Fuel Injection in the CI Engine

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Fuel Injection in the CI Engine Powered By Docstoc
					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.
    Objectives of the Injection System
The injection system of the compression ignition
    engine should fulfil the following objectives
    consistently and precisely:
1. Meter the appropriate quantity of fuel, as demanded
    by the speed of, and the load on, the engine at the
    given time.
2. Distribute the metered fuel equally among cylinders
    in a multi-cylinder engine.
3. Inject the fuel at the correct time (with respect to
    crank angle) in the cycle.
4. Inject the fuel at the correct rate (per unit time or
    crank angle degree).
5. 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.
6. Begin and end injection sharply without dribbling
    or after injection.
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.
1. Pumping elements to transfer the fuel from the tank to
   the cylinder, along with the associate piping and
   hardware.
2. Metering elements to measure and supply the fuel at
   the rate as desired by the speed and load conditions
   prevailing.
3. Metering controls to adjust the rate of the metering
   elements for changes in load and speed of the engine.
4. Distributing elements to divide the metered fuel
   equally among the cylinders in a multi cylinder
   engine.
5. Timing controls to adjust the start and stop of
   injection.
6. Mixing elements to atomize and distribute the fuel
   within the combustion chamber
 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.
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.
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.
     Fuel Injection Systems
There are two main classifications for
   fuel-injection systems, namely
1. air injection which had become
   obsolete but now some interest has
   been shown by researchers (however
   very high pressure is required for air)
   and
2. solid (or airless) injection systems.
The airless, mechanical, or solid injection
systems consist of three types.

1. Individual pump system: This consists of a
   separate metering and compression pump
   for each cylinder.
2. 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).
3. Common rail system: A single pump for
   compressing the fuel, plus a metering
   element for each cylinder.
   
   m f  CD An 2 f p

                       
m f  CD An   2 f p
                      360 N
Spray Structure
   The Sauter Mean Diameter
• If ΔNi is the fraction of droplets counted in
  size interval Δdi, then the Sauter Mean
  Diameter SMD is given by
                              

                              N d i   i
                                           3


                    SMD      i 1
                               

                              N d
                              i 1
                                      i   i
                                           2
                   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,

1. A primary stage (a metal- edge filter to remove
   coarse particles, larger than 25 microns).

2. 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.
 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
                    V f  Cd 2 gh
where h is the pressure difference between
 injection and cylinder pressures, measured in
 meters of fuel column.
     The volume of fuel injected per second, Q, is given by
           .
               2                60   N i 
          Q   d  V f            
              4               360 N   60 

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,
     θ is the duration of injection in crank angle
     degrees,
     Q is expressed usually in mm3/degree crank
     angle/liter cylinder displacement volume

				
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posted:10/22/2011
language:English
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