The Best Tutorial On Piston For Marine Engines

Piston for Marine Engines

In this post on Piston for Marine Engines, we will discuss the pistons used in marine Engines, trunk type piston, Piston material, Overhauling Process of Piston, problems in pistons. All the procedures have been explained in easy to understand language for the marine engineers and engineering cadets.

The purpose of Piston

The Piston for Marine Engines is the main component of the Engine.

1. The piston transmits the force of expanding gas in the cylinder via connecting the rod to the crankshaft.
2. Piston helps to convert chemical energy into mechanical energy.
3. A piston is supported by piston rings to provide the best seal between piston and cylinder liner walls. Thus, the piston contributes to the efficiency and economy of the Engine.
4. Piston takes the side thrust by forming the guide and bearing to the small end of the connecting rod.

Piston for Marine Engines being the central reciprocating part of the Engine, an imbalance is generated. This imbalance manifests in the form of vibration. This vibration causes the Engine to be perceivable severe. Wear is eventually resulted between the cylinder and piston rings, resulting in a reducing the efficient life of the mechanism.

Piston Parts identification

The Piston for Marine Engines crown is the upper portion of the piston. Piston crown comes in contact with the burning gases is heat-resisting steel.

Ring grooves are machined on the circumference of the upper part of the piston crown.

The part below the ring groove is known as the piston skirt. This part is made of cast iron the portion between the ring grooves separation is known as the lands.

The piston pin is held in the bosses of the reinforced parts.

Piston Qualities

1. The piston should be strong enough to withstand the forces developed by the combustion process.
2. The design should curtail the piston weight to the minimum possible level to minimize inertial created due to the reciprocating mass.
3. It should have excellent heat conductivity to reduce the risk of detonation, allowing a higher compression ratio.
4. Piston Material should have low expansion and arrangement to qualify for various expansion rates of a Cylinder block and an aluminum piston.
5. Properly designed skirt to give uniform bearing under working conditions

Piston Design

Piston’s design is complex concerning design. A piston is built in two parts as crown and piston skirts. The material of the piston crown is corrosion resistant and high strength since the crown is always in contact with the combustion gases temperature

Types of pistons fitting arrangements

There are following two types of pistons

1. Crosshead Piston
2. Trunk type piston

Crosshead Piston

In the crosshead type piston, the piston is indirectly connected to the crankshaft. The piston connected to the crosshead bearing through connecting rod is connected to the crankshaft by the piston rod. Via crosshead guides, vertical stress is conveyed, reducing the side thrust on the piston.

Trunk Type Piston for Marine Engines

The term trunk piston is generally used for the pistons in four stroke medium speed engines.In the Trunk type piston, the piston skirt is long and attached to connecting rod by a small end bearing rotating on the piston pin.

Different between Trunk Type Crosshead and Piston Engine

Trunk type engine

1. Trunk-type Piston for Marine Engines has only connecting rod.

2. There is no diaphragm

3. In the trunk-type engine, the upper end of the connecting rod, known as the small end, is connected with the piston directly through the gudgeon pin and small end bush assembly.

4. In the trunk-type engine, no separate lube oil is used for the cylinder liner.

5. Piston skirt takes the transverse thrust

6. Cross head assembly is not required to connect the connecting rod and piston.

7. High power is produced at medium and high speed.

8. Less space is required.

9. Reduced engine manufacturing cost

10. Less height for similar power and speed.

11. Can use low-grade oil efficiently.

Crosshead Type Engine

1. In crosshead type engine, both connecting rod and piston rod are used,

2. Cylinder space and crankcase is separated by a diaphragm

3. In the crosshead type, the top part of the connecting rod is attached to the crosshead consisting of the block slipper. The assembly is connected to the lower part of the piston rod.

4. The lubrication system for the cylinder and crankcase is different.

5. Transverse force caused by oscillation of connecting rod is taken by crosshead guide assembly.

6. Cross head assembly connects the connecting rod and piston rod.

7. Crosshead type engine develops high torque at minimum speed.

8. Height and space are required more than the trunk piston-type engine.

9. Manufacturing cost is higher than the trunk type engine.

Advantages of Crosshead type engine

1. At lower rotational speed will develop high power than the trunk engine

2. Lube oil contamination by combustion product is less than the trunk type engine

3. The lubricating oil cost is lower than the Trunk-type engine.

Lubrication of Trunk type pistons

he piston is cooled by the lubricating oil supplied by the crankshaft. The shaking effect of lubricating oil cools the crown and flows out through the centre of the joint screw. After cooling, the piston crown oil scatters on the cylinder wall and flows down to the sump after cooling down the cylinder liner wall.

Procedure for Inspection of Piston for Marine Engines

Overhauling Process of Piston

Pre-overhauling preparation

1. Shut and lock the air starting system

2. Open the indicator cock

3. Before shutting down, the lube oil pumps cool down the engine sufficiently.

4. Disconnect fuel and cooling water.

5. All the measuring and inspection tools should be ready inadequately calibrated, and working conditions

6. Check the availability, workability, and validity of lifting and strops and shackles.

7. Personnel involved in the maintenance should be well versed with the working procedures.

8. Place the safety symbols as per the manufacturer’s recommendations in the vicinity of the maintenance area.

9. Confirm the availability of spares.

10. Check the workability of hydraulic tools.

11. Ensure the mobilization permission.

12. Undertake the risk assessment.

13. Carry out the ventilation of the crankcase with permission to undertake the job.

14. Rope off the working vicinity.

Overhaul Procedure of Piston for Marine Engines

In the flame mounted liner carry out the following

1. Clean the cylinder liner top of coking as the piston may stick in the deposits

• Turn the piston to BDC
• Insert and mount the tool above the piston to dismount the flame ring
• Place an old used piston ring on top of the flame ring removal tool
• Mount the cylinder liner arresting tool on one of the cylinder head studs.
• Turn the engine slowly to push out the flame ring.

2. Dismount the crankcase cover for the cylinder section in question. Dismount the platform, if any, in order to facilitate access

3. Mount the supporting device on the crankcase cover frame.

4. Place the mandrel in the guide sleeve and push forward until the shoulder of the mandrel is resting on the supporting device.

5. Turn the crankshaft until the center of the connecting rod screw is on a level with the mandrel top

6. Then the crank throw is in a position where both upper and lower connecting rod screw can be loosened, see fig

7. Clean the thread hole in the piston top and mount the lifting device

8. Mount the pipe for retaining of the cylinder liner

9. Grab the tackle in the lifting tool for the piston and tighten.

If it is necessary to turn the crank throw somewhat to achieve a better access to the connecting rod screws, the wire must be loosened before turning the crank[1]shaft and then tightened-up again in the new position

10. Pull out the mandrel of the guide sleeve and dismount the supporting tool

11. Turn the supporting tool so that the two supporting grooves for the wrench torque extension turns left to the middle of the supporting too

12. Mount the supporting tool

13. Loosen and dismantle one of the upper connecting rod screws.

14. Mount the guide pin in the thread hole and tighten with a screwdriver

15. Mount the holding-up tool on the cover frame of the crank housing on the opposite side of the engine

16. Press the forked rod tool against the connecting rod and lock it in this position with the locking screw on the tool

17. Dismantle the remaining screws and the bearing cap.

Note: The purpose of the guide pin is to prevent any damage to the crank journal, joint faces, or bearing surface to occur during the dismounting of the bearing cap.

18. Loosen the locking screw and release the forked rod tool, see fig. 8 in order for the connecting rod to swing free of the crank-pin when the piston and the connecting rod are carefully pulled out.

19. The upper bearing shell is carefully dismounted.

19. Pull out the piston and the connecting rod throughout the cylinder liner and out of the engine. The connecting rod is carefully guided in order not to damage the cylinder liner skirt

Separation of Piston from connecting rod

1. Place the piston and connecting rod with care on wooden supports to avoid damage to the piston and rings

2. Mount the bearing cap with hand-tightened screws to protect the serrations of connecting rod joint surfaces during assembly handling.

Remove the shackle and lifting tool from the piston

4. Lay the piston and connecting rod assembly in a vertical position resting on the top face of the piston

5. Use the wooden wedges to arrest the connecting rod movements from swinging out and thus impact the piston skirt during the lift in a vertical position.

6. Place the wire around the big end of the con rod, attach a tackle and tighten the wire rope

7. Remove the securing ring, push out the piston gudgeon pin and lift away the con rod.

Removal of the Piston Ring

1. Remove the Piston and scraper ring with the help of ring opener tool

2.Clean and inspect the rings to ascertain for the acceptance of re-use.

3. Clean the piston both from inside and outside.

4. Inspect the piston ring and scraper ring groove for wear and damage.

Important checks on Piston for Marine Engines

1. Top part of the piston for any burning symptoms.

2. Piston crown side wall and ring grooves for any wear marks

3. The top part of the piston for cracking caused due by the thermal ad mechanical stress

4. Entire piston for high-temperature corrosion. 5. for hot corrosion at the upper surface and acid (cold)corrosion at the lower part.

Piston Ring Grooves and Piston Rings

Inspection of the ring grooves and piston rings of Piston for Marine Engines

1. Clear and free movement of the piston rings in the grooves

2. Ring clearance/ but clearance

3. Any wear, stepping, and scuffing at grooves.

Piston Skirt/ Sidewalls

Inspect the piston skirt for the following

1. Any rubbing marks

2. Wear of wear rings

Cooling Water section

1. Scaling due to poor water quality

2. High-temperature congestion.

Condition for change of Piston for Marine Engines

Change the piston in case of the following abnormalities.

1. The wear limit on the testing mandrel has exceeded

2. The clearance between the new piston/ scraper ring and the ring groove has exceeded more than the manufacturer’s specified limit.

Conclusion

Hope the post on Piston for Marine Engines has been useful to the engineers and engineering students.

Good Luck!!