Docstoc

types-of-scavenging

Document Sample
types-of-scavenging Powered By Docstoc
					Types of scavenging

   Loop or cross scavenge
          Uniflow.
                 • Cross flow
• The main difference between the two
  types is that uniflow requires an exhaust
  valve or piston to operate. Loop or cross
  flow relies on the piston to open and close
  exhaust ports
After ignition of the fuel the piston travels
  down the liner uncovering firstly the
  exhaust ports. The exhaust gas at a
  pressure above atmospheric is expelled.
  This is often referred to as blowdown and
  its effect can be seen on the power card
  for all the types of scavenging as a rapid
  drop in cylinder pressure towards the end
  of the cycle.
                 Loop

• The method of loop scavenging is similar
  to the cross flow except the exhaust and
  scavenge ports may be found on the same
  side
• Some 2 stroke engines do not have
  exhaust valves; As well as scavenge
  ports in the cylinder liner, they are fitted
  with exhaust ports located just above the
  scavenge ports. As the piston uncovers
  the exhaust ports on the power stroke,
  the exhaust gas starts to leave the
  cylinder. When the scavenge ports are
  uncovered, scavenge air loops around
  the cylinder and pushes the remaining
  exhaust gas out of the cylinder. This type
  of engine is known as a loop scavenged
  engine. Note that the piston skirt is much
  longer than that for a uniflow scavenged
  engine. This is because the skirt has to
  seal the scavenge and exhaust ports
  when the piston is at TDC.
• Although simpler in construction with less
  moving parts, these engines are not as
  efficient or as powerful as uniflow
  scavenged engines. The scavenging of
  the cylinder is not 100%, and thus less fuel
  can be burnt per stroke.
• All modern large 2 stroke crosshead
  engines now being built are of the uniflow
  scavenged type.
Uniflow
UNIFLOW
    • 2 stroke engines with
      an exhaust valve
      mounted in the
      cylinder head are
      known as uniflow
      scavenged engines.
      This is because the
      flow of scavenging
      air is in one (uni)
      direction.
MAN B&W MC series uniflow
    scavenged engine
The scavenge air enters through the
  scavenge ports in the lower part of the
  cylinder liner, the exhaust gas is expelled
  through the centrally mounted exhaust
  valve in the cylinder cover. The scavenge
  ports are angled to generating a rotational
  movement of the rising column of air.
Air is forced out of the cylinder by the rising
  piston leading to low flow resistance, the
  effect is often compared to squeezing the
  contents out of a tube.
Disadvantages of the loop/crossflow method of
                     scavenging
The greatest disadvantage of this system, and
  the one that has led to the abandonment of
  its usage where once it was widespread is its
  inefficiency in clearing the cylinder of all
  combustion products.
Following the exhaust blowdown the scavenge
  ports are opened.
The period available for scavenging is limited
  to the recovering of the exhaust ports and is
  only at its most effective until the closing of
  the scavenge ports.
Therefore, high air velocities are designed in,
  the air entering through steeply angled
  shaped ports.
The possibility exists for the scavenge air to
  shortcut directly to the exhaust ports, a
  situation which worsens with blockage of the
  scavenge ports due to carbon build up.
Due to the inefficiencies above there is a high
  volume of scavenge air requirement with this
  design.
This has led to the complicated underpiston
  effect designs to augment the turboblower
  output with some engine power being
  absorbed dropping cycle efficiency.
A problem with having fixed ports is that
  difficulty is encountered with port timing.
On the piston down stroke the exhaust port is
  opened followed by the scavenge port to
  make effective use of blowdown.
However, the same timing for closing the
 ports means that the effective
 compression stroke is reduced.
To try to remedy this differing means of
 closing the exhaust ports before the
 piston covered the ports was tried, One
 such method was by engine driven
 rotating valves which opened and
 closed the exhaust ports.
All the designs increased complexity
 and often proved unreliable due to the
 arduous conditions they had to operate
 in.
To prevent exhaust gas entering the cylinder
 under the piston as the piston moves up to
 TDC, extended piston skirts are fitted.
This adds to the reciprocating mass and
 increases load on the crosshead bearing.
 The small amount of side thrust not
 absorbed by the crosshead is spread over
 the larger of the skirt reducing loading and
 wear on the liner, however, problems of
 increased lubrication requirements for the
 increased surface area largely negate any
 advantage.
The requirement for both the exhaust and scavenge
  ports being fitted into the liner makes for a more
  complicated design with increased liner lubrication
  difficulties especially in way of the exhaust ports.
  This region suffering not only the washing away
  effect of the gas flow but also contamination from
  combustion products and increased temperature.
  Cylinder lubricating oil volume demand is
  therefore higher with this design.
In an attempt to improve the scavenge efficiency
  shaped pistons have been used which produce a
  combustion chamber shape not the best for
  efficient combustion.
Asymmetrical piston designs can also lead to
  excessive thermal loading and complicated
  strengthening and cooling designs.
One advantage of the Loop method of
 scavenging is that it does away with the
 requirement for an exhaust valve or
 opposed piston.
This means that all the extra running gear
 associated with this can be omitted.
That means, simpler cylinder cover design,
 simpler and less stressed camshaft and
 camshaft drive train.
Where an exhaust valve is fitted in the
 cover, there is increase thermal stressing
 especially in way of the valve where higher
 temperatures are encountered.
  Additional advantages of the Uniflow method
The fitting of an exhaust valve does give a major
  advantage in that the timing of opening and
  closing of the valve can be altered which is
  used to its fullest with modern designs with
  'Variable exhaust timing' control fitted.
This means that the effect of the scavenge air
  inertia entering the cylinder, and optimising the
  closing of the valve to increase the effective
  compression (which starts when the exhaust
  valve is closed) can all be taken into account
  for the varying loads and engine speeds.
The increased scavenging efficiency with this
  type of scavenging creates greater scope for
  increased economy and so all modern designs
  are based on this design.
The opposed piston design once in favour due to its
   inherent dynamic load balancing has now largely
   disappeared due to its increased mechanical
   complexity.
There is a minimum air demand as the ingressing air
   pushes the combustion products ahead of it with
   little requirement for scavenging by dilution.
As the air flow is symmetrical rising up the liner the
   thermal influencing on the liner walls, cylinder
   covers and piston crowns is also symmetrical.
this allows simpler oil cooling of the piston crown
The reduced number of ports (no exhaust ports), and
   reduced size of the scavenge ports (due to reduced
   air volume requirements), this reduces the problems
   of liner lubrication allowing reduced oil consumption.