models for design & selection of injection system p m v subbarao professor mechanical...

Models for Design & Selection of Injection System

P M V SubbaraoProfessor

Mechanical Engineering Department

Mathematical Tools for Sizing of Hard Ware As per

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Spray Formation

• Spray formation is explained as Breakup Mechanism, described as:

• Stretching of fuel ligament into sheets or streams.

• Appearance of ripples and protuberances.

• Formation of small ligaments or holes in sheets.

• Collapse of ligaments or holes in sheets.

• Further breakup due to vibration of droplets.

• Agglomeration or shedding from large drops.

• The flow parameters of a jet:

• Jet Reynolds number

• Jet weber number

• Ohnesorge number

Spray Structure

Distribution of Droplets in A Spray

Distribution of Droplets in A Spray

Distribution of Droplets in A Spray

Sauter Mean Diameter

• The representative diameter is defined as Sauter mean diameter (SMD).

• Introducing the definition of SMD:

Where is the liquid surface tension, L is the liquid viscosity, A is the air density, L is the liquid density,pL, is the injection pressure differential across the nozzle, is the half spray angle and t is the film thickness, given by

where do is the discharge orifice diameter and FN is the nozzle flow number defined by

Post Break-up Kinematics

• After break-up, the fuel jet behaves as a gas jet and follows the moment conservation law.

• jet centreline (um) is

According to the cylindrical free turbulent jet theory, the velocity distribution at the jet cross-section is

where u is the velocity of fuel reaching the point (r, z).

ce is the entrainment factor.

The spray angle

The velocity of the fuel zone reaching the point (, s) is

Penetration of Fuel Spray

• The spray penetration length and spray penetration rate from a fuel injector are the parameters used to judge fuel spray performance.

• The merits of high or low penetration largely depend on engine design and geometry.

• Shorter spray penetration may be of an advantage where it reduces fuel impingement, but in larger engines may inhibit maximum air utilisation.

Influence of Injection Pressure on Spray Penetration Length : HS Diesel

Influence of Ambient Pressure on Spray Penetration Length : HS Diesel

Model to Design a Spray

SOI : 11o BTDC

Impingement of Spray

• After the fuel jet has impinged the opposite chamber wall, the movement of the wall jet is complicated.

• Many experimental formulae have been suggested in the design methodologies.

• One such formulae used to calculate the velocity of the zone that has impinged on the wall :

Spray Velocity Distribution in A Two Hole NozzleEffect of Swirl

Spray Velocity Distribution in A Two Hole NozzleEffect of Swirl

Air entrainment and mixing

• The rate of air entrainment into each zone is calculated on the basis of the momentum conservation principle.

• The rate of entrainment relates not only to the fuel but also to the air–fuel ratio in the quiescent combustion chamber.

• It is observed that the rate of air entrainment increases with the increase of air–fuel ratio in the cylinder.

• The entrainment factor ce is linearly related to the equivalence air–fuel, .

Where a and b are constants for a certain engine type.

• u The relation takes some account of incomplete mixing for low air–fuel ratios.

• The relationship given above also improves the predictive ability of the model for transient operating conditions, for which the air–fuel ratio is low and the fresh air in the cylinder may be consumed by entrainment of fuel zones.

Idealized spray vaporization

• The spray is divided into an initial "cool” and "hot" zone.

• Within the cool zone, heat transfer is restricted to radiation from the flame front to the droplet surface.

• In the hot zone, heat transfer takes place both by radiation from the flame front and by turbulent convection.

• The reduction in droplet diameter due to vaporization follows the "d2 law":

dt

dd 2

where is the evaporation constant for forced convection.

The evaporation in forced convection

• The evaporation constant in forced convection

3/15.00 Re276.01 Sc

Bc

k

pf

1ln8

0

Where, k is the thermal conductivity of the gas, Cp is the specific heat of the gas, f is the density of the fuel and B=the transfer number

01 QY

TTch

B fpfg

hfg = latent heat of vaporization per unit mass of fuel, T= temperature of gas surrounding the droplet, Tf= temperature of drop surface, Q = heat of reaction, Y0=mass fraction of oxidant in the surrounding atmosphere and stoichiometric mixture ratio.

Ignition Delay

• The sum of times required for sub process.

• The most widely reported correlation relating the ignition delay to the ambient gas condition is given by the relation

where τ is the ignition delay, Pg and Tg are the ambient gas mean pressure and temperature before autoignition takes place, A, B and n are experimental constants.

Arhenius-type equation for Ignition Delay

• An Arhenius type equation for Ignition delay is:

Tp

pg

ref

cylid

6000exp100.4 04.1

5.2

3

Tpcyl

pid

6000exp18

2

p :Premixed air fuel ratio.

Symptoms to be Sensed to Predict Auto Ignition

Effect of Gas Temperature on Ignition Delay

Effect of Equivalence Ratio on Ignition Delay

The flammability limits versus the number of carbon atoms in alkanes

Development of Injection Pressure & Injection System in CI Engines

Fuel Injection System

• The fuel is to be introduced into the cylinder of a diesel engine through a nozzle with a large pressure differential across the nozzle orifice.

• The cylinder pressure at injection is typically in the range of 50 to 100 atm.

• Fuel injection pressures in the range of 200 to 1700 atm are used depending on the engine size and type of combustion system employed.

• These large pressure differences across the injector nozzle are required so that the injected liquid fuel jet will enter the chamber at sufficiently high velocity to

• Atomize into small-sized droplets to enable rapid evaporation

• Traverse the combustion chamber in the time available and fully utilize the air charge.

European Standards

Types of CI Engine Injection Systems

• Fuel-Injection Systems

• Unit Injector System (UIS) – Single-Cylinder CI Engine.

• Unit Pump System (UPS) – Multi-cylinder CI Engine.

• Common Rail Injection System (CRS) – Multi-cylinder CI Engine.

• The Unit Injector System (UIS) and the Unit Pump System (UPS) are among the most significant innovations in this field.

• They inject precisely the right amount of fuel individually into each cylinder, at very high pressure, and at exactly the right moment in time.

• This results in considerably more efficient combustion than is the case with conventional injection systems.

• This, in turn, equates to higher output, less fuel consumption, and lower levels of noise and exhaust-gas emissions.

Unit Injector System

Functional Principle of Modern Unit Injection System

Actuation of Solenoid Valve

Actuation of Injector Nozzle

Unit Pump Diesel Injection System

Common Rail Diesel Injection System

The Common Rail Diesel Injection System delivers a more controlled quantity of atomised fuel, which leads to better fuel economy; a reduction in exhaust emissions; and a significant decrease in engine noise during operation.

Common rail diesel injection system

• In the Common Rail system, an accumulator, or rail, is used to create a common reservoir of fuel under a consistent controlled pressure that is separate from the fuel injection points.

• A high-pressure pump increases the fuel pressure in the accumulator up to 1,600 bar .

• The pressure is set by the engine control unit and is independent of the engine speed and quantity of fuel being injected into any of the cylinders.

• The fuel is then transferred through rigid pipes to the fuel injectors, which inject the correct amount of fuel into the combustion chambers.

Injectors for CRDI

• The injectors used in Common Rail systems are triggered externally by an Electronic Diesel Control, or EDC unit.

• EDC controls all the engine injection parameters including the pressure in the fuel rail and the timing and duration of injection.

• Diesel fuel injectors used in Common Rail injection systems operate differently to conventional fuel injectors used in the jerk pump system.

• Some common rail injectors are controlled by a magnetic solenoid on the injector.

• Hydraulic force from the pressure in the system is used to open and close the injector, but the available pressure is controlled by the solenoid triggered by the Electronic Diesel Control unit.

• Some injectors use Piezo crystal wafers to actuate the injectors.

• These crystals expand rapidly when connected to an electric field.

• In a Piezo inline injector, the actuator is built into the injector body very close to the jet needle and uses no mechanical parts to switch injector needles.

• The electronic diesel control unit precisely meters the amount of fuel injected, and improves atomization of the fuel by controlling the injector pulsations.

• This results in quieter, more fuel efficient engines; cleaner operation; and more power output.