Sunday, October 20, 2013

Pistons-their types & Piston Rings

 The Piston comprises of two pieces; the crown and the skirt. The crown is subject to the high temperatures in the combustion space and the surface is liable to be eroded/burnt away. For this reason the material from which the crown is made must be able to maintain its strength and resist corrosion at high temperatures. Material used is mostly Steel, alloyed with chromium and molybdenum. The crown also carries the 4 or 5 piston ring grooves which may be chrome plated.
There r basically 2 types of pistons used in IC Engines:
1) Trunk type
2) Crosshead type
Among them, Trunk pistons are long, relative to their diameter. They act as both piston and also as a cylindrical crosshead. As the connecting rod is angled for part of its rotation, there is also a side force that reacts along the side of the piston against the cylinder wall. A longer piston helps to support this.

The cast iron skirt acts as a guide within the cylinder liner. It is only a short skirt on engines with an exhaust valve (known as uniflow scavenged engines), as unlike a trunk piston engine, no side thrust is transmitted to the liner (that's the job of the crosshead guides).
A forged steel piston rod is bolted to the underside of the piston. The  other end of the piston rod is attached to the crosshead pin.
Pistons are cooled either using water or the crankcase oil. Water has a better cooling effect than oil, but there is a risk of leakage of water into the crankcase.

Modern engines have oil cooled pistons. The piston rod is utilised to carry the oil to and from the piston. The rod is hollow, and has a tube running up its centre. This gives an annular space which, with the central bore, allows a supply and return. The MAN B&W piston has an 8mm thick heat resisting layer of a hard nickel-chrome alloy called Inconel welded to the hottest  part of the crown to resist the "burning" of the piston crown.  

Cooling
Water Cooled PistonOil Cooled Piston
High specific heat capacity therefore removes more heat per unit volumeLow specific heat capacity
Requires chemical conditioning treatment to prevent scalingDoes not require chemical treatment but requires increased separate and purification plant
Larger capacity cooling water pump or separate piston cooling pump and coolers although less so than with oilLarger capacity Lube oil pump, sump quantity and coolers
Special piping required to get coolant to and from piston without leakNo special means required and leakage not a problem with less risk of hammering and bubble impingement.
Coolant drains tank required to collect water if engine has to be drained.Increased capacity sump tank required
Pistons often of more complicated designThermal stresses in piston generally less in oil cooled pistons
Cooling pumps may be stopped more quickly after engine stopped Large volumes of oil required to keep oxidation down and extended cooling period required after engine stopped to prevent coking of oil


An alternative method of cooling uses a nozzle plate and nozzles. Note that the oil goes up the annular space formed between the oil tube and the bore in the piston rod, and returns down the centre

For a modern slow speed engine steel forging or castings of nickel-chrome steel or molybdenum steel are common. The weight of the material is not normally a governing factor in this type of engine although resistance to thermal stress and distortion is. Efficient cooling is a required to ensure the piston retains sufficient strength to prevent distortion.The high thermal conductivity of aluminium alloys allied to its low weight makes this an ideal material. To keep thermal stresses to a reasonable level cooling pipes may be cast into the crown, although this may be omitted on smaller engines.Where cooling is omitted, the crown is made thicker both for strength and to aid in the heat removal from the outer surface.
  
Piston rings

Rings must have sufficient spring so that they will provide an initial seal with the liner. As pressure builds up gas acting on the back face of the ring increase the sealing effect.They must not crack under high temperature and pressure ranges. Rings are generally of spherical graphite cast iron because of the strength and limited self lubricating properties.  
The ring axial depth must be sufficient to provide a good seal against the liner but it must not be so great so that an oil wedge does not form.Radial clearance must be sufficient between groove and ring back to allow a gas cushion to build up. The butt clearance must be sufficient to allow for thermal expansion. If insufficient the rings may seize and if excessive can lead to excessive blowpast
Grooves are sometimes coated with chromium to restrict deposit build up. For reconditioning the bottom face of the groove is generally provided with a replaceable steel wear ring. As the rings maintain the gas seal there is a desire to position the top or firing ring as close to the piston crown as possible. However ,since the crown is highly stressed, thermally, this results in distortion of that zone. There is thus a desire to position the ring a long distance away from the crown. A compromise position is decided upon in each engine design.
Lubricating oil must spread over the liner surface by the rings, this helps to combat acidic products of combustion.

Faults leading to ring collapse

Ring faults

 The top piston ring is moved further down the piston. This allows the crown to enter deeper into the crown reducing temperature and pressure on the liner. The top piston ring is a 'Controlled Pressure relief' (CPR) ring. This design has several oblique shallow grooves in the piston ring face allowing some gas presure to pass through to the 2nd ring thereby reducing load on the top ring. To reduce blowpast an 'S' type joint is formed n the ring ends
 
Types of Piston Rings

Compression Rings:
The main functions of compression rings are to seal the combustion chamber from the crankcase and transfer heat from the piston to the cylinder. However, they also play an important part in controlling engine oil consumption.
Oil Scraper Rings:
 Practically all of the scraper rings used are rings with a step recessed into the bottom outer face. This assures extremely effective oil scraping .
Oil Control Rings:
The main function of the rings in the bottom piston groove is to scrape oil from the cylinder wall, i.e. oil control rings play a major part in regulating the oil economy of the internal combustion engine. Oil Control Rings regulate and limit oil consumption. They scrape off excess lubricating oil from the cylinder walls and return it to the crankcase. They are designed to provide a thin oil film to ensure piston and ring lubrication

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