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What is meant by overlap in diesel engines?
The time when both the inlet and exhaust valves are open during the engine cycle.
What is scavenging in diesel engines?
The process of removing exhaust gases from the cylinder and replacing them with fresh air.
What is afterburning in diesel engines?
The combustion of unburned fuel in the exhaust system after the main combustion process.
What is compression in the context of diesel engines?
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What does expansion mean in diesel engines?
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Why are air coolers fitted after main engine turbochargers?
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What are the effects of undercooling the charge air on the engine?
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What are the THREE clearances necessary for efficient piston ring functioning?
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Why are piston rings necessary?
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Why are the clearances sketched in part (a) necessary?
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Sketch an overspeed trip of the centrifugal type.
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How does the overspeed trip sketched in part (a) operate?
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What is meant by cloud point in fuel terms?
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What is compatibility in fuel terms?
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How can fuel system and engine-related problems be avoided with reference to cloud point and compatibility?
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Describe a coalescer type distillate fuel oil filter.
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How can the lubricating oil of a large medium-speed diesel engine be assessed on board?
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What checks should be carried out if a large medium-speed marine diesel engine will not start?
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List FOUR safety devices fitted to the air start system.
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Describe how to calibrate a diesel engine cylinder liner, already removed from the engine.
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What precautions should be taken during initial startup when putting a new liner into service?
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Sketch an epicyclic planetary reduction gear.
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Marine Diesel Engineering Terms
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Overlap: Refers to the timing of the intake and exhaust valve events in an internal combustion engine, specifically when the exhaust valve starts closing before the intake valve fully opens. This creates a period of overlap where both valves are partially open.
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Scavenging: The process of clearing the exhaust gases from the cylinder of an internal combustion engine before the induction of the fresh air charge. Efficient scavenging maximizes combustion efficiency in diesel engines.
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Afterburning: The combustion of unburnt fuel in the exhaust system of an internal combustion engine, contributing to emissions and heat loss. It is also known as after combustion.
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Compression: The process in an internal combustion engine where the piston moves from bottom dead center (BDC) to top dead center (TDC), compressing the air inside the cylinder. This process heats the air, aiding ignition.
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Expansion: The power stroke in an internal combustion engine. The high-pressure gas created by combustion expands, pushing down on the piston and converting thermal energy into mechanical energy.
Air Cooler After Turbochargers
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Why air coolers are fitted after main engine turbochargers: Air coolers are fitted to lower the temperature of the compressed air delivered from the turbocharger before it enters the engine cylinders. This is beneficial for the engine as it reduces thermal loading and improves combustion efficiency. Cooler intake air increases air density allowing the engine to consume more oxygen, leading to increased power output and reduced NOx emissions.
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Effects of undercooling the charge air on the engine: Undercooling the charge air signifies that the air temperature is significantly lower than the optimal range. While cooler intake air is generally beneficial, undercooling can lead to problems such as increased wear on cylinder liners and piston rings due to the increased thermal shock, as well as condensation in the intake manifold, potentially leading to corrosion and performance issues.
Piston Rings
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Clearances for efficient piston ring functioning:
- Radial Clearance: The space between the top of the piston ring and the cylinder liner. Ensures the ring can expand for sealing.
- Axial Clearance: The space between the ring and the ring groove in the piston. Allows the ring to move back and forth for proper scraping action.
- Side Clearance: The gap between the ring ends. Ensures the ring can expand without binding and maintain proper sealing.
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Why piston rings are necessary: Piston rings act as sealing mechanisms, preventing gases from escaping the combustion chamber and oil from entering. There are two main types: compression rings and oil control rings. Compression rings are crucial for sealing the combustion chamber. Oil control rings scrape excess lubricating oil off the cylinder wall, preventing oil consumption.
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Importance of piston ring clearances:
- Radial Clearance: Allows the ring to expand and contract with temperature changes.
- Axial Clearance: Enables the ring to move back and forth, ensuring proper sealing and scraping action.
- Side Clearance: Prevents the ring from binding, allowing the piston to move freely and prevents excessive wear.
Overspeed Trip
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Sketch: A centrifugal overspeed trip utilizes a rotating weight attached to a spring-loaded mechanism. As the engine speed increases, the centrifugal force on the weight overcomes the spring force, ultimately actuating a trip mechanism which shuts down the engine.
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Operation: The overspeed trip is a safety device designed to shut down the engine if it exceeds a predetermined speed. It is activated through a centrifugal force mechanism that senses engine speed. If the engine speed surpasses the set limit, the centrifugal force on a rotating weight overcomes the spring force, pulling a lever that engages the trip mechanism. This can shut down the fuel supply or trigger another safety mechanism, preventing damage to the engine.
Fuel Terms
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Cloud Point: The temperature at which wax crystals first appear in a diesel fuel, indicating its suitability for use in cold climates. Lower cloud point fuels are preferred in colder temperatures.
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Compatibility: The ability of different batches of fuel to be mixed without adverse reactions, such as precipitation or phase separation. Compatibility is crucial for maintaining fuel quality and avoiding problems in the fuel system and engine.
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Avoiding fuel system and engine problems: To avoid fuel system and engine problems, it is important to use the fuel that is compatible with the engine and fuel handling system. It's also important to ensure the fuel meets the proper specifications, including the cloud point. Proper fuel filtration and handling are crucial to minimize contamination and potential filter clogging.
Coalescer Type Distillate Fuel Oil Filter
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Description: This type of filter uses a coalescing medium to remove water and solid contaminants from the fuel. It separates water droplets from the fuel using a hydrophilic material and a filter element. This process of combining small water droplets into larger droplets makes them easier to remove by gravity, sedimentation, or further filtration.
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Sketch: The filter typically contains a chamber where the contaminated fuel passes through a coalescing medium. This medium can be made of materials like polypropylene or polyethylene, which attract water and allow it to collect. The filter also has a sediment bowl, allowing the water and solid contaminants to settle and be drained.
Lubricating Oil Assessment Onboard
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Assessment methods: There are several onboard methods to assess the lubricating oil of a large medium speed diesel engine:
- Visual inspection: The color, clarity, and consistency can indicate potential contamination.
- Oil analysis kit: Includes tests for viscosity, water content, contamination levels (such as soot, metallic particles, and wear debris).
- Spectrometer analysis: A more advanced analysis method that can identify wear particles and potential issues with oil degradation.
Air Starting System
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Checks for a non-starting engine:
- Air pressure: Check the availability and pressure of the starting air system.
- Air starting valve operation: Make sure the air starting valve is open and functioning correctly.
- Starting air line: Ensure the starting air line is clear and not obstructed.
- Engine turning: Confirm that the engine is turning over.
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Safety devices:
- Overspeed trip: To prevent excessive speed while starting.
- Starting air pressure relief valve: To protect the air starting system from excessive pressure.
- Low starting air pressure switch: To prevent starting if the pressure is inadequate.
- Automatic shutdown solenoid: To shut down the engine in case of a failure in the starting system.
Engine Cylinder Liner Calibration
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Calibration process: After removing the cylinder liner from the engine, it is carefully measured and calibrated to ensure it meets the specifications. This process includes checking the diameter, roundness, and taper of the liner. A specialized tool called a bore gauge is often used to measure the diameter at various points.
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Initial startup precautions:
- Lubrication: Ensure the liner is properly lubricated before starting the engine, as the new liner may be prone to wear.
- Engine speed: Start the engine slowly and gradually increase the speed to allow the liner to bed in and prevent any excessive wear.
- Engine load: Avoid overloading the engine during initial startup, allowing the liner to acclimate.
Epicyclic Planetary Reduction Gear
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Description: An epicyclic planetary reduction gear is a type of gear system containing multiple gears engaged in a series of concentric rings and interconnected planets. It is a common gear system used in modern marine diesel engines.
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Sketch: The gear setup includes a sun gear (the central gear), planet gears that mesh with both the sun gear and the ring gear, and a ring gear that surrounds the entire assembly. The rotation of one gear (typically the sun gear) causes the other gears to turn in a specific relation, resulting in gear reduction and a change in rotation direction.
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