Design of Port Injection Systems for SI Engines

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Design of Port Injection Systems for SI Engines

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Methods Means to Control Premixed Combustion..
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Port Fuel Injection System
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Physical Models for Spray Characterization
Entropy of a group of droplets
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 droplet distribution.
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Physical Constraints

• The following physical and mathematical
  constraints must be obeyed
• The sum of all probabilities must be unity

(ii) the mass flow of sprayed liquid must be
equal to the mass of all droplets produced per
unit time
where n is the total number of droplets produced
per unit time and mL is the liquid mass flux.
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Simplified Engineering fuel Evaporation model

• The comprehensive fuel spray model predicts
  individual motions of liquid fuel droplets and
  evaporation of each droplet.
• It also includes a more detailed treatment of
  in-cylinder 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.

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Sauter Mean Diameter

• Introducing the definition of SMD

where dnozz is the nozzle diameter, µf , µg are
the fuel and gas dynamic viscosity, respectively,
Re the Reynolds number and We the Weber number.
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Fuel Droplet Dynamics Prediction of Trajectory
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Details of Heat and Mass Transfer across Droplet
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Evaporation Process
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Droplet dynamics

• During the spray penetration, there is a drag
  force exerted on the droplets from the
  surrounding gases, which tends to decrease the
  relative velocity between the drop and the gas
  flow.
• From the Newton's Second Law, the equation is

Where d is the droplet diameter, ug and uf are
the velocities of the gas flow and the liquid
fuel droplet, respectively. And the drag
coefficient CD is given as
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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
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The heat flux available for heating up the
droplet is
where the ambient temperature, T8 and ? the
correction factor to account for the effect of
evaporation on heat transfer. The gas phase
temperature is evaluated from the one-third rule
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Port Fuel Injection System Spray Wall
Impingement
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Control of Wall Wetting in Port Injection
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Geometrical Features of Fuel Spray
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Model to Design a Spray
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Wall Wetting in Valve Regime
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Droplet Impingement Process
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Droplet Impingement on Film
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Film Height And Surface Angle Distribution
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Simulated Experiments
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Variation of Volume Fluxes
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Spray Wall Impingement

• In an engine system, a liquid fuel spray may
  impinge on the solid walls, either on the smooth
  sidewalls in ducts, manifolds and cylinders.
• A Model is needed to find the impingement site
  with the given fuel spray cone angle and
  impingement incidence angle.
• This model estimates the nominal area covered by
  the spray cone angle from the duct geometry.
• For each impingement site, the impingement
  probability and the passing-by probability then
  are assumed to be proportional to the spray
  covered wall area and the downstream flow
  cross-sectional area.

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Regimes of Wall Impingement

• The droplets in the cylinder can be discharged to
  the exhaust duct, when the exhaust valve opens.
• It is assumed that the droplets are uniformly
  distributed within the cylinder.
• There exists a wide range of spray wall
  impingement regimes, which have been identified
  as, Adhesion, Rebound, Spread and Splash.
• The outcomes of impingement depend on the
  incoming droplet conditions droplet velocity,
  size and temperature, incidence angle, wall
  temperature, surface roughness, wall film
  thickness
• and fluid properties, such as viscosity and
  surface tension.

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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