21–22 Best Review On |Diesel Engine Piston Rings
Diesel Engine Piston Rings
Introduction: — In this post on Diesel Engine Piston Rings, we will discuss about the complete details of Piston rings, types of piston rings, material of piston rings and checks on piston rings.
What is Piston Ring?
A piston ring is a split ring that can expand and provides a seal between the cylinder wall and piston.
Piston ring performs the following functions.
- Sealing of the combustion chamber by outward expansion due to the gas pressure acting behind them.
- Prevents the passing of gases and combustion products.
- Prevention of the gases and combustion products passing the piston.
- It also conducts heat away from piston to liner.
- It also controls the cylinder oil.
Piston Ring material
Cast iron is a common material for the manufacturing of piston rings. Cast iron doesn’t lose its original shape under the effect of heat, load, and various other forces.
Facts about Piston rings
Engine design and material of cylinder liner will decide the size and composition of the piston ring.
The effectiveness of a piston ring for leak tightness and sealing decides the overhaul period of a cylinder.
The output generation of an engine depends upon the function of a piston ring.
Piston ring surface should have a complete contact with the liner surface and its complete depth and periphery.
Faulty function of the piston ring is the origination of troubles in Diesel engines.
Types of Piston Rings
- Top Piston Ring/Compression ring,
- Wiper ring/2nd ring
- Oil ring.
Top Piston Ring/Compression ring
The compression ring/Fire ring fits in the top grove of the piston nearest to the piston crown. Function of the top ring is to provide the sealing to combustion chamber and prevents leakage of gases during the combustion process.
Pressure of the combustion gases during the ignition of air fuel mixture in power stroke acts on the piston head and forces the piston downward.
Gases at pressure enter the piston ring groove through the gap between the cylinder liner wall and the piston. This combustion gases pressure forces the piston towards the cylinder wall and forms the sealing. The gas pressure on the piston ring is equal to the combustion gas pressure.
Compression ring has the direct in contact with the combustion gases, and has the direct exposure to the maximum operating temperature and the acidic corrosion of the gases due to sulphur.
The top ring/ Compression ring transfer the about 70 to 75 % of the heat from the piston to cylinder liner.
Sealing of combustion chamber by a compression ring is through the following forces.
- Inherent Pressure;- Inherent pressure is the tangential force due the springing action of the ring that expands the ring outward, due the material properties and the design of the ring. The ring free gap will determine the inherent pressure. Greater the free ring gap more the inherent pressure. Free gap of a piston ring is the gap between the two ends of a piston ring in uncompressed position.
- Applied pressure.;- Applied pressure is due the application of pressure by combustion gases on the piston ring.
Piston ring Designs
- Taper faced compression ring;- This design of ring has the taper angle of approximately 10 on the running surface The purpose of providing the taper is to have the mild wiping action to avoid the excess lubricating oil passing to the combustion chamber.
- Barrel faced compression ring;- curved running surface on the ring provides the positive lubrication between the piston ring and the cylinder liner wall. This ring also has a wedge effect to ensure the proper oil distribution throughout the piston stroke. The curved surface reduces the possibility of oil film breakage due to the pressure on the ring by tilting action of the piston.
Wiper Ring/Scraper ring/2nd ring
This piston ring with the running tapered face has the placement in the 2nd ring groove between the compression ring and the Oil ring.
This ring supports the top ring in sealing the combustion gases as well as controls the oil. The tangential force of the piston ring holds the ring against the cylinder wall. The ring has the undercut at the bottom side which scrapes the oil downward to engine sump. This ring also provides the constant oil film for the lubrication on the running surfaces of compression ring.
The wrong installation of wiper ring will cause excessive lube oil consumption since it will wipe up the oil towards the combustion chamber.
Oil scraper Ring
Placement ofthis ring is in the groove below the 2nd groove near to the crankcase. This ring wipes the excess lubricating oil from the cylinder wall to sump in downward movement off the piston. The oil returns back to sump via ring opening to the engine sump.
An oil ring consists of two thin operating surfaces. Slots cut in to the centre will lead the excess oil back in to the oil sump. All the features are available in single piece oil ring. Every oil ring has a expander for the additional radial pressure on the ring. This helps in keeping the ring in contact with the cylinder wall. Oil rings are with highest inherent pressure of the three rings in piston.
Oil ring has the three pressures as below.
- Piston ring pressure
- Expander pressure
- High pressure by the running surface of the thin rails.
During the manufacturing of a ring, a gap is cut on the machined out ring from a casting drum. The free width of the gap is as per the pre-calculated dimensions to meet the working clearance.
It is not possible to design a complete round ring. It is general practice to coat for the running in process. The ring achieves a round shape only on completion of the running-in period. After running for the period the ring will efficiently seal the surface.
Normally rings have two coatings. The outer coating will be of soft aluminium bronze to run in and proper bedding of ring in a short period. The secondary coat is hard and scuffs to provide the strength to ring for long lasting life. The hard coat increases the time between overhauls.
Type of joint in piston rings
As per the above fig types of joints for piston rings are as under.
- A is the plain joint
- B & C are the bevel joints
- D is the stepped or overlap joint.
Due to the working of a ring under severe condition of pressure, heat and sudden reversal the suitable and most popular rings are of A and B © types.
The ring of D design is costly but has the best sealing effects. Apart from the high cost, the overlap joint type rings have a serious drawback of breaking due to thin ends. Many cylinder liners had serious damage due to the broken piece entering the working surface and causing scuffing.
Type of Diesel Engine Piston Rings Clearances
An important point to be given a consideration that the piston expands outward and the piston ring expands inward. Both the deformations will tend to decrease the clearance between bottom of the grove and the inner surface of the piston rings.
Other point is that in few cases piston crown expands in a way that the width of the upper ring groove will be reducing considerably.
Butt clearance of Diesel Engine Piston Rings
- it is the cut that is made in the ring such that due to thermal expansion the ring doesn’t collapse. Maintain the butt clearance in the range of 0.4 to0.5% of the cylinder liner internal diameter.
It has high importance for the following reasons.
- If all the butts get aligned the combustion gases blow by or blow pass takes place which might lead to scavenge fire.
- If the butt clearance is less than the specified limit, end of piston ring will touch due to the thermal expansion. Ring will expand against the cylinder wall and will increase the friction.
The increase in friction will result in breakage of piston ring.
it is the vertical distance between piston ring and piston ring groove.
Importance of axial clearance
1. It allows pressure to build up behind the ring
2. it allows oil to flow into the groove
3. It allows pressure to build up behind the ring.
Vertical Clearance of Diesel Engine Piston Rings
A new set of piston and piston ring will have the normal vertical clearance. After a prolonged run the vertical clearance will increase due to the wear down in ring groove and piston rings. Inadequate clearance will cause the following.
- It is restrict the free movement during operation.
- Less vertical clearance will cause rings to stick in the groove and break as they come up to operating temperature.
- Excessive clearance of the ring will cause the hammering and ring groove wear.
Piston Ring Clearances measurement Procedure.
To measure butt clearance insert the rings in the bottom of the liner and ensure that they are in line with the linear axis.
Use the feeler gauge to measure the clearance between join of piston rings.
Carry out the measurement in 12o clock, 3o clock, 6o clock and 9o clock positions.
Axial clearance is the clearance from the top of the ring to the top of ring groove.
If the clearance is less than the minimum limit combustion gases cannot flow behind the rings. To measure axial clearance feed the ring in the respective groove such that it rests on lower surface of the groove.
Use the feeler gauge to measure the readings between upper face of the ring groove and the piston ring.
Carry out the measurement in 12o clock, 3o Clock, 6o clock and 9o clock positions.
The clearance between inner diameter of the ring groove and inner diameter of the piston ring is known as the back clearance.
Thickness of the ring should be less than the groove depth so that the outer ring face is inside the ring groove.
Measure the depth of ring groove for the measurement of back clearance.
Use Vernier calliper to measure the thickness of ring.
Take both these measurements in at least six different positions around the groove circumference. Back clearance will be difference of these two readings. Replace the rings in case the back clearance is more than the limits.
Measure the free gap of all the piston rings before dismantling the rings.
Diesel Engine Piston Rings clearance calculation formulae
Important checks before the fitment of Diesel Engine Piston Rings
For the achievement of the best results of Diesel Engine Piston Rings, carry out the following points. These points are in the light of past experience.
- Try out the ring in the groove for which it is meant and ensure that it has the required clearance in vertical direction, and is free to contact at back and bottom of the groove.
- Measure the butt clearance at the smallest diameter of the cylinder liner.
- Remove the ridge from the cylinder liner top reversal point of piston ring. A new ring may foul with this ridge and is likely to damage the cylinder liner with heavy knocking sound.
- Remove any ridge formation in the groove which may hinder the free movement of the piston ring.
- Always follow the manufacturer’s recommendations on fitment of the rings. Ensure the correct upward and downward position of the ring, failing to do so will have serious consequences of high lubricating oil consumption and loss of compression pressure.
- Before the fitment of ring with coil spring loaded expander carry out the following.
- Dismantle the coil spring.
- Assemble the coil spring in the groove.
- Assemble the ring in such a way that the ring joint is approximately 180 0 offset to the spring joint.
- To prevent the leakage from the coinciding ring joints, offset the ends of the adjacent rings by 180 0 opposite to each other before springing in.
Conclusion:- I hope this post on Diesel Engine Piston Ringshas been useful to the readers.