Analysis of Port Injection Systems

P M V SubbaraoProfessor

Mechanical Engineering Department

Multi-physics Phenomenon of Fuel-Air Mixing…...

The Need for Multi Physics Processes Modeling in Port Injection

Early dynamometer tests indicated that numerous engine performance and emission parameters were significantly influenced by both;the cone angle of the spray plume and its direction relative to the back face of the intake valve.

first large-scale introduction ofPort Fuel Injection (PFI) in the mid-1980s.

Metered fuel is introduced into each engine intake runner individually with a low-pressure, port fuel injector, by means of a brief injection event during each engine cycle.

New Chapter in Fluid Mechanics

Arbitrary pulsating Micro pipe flows

fully developed axisymmetric pulsatinglaminar pipe flow

Transient Response of Injector

Physics of Spray Formation

• The combination of these two geometric parameters was denoted as spray “targeting” among fuel systems engineers.

• It soon emerged as a prime consideration in selecting an injector design for minimizing unburned hydrocarbon emissions.

• Made the engineers to understand physics of 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.

Schematic of a spray breakup

•The flow parameters of a jet:•Jet Reynolds number•Jet weber number•Ohnesorge number

Philosophical Models for Spray Characterization

Entropy of a group with a random behavior: i

ii PPKS ln

where S is the information entropy, the name used when the information concept is applied to problems in physics and engineering. In this equation K is a constant and Pi is the probability of the occurrence of a certain result, in terms of number fraction.Maximum feasible entropy corresponding to physical conditions will decide the behavioral distribution.The behavioral parameters for spray are diameter and velocity of Droplet group created by a spray.

Time Available to Attain Maximum Entropy

• The fuel injection pulse width in engine should be reduced because of the injection pressure and mass flow rate increase.

The impact of Injection Pressure on Volumetric Efficiency

Injector Capacity : Maximum Fuel Flow Rate

• Flow rate is the most important factor to consider when selecting a fuel injector.

• EFI tunes are written for a specific injector flow rate and, generally, the fuel injector should match this.

• The flow rate is a measure of how much fuel an injector will spray at a 100% duty cycle (wide open) at a specific base fuel pressure.

• Most fuel injector flow ratings are measured at a base fuel pressure of 3 bar and is about 10.5 cc/minute.

• It is possible to use a different injector size than the fueling map was written for by changing the fuel pressure.

Sizing of Injector

• While fuel injectors are rated at a 100% duty cycle, it is common practice to size the fuel injectors based on a 80% duty cycle.

• Since the flow rate is measured at a static fuel pressure on a new injector, and does not take into account the opening and closing of the injector, the 80% duty cycle maximum gives a factor of safety to account for actual driving conditions and a reduction in flow rate caused by deposits.

• This factor of safety is important because a lean fuel mixture will result in excessive heat and can potentially lead to engine damage.

Control of Mass Flow Through an EFI

• The mass flow through an EFI is selected based on three main factors.

• Engine rpm,

• Throttle position, and

• Oxygen remaining within the exhaust.

• The distributor triggering contacts relay the engine speed to the Electronic Control Unit (ECU).

• The load on the engine is taken from the intake air pressure sensor.

• These factors influence how long the injectors remain on for.

Exclusive Operations of EFI

• When the engine is first started, the starter is on; additional gasoline is pumped through the intake manifold.

• When the car is accelerating, throttle position is closer to wide open, the amount of time the injectors remain on is longer.

• When decelerating the fuel injectors remain turned off until the rpm’s drop below 1,000, when the injectors open again to allow the engine to run in the idle mode.

• The accuracy of the correct amount of fuel used and stepping up the voltage increases proper timing.

• A square wave is used to drive the electronic control unit. • The square wave is either on or off, 1 or 0. • The major concern is to reduce the lag between when the

electric pulse is received and when the solenoid is discharged. • By increasing the pressure of the fuel coming into the injector

and reducing the air gap you can accomplish this goal of reducing lag.

• The air gap is based on the distance between the needle of the fuel injector when it’s fully closed compared to when it’s fully open.

• The more pressure you have the less time it takes for the correct amount of fuel to be distributed and also reduces the size of the hole needed to allow the fuel into the manifold.

Injection Pulse

• The length of time an injector is open and squirting fuel is called the "pulse width," and it is measured in milliseconds (MS).

• As rpm increase, an injector can only be held open for so long before it needs to be held open again for the next engine revolution -- this is called its "duty cycle."

• Even though a fuel injector's flow rate is measured at its maximum duty cycle (100%), fuel injectors should never be operated at 100% duty cycle.

• Instead, a typical maximum duty cycle is around 80%.

• Fuel pressure plays a big role in the operation of a fuel injection system.

• Because the injector is essentially a gate valve for fuel delivery, increasing fuel pressure can allow you to cram more fuel into the intake tract for a given injector pulse width.

Injector Electrical Input

Flow Characteristics of EFI

Actuation of EFI

Speed Density Electronic MPFI

Speed Density Electronic MPFI

• Speed-density system uses engine speed, manifold pressure and air temperature to calculate the engine air flow.

• The electrically driven fuel pump delivers the fuel through a filter to the fuel line.

• A pressure regulator maintains the pressure in the line at a fixed value (270 kPa).

• Branch line leads to each injector, the excess fuel returns to the tank vial a second line.

• Typical injection times for automobile applications range from 1.5 to 10 ms.

• The appropriate coil excitation pulse duration or width is set by the electronic control unit (ECU).

• In speed-density system the primary inputs to the ECU are the outputs from the manifold pressure sensor, the engine speed sensor and temperature sensors installed in intake manifold.

• For warm-engine operation, the mass of air per cylinder per cycle is:

• The ECU forms the pulse which excites the injector solenoids proportional to mass flow air measured.

• Addition temperature signals are used to find out cold start or hot engine cylinder conditions.

ia

divdiiava TR

VpVpTNm

,

Electronic MPFI with Air-flow Meter