Test # 1 Flashcards
(26 cards)
Piston stroke
the distance the piston travels from BDC to TDC
Swept volume
The volume of the cylinder enclosed within the stroke
Clearance volume
the volume between the cylinder head and the top of the piston at TDC
Total Cylinder volume
the volume between the cylinder head and the top of the piston at BDC
Compression ratio
(swept + clearance volume)/(clearance volume).
Generally 12:1 - 16:1 for a diesel engine. Depends on the air temperature required at the end of the compression stroke necessary to ensure spontaneous ignition of the injected fuel
Volumetric efficiency
(air drawn in during induction stroke)/(swept volume).
A measure of the efficiency of a 4 stroke engine and air compressors. Generally about 90% for a 4 stroke engine, measure of the thoroughness with which the induction stroke recharges the cylinders with pure air.
Scavenge efficiency
(pure air trapped in cylinder)/(total volume of air and exhaust gases in the cylinder at the end of the scavenge period).
Measurement of the thoroughness with which the cylinder is scavenged of exhaust gases and recharged with pure air prior to compression stroke.
Indicated power
power produced within the cylinders of the engine
Brake power
power available for doing work at the output shaft. Indicated power minus losses due to friction, pump drivers, and blowers
Mechanical efficiency
ratio of work done by the engine at the flywheel to the work done in the cylinders (brake power)/(indicated power)
Brake thermal efficiency
work done at the flywheel in heat units to the energy available at the cylinders from the fuel. Energy losses due to exhaust, cooling water, radiation
Specific fuel consumption
measure of the fuel efficiency of any prime mover that burns fuel and produces rotational power. It is the rate of fuel consumption divided by the power produced. Typically used to compare the efficiency of internal combustion engines and directly compare the fuel efficiency of different engines.
During a regular inspection, it was noticed that some of the main engine hold down bolts are loose. What could be the possible causes and describe what could happen if the bolts were not retightened.
Hold down bolts are very strong and are responsible for maintaining the engine location in relation to the shaft line, ensuring proper alignment.
The tightness of the hold down bolts must be verified at regular intervals, especially after grounding, extreme heavy weather, or any other instance where the hull may have been subjected to high stresses.
If the hold down bolts become loose, fretting can happen. Fretting occurs when 2 parts under load are allowed to move or rock on one another. The rubbing surfaces then wear, causing irregular surfaces, creating looseness between the parts and spot or uneven loading which can lead to misalignment and serious damage to the engine.
Describe how resin based chocks are installed and list their advantages and disadvantages
Advantage: savings in time and manpower, torque required to tighten the hold down bolts is 1/4 of that for metallic chocks.
Disadvantage: cannot be used in high temperature environments (max 80C, rule of thumb ambient temp 60C)
Installation: once the alignment of the engine is correct, dams are placed between the bedplate and the foundation to contain the epoxy liquid. The dams are a series of boxes with holes left for the hold down bolts. Most often, steel flat bar is tack welded to the foundation and then closed cell foam is used to seal the dams. A release agent is applied to the bolts or spacers that are used to keep the bolt holes clear.
A liquid resin and the hardener are mixed and poured into the dams. The dams are filled until the epoxy touches the underside of the engine feet or the underside of the bedplate. A chemical reaction between the resin and the hardener will take place and the epoxy will solidify. There is no loss of volume during solidification. This is important to guarantee that when the hold down bolts are tightened, the engine or bedplate will not move, thus ensuring proper alignment.
The surface area of an epoxy chock is greater than a metallic chock, because epoxy cannot support the same load per unit area. Therefore, increased area means greater support surface for the engine or bedplate.
Describe in detail the points considered when designing a bedplate
- Strength: capable of resisting the forces created by combustion
- Shock resistant: the loads on the bedplate are cyclic in nature; this results in stresses on the bedplate which can result in fatigue. The bedplate must resist these cyclic stresses and fatigue.
- Rigidity: rigid to maintain the alignment of the crankshaft as other components that make up the engine.
- Weight: as light as possible; an acceptable ratio of weight to power must be maintained.
- Simple design: reduce fabrication cost
- Accessibility: must provide easy access to gears, crankshaft, main bearings, connecting rods, etc. for easy maintenance of components
- Oil tight/air tight: prevent oil or oily vapor from leaking into the engine room
With the aid of a simple sketch, describe the construction of a welded bedplate.
Advantage: lighter and less expensive to build
Disadvantage: stresses formed during welding so must be heat treated, bedplate may crack during welding due to improper construction techniques
Two longitudinal girders are attached together by means of transverse girders. The transverse girders are made in such a way as to include the seating for the crankshaft main bearings. The transverse girders are responsible for maintaining the transversal rigidity of the bedplate and providing a seating arrangement for the main bearings. The lower section of the crankcase or oil sump is fabricated by welding a steel plate to the entire length of the bottom or underside of the bedplate and by welding steel plates between each transverse girder.
The steel plates used must meet very high standards and are subjected to destructive and non-destructive testing to verify the quality.
Once the plates are welded together, the entire bedplate is heat treated to eliminate the stresses produced during welding. The finished bedplate is placed in a large furnace and heated to a high temperature before being slowly cooled.
Once assembled, the top and bottom surfaces will be milled flat and smooth. The upper surface is used to support the engine frame and other components of the engine, and the alignment of all of these depends on this surface being smooth and true. The bottom surface will support the bedplate on the ship’s structure and is directly responsible for the longitudinal and transverse alignment of the engine. The seating area for the crankshaft’s main bearings will be line bored in one continuous operation to ensure all bearings will be concentric and at the same height. Thus the crankshaft will be level within the bedplate.
Describe in detail the purpose and construction of tie bolts fitted to crosshead engines
Purpose: maintain the main components of the engine structure (bedplate, monoblock, cylinder block) under compression. Tie bolts will transfer all the forces generated during the compression and combustion to the bedplate.
Material: good quality mild steel, may be in 2 pieces for very large engines.
Construction: rod threaded at both ends, outside diameter of the tie bolt is less than the diameter of the root of the threads.
Assembly: two tie bolts are fitted to each transverse girder of the bedplate and are installed as close to the center axes of the main bearing as possible to reduce the bending moment of the bearing saddle. Tie bolts are held in place by two nuts: one under the main bearing saddle and one at the top of the engine (upper surface of the cylinder block). If tie bolt is in 2 pieces, the joint will be where the A frame meets the entablature. Tie bolts are normally tightened by hydraulic force, must be done gradually and evenly tightened to prevent distortion.
For large bore high horsepower engines, the tie bolts will be very long and special supports must be put in place over the whole length of the rod to stop vibration.
How often are tie bolts checked? What will happen if a tie bolt is loose?
Tie bolts are checked at regular intervals listed in the engine manual, by the manufacturer, classification society and TC.
If a tie bolt is loose or broken, the engine must not be started until the tie bolts are tightened or replaced. If the engine is started with loose tie bolts, fretting will occur between the frames, bedplate and cylinder block, causing misalignment and faster wear of the bearings.
Conversely tie bolts must not be over tightened because they may fail.
What are the causes of cylinder liner wear?
- Corrosion: heavy fuel contains high sulphur; if liner is over-cooled, the combustion gases will condense and create sulphuric acid in greater quantity than can be neutralized by the lubricating oil. Sulphuric acid leads to a higher rate of wear. Corrosion mainly occurs in the upper 10% of the cylinder, can cause small particles to flake off that add to the abrasion.
- Adhesion: caused by lack of hydrodynamic lubrication between the piston rings and the cylinder liner. Reduced by the use of good quality lubricating oil in the cylinders, ensuring that the lubricating system is in good working order, liners and piston rings replaced at regular maintenance intervals, preventing the engine from running in an overloaded condition.
- Abrasion: abrasive particles enter the liner from the fuel itself or as a by-product of corrosion or adhesion. Will mix with the lubricating oil and form a grinding compound leading to wear to the piston rings and the liner. Mainly takes place in the upper section of the liner where the piston rings are under the greatest pressure.
Describe in detail the forces acting on a cylinder liner.
- Internal gas pressure inside the cylinder (hoop tensile stress)
- Pressure of cooling water circulating on the external surface of the liner (compressive hoop stress)
- Temperature different between the internal surface of the liner (higher) than the external surface (cooler): differing rates of expansion (thermal stress)
- Friction of the piston rings sliding over the internal surface of the cylinder liner
What is a wet liner? List advantages and disadvantages
Cooling water comes into direct contact with the liner.
Advantage: better cooling of the liner
Disadvantage: the outside surface can erode due to the vibration of the liner, the shock of combustion, or the impact of the piston during combustion; and erosion caused by the water flowing on the outside of the liner. To minimize erosion:
1. Water treatment added to the jacket cooling water
2. Outside surface of the cylinder liner is coated to reduce the negative effect of the water
3. Liner may be thicker
What is a dry liner? List advantages and disadvantages
Cooling water does not come in direct contact with the liner, bore < 150 mm, minimum thickness 1-1.5 mm. The liner must make full contact with the cylinder block or certain areas of the liner will become hotter than the rest, called hotspots. Maintained in place by interference fit or by the use of an epoxy.
Advantage: no erosion due to cooling water
Disadvantage: construction of the cylinder block is more complex, installation and removal of the liner is more complicated and difficult.
Describe how are cylinder liners are typically lubricated in trunk and crosshead type engines.
Trunk type: occurs by splash lubrication. As the crankshaft turns and the piston moves towards TDC, the crankpin will hit the oil in the crankcase and throw it up and onto the cylinder liner. Tends to excessively lubricate the cylinder liner, so scraper rings must be fitted to the pistons to remove oil from the surface of the cylinder liner and return it to the crankcase.
Crosshead type: need a lubricating system composed of a cylinder lubricating oil tank, an oil pump, and cylinder lubrication quills (injectors). Quills inject oil at a set point in the stroke, usually on the compression stroke as the piston is travelling up from BDC. The quills are fitted around the periphery of the liner and the oil pump is driven off the cam shaft. The quills are fitted with non-return valves to ensure that combustion gases will not pass backwards into the cylinder lubricating system.
What would happen if an engine is run with the cylinder liners worn beyond an acceptable limit?
Increase wear, reduce efficiency, improper sealing resulting in combustion gas leaking into crank case
