Glossary Flashcards
Thrust Bearing
Two rings fore and aft of the collar secured to the hull rather than the shaft. Act as stoppers and in doing so transfer the thrust from the shaft to the hull.
Thrust bearing Rings
Referred to as the housing they are lined with easily replaceable alloy pads which greatly reduce the coefficient of friction. These alloy pads contain grooves through which lubricating oil flows.
Lubrication
Reduces friction, removes heat. prevents corrosion and cleanses.
Internal combustion engines
Requires oxygen, a combustible and heat to start ignition in order to function.
Compression Ignition Engine
Air is compressed and the temperature increases to the point where the fuel combusts.
Flash Point
The temperature at which the rate of vapour creation would result in momentary ignition.
Fire Point
The temperature at which the rate of vapour would create a continuous fire upon ignition
Autoignition Temperature
The point at which a spark is no longer necessary for combustion to occur
Cycle (5 Processes Of)
1) Induction
2) Compression
3) Combustion
4) Expansion
5) Expulsion
Engine Stroke
The full distance through which the piston passes.
Two Stroke Engines
Take 2 strokes to complete a cycle
4 Stroke Engines
Require 4 strokes to complete a cycle
Top Dead Center
Highest position the piston reaches within the cylinder
Bottom Dead Center
The lowest position the piston the piston reaches within the cylinder
3 Forms of Scavenging
1) Loop
2) Cross
3) Uniflow
Crosshead Pistons
Collect soot, preventing it from entering the crankshaft system. Allows for the use of a different form of lubrication on the piston head.
Trunk Piston Engine
For Higher speed engines
Back Plate
Provides support and rigidity, carries the seating for the crankshaft and main bearing housing.
A-Frame
Fabricated strength members which straddle the bed plate, carry the guides for crosshead engines.
Cylinder Liner
Easily replaceable and made of materials with good cooling properties. For two stroke engines, ports are cut into the liner
Scavenge Diaphragm
Only possible in crosshead engines, seals off the piston from the crankcase.
Cylinder Head
Houses:
1) The exhaust valve (4 stroke and uniflow)
2) The intake valve (4 stroke)
3) Fuel injector
4) Relief valve (removes excess pressure)
5) Air starting valve
Tie Bolts
Long Bolts connecting the cylinder block to the engines A frame. Constantly under tension, they increase the strength of the material as their compression counteracts tensile strain produced by the engine.
Bedplate
Only for large two stroke engines, a platform which supports structural components while withstanding heavy fluctuating stresses.
Cylinder Liners
Must:
1) readily transfer heat
2) retain lubricating oil
3) have high mechanical strength
Cylinder Liner Mantainence
Replaced after a wear-down of 2%
The internal diameter is measured fore/aft and port/starboard several times along the liner
The typical rate of weardown is 0.01mm per 1000 hours.
Cylinder Liner Weardown
Caused by:
1) Frictional Wear
2) Corrosion: water and SO3 produced via combustion bond to form sulphuric acid which condenses on the liner.
3) Abrasion: hard particles from combustion or lubricating oil and become trapped between the liner and piston ring.
4) Adhesion/Scuffing: Excessive heat or gas pockets due to the lack of lubrication
Piston Cooling
Limits thermal stressing and thermal expansion
Oil cooled pistons
1) Lower thermal capacity than water
2) Lower maximum safe temperature than water (above 56 degrees oil carbonises)
3) Oil is supplied under pressure via telescopic pipers or swinging links
Water Cooled Pistons
1) danger of water leaking into the combustion chamber and contaminating fuel
2) Inhibitors needed to prevent corrosion
3) separate cooling system is necessary
4) Temp of 70 degrees can be sustained
Thermal Stressing
Reduced by lowering the temperature gradient between material and coolant
Top Piston Ring
aka fire ring as it is exposed to combustion
Pistons Rings
The only part of the piston in contact with the cylinder lining
More than one ring is necessary as each ring has a small gap left from when it was inserted onto the piston ring. Multiple rings also help to distribute oil more effectively and act as reserve compression rings
Scraper rings
Downward wedge on lip means that on the downward stroke they scrape of oil from the cylinder liner, which they then distribute on the upward stroke
Crosshead
Forged steel block secured to the foot of the piston rod. Transfers the transverse component of the pistons movement to its guides.
Top end bearing
Transmits full gas load from the piston to the connecting rod and crosshead
Bottom end bearing
Transmits similar loads to those in the crankshaft
Crankshaft
Receives and transmits the full power of the engine. Subject to high torsion and bending stresses.Either semi-built or welded. Statically built and dynamically balanced
Flywheel
Solid wheel shaped to have a high mass and high inertia to store energy while the engine is running. Absorbs the energy at peak torque and returns it during low torque and deceleration. Smooths out fluctuation in engine speed
Main Bearings
Support the crankshaft at each journal, subjected to heavy loads fluctuating in magnitude and direction
Fuel Injectors
Atomise the fuel by passing it through tiny orifices under high pressure. The usual pressure is 250 bar.
Spring Loaded Fuel Injector
Pressure is set by adjusting the resistance of the spring
Hydraulic Fuel Injector
Pressure is created by a fuel pump, the injector creates no pressure itself
For Crisp Combustion
1) Viscosity (resistance to flow) : specific to the injector
2) Atomisation: Breaking fuel into fine particles (increases surface area allowing for more effective combustion)
3) Penetration: The distance that fuel droplets travel into the compressed air.
4) Turbulence: Provision of air among the particles
Fuel Pumps
One fitted per cylinder. Driven by the engine. Delivers the correct amount of fuel to the engine at the correct pressure and time.
Turbocharger
Increase the mass of air which enters the engine, as a greater volume of available air means complete combustion is more likely. Uses the energy wasted in the exhaust.
Pulsed Turbochargers
1) More efficient at low load
2) Highly responsive to change in speed
3) utilises the the thermal energy of exhaust gas and kinetic energy during the blowdown period.
4) direct connection between each cylinder and the turbocharger system
5) Earlier opening of exhaust valve maximises the pulse.
6) Limited to three cylinder per exhaust branch to stop interference with scavenging.
Constant Pressure Turbochargers
1) every cylinder exhausts into a large receiver
2) No need for earlier opening of exhaust
3) turbocharger nozzle can be calibrated
4) More efficient at a higher load
Air Starting System
1) High pressure air stored in ‘bottles’
2) Master Valve leads to individual air starting valves
3) Air starting valves are in the cylinder head
4) Distributor opens the appropriate valve
5) Bottles should have enough air for 12 consecutive starts
Lubricating Oil System
Operates 24/7 to oppose condensation.
