of fuel injected is regulated by the effective stroke

of plunger



• Common Rail System

- In this, a high pressure pump supplies fuel to a header

- High pressure in the header forces fuel to each of the nozzle

- At given instant, the fuel is injected in a specific cylinder by

a mechanically operated valve, by means of a push rod and

rocker arm. The amount of fuel is regulated by varying the

length of the push rod stroke.

- The pressure in header is as

per injector design for pene-

tration & dispersion of fuel in

combustion chamber



• Distributor System

- In this system, the pump pressurizes, meters and times the

fuel by supplying it to a rotating distributor for injection to

each cylinder. A uniform distribution of fuel is automatically

ensured because of one metering element.


• Fuel Feed Pump

- It is of spring loaded plunger type. The plunger is actuated

through a push rod from the cam shaft.

- At the minimum lift position of cam the spring force on the

the plunger creates suction causes fuel flow from the main

tank into pump. When cam turn to maximum lift position the

plunger lifts upwards. At the same time inlet valve is closed

and fuel is forced through the outlet valve.

When the operating pressure gets released, the plunger

return spring ceases to function resulting in varying of the

pumping stroke under varying engine load according to the

quantity of fuel required by the injection pump.


• Fuel Feed Pump


• Injection Pump

The main objective of the fuel- injection pump is to deliver

accurately metered quantity of fuel under high pressure at

the correct instant to the injector fitted on cylinder.

Injection pumps are of two types;

- Jerk Type Pumps

- Distributor Type Pumps

• Jerks Type Pumps

- It consists of a reciprocating plunger inside a barrel.

- The plunger is driven by a cam


• Jerks Type Pumps


• Distributor Type Pumps

- This pump has only a single pumping element and the

fuel is distributed to each cylinder by means of a rotor.


• Distributor Type Pumps

- There is a central longitudinal passage in the rotor and

two sets of radial holes located at different heights. One

set is connected to pump inlet via central passage

whereas the second set is connected to delivery lines

leading to injectors of various cylinders.

- The fuel is drawn into the central rotor passage from

the inlet port when the pump plunger move away from

each other.

- Wherever, the radial delivery passage in the rotor

coincides with the delivery port for any cylinder the fuel

is delivered to it..

- Main advantage lies in its small size and light weight


• Injection Pump Governor

- Mechanical governor


• Injection Pump Governor

- Pneumatic governor


• Fuel Injector

- Injector assembly consists of ;

I) nozzle/needle valve

ii) compression spring

iii) nozzle

iv) injector body


• Fuel Injector

- Nozzle should fulfill the following functions

i) Atomization

ii) Distribution of fuel :

Factors affecting fuel distribution

# Injection pressure : dispersion, penetration

# Density of air in the cylinder: resistance to movement

# Physical properties of fuel: self ignition temp., vapour

press., viscosity, etc.

iii) Prevention of impingement on walls: decomposition

iv) Mixing : in case of non-turbulent type of combustion

chamber


• Fuel Injector

- Types of Nozzles

I) Pintle Nozzle ii) Single hole nozzle

iii) multi- hole nozzle iv) Pintaux nozzle


• Spray Formation

- At the start of injection the pressure difference across orifice is

low and single droplets are formed

As pressure difference increases following occurs :

- A stream of fuel emerges

- Stream encounter aerodynamic resistance from dense

air( 12 to 14 times ambient pressure ) and breaks into a

spray at the break-up distance

- With further increase in pressure the break- up distance decreases

and the cone angle increases until apex of the cone practically

coincides with the orifice


• Spray Formation


• Typical Comments/ Observation on Quantity of Fuel

Injected

• Fuel Injector should develop a pressure higher than the

highest desired pressure in the engine

• Also make additional pressure available to impart high

velocity head & adequate atomization

• Fuel particle size & Velocity has an Optimum value such

that the momentum is adequate for the depth of

penetration of spray in the compressed air in cylinder

• Excessive atomization is not always conducive to proper

mixture formation



Injected

– Spray has compact & denser core, surrounded by a cone of fuel

droplets of various sizes & vaporized liquid

– Larger droplets provide higher penetration

– Smaller droplets required for quick mixing & evaporation of fuel

– Droplet size ( 5 micron ) depends upon :

- decrease with increase in Inj. Press & Air density

- increase with increase in fuel viscosity & size of orifice



Injected

– Rate of Fuel Injected / degree is a function of ;

Injector camshaft velocity, Plunger dia, flow area of tip orifice

– Increasing Rate of Injection decreases duration of Injection &

subsequently Injection timing ( Instant & duration )

– Higher rate retards Injection timing – improve fuel economy w/o

smoke

– Increase in Injection rate increases load on injector push rod &

cam and affect durability

• Computation for Fuel Injection in CI Engines

Quantity of Fuel and Size of Nozzle orifice


• Computation for Fuel Injection in CI Engines

Applying the Bernoulli equation upstream (Injector ) &

downstream (cylinder) of Fuel Injector :

Pinj / ρf + V2

inj / 2 = Pcyl / ρf + V2

cyl/ 2

Pinj ; Injection Pressure

ρf ; density of fuel ( same being incompressible)

Vinj ; Velocity at section inside injector to be very small

compared to inside cylinder hence negligible

Pcyl ; Cylinder pressure at the time of injection

Vcyl ; Velocity at section inside cylinder at injector tip

Vcyl = √ 2 ( Pinj - Pcyl ) / ρf


• Calculations for Quantity of Fuel and the Size of Nozzle

Orifice

• Actual Fuel jet velocity at exit of orifice, Vf = Cd x Vcyl ,

where Cd is flow discharge coefficient of nozzle orifice

Vf = Cd √ 2 ( Pinj - Pcyl ) / ρf

• Velocity of Fuel through nozzle orifice in terms of fuel

column; Vf = Cd √ 2gh

• The volume of fuel injected per second ( rate of inj.), Q

Q = [ area of all orifice] x [ fuel jet velocity] x [time for one

injection] x [ number of injections per sec.for one orifice]

Q = [(π/4 .d2 x n)] x[Vf] x [(Ө / 360) x( 60/ N )] x [(Ni / 60)]


Q = [(π/4 .d2 x n)] x[Vf] x [(Ө / 360) x( 60/ N )] x [(Ni / 60)]

where ;

Vf = fuel jet velocity, m / s n = number of orifices

d = diameter of one orifice, m Ө =duration of injection,º CA

N = rpm Ni = no. of injections/ minutes

Ni = N/2, for four stroke engine, Ni= N, for two stroke engine

Ni ≡ no. of cycles

Alternatively;

Fuel consumption / hr = bsfc x Power

Fuel consumption/cycle = Fuel consumption per minutes/ Ni

Nozzle orifice area, A = [vol.of fuel inj /cycle] / [Inj Vel x inj time]

= π/4 .d2


• Functional Requirements of an Injection System

- Accurate metering /Precise timing / Proper control of rate

/atomization/spray pattern /Uniform distribution /

equal quantities /No lag

• Fuel Injection system : Pump, Nozzle System and other

components

• Spray Formation

fuel injection systems

Engineering

fuel economy

liquid fuel

fuel tank fuel feed

fuel compression pump

fi system injection

fuelair mixture

demand requirements

dribbling of fuel droplets