Analysis of Fuel Injection & Related Processes in Diesel Engines

P M V SubbaraoProfessor

Mechanical Engineering Department

Special Behavioral Issues of Teen Combustion ….

Development of Injection Pressure & Injection System in CI Engines

Types of CI Engine Injection Systems

• Air assisted 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 Injection System (Mechanical)

Unit Injector System (Electronic)

Functional Principle of Modern Unit Injection System (Electro-Mechanical)

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.

History of CRDI

• The common rail system prototype was developed in the 1960's by Robert Huber of Switzerland.

• The technology was further  developed by Dr.Marco Ganser at the swiss Federal Institute of Technology in Zurich.

• The first successful usage in production vehicle began in Japan in the mid-1990's by Dr.Shohei Itoh & Masahina Miyaki of the Denso Corporation.

Second Generation (Electronically Controlled) CRDI

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, (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.

The Third Generation Diesel Injection Systems: The HEUI (Hydraulically Actuated Electronically Controlled

Unit Injector) technology

Thermo-Fluid Dynamics of Injection• The basic processes are as follows.

• The fuel injected into a combustion chamber gets divided into many zones.

• Events in each zone are:– droplet break-up,

– evaporation,

– air–fuel mixing,

– ignition,

– heat release,

– heat transfer and

– formation of exhaust emits.

• Each zone obtains its own zonal temperature and compositions.

• The mass, internal energy and mole quantity of NOx of each zone are different.

• The final effect of combustion is cylinder-averaged temperature, air–fuel ratio and NOx concentration.

Mass flow rate through Nozzle

pACm fnDf

2

N

dpACm fnDf

3602

Fuel Exiting into High Pressure& Temperature Air Enviroment : An unexplored Fluid Mechanics

Coefficient of Discharge

Nurick’s Number, K

21

1

pp

ppK v

pv is the vapor pressure of the fuel.

Optimal Design of Nozzle Hole

Distribution of Droplets in A Spray

Distribution of Droplets in A Spray

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

Simultaneous Occurrence of Multiple Process in CI Engines

-10

Start ofinjection

End ofinjecction

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