Thursday, March 8, 2012

Piston Rings

What are the uses of  Piston Rings?

  1. Sealing the combustion/expansion chamber.
  2. Supporting heat transfer from the piston to the cylinder wall.
  3. Regulating engine oil consumption

     The first question came into mind when I saw the use of piston ring is for sealing the combustion is 

" A Piston skirt with a diameter same as the inner diameter of the combustion chamber could have done this? Then why Piston Rings are efficient?" 
Then I came to the conclusion that using a large surface of the piston to collide with the bore will cause comparitively more friction than collision of piston rings with the bore. But definitely the ring may wear faster. To avoid friction many coatings have been given.

What are the Coating Materials?

  • Chromium (CKS] or GDC which are wear resistant peripheral coatings)+
  • Nitriding and Nitrocarburizing (Enhancing the performance of piston rings in gasoline engines)
  • PVD Coatings
  • Thermal Spray Coating (Chiefly for Diesel Engines)

How many Rings will be there in a piston?

     It depends. But most of the pistons I have seen has three rings.The First two rings is for the sealing the Compression and the last ring is for controlling which lubricates the Piston Skirt.

How many types of Piston Rings are there?

There are many type of Piston Rings Based on the application for which it is used.

Types of Compression Rings

Rectangular Ring:
A piston ring with a rectangular cross section. This ring with its geometrically simple shape performs the necessary sealing functions under normal operating conditions. With a peripheral coating and appropriate barrel face the rectangular ring is today used mainly in the top groove in passenger car gasoline and diesel engines. Besides service in internal combustion engines, rectangular rings are commonly used as rotary shaft seals, e.g. transmission seals .
Taper Faced Ring: 
Owing to the tapered running face the ring contacts the cylinder bore with its bottom outer edge. This shortens running-in and improves oil scraping. The gas forces acting initially at the running face provide a degree of pressure relief (especially when used in the top groove). Taper faced rings are chiefly installed in the second groove in passenger car gasoline and passenger car and truck diesel engines. In passenger car gasoline engines they are also used in the top groove.
Internally Bevelled or Stepped Ring:
By providing an edge relief on the top side of rectangular and taper faced rings a twist effect is achieved which, in all operating phases without gas pressure loading, brings the ring into bore contact only with its bottom outer edge while the inner edge contacts the bottom groove side (positive twist). This helps to improve oil consumption control. Under operating conditions the gas pressure forces the ring flat against the piston groove, creating an additional dynamic behaviour of the ring. Rings of this kind are used in the top and second groove of passenger car gasoline and passenger car and truck diesel engines.
Taper Faced Ring with Inside Bottom Bevel or Step:
In the installed condition this edge relief causes a negative twist, i.e. in the opposite direction to a ring with the relief on the top side. The taper must be larger than on a taper faced ring without twist or with positive twist so that the top outer edge is prevented from contacting the cylinder wall.
The effect of the negative twist is to make the ring contact the groove and create a seal with its outer bottom side and its inner top side. This type of ring is installed in the second groove in passenger car gasoline and passenger car and truck diesel engines.
Keystone Ring:
compression ring with a wedge cross section. With its tapered sides, radial movement of the ring in engine operation will cause the axial clearance in the groove to increase and decrease. This greatly reduces ring sticking, as the ring continuously works its way free of the combustion residues. These rings are designed with an overall side angle of 6° bzw. 15°, the larger angle being more effective against the tendency to coking. The keystone ring is used in the top groove in passenger car and truck diesel engines where ring sticking must be expected.
Half Keystone Ring:
compression ring with only the top side tapered. Like on the keystone ring, the tapered side (keystone angle 7°) causes the axial clearance to vary as the ring moves radially, and thus reduces ring sticking. Owing to its asymmetrical cross section the ring has a positive twist when installed.
half keystone ring is used in the top groove of passenger car and truck diesel engines when arectangular ring is no longer adequate in regard to ring sticking but a keystone ring is not yet warranted. Another application is in 2-stroke gasoline engines, e.g. for snowmobiles and ultralight aircraft
L-Shaped Compression Ring:
This ring is used mainly in small 2-stroke gasoline engines as a "head land" ring, the vertical arm of the L being flush with the top edge of the piston crown. With gas pressure acting behind the vertical arm, this ring will also seal when in contact with the top side of the piston groove.
Besides being used in 2-stroke engines, in some cases it has been installed in automotive diesel engines in order to minimize crevice volume in the combustion chamber

Types of Scraper Rings

Napier Ring:
This can be installed in the second groove in passenger car gasoline and passenger car and truck diesel engines, but has been almost entirely replaced by the taper faced Napier ring. It is now occasionally used only in the compressors of air brake systems.
Taper Faced Napier Ring:
Like on the taper faced ring, the periphery of this ring is designed with a taper in order to improve oil scraping and quicken running-in. These rings are used on the same applications as Napier rings.
Taper Faced Closed Gap Scraper Ring::
On this ring the step runs out at the gap and is designed without an undercut. It seals better against blow by than the Napier and taper faced Napier rings with a continuous step. The applications for this ring are the same as for Napier and taper faced Napier rings.

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