Fuel Evaporation in Ports of SI Engines

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Fuel Evaporation in Ports of SI Engines

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Measure of Useful Fuel ..
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Wall Wetting in Valve Regime
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Variation of Volume Fluxes
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Film Dynamics

• There are two different forces exerted on a fuel
  wall film.
• On the gas side, the gas flow tends to drive the
  film moving along the same direction.
• On the wall side, the viscous friction tends to
  resist the film movement.
• The force balance on the film gives the equation
  for the film motion

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tg and tw are the driving force on the gas side,
the viscous stress on the wall side. timp the
momentum source per unit film area due to the
impingement.
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Deposited Film Mass Fraction
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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.
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In-cylinder HC Concentration During Compression
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Partial-heating of the Intake Port
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Surface temperature of the Port under different
heating powers.
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Effect of PH on In-cylinder HC Concentration
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Increment of in-cylinder HC concentration
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The injection technology Gasoline Fuel Sprays
in Port
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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.

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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.

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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.

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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 rpms 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.

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• 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 its fully
  closed compared to when its 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.

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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.

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Injector Electrical Input
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Flow Characteristics of EFI
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Actuation of EFI
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Speed Density Electronic MPFI
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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.

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• 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.

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Electronic MPFI with Air-flow Meter