Fuel Evaporation & Mixing Process in Port Injection Systems

P M V SubbaraoProfessor

Mechanical Engineering Department

Locally Correct Air/Fuel Mixture is Essential …...

Engineering Models for Droplet Evaporation

• In a simplified engineering model a representative diameter for the entire group is defined to compute evaporation rate.

• Equivalent diameter of same number of uniformly sized droplets having same total surface area

Droplet Evaporation Models

The mass transport model must describe a process in which the vapors evaporated from the surface of the droplet, are transported by coupled diffusion and convection to the ambient air. It is assumed that far away from the droplet surface the concentrations of the evaporating components are zero.

Maxwell–Stefan diffusion and convection theory

• The molecular flux at the droplet surface depends on the Maxwell–Stefan diffusion coefficient and the concentration gradient of molecules in the gas phase over the surface.

• The governing equation that describes the Maxwell–Stefan mass transport is as follows:

0

VJt

Where is the fuel density, is the mass fraction of evaporated fuel j is the diffusion-driven mass flux and V is the velocity vector.

Simplfied Droplet evaporation

• The droplet evaporation rate is given by

where d is the droplet diameter, DAB the gas diffusivity, Sh* the

non-dimensional Sherwood number, and Bm the mass transfer

number. The mass transfer number BM

is equal to:

Heat balance of evaporating droplet• Assume a quasi-equilibrium conditions around the droplets.

• The temperature of the droplet surface – and also the temperature of the whole droplet – are to be established for accurate prediction of fuel evaporation rate.

• Balancing the heat required for evaporation, the heat content of the droplet and the heat gained by conduction from the warmer surrounding air to the droplet is to be developed.

• For global energy balance calculation in evaporating systems the evaporation enthalpy and specific heat capacity, and thermal conductivity of air are necessary.

• The instantaneous temperature of the fuel is computed using:

evapdropletp

evapcondttt mmc

HQTT

@@

Film Evaporation

• The film vaporization rate is determined by

where hD is the mass transfer coefficient, and BM the mass transfer number, as described above. For the wall film, the energy equation is described by:

where h and kf are the heat transfer coefficient and the liquid fuel

thermal conductivity. The terms in right hand of equation are the heat transfer rate from gas to fuel film on the gas side, the heat transfer rate used for vaporization and the heat transfer from wall to the film respectively.

Analysis Of Mixture And Wall Film BehaviorThe injection system configuration, injection timing and coolant temperature exert an influence on combustion stability and duration.

Change of injection affects mixture formation in the intake port, wall film location and amount, size andamount of droplets, and finally mixture distribution in the combustion chamber.

System 1 : Case A

SOI 240[deg] BCTDCWater Temp. 80C

COV of NMEP 3.1[%]

System 1: Case B

SOI 380[deg] BCTDCWater Temp. 80C

COV of NMEP 1.8[%]

System3: Case F

SOI 240[deg] BCTDCWater Temp. 80C

COV of NMEP 8.6[%]

In-cylinder HC Concentration During Compression

Port Fuel Injection System : Spray Wall Impingement

Partial-heating of the Intake Port

Surface temperature of the Port under different heating powers.

Effect of PH on In-cylinder HC Concentration

Increment of in-cylinder HC concentration

Closing Remarks on Baby Care Philosophy

Occurrence of Heat Addition in CI Engine: A Teen Care Event.

Rudolf Christian Karl Diesel is Always

right ?!?!?!

Schematic of a diesel spray & Transport Process

Simultaneous Occurrence of Multiple Process in CI Engines

-10

Start ofinjection

End ofinjecction

Events in CI Combustion

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.

Development of Injection Pressure & Injection System in CI Engines

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.

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, 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.

Diesel fuel injection nozzles

Sac type VCO-type

The holes in a modern injection system are very small, typically 50 – 250 μm, and they are manufactured using a complicated EDM (Electro Discharge Machining) process.