Fuel Injection in the CI Engine

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Fuel Injection in the CI Engine

• For the compression ignition engine, it is very
  important to promote a means of injecting fuel
  into the cylinder at the proper time in the
  cycle. This is so because the injection system
  starts and controls the combustion process.

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Objectives of the Injection System

• The injection system of the compression ignition
  engine should fulfil the following objectives
  consistently and precisely
• Meter the appropriate quantity of fuel, as
  demanded by the speed of, and the load on, the
  engine at the given time.
• Distribute the metered fuel equally among
  cylinders in a multi-cylinder engine.
• Inject the fuel at the correct time (with respect
  to crank angle) in the cycle.
• Inject the fuel at the correct rate (per unit
  time or crank angle degree).
• Inject the fuel with the correct spray pattern
  and sufficient atomization as demanded by the
  design of the combustion chamber, to provide
  proper penetration also.
• Begin and end injection sharply without dribbling
  or after injection.

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• To accomplish these objectives, a number of
  functional elements are required. These
  constitute together, the fuel injection system of
  the engine. These elements are as follows.
• Pumping elements to transfer the fuel from the
  tank to the cylinder, along with the associate
  piping and hardware.
• Metering elements to measure and supply the fuel
  at the rate as desired by the speed and load
  conditions prevailing.
• Metering controls to adjust the rate of the
  metering elements for changes in load and speed
  of the engine.
• Distributing elements to divide the metered fuel
  equally among the cylinders in a multi cylinder
  engine.
• Timing controls to adjust the start and stop of
  injection.
• Mixing elements to atomize and distribute the
  fuel within the combustion chamber

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Function of fuel injection equipment

• The function of fuel injection equipment is to
  supply the engine with fuel in qualities exactly
  metered in proportion to the power required and
  timed with utmost accuracy, so that the engine
  will deliver that power within the limits
  prescribed for fuel consumption, exhaust smoke,
  noise and exhaust emissions.
• The fuel must be injected through suitable
  nozzles at pressures high enough to cause the
  required degree of atomization in the combustion
  chamber and to ensure that it mixes with
  sufficient air for complete combustion in the
  cycle time available.
• In multi cylinder engines the periods of
  injection, the timing and the delivered quantity
  must be accurately metered to ensure an even
  balance between the cylinders.

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• For an engine developing 3kW at 60rev/s, of
  cylinder capacity 0.2 liter the fuel delivery at
  full load would be approximately 10mm3 in 1.2ms,
  repeating this 30 times every second. At no load
  the quantity will be reduced to approximately to
  3mm3.
• In general terms the injection period and the
  pressure increase with engine size small direct
  injection (DI) engines will have a period about
  25 degrees crank travel and an injection pressure
  exceeding 400bar whilst large engines may have
  periods approximating 40degrees with pressures in
  excess of 1000 bar. Engines required to meet
  future limits of exhaust NOx emissions will need
  shorter injection periods with corresponding
  higher injection pressures.

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• The equipment for a six cylinder medium-sized
  high speed turbo charged vehicle engine
  developing 110kW at 43.3rev/s will have a full
  load delivery of 65mm3 with an injection period
  of approximately 26degrees crank travel. The
  nozzle will have a total orifice area of
  approximately 0.247mm2 (equivalent to four holes
  of 0.28mm diameter) and the peak injection
  pressure will be about 450 bar. To meet a NOx
  emission standard of 10g/kWh the injection period
  will have to be reduced to about 23 degrees crank
  angle for the same hole diameter. This will
  increase the probable peak line pressure to 650
  bar.

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Fuel Injection Systems

• There are two main classifications for
  fuel-injection systems, namely
• air injection which had become obsolete but now
  some interest has been shown by researchers
  (however very high pressure is required for air)
  and
• solid (or airless) injection systems.

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The airless, mechanical, or solid injection
systems consist of three types.

• Individual pump system This consists of a
  separate metering and compression pump for each
  cylinder.
• Distribution system This consists of a single
  pump for compressing the fuel (which may also
  meter), plus a delivery device for distributing
  the fuel to the cylinders (which may also meter).
• Common rail system A single pump for compressing
  the fuel, plus a metering element for each
  cylinder.

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Spray Structure
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The Sauter Mean Diameter

• If ?Ni is the fraction of droplets counted in
  size interval ?di, then the Sauter Mean Diameter
  SMD is given by

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Fuel Filters

• A low pressure (2.5 bar) transfer pump or fuel
  feed pump is required to lift the fuel from the
  tank, to overcome the pressure drop in the
  filters, and to charge the metering or pressuring
  unit. Three filters are recommended, namely,
• A primary stage (a metal- edge filter to remove
  coarse particles, larger than 25 microns).
• A secondary stage (a replaceable cloth, paper or
  lint element to remove fine particles from about
  4 to 25 microns) and
• 3. Final stage (a sealed, non-replaceable
  element) to remove fine particles that escaped
  the secondary stage.

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Quantity of Fuel and the Size of Nozzle Orifice

• The quantity of fuel injected per cycle is
  dependent on the power output of the engine. The
  size of droplets depend on the velocity which
  should be of the order of 400 m/s. As mentioned
  earlier, this velocity is given by
• where h is the pressure difference between
  injection and cylinder pressures, measured in
  meters of fuel column.

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The volume of fuel injected per second, Q, is
given by
where d is the diameter of one orifice in m,
Ni is the number of injections per minute,
N/2 for a 4-stroke engine, N is the
engine speed in rev/min, ? is the
duration of injection in crank angle degrees,
Q is expressed usually in mm3/degree crank
angle/liter cylinder displacement volume