Main Engine Operations Page 15-19 Flashcards
What begins the induction stroke in a 4-stroke cycle diesel engine?
The intake stroke begins at top dead centre, and as the piston moves down, the intake valve opens.
What happens during the compression stroke of a 4-stroke cycle diesel engine?
The piston rises from bottom dead centre to compress the fuel and air mixture, which is ignited at the end of the stroke.
What occurs during the ignition/power stroke in a 4-stroke cycle diesel engine?
The fuel and air mixture is ignited, burns, expands, and forces the piston down.
What is the function of the exhaust stroke in a 4-stroke cycle diesel engine?
The exhaust stroke pushes burnt gases out of the combustion chamber through the exhaust port.
Why is the correct tappet clearance essential on the inlet and exhaust valves of a 4-stroke diesel engine?
It allows for thermal expansion of the valve, tappet, and rocker gear during running service.
What is the purpose of providing clearance between the valve stem, tappet, and rocker arm?
To ensure the fit accommodates thermal expansion when the engine is up to temperature.
What must be accommodated with a positive tappet clearance in a 4-stroke diesel engine?
Differential expansion of the intervening cylinder blocks compared to the oil bathed pushrods.
Describe with the aid of sketches, the operation of 4-stroke cycle diesel engines (16)
Induction/Inlet - The intake stroke begins at top dead centre, and as the piston moves down, the intake valve opens. The downward movement of the piston creates a vacuum in the cylinder, causing a fuel and air mixture to be drawn through the intake port into the combustion chamber. As the piston reaches bottom dead centre, the intake valve closes.
Compression - The compression stroke begins with the piston at bottom dead centre and rising up to compress the fuel and air
mixture. Since both the intake and exhaust valves are closed, there is no escape for the fuel and air mixture, and it is compressed to a fraction of its original volume. At this point, the fuel and air mixture is ignited
Ignition/Power - The power stroke begins when the fuel and air mixture is ignited, burns and expands and forces the piston
down. The valves remain power stroke ends as the piston reaches bottom dead centre.
Exhaust - The exhaust stroke begins when the piston nears the end of the power stroke and the exhaust valve is opened. As
the piston moves upward towards top dead centre, it pushes the burnt gases, resulting from the ignition of the fuel and air
mixture, out of the combustion chamber and through the exhaust port. As the piston reaches top dead centre, ending
the exhaust stroke, the exhaust valve closes, and the intake valve opens to begin the intake stroke for the next cycle
. a) Explain why the correct tappet clearance is essential on the inlet and exhaust valves of a 4-stroke diesel engine (8)
Both inlet and exhaust valves actuated together, this is done by means of a tappet. A tappet clearance is ensured and must be
correct to allow for thermal expansion of the valve, tappet and rocker gear during running service. A clearance is provided
between the valve stem, tappet and rocker arm for this purpose.
• To ensure that the valves close fully and at the correct time.
• To ensure open at correct time especially when the engine is upto temperature and the valve stem has expanded from its cooled length.
• Differential expansion of the intervening cylinder blocks compared to the oil bathed pushrods needs to be
accommodated with a positive tappet clearance to ensure that the valves are never held of their seats for any operating scenario/temperature regime.
Describe with the aid of sketches, the operation of 4-stroke cycle diesel engines
What happens if tappet clearance is too small?
The valve will open early and close late, potentially not fully closing. This causes poor compression, blow past of hot gases, and eventual wear out of the valve and seat face, leading to lower power output.
What happens if tappet clearance is too big?
Valves will open late and close early, resulting in reduced lift. There will be more noise and wear from the impact of working surfaces, poor scavenging of exhaust gas, and reduced power.
Describe the scavenging process in a 2-stroke diesel engine.
Scavenging begins with the piston at BDC, where intake ports are uncovered and the exhaust valve is open. A supercharger forces air into the cylinder, expelling gases from the previous cycle and admitting clean air for the new cycle.
What occurs during the compression stroke of a 2-stroke diesel engine?
As the piston moves towards TDC, it covers the intake ports and the exhaust valve closes, sealing the upper cylinder. The air is compressed at high pressure and temperature, generating heat.
What happens during the power stroke of a 2-stroke diesel engine?
Just after TDC, atomized fuel is injected, and the intense heat causes the fuel to ignite.
What are the constructional differences between 2-stroke slow speed and 4-stroke medium speed engines?
2-stroke engines have no intake valves (scavenge ports instead), a crosshead bearing, one large exhaust valve, stuffing boxes, and are larger in size. 4-stroke engines have multiple inlet and outlet valves, a gudgeon pin connecting piston to connecting rod, and require a camshaft for lubrication and fuel pumps.
What is a unique feature of some 2-stroke electronically controlled engines?
Some 2-stroke ME electronically controlled engines are camshaft-less, controlling lubrication and fuel pumps electronically, unlike 4-stroke engines which need a camshaft.
What aids scavenging in 2-stroke engines?
2-stroke engines have auxiliary blowers to aid in scavenging, whereas 4-stroke engines utilize multiple inlet valves.
Describe with the aid of sketches, the operating principle of the 2-stroke diesel engines
(16)
During the exhaust and intake stroke, scavenging begins with the piston at BDC. At this point intake ports are uncovered and exhaust valve is open. Super charger forces air into cylinder expelling the gases from previous cycle and clean air admitted for new cycle.
Power
On the compression stroke As the piston moves towards TDC, it covers the intake ports, the exhaust valve close and seal upper cylinder as piston continues upwards the air is compressed at high pressure and temperature where heat is generated by compression.
ScAvenge On the power stroke Just after TDC compression stroke ends atomised fuel is injected at this point and the intense heat causes fuel mto ignite
Describe with the aid of sketches, the operating principle of the 2-stroke diesel engines
(16) sketch
What are the components of a medium speed diesel engine’s piston and connecting rod arrangement?
Wrist Pin, Piston crown, Top compression ring, Second compression ring, Oil control ring, Snap ring, Piston skirt, Connecting rod shaft, Connecting rod cap, Bearings, Connecting rod bolts.
What is the constructional difference between a slow speed 2-stroke and a medium speed 4-stroke diesel engine regarding the piston and crankshaft connection?
The 2-stroke has a crosshead piston while the 4-stroke has a trunk piston. The 2-stroke engine uses a crosshead to connect the piston to the connecting rod, whereas the 4-stroke connects the piston to the connecting rod directly using a gudgeon pin.
What type of ports do 2-stroke and 4-stroke engines have?
The 2-stroke always has scavenge ports, whereas the 4-stroke has an inlet valve.
What is the difference in connecting rod structure between 2-stroke and 4-stroke engines?
The 2-stroke has both a connecting rod and a piston rod, while the 4-stroke only has a connecting rod.
How do the cylinder liners differ between 2-stroke and 4-stroke engines?
The cylinder liners on a 4-stroke are considerably shorter to allow transverse movement of the connecting rod.
What is the difference in piston skirt length between 2-stroke and 4-stroke engines?
Piston skirts on 4-stroke engines are considerably longer to absorb side loading against the cylinder liner created by the rotating crankshaft.
What components seal and separate the cylinder from the crankcase in a 2-stroke engine?
A stuffing box and diaphragm are used to seal and separate the cylinder from the crankcase.
What is the function of the diaphragm in a 2-stroke crosshead engine?
The diaphragm plate separates the crankcase from the cylinder and scavenges, preventing lube oil from being carried upwards and stopping contaminated cylinder oil from going downwards.
How does the stuffing box function in a 2-stroke engine?
The stuffing box allows the piston rod to pass through while preventing pressurized scavenge air from leaking into the crankcase.
What is a power indicator card in a slow speed marine diesel engine?
A graphical representation used to assess the power developed in the cylinder.
How can the power developed in the cylinder be assessed using the power indicator card?
Using the formula: P = pi * l * A/n, where Pi is indicated pressure, l is the indicator length of cylinder stroke, A is the stroke area, and N is the number of power strokes.
How is power calculated for a 2-stroke engine?
Power = Pi * A * N * number of cylinders.
How is power calculated for a 4-stroke engine?
Power = Pi * A * (N/2) * number of cylinders.
- With reference to medium speed diesel engines, sketch and label a typical arrangement for the piston and connecting rod (
. Sketch a typical power indicator card for a slow speed marine diesel engine (6)
b) Explain how the card may be used to assess the power developed in the cylinder (10)
P=pi ℓ A n pi=A/ℓ x K
Pi= indicated pressure
ℓ= being the indicator length of cylinder stroke
A= being the stroke area (area under curve)
N= being the number of power strokes.
2 stroke engine power= Pi l A N x no. of cylinders
4 stroke power= Pi l A (N/2) x no. of cylinder
What are chocks used for in an engine?
Chocks securely hold the engine and prevent movement whilst maintaining initial alignment.
What is the purpose of the bed-plate in an engine?
The bed-plate is the base of the engine, rigid and strong, to support the whole engine whilst resisting stress forces maintaining crank shaft alignment.
What do tie-rods do in an engine?
Tie-rods maintain compression between all components through the engine and transmit the compressive and explosive forces acting on the cylinder heads throughout the engine structure.
What is the function of the entablature in an engine?
The entablature is the cast iron housing of the cylinder blocks, housing the cylinder liner inserts, incorporating the scavenge space and jacket water cooling passages.
What is the role of holding down bolts in an engine?
Holding down bolts securely hold the engine in its original and correct position, whilst in operation and when static.
What does the camshaft do in an engine?
The camshaft drives the fuel pumps, cylinder lubricators and maintains engine timing.
- Draw a line diagram of a main lubricating oil system for a large two-stroke, crosshead type diesel engine. Label all principle
components in the system (16)
What is the first step in starting a small diesel engine with a starting handle?
Check oil and cooling water level (if required)
What should you do to ensure sufficient fuel for operation?
Dip fuel tank to ensure sufficient fuel for the operation
What is the importance of checking air filters before starting the engine?
Check air filters make no blockage.
What must be verified about the engine’s gear before starting?
Check that the engine is not in gear and free to rotate.
What must be done with the decompression mechanism before using the starting handle?
Check that the decompression mechanism has been operated.
What is the procedure for cranking the engine?
Crank handle slowly and build up pressure; once speed is picked up, close compression lever.
What is one reason that could cause difficulty in starting the engine?
No fuel due to empty fuel tank or blocked filters.
What can cause low compression in the engine?
Worn piston rings, burnt exhaust gases, incorrect adjustment of decompression mechanism.
What type of mechanical failure could prevent the engine from starting?
Mechanical breakage of one of the moving components of engine, broken drive of fuel pump, or mechanical breakage of fuel pump.
What can cause issues with the engine’s valves?
Wrong tappet clearances, burnt exhaust valves not opening.
What should be checked if the engine is started hydraulically?
Make sure hydraulic reservoir is up to pressure.
What is the first step in starting a small diesel engine with a starting handle?
Check oil and cooling water level (if required)
What should you do to ensure sufficient fuel for operation?
Dip fuel tank to ensure sufficient fuel for the operation
What is the importance of checking air filters before starting the engine?
Check air filters make no blockage.
What must be verified about the engine’s gear before starting?
Check that the engine is not in gear and free to rotate.
What must be done with the decompression mechanism before using the starting handle?
Check that the decompression mechanism has been operated.
What is the procedure for cranking the engine?
Crank handle slowly and build up pressure; once speed is picked up, close compression lever.
What is one reason that could cause difficulty in starting the engine?
No fuel due to empty fuel tank or blocked filters.
What can cause low compression in the engine?
Worn piston rings, burnt exhaust gases, incorrect adjustment of decompression mechanism.
What type of mechanical failure could prevent the engine from starting?
Mechanical breakage of one of the moving components of engine, broken drive of fuel pump, or mechanical breakage of fuel pump.
What can cause issues with the engine’s valves?
Wrong tappet clearances, burnt exhaust valves not opening.
What should be checked if the engine is started hydraulically?
Make sure hydraulic reservoir is up to pressure.
- With reference to a small diesel engine that requires the use of a starting handle:
a) State the procedure for starting (8)
• Check oil and cooling water level ( if required)
• Dip fuel tank to ensure sufficient fuel for the operation
• Check air filters make no blockage.
• Check that the engine is not in gear and free to rotate.
• In order to use the starting handle check that the decompression mechanism has been operated and rotate the engine to
check engine is free to operate.
• Crank handle slowly and build up pressure once speed is picked up close compression lever
- With reference to a small diesel engine that requires the use of a starting handle:
b) State EIGHT reasons that would cause difficulty to start (8)
• No fuel due to empty fuel tank or blocked filters, faulty injector
• Low compression due to worn piston rings, burnt exhaust gases, incorrect adjustment of decompression mechanism
• Mechanical failure due to mechanical breakage of one of the moving components of engine, broken drive of fuel pump or
mechanical breakage of fuel pump.
• Wrong tappet clearances, burnt exhaust valves not opening
• Blocked filters
• If engine is started hydraulically make sure hydraulic reservoir is up to pressure.
