NUST

SUBMITTED TO

LEC MUBASHIR GULZAR

SUBMITTED BY

NS MUDASSAR HUSSAIN

DE-32(MECHANICAL ENGINEERING)

SYNDICATE:B

DATE: FEBRUARY 18TH, 2012

BENEFITS AND OPERATIONS OF PISTON RINGS AND PISTON SKIRT

BENEFITS AND OPERATIONS OF

PISTON RINGS AND PISTON SKIRTS

PISTO N RINGS Description: P iston rings are metal rings that fit into circumferential grooves around the piston

and form a sliding surface against the cylinder walls. They are split to allow for installation and

expansion, and they exert an outward pressure on the cylinder wall when installed. They fit into

grooves that are cut into the piston, and are allowed to float freely in these grooves. A properly

formed piston ring, working in a cylinder that is within limits for roundness and size, will exert

an even pressure and a solid contact with the cylinder wall around its entire circumference.

History: In the early steam engines no piston rings were used. The temperatures and the steam

pressures were not as high as the corresponding parameters in today’s internal combustion

engines, and the need for considering thermal expansions and clearances was smaller. Increasing

power demands required higher temperatures, which caused stronger heat expansion of the

piston material. This made it necessary to use a sealant between the piston and the cylinder liner

to allow a decrease in the clearance in cold conditions, i.e. when the clearances were at their

maximum. Keeping the clearance between the piston and liner wall at a minimum considerably

reduces the combustion gas flow from the combustion chamber past the piston.

The first piston rings used in an engine had the sole task of sealing off the combustion chamber,

thus preventing the combustion gases from trailing down into the crankcase. This development

increased the effective pressure on the piston. Ramsbottom and Miller were among the pioneers

to investigate the behaviour of the piston rings in steam engines. Ramsbottom, in 1854,

constructed a single-piece, metallic piston ring. The free diameter of the ring was 10 per cent

larger than the diameter of the cylinder bore. When fitted in a groove in a piston, the ring was

pressed against the cylinder bore by its own elasticity. Previous piston rings had consisted of

multiple pieces and with springs to provide an adequate sealing force against the cylinder bore.

Miller, in 1862, introduced a modification to the Ramsbottom ring. This modification consisted

of allowing the steam pressure to act on the backside of the ring, hence providing a higher

sealing force. This new solution enabled the use of more flexible rings, which conformed better

to the cylinder bore (Priest and Taylor, 2000).

In the early days, the ring pack was lubricated solely by splash lubrication; i.e. lubrication by the

splashing of the rotating crankshaft into the crankcase oil surface. Subsequently, when the

combustion conditions became even more demanding, i.e. with higher temperatures, pressures

and piston speeds, oil control rings were introduced. A proper lubricant film on the piston, piston

rings and liner wall was required in order to prevent damage. The oil control rings were, and are,

especially designed to appropriately distribute the oil on the cylinder liner and to scrape off

surplus oil to be returned to the crankcase.

Classif ications: There are two basic classifications of piston rings.

The Compression Ring: The compression ring seals the force of the exploding mixture

into the combustion chamber.

The Oil Control Ring: The oil control ring keeps the engine’s lubricating oil from

getting into the combustion chamber.

Most pistons have two compression rings and one oil

control ring. Most passenger car engines have oil

rings with two rails and an expander spacer as shown

here.

Compression Ring: The purpose of the compression ring is to hold the pressure from the power

stroke in the combustion chamber. There are many different cross sectional shapes of piston

rings available. The various shapes of rings all serve to preload the ring so that its lower edge

presses against the cylinder wall. This serves the following functions:

The pressure from the power stroke will force the upper edge of the ring into contact with

the cylinder wall, forming a good seal.

As the piston moves downward, the lower edge of the ring scrapes, from the cylinder

walls, any oil that manages to work past the oil control rings.

On the compression and the exhaust strokes, the ring will glide over the oil, increasing its

life.

As shown in figure above a second compression ring is also used. The primary reason for using it

is to hold back any blow-by that may have occurred at the top ring. A significant amount of the

total blow-by at the top ring will be from the ring gap. For this reason, the top and the second

compression rings are assembled to the piston with their gaps 60 degrees offset with the first ring

gaps.

Oil Control Rings: The oil control rings serve to control the lubrication of the cylinder walls.

They do this by scraping the excess oil from the cylinder walls on the down stroke. The oil then

is forced through slots in the piston ring and the piston ring groove. The oil then drains back into

the crankcase. The rings are made in many different configurations that can be one-piece units or

multipiece assemblies. Regardless of the configuration, all oil control rings work basically in the

same way.

Piston Ring Expanders: Expander devices are used in some applications. These devices fit

behind the piston ring and force it to fit tighter to the cylinder wall. They are particularly useful

in engines where a high degree of cylinder wall wear exists.

Conf igurations: Piston rings are arranged on the pistons in three basic configurations. They are:

The three-ring piston has two compression rings from the top, followed by one oil control

ring. This is the most common piston ring configuration.

The four-ring piston has three compression rings from the top, followed by one oil

control ring. This configuration is common in diesel engines because they are more prone to

blow-by. This is due to the much higher pressures generated during the power stroke.

The four-ring piston has two compression rings from the top, followed by two oil control

rings. The bottom oil control ring may be located above or below the piston pin. This is not a

very common configuration in current engine design.

Configurations of Piston Rings

Benef its: Following are the benefits of piston rings.

They maintain a gas-tight seal between the piston and the cylinder wall to prevent

blow-by of the gases.

Provide a path for conducting heat from the piston for the cylinder walls.

Control the quantity of oil reaching the piston crown and rings, allowing sufficient to

ensure lubrication but limiting an excessive quantity which would cause an increase in oil

consumption and carbonization.

PISTO N SKIRTS The main task of the piston is to convert thermal energy into mechanical work. Furthermore, the

piston rings seal the combustion chamber from the crankcase and transfers heat to the coolant.

The piston skirt acts as a load-carrying surface, which keeps the piston properly aligned within

the cylinder bore.

P iston skirt is fitted in both two stroke and four stroke engines. The diameter of the skirt is

usually kept slightly larger than that of the piston. This is done to prevent damage to the liner

surface due to the piston movement.

Soft bronze rings are also fitted in the piston skirts. These bronze rings help during the running-

in of the engine, when the engine is new, and can be replaced if necessary. In two stroke engines

having loop or cross scavenging arrangements the skirts are slightly larger as these helps in

blanking off the scavenge and the exhaust ports in the liner. In four stroke or trunk piston engines

the skirt has arrangement for gudgeon pin, which transmits power from the piston to the gudgeon

pin or top end bearing. As there are no cross head guides in four stroke engines, these skirts help

in transferring the side thrust produced from the connecting rod to the liner walls.

DESIGN CO NSIDERATIO NS AND CONFIGURATIONS During manufacture, the surface of the piston skirt is intentionally left slightly rough with small

machine marks less than 0.001* deep. This somewhat rough surface helps retain lubrication.

Aluminum pistons are often coupled with cast iron cylinder blocks. Aluminum expands at about

twice the rate of cast iron. Three things can be done to minimize the differences in expansion:

Most piston skirts are tapered as shown in Figure below. They are larger in diameter at the

bottom because the bottom of the piston runs cooler than the top so it does not expand as much.

At operating temperature, the piston will fit the cylinder wall correctly. Some pistons are not

tapered but instead are barrel shaped, which means they are smaller at both the top and bottom.

P iston skirt taper.

The piston skirt is cam ground. This allows a cast piston lo fit the cylinder with only 0.0005 to

0.0025 of clearance. The smaller the clearance, the less chance of piston slap. Expansion takes

place perpendicular to the piston skirt.

Piston Skirt Conf igurations: Figure A. shows slipper-skirt and trunk-skirt pistons. A full-skirt

piston used on longer stroke engines is known as a trunk position. Most of today’s automotive pistons are slipper-skirt pistons. A slipper-skirt is designed to clear the crankshaft counterweights

on engines with short strokes. The surfaces of the piston skirts that are 90 degrees to the piston pin are called thrust surfaces.

The slipper-skirt piston is cut away to clear

the crankshaft counterweights.

Slipper and trunk skirt pistons

BENEFITS Piston skirts have different functions for different engines. In large cross head two stroke

engines with uniflow scavenging these skirts are short in length and are fitted to act as a guide

and to stabilize the position of the piston inside the liner.

The piston skirt is the extension of the side profile of piston which controls the piston

movement in the bore preventing it from wobbling around and controlling the angular forces

present on the piston walls from the angular rotation of the crankshaft.

REFERENCES [1]. Andersson, Peter, Tamminen, Jaana & Sandstrom, Carl-Erik. P iston ring tribology. A

literature survey. Espoo 2002. VTT Tiedotteita – Research Notes 2178. 105 p.

[2]. Departmet of the US Army. (1985). Principles of Automotive Vehicles, TM 9-8000.

Washington, DC.

[3]. Gilles, T. (2010). Automotive engines: Diagnosis, Repair and Rebuilding. (6th ed., p. 469).

NY: Delmar International Inc.

[4]. Gilles, T. (2011). Automotive service: Inspection, maintenance, repair. (4th ed., pp. 271-

273). NY: Delmar International Inc.

[5]. Introduction to piston design for forced induction engines. URL:

http://horsepowercalculators.net/category/supercharger-power-parts [Archived in Superchargers

Power Parts]

[6]. Kaushik, M. (2010). Piston skirt, Piston Rod and Trunk Piston. Retrieved from

http://www.marineinsight.com/tech/main-engine/piston-skirt-piston-rod-and-trunk-piston/

[7]. Leeming, D. J. and Hartley R. (1989). Heavy Vehicle Technology. (2nd ed., p. 43). London,

England: Stanley Thornes(Publishers) Ltd.

[8]. Pulkrabek, W. W. (2007). Engineering Fundamentals of the Internal Combustion Engine.

(2nd ed., p. 23). Delhi: Dorling Kindersley (India) Pvt. Ltd.