Flow Pattern Of Coolant:
Figure below show a piston rod which has a through going
bore for the cooling Oil outlet pipe, which is secured to the funnel of the
cooling element. The cooling oil is supplied through a telescopic pipe
connection on the crosshead and passes through a bore in the foot of the piston
rod and on through the bore in the piston rod to the cooling element.
Four angled bore in the cooling element give the oil a
rotary movement inside the Piston crown. The oil is passed on through a number
of milled grooves in the upper edge of the cooling element to the funnel and
the outlet pipe in the piston rod. From a bore in the piston rod foot the oil
is led through a discharge spout to a slotted pipe in the engine frame and
through a control device for checking of flow and temperature.
In this way, the flow is such that piston-cooling oil enters
at the lowest part of the cooling space and leaves from the upper most part.
The flow direction is arranged in this manner so that the piston is always
partially full of coolant and the underside of piston crown is always in
contact with it. This is particularly important in slow speed propulsion
engines as when the piston is running at dead slow speed the coolant in piston
is not ‘shaken up’ in the way it is done when engine is running at full speed.
If the coolant flow took place in the opposite direction, it would be possible
at very slow speed for the coolant to drain from the piston and loose contact
with the crown. The piston could become overheated.
Some water-cooled pistons have the outlet for the water at
approximately half the Cooling space height. When running slow, the piston is
half full of water and piston movement agitates the water in the piston the
water gets splashed on the underside of the crown. The splash method of cooling
is called ‘cocktail shaker cooling’ (See drawing in page 2).
This piston consists of 3 main parts, which are piston
crown, cooling element and piston skirt. The cooling element is tightened to
the upper end of the piston rod and transmits the combustion pressure from the
piston crown to the piston rod. Between the piston crown and skirt, which are
assembled with a number of screws, there is a heavy clamping ring, which
presses the piston rod and the cooling element against the crown.
Latest Development:
The GFCA, GB and MC pistons shown the figure below reflect
the gradually increased firing pressures which are inevitable result of the
demand for ever decreasing fuel consumption figures. The material for the
crowns remains the same well proven chrome-molybdenum steel, which has
excellent strength against thermal stresses and a reasonable temperature
rating.
The GF pistons were originally rated for a firing pressure
of 84.3 bar. This pressure has been increased first to 87.3
bar and finally to 95 bar justified partly by the reduced thermal load, which
is a reflection of the higher efficiency of constant pressure turbo charging,
and partly by the excellent service record with an almost non existent failure
rate.
The utter simplicity of the GF (CA)
piston crown gives low production cost which is important, as pistons are
components with a finite service life. The simplicity, however gives way to
some minor deficiencies, which are the result of the flexible connection to the
piston rod, which in turn is necessary because of the thermal deformation of
the crown. These deficiencies do not limit the useful lifetime of the crown.
The GB pistons, rated to 112 bars are the next step. With
this design the mentioned deficiencies are cured by the firm connection to the
piston rod. The changed cross-section gives adequate strength to limit the
mechanical stress level to the same as for the GFCA pistons, giving the same
very low failure rate.
The MC piston crown, like the GB type, is rigidly bolted to
the piston rod at its inner support, which has a smaller diameter relative to
the bore, than the GB type. A new structural member shaped as an inverted
truncated cone, transmits a considerable part of the gas force to the rod, and
leaves a toroidal cooling chamber outside the supporting ring thus making a
reasonably thin plate possible.
A consequence of higher firing pressure is higher thermal
efficiency. This is reflected in the relatively small heat losses, which
include the piston, thus oil cooling is sufficient even for the biggest bores.
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| L90GFCA. L90GB. L90MC |
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| Ad/B&W design latest Piston, Piston rod, and
Stuffing box (Bore cooling system used).
|
1 comment:
I need a supplier of marine aftermarket pistons and I am lucky that I get it for a cheap amount.
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