Questions and Answers
What is the primary function of a piston skirt?
What is a garter spring primarily used for in a piston assembly?
Which type of piston ring is specifically designed to control engine oil consumption?
Which characteristic best describes a dry cylinder liner?
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What is the main purpose of honing a cylinder liner?
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Why are inlet and exhaust valves typically equipped with two springs?
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How does a valve rotator function in an engine, and what type of fuel is it primarily used with?
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What is critical vibration in an engine, and why is it important to manage?
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What is the key difference between indicated power and brake power in an engine?
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What characterizes a dual fuel engine?
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In a 4-stroke engine cycle, which stroke is responsible for power generation?
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What is scavenging in a 2-stroke engine?
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What is a stoichiometric mixture in combustion engines?
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What are the typical strokes and bores for low-speed engines compared to high-speed engines?
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What is the purpose of valve overlap in an engine?
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Which types of bearings are most commonly used in engines?
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Study Notes
Dual Fuel Engine
- A dual fuel engine operates on two different types of fuel, typically a combination of gas and diesel.
Engine Speeds
- Low-speed engines: Typically operate at speeds less than 300 RPM.
- Medium-speed engines: Operate in the range of 300 to 1200 RPM.
- High-speed engines: Operate at speeds greater than 1200 RPM.
Strokes and Bores
- Low-speed engines: Often have longer strokes (up to 300 mm) and larger bores.
- Medium-speed engines: Shorter strokes compared to low-speed, with bores around 200-400 mm.
- High-speed engines: Short strokes and smaller bores (usually less than 200 mm).
Engine Numbering
- An engine is numbered based on its type, model, and configuration, often including parameters like displacement and cylinder arrangement.
Heat Engine
- A heat engine converts thermal energy to mechanical work. An example is a steam engine.
Air Standard Cycle in Diesel Engines
- The air standard cycle for diesel engines usually follows the Diesel cycle, involving adiabatic compression, constant pressure heat addition, adiabatic expansion, and constant volume heat rejection.
Indicated-Power vs. Brake-Power
- Indicated power measures the total power produced within the cylinder, whereas brake power measures the output power available at the crankshaft after losses due to friction.
Parasitic Load
- Parasitic load refers to energy consumed by components when the engine is not producing useful work. Examples include alternators and water pumps.
Stoichiometric Mixture
- A stoichiometric mixture achieves optimal combustion efficiency. Gasoline typically has a ratio of 14.7:1 (air to fuel), while diesel ranges from 14:1 to 18:1.
Combustion Engine Description
- A combustion engine can be described by its cycle (e.g., Otto, Diesel) and configuration (e.g., inline, V-type).
Petrol in CI Engines
- Petrol cannot be used in a Compression Ignition (CI) engine due to its lower ignition temperature and octane rating leading to pre-ignition.
4-Stroke Engine Cycle
- The 4-stroke cycle consists of intake, compression, power, and exhaust strokes, with each stroke completing during two revolutions of the crankshaft.
Power Stroke in 4-Stroke Engine
- The power stroke occurs during the combustion process when the mixture ignites and forces the piston down.
Revolutions per Power Stroke in 4-Stroke Engine
- One power stroke occurs every two revolutions of the crankshaft; the camshaft rotates at half the crankshaft speed.
Revolutions per Power Stroke in 2-Stroke Engine
- In a 2-stroke engine, one power stroke occurs every revolution of the crankshaft; camshafts are generally not used.
2-Stroke Engine Cycle
- The 2-stroke cycle involves a single crankshaft revolution for both intake and power strokes, using scavenging techniques for gas exchange.
Scavenging
- Scavenging is the process of clearing exhaust gases from the cylinder after combustion to make room for fresh air-fuel mixture.
Scavenging Methods
- Loops Scavenging: Efficient for high-speed engines; drawbacks include loss of fresh fuel.
- Cross Scavenging: Better mixing of air and fuel; can lead to increased exhaust pollution.
Engine Emissions Pollutants
- Major pollutants include carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM).
Hazards of Emissions
- CO can cause suffocation, HC contributes to smog and health issues, NOx is linked to respiratory problems, and PM can lead to heart disease.
Engine Valve Overlap
- Valve overlap occurs when both the intake and exhaust valves are open at the same time, improving scavenging and cylinder filling.
Timing Diagrams
- Timing diagrams visually represent valve operation phases in both 4-stroke and 2-stroke engines.
Differences Between 2 and 4-Stroke Engines
- 2-stroke engines produce power every crankshaft revolution, while 4-stroke engines do this every two revolutions. 4-stroke engines have more components, leading to complexity.
Engine Securing to Ship
- Engines are secured to ships using bedplate and chocks to minimize vibration and stabilize during operation.
Bedplate Purpose
- Acts as a foundation for the engine and distributes engine loads; typically made of cast iron or steel.
Chock Materials
- Common materials for chocks include rubber (good vibration dampening, but wear over time) and metal (durability, but less vibration dampening).
Earthing the Bedplate
- Earthing prevents electrical hazards by connecting the bedplate to the ship's hull, dissipating electrical currents.
Dry vs. Wet Sump
- Dry Sump: Oil is stored in an external reservoir; advantages include lower engine height.
- Wet Sump: Oil is stored in the oil pan; simpler but may lead to oil starvation during heavy maneuvers.
Engine Bearings
- Types include plain (for low friction), roller (for higher load), and ball bearings; each offers different performance characteristics depending on application.
Plain Bearings Materials
- Common materials are bronze and aluminum-based alloys, often constructed to ensure durability and resistance to wear.
Split Plain Bearings
- Splitting allows for easier installation and removal without needing to disassemble the entire engine.
Conrod Plain Bearins
- Larger bottom half for load distribution and to accommodate the crankshaft design, reducing stress.
Marine Palm
- A configuration that connects the conrod to the piston, allowing for multi-directional flexibility during operation.
Oblique Conrods
- Used for compact engine designs and reducing overall engine height, improving weight distribution.
Piston Skirt Purpose
- Provides stability and aligns the piston in the cylinder; absence can lead to excessive piston wear and noise.
Garter Spring Function
- A garter spring helps hold the piston rings in place and is typically located at the top of the piston.
Piston Rings Types and Functions
- Types include compression rings (seal combustion gases), oil control rings (regulate oil), and scraper rings (remove excess oil).
Cylinder Liners Types
- Two types:
- Cast iron liners (durability, low cost)
- Composite liners (lighter, enhanced thermal expansion properties).
Purpose of Honing
- Honing provides a smooth surface finish on liners to improve sealing and lubrication, typically achieved through abrasive processes.
Medium-Speed Engine Cylinder Head Ports
- Four ports facilitate efficient gas exchange, allowing separate paths for intake and exhaust gases.
Timing Marks on Camshaft Drive
- Timing marks ensure correct alignment between the crankshaft and camshaft, crucial for synchronized valve operation.
Valve Bridge/Yoke Arrangement
- This arrangement ensures even distribution of force across the valve springs, leading to consistent valve operation.
Valve Operation Arrangements
- Spring-loaded, pneumatic systems, or hydraulic actuation are some ways valves can be opened and closed.
Double Springs for Valves
- Inlet/exhaust valves use two springs to provide redundancy, enhancing reliability and ensuring proper closure under various conditions.
Valve Rotator Function
- Valve rotators promote even wear and reduce seat deposits; commonly used with fuels that create high deposits, like diesel.
Differences in Low-Speed and Medium-Speed Engines
- Low-speed engines typically feature larger components and a simpler design facilitating lower operational speeds.
Crosshead Function
- The crosshead connects the piston to the crankshaft, allowing for vertical piston movement and reducing side loading on the piston.
Piston Rod Connection in Low-Speed Engines
- Piston rods are typically attached using a bolted or pinned connection, ensuring robust linkage to the crosshead.
Stuffing Box Function
- Ensures a seal around the moving parts (like shafts), preventing leaks of fluids or gases and maintaining pressure.
Crankshaft Construction Methods
- Methods include:
- Forged crankshafts (high strength).
- Cast crankshafts (resistance to fatigue).
- Built-up crankshafts (combined sections for flexibility).
Crankshaft Witness Marks
- Witness marks indicate the original alignment and help in assessing wear or misalignment during maintenance checks.
Oil Control Rings in Low-Speed 2-Stroke Engines
- Generally, low-speed 2-stroke engines do not require oil control rings due to their design often using oil mist for lubrication.
Piston Ring Ends Sketching
- Piston ring ends can be shaped as butt, angled, or tapered to facilitate proper sealing and oil control.
Piston Ring Shape Purpose
- Specific shapes help optimize combustion sealing and accommodate thermal expansion while reducing blow-by gases.
Low-Speed Engine Exhaust Valve Function
- The exhaust valve allows spent combustion gases to exit the cylinder after power is produced, preventing back pressure.
Fuel Injector Location in Low-Speed Engines
- Fuel injectors are typically located around the cylinder head, directing fuel into the combustion chamber.
Golden Rule of Vibration
- The rule states that vibrations should be minimized to avoid engine damage and improve operation efficiency.
Leaf Spring Damper Function
- Leaf spring dampers limit torsional vibrations, enhancing stability and performance of rotating systems.
Viscous Damper Function
- Viscous dampers limit vibrations by utilizing fluid damping principles, effectively absorbing energy from oscillations.
Moment Compensator Function
- A moment compensator balances forces during engine operation, reducing excessive shaking and wear.
First vs. Second-Order Moments
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Description
Test your knowledge on the components and functions of engine piston assemblies. This quiz covers the roles of the piston skirt, garter springs, oil control rings, and more. Challenge yourself to see how well you understand these crucial elements of engine mechanics.
