Piston

0

What is piston ?

A Piston is an important part of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar devices.

Piston

Or,

It is components of the combustion chamber of a marine diesel engine that converts the exhaust gas forces into mechanical power by reciprocating motion.

Or,

It is a part of diesel engine which forms the lower part of the combustion chamber.

From mariner point of view,

A piston is a part of marine Diesel Engines that forms lower part of Combustion chamber and also helps to transfer the energy transfer produced within the combustion chamber of the engine to the external part of the engine via crankshaft where it can be channeled for valuable uses like rotating the propeller in the sea water to move the ship or to run an alternator to produce electricity in case of smaller plant.

Or,

Generally, we can say that

It is a disc or cylindrical part tightly fitting and moving inside a cylinder, either to compress or move fluids collected in a cylinder, such as air or water, or to convert the energy supplied by a fluid entering or expanding within a cylinder, such as compressed air, explosive gases, or steam, into a rectilinear motion normally converted into rotary motion by means of a connecting rod.

Difference between Two stroke and Four Strike Engine

What is the function of piston ?

As we know that it is moving and integral component of Internal Combustion engine enclosed in the cylinder liner and made gas tight seal by piston rings.

1.It is used to seal the combustion chamber ( cylinder liner ) of IC Engine so that combustion takes place inside enclosed spaces. ( As we know it forms the lower part of the combustion chamber )

2.Transfer forces / Energy

A.In Internal Combustion Engine,it’s purpose is to transmit the force from the expansion of the gas in the cylinder to the crankshaft through the piston rod and/or the connecting rod. or (transfer Chemical energy produced by combustion of gases to mechanical energy to crankshaft. )

B.In certain engines (2-stroke engine ) the piston often serves as a valve by covering and uncovering the cylinder’s ports.

C.In a reciprocating pump, the function is reversed. Forces and Energy from crankshaft transferred to piston for compressing and expelling fluid in the cylinder.

What Does a Piston Do ?

It is used to transfer chemical energy to the crankshaft via connecting rod , forms lower part of combustion chamber ,seal cylinder liner and transfer heat to liner via piston rings.

As we know that in any Internal Combustion of Diesel Engines,four process occurs I.e Suction,Compression, Expansion and Discharge.

These four roles in Diesel Engine performed with the help of Piston.

Thus,We can say that it helps in Combustion of fuel.

How Does a Piston Work ?

Here,We will Explain How does a piston Works in Both 2- stroke and 4-stroke Engine in details.

In 4-stroke engine

Four stroke

1.Suction :-When Piston moves Down , inlet valve open and allow charge air to enter & fuel injection takes place.

2.Compression :- Due to rotation of crankshaft piston moves up ( Both inlet and outlet closed ) to compress the fuel and air mixtures. When temperatures reaches combustion if fuel takes place.

3.Power/Expansion :- When Combustion takes place,gas will expand and it moves Down.

4.Exhaust :- It is final stage in which Due to rotation of crankshaft piston moves up (exhaust valve open ) .It forces out the gases to exhaust manifold.

Thus ,all the process of combustion takes place continuously ( 100 or 1000 times per minute )

2-stroke engine

In two stroke engine ,all process of combustion takes place in two stroke.

Two stroke

1.Suction / Compression :- Piston is designed such that when it moves Suction takes place and simultaneously compress fuel and air mixture.

2.Power / Exhaust :- In this stroke piston go downs and simultaneously exhaust gas push out in exhaust manifold.

All parts of Diesel Engine

Bed plate

A -Frame

Tie -bolt

Cylinder cover

Cylinder liner

Gudgeon pin

Connecting rod

Cross headGuide and guide shoe

What are the Different types of piston ?

Generally,we used two types of piston arrangement for Internal Combustion Engine.

1.Crosshead Piston :- It consists of Crown, Skirt and piston rod connected to crosshead used to transfer side thrust to engine structure.

Piston

In larges 2-stroke engines , due to movement of connecting rod it creates two forces

a.Axial

b.Transeverse

Thus,Large slow speed Diesel engines requires additional support for the side Force / Thrust on the piston.

These engines use typically crosshead pistons.

2.Trunk type piston :- It Consists of an elongated skirt piston to absorb the side thrusts and connected to Con. rod by a small end rotating bearing.

Piston

Relative to their diameter, trunk pistons are long. They operate as a piston as well as a cylindrical crosshead. There is also a side force that interacts against the cylinder wall along the side of the piston as the connecting rod is bent for most of its rotation. To support this, a longer piston helps.

In addition to the rings between the gudgeon pin and crown, a characteristic of most trunk pistons, especially for diesel engines, is that they have a groove for an oil ring below the gudgeon pin.

On the basis of shape of crown,it us divided in

1.Flat Piston Type:-Flat surface Crown is very easy and simple to manufacture.This is mostly used in Petrol engine.It is rarely used in spark ignition diesel engines.

Flat piston type

2.Concave :- This type of engines have concave crown structure.It is mainly used in diesel engines.

3.Convex :-The Convex top engine piston has a raised ball shape, which has a high power. It can improve gas exchange process and act as the guide function. In the two-stroke gasoline engines of motorcycles, the Convex top engine piston is commonly used.

Piston Materials / What the piston made of ?

Properties of Materials

Materials used must will require similar properties to those for cylinder liners and covers.

1.Rigidity to withstand high pressure :- The crown must withstand the high gas load and transmit the force from this to
the piston rod.

2.Higher Mechanical Strength and higher fatigue strength :- It must have a long fatigue life to survive the fluctuating mechanical and
thermal stresses; its surface is exposed to very hot combustion gases followed rapidly
by cool scavenge air during each engine cycle.

3.Low coefficient of Expansion :-The metal must resist high temperature
creep, corrosion and erosion; and readily conduct heat for cooling but have limited
thermal expansion so that working clearances are maintained with the liner and piston
rings.

4.High surface properties I.e Hardness and anti – corrosive.

5.Correctly formed skirt to give uniform bearing under working conditions.

6.Silence in operation.

Material used :-

Materials and design will depend on the engine rating, size, speed and fuel.

The most common material used are :-

1.Cast Iron

2.Alluminium alloys

1.Cast iron used for for moderately rated engines whose piston speed is below 6 m/s.

Cast iron is also used for the piston. Cast iron is a universal material in the early years because it possesses excellent wearing quality, coefficient of expansion and general suitability in manufacture.

2.Aluminum alloy are used for those engines whose piston speed is more tha 6 m/s.

But due to the reduction of weight in reciprocating parts, the use of aluminium for piston was essential. To obtain equal strength a greater thickness of metal is necessary, the same of the advantage of the light metal is lost. Aluminium is inferior to cast iron in strength and wearing qualities, and its greater coefficient of expansion necessitates greater clearance in the cylinder to avoid the risk of seizure.

The heat conductivity of aluminium is about three times that of cast iron, and this combined with the greater thickness necessary for strength, enables and an aluminium alloy piston to run at much lower temperatures than a cast iron one (200°C to 250°C as compared with 400° to 450°C).

As a result, carbonised oil does not form on the underside of the piston, and the crankcase, therefore, keeps cleaner. This cool running property of aluminium is now recognized as being quite as valuable as its lightness indeed, pistons are sometimes made thicker than necessary for strength in order to give improved cooling.

Piston Parts

What are the parts of a piston ?

Piston feature include the following :-

  1. Head or crown
  2. Lands
  3. Ring groves
  4. Oil hole
  5. Separating web
  6. Piston pin or gudgeon pin hole
  7. Piston skirt

1.Head or Crown

It is also known as the piston crown or dome. This part directly comes into contact with the combustion gases. As a result, it gets heated to extremely high temperatures. In order to avoid melting, this made using special alloys, among them steel alloy.

Usually, the piston head is built with channels and cavities. They help to create a swirl that improves combustion. Different types of piston heads are used in various engines. The explanations for the variations are different. The chosen configuration of the crown or head depends on a variety of factors, such as expected performance and type of engine.

The head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.

Piston Head Functions

The crown, as is also known, forms a surface that absorbs the pressures, temperatures and other stresses of the expanding gases. Purpose of this is :-

  • Creating swirl to make combustion uniform and regulate knocking.
  • Act as a heat barrier between the combustion chamber and the lower piston parts.
  • Contain the pressures resulting from knock in the cylinder.
  • Convert the pressure developed in ignition stroke to a downward force – carry that force to the       piston rod.
  • Sustain pressure waves generated by occasional knocking.

2.Lands :-Ring lands are the two parallel surface of groove of rings that act as the piston ring sealing surface.

On the Circumference of The top of the piston there is a grooves cut to fit the piston rings. The bands between the grooves are known as ‘lands’. The function of the lands is to sustain the rings against the gas pressure.

The lands did the work of guideing the rings so that they rotate freely in a circular direction.The supporting webs transmit the explosion force directly from the crown to the piston-pin bosses.  Lands relieves the large loads from the ring grooves.

3.Ring Grooves :-A ring groove is a recessed area that is used to hold a piston ring, located around the perimeter of the piston.

4.Piston Rings :- Piston rings are pieces of split rings that are placed on the recessed area of the piston. In a typical car engine, there are usually 3 piston rings. The number varies and there may also be one ring for a piston. The piston ring lands are the areas or surfaces between these rings. The ring mounting grooves are built to maintain the position of the piston ring, and you can hear something like a tapered house.

Piston rings

The split design of the piston ring has several advantages. It allows the spring action, which helps the rings to maintain the correct piston ring gap. The split also makes it easy to instal the piston ring. In order to ensure constant spring under heat, load, pressure and other conditions, manufacturers prefer cast iron or steel parts for piston ring material.

Piston Ring Function

The main function of piston rings is to seal off the combustion chamber and control/regulate the lubricating oil used. The rings also dis the work of conducting heat away to the cylinder liner. As we earlier mentioned that , most of the vehicle engine uses three rings; two upper compression rings and one lower oil ring. For better understanding the different rings are explained below.

  • Compression ring– This ring is fitted at the top side and nearest the combustion chamber. It is also called the sealing or gas or pressure ring. This ring prevents combustion gases from leaking to crankcase.This rings also help in transferring heat from the piston to the walls of the cylinder liner.
  • Scraper/wiper ring– Scraper ring is located between the compression and oil rings. It has a tapered surface and serves the function of both rings: sealing the combustion chamber and wiping oil off the piston cylinder walls.
  • Oil control ring– Oil Control ring is the lower ring and It consists of two thin surfaces, with holes all around. The slots on this ring allow oil to flow back into the sump. As its name suggests, the function of oil control ring is to remove excess oil from the cylinder walls. It is done as the pistons works back and forth.

4.Oil holes:-

6.Piston pin or Gudgeon pin :-

A Gudgeon pin bore is a hole in the side of the piston perpendicular to piston travel that receives the gudgeon pin. 

Gudgeon pin

A Gudgeon pin is a hollow shaft which is used to connect the small end of the connecting rod.

It is known as wrist pin or Gudgeon pin. The gudgeon pin is the hollow or solid shaft  in the partion of the skirt. The piston rod pivots on this pin, held in the piston ring bushing. For tensile strength, gudgeon pins are usually built from alloy steel and machined to fit the piston bearing. Through Holes inside the connecting rod oil delivered to the wrist pin, helping to reduce friction.

Gurgaon pin assemblies and mounting styles vary. They can be categorized into 3 designs:

1. free to revolve in both piston and connecting rod,

2. clamped to connecting rod, and

3. rigidly mounted to piston bosses.

Piston Pin /Gudgeon pin Functions

A gudgeon pin forms the connection or pivotal point of the connecting rod and pistons. It provide bearing support and also help pistons to work properly. In other words, the gudgeon pin facilitates the back and forth movement of the pistons.

As we have seen, gudgeon pins/wrist pin use three methods to mount on the piston assembly. These give rise to the following types of pins.

  • Stationary/fixed pin– This type of pin attaches to the bosses of the piston via a screw and The pistons rod then pivots on the pin.
  • Semi floating– This type of pin attaches to the connecting rod in the middle, and this pin ends move freely within the piston bearing and at the bosses. 
  • Full floating– In this of pin assembly, the pin is not attached to the pin or piston connecting rod. Instead of this, it is secured by plugs, clips, or snap ring attached to the piston bosses. The pin can then oscillate at the bosses as well as the rod.

7.Piston Skirt :-The skirt is the piston portion closest to the crankshaft that helps the piston to align as it moves in the bore of the cylinder.

Piston skirt

Some skirts have profiles cut into them to decrease the mass of the piston and also to provide clearance for the rotating crankshaft counterweights.

Skirt is fitted in both engine i.e 2 – stroke and 4 – stroke engines. But It has different function for different engines.

In large crosshead 2 – stroke engines with uni-flow scavenging, skirts are short in length and it is fitted to act as a guide and to stabilise the position of the piston inside the cylinder liner.

Skirt is generally made up of cast iron.

Note :-The skirt Diameter 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.on skirt,Bronze rings are also fitted.These bronze rings on skirt help during the running – in of the IC Engine ,when the engine New and it can be replaced according to need.

In 2 – stroke engines having loop or cross scavenging types /arrangements, the skirts are slightly larger.It is because , it helps in blanking off the scavenge and the exhaust ports in the cylinder liner.

In 4 – stroke or trunk type piston engines, the skirt has arrangement for gudgeon pin, which transmits power from the piston to the gudgeon pin or top end bearing. In trunk type ,there are no cross-head guides and , these skirts help in transferring the side thrust produced from the connecting rod to the walls of the cylinder liner.

There are two main types of piston skirts:

  • Full skirt

Full Skirt is also known as solid skirt. The full skirt features a tubular shape and commonly used in the IC engines of large automobiles. e.g 2 stroke marine Diesel Engine.

  • Slipper skirt

Slipper skirt is used on pistons motorcycles and some cars. It has part of the skirt cut away to leave only the surfaces on the back and front of the cylinder wall. Advantage of this type of skirt is, it helps to reduce weight and minimize the contact/touch area between the cylinder wall and the piston.

8.Piston Rod :-

Piston Clearance

Pistons are normally small in diameter than the bore of the cylinder. The area between the cylinder and the cylinder wall is called the piston clearance.

Or

Piston clearance is the clearance or gap between piston and metal cylinder, to avoid damage due to excessive expansion of piston on getting heated during combustion. It is also known as piston to bore clearance.

Clearance is Essential for The Following Reasons

  1. It provides a space for a film of lubricant between the piston and cylinder wall to reduce friction.
  2. It prevents piston seizure: Due to very high operating temperature, piston and cylinder block expand. The cylinder is getting cooled faster than the piston, hence enough clearance should be given for the piston to expand, failing which the piston seizure will result.
  3. If there is no clearance between the piston and cylinder, it will be difficult for the piston to reciprocate in the cylinder
  4. Generally, piston is made up of cast aluminium alloy for good thermal conductivity. On heating, aluminium expands more than the metal cylinder. So proper piston clearance is necessary to maintain free piston movement in the cylinder.

The piston clearance depends upon the size of the cylinder bore and the metal used in the piston. But it is generally 0.025 mm to 0-100 mm. In operation, this clearance is filled with oil so that the piston and rings move on films of oil.

If the clearance is too small, there will be loss of power from excessive friction, severe wear and possible seizure of the piston in the cylinder.

  • piston will seize inside the cylinder on more expansion
  • piston will get too tight in the cylinder, resulting in excessive friction loss
  • piston can damage the cylinder wall

If the clearance is too much

piston will move back and forth very freely, resulting in engine knock and may even damage the piston skirt

large clearance may also reduce the sealing property of compression rings to seal the compression chamber

Piston slap will occur if the piston clearance is too much. Piston slap means sudden tilting of the cylinder as the piston is down on the power stroke.

Piston clearance

The piston shifts from one side of the cylinder to the other with sufficient force to produce a distinct noise. As the piston warms up the clearance is reduced and the noise usually disappears. In order that fixed clearances may be used without risk of seizure special alloys have been introduced and many designs of the piston are in use.

These special designs involved cam grinding to non-circular forms, semi-flexible skirts incorporating obliques slits, controlled distribution and the like expedients.

2 Stroke Marine Diesel Engine Piston

The piston of 2 – stroke marine diesel consists of two pieces : Crown and Skirt.

The crown is exposed to high temperatures in the combustion chamber and the surface is likely to be eroded & burned away.

Due to this reason, the materials of crown is must be capable of preserve it’s strength and resist corrosion at high temperature.

That’s why steel alloyed with chromium and molybdenum is used in formation of crown.Some pistons have a special alloy welded to the hottest part of the crown to try and reduce the corrosion caused by burning fuel.

The crown also has four to five piston ring grooves that can be chrome plated.

The Skirt of Cast Iron acts as a guide in the cylinder liner.

The underside of the piston is bolted to a cast steel piston rod. The other end of the piston rod is connected with the cross-head pin.

The Cooling of Pistons is necessary for removing excess heat from combustion and to limit thermal stressing.It also limit thermal expansion to maintain clearance. Cooling us carried out by circulating coolant inside internal passage of it.It done either by water or crankcase oil.

Water has advantage over oil (thermal capacity ) but there is risk of leakage of water in the crankcase.

Now,In modern engines oil cooled pistons are used.To carry the oil to and from the piston, the piston rod is used. The rod is hollow, and the tube is running up its centre.

The MAN B&W piston has an 8mm thick heat resisting coating of a hard nickel-chrome alloy called Inconel which is welded to the hottest part of the crown to resist the “burning” of the piston crown.

4-Stroke Marine Diesel Engine Piston

The pistons for medium-speed piston engines that burn residual fuel are composite pistons. ( i.e. crown and skirt are made of different materials.)

The crown is a heat resisting steel forging which may be aThe Crown must be heat resisting that’s why it is steel forging and alloy of Chromium, molybdenum & nickel.It is used to maintain strength at high temp. and resist corrosion.

It is designed to form a combustion chamber with cutouts to allow the valves to be opened. The topland (the area between the top ring and the top of the piston) may be tapered to allow the expansion to be larger where the piston is hottest.

The skirt can be either a nodular cast iron or forged or cast aluminium alloy.

Aluminium has the advantage of

  • Being light
  • With low inertia,
  • reducing bearing loading.

But, Aluminium has a lower coefficient of expansion than steel that’s why greater clearance must be allowed during manufacturing.

This means that the clearance of the piston skirt in the liner is greater than that of the cast iron when running at low loads. The skirt transmits the side thrust to the liner due to the varying angularity of the con rod. Too big a clearance will cause the piston to tilt.

The piston pin for the small end bearing of the con rod is located in the skirt of the piston. The piston pin floats in the skirt of the piston and is placed in placed by circlip . Depending on the material used for the skirt ( cast aluminium), a bushing can be used for the pin.

The piston rings can be mounted in the crown or in both the crown and the skirt. Normally, the rings are chrome plated or plasma coated to tolerate wear. Since the liner is lubricated, the oil scraper ring (oil control) is fitted to the skirt of the piston.

The piston is cooled with oil. This is done by a variety of means; the easiest is for a jet of oil to be guided upwards from a hole in the top of the con rod to the underside of the crown. A more effective approach is the use of the oil catcher as seen in the image above. This directs the oil into the cooling chambers at the bottom of the crown where the cocktail shaker effect of the reciprocating piston ensures a positive cooling effect. It is rare for the temperature of oil return to be controlled.(In comparison to the 2-stroke slow-speed crosshead engine, where both temperature and quantity are monitored).

Some engines are equipped with one-piece pistons made of either cast iron or aluminium alloy silicon. They can not be used with residual fuel because higher temperatures cause the piston crown to burn. Aluminum also suffers from carbon build-up above 300o C. Ring grooves in aluminium pistons typically take the shape of a chrome plated cast iron insert.

Piston Engines Types

InLine Engine is a very simple and traditional type of engine design. In this kind of engine construction, as seen in the diagram, the cylinders are mounted in a straight line. Manufacturers often refer to this engine as a ‘Straight Engine’ as all the cylinders are in a straight line.

In-line engines can have up to 2, 3, 4, 5, 6 or 8 cylinders. Although the manufacturers refer to the four-cylinder in-line engine as the Inline-4 engine. They usually denote either I4 or L4 (longitudinal 4) in the car nomenclature.

Rotary engines

Rotary engine, internal combustion engine in which the combustion chambers and cylinders rotate with the driven shaft around the fixed control shaft to which the pistons are attached; the combustion gas pressure is used to rotate the shaft.

Some of these engines have toroidal (doughnut-shaped) cylinder-sliding pistons; others have single and multiple-lobed rotors.

Vee engines

A V engine, often referred to as a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks that are connected to a common crankshaft, usually with the same number of cylinders in each bank. These cylinder banks are arranged at an angle to each other, so that, when viewed from the front of the engine, the banks form a ‘V’ shape.

Typically, V engines are shorter in length than equivalent inline engines, but the trade-off is larger in width. V6, V8 and V12 engines are the most common configurations for six, eight or twelve-cylinder respectively.

Radial engines

The radial engine is a reciprocating type of internal combustion engine arrangement in which the cylinders radiate outwards from the central crankcase like the spokes of the wheel. It looks like a stylized star when viewed from the front.

The radial piston engine consists of one or more rows of odd-numbered cylinders mounted around the central crankshaft.

This type if engines mainly used in aeroplane.

Opposite piston Engine

The Opposite-piston engine is a piston engine in which each cylinder has a piston at both ends and no cylinder head. Oil and diesel engines opposed to piston were often used in large-scale applications, such as ships, military tanks and factories.

Advantage

  • No cylinder head
  • Creating uniflow scavenging
  • Reduced height of the engine.

What is a piston slap?

The piston slap is a sound produced when the piston skirt touches the cylinder wall as the piston pivots slightly in its upward or downward direction.

This will happen if the piston’s skirt area is worn to increase the clearance between the piston and the wall of the cylinder, or if the piston has been machined for a loose fit in the bore. This is often done , especially when forged aluminium pistons are used when the engine is built for high performance , e.g. on a dragster. The reason for this is that the piston is expected to expand when it heats up, reducing clearances in the bore.

Piston slap is heard more often and it is more noticeable on a cold engine.

What is top-high land? And why? In which type of mc engine 


Top land is the annular area around the piston and above the uppermost ring.
In order to protect the piston rings from the thermal load from combustion the height of the piston top land was increased (Figure 1). The resulting increased buffer volume between the piston crown and the cylinder wall improves conditions for the rings and allows longer times-between overhauls. The high top land was first introduced in the mid1990s, the positive service experience leading to its use for all new engine types.
it’s a development of mc- c engine.