M/E 2&4 Stroke Flashcards
- a) Explain why the correct tappet clearance is essential on the inlet and exhaust valves of a 4- stroke diesel engine.
a) The tappet clearance ensures that the valve is closed when it is supposed to be, as the valve spindle expands in length during operation due to the high temperature. If there was no clearance the valve would expand against the fixed piston of the rocket arm, while cam on base circle and would therefore expand into the cylinder causing loss of compression and burning of the valve and seat.
b) State the results of the tappet clearance being:
i) too large
ii) too small
bi) If the tappet clearance is too large the valve will open late and close early
bii) If the tappet clearance is too small the valve will open early and close later.
Explain the constructional differences between 2 stroke low speed and 4 stroke medium speed diesel engines.
In a 2 stroke engine, the air inlet to the cylinder is usually in the form of inlet ports in the cylinder on the exhaust is usually by a centrally located exhaust valve in the cylinder head. In 4 strokes the inlet and exhaust is in the form of valves in the cylinder head. In a 2 stroke the scavenge space is separated from the crankcase by a separating diaphragm whereas in a 4 stroke the crankcase is exposed to the piston skirt and cylinder liner. The force from the piston in a 2 stroke is transmitted via the piston – piston rod – crosshead bearing – connecting rod - crankshaft arrangement whereas
John Willison IAMI Revision 5
in a 4 stroke the arrangement is piston – gudgeon pin – connecting rod – crankshaft. The stroke to bore ratio in a 4 stroke is approx. 2:1 whereas 2 stroke is approx. 4:1. Large stroke to bore ratio in 2 strokes separates crankpin and main journal giving large throws, In a 4 stroke the crankpin and main journal gives an overlap. Lubricating oil drilling connection min journal and crankpin in 4 strokes are not present in 2 strokes.
- a) Sketch a power indicator card for a slow speed marine diesel engine. (DONT DO?)
b) Explain how the card may be used to asses the power developed in the cylinder.
Indicated power is found with the equation:
ip= PLAN
P – the indicated mean pressure
L – the cylinder stroke length
A – area of piston
N – the number of power strokes per second.
- Describe the function of each of the following components of a diesel engine:
a) Chocks
a) To securely hold the engine and prevent movement whilst maintaining initial alignment.
Describe the function of each of the following components of a diesel engine:
b) Bedplate
b) The base of the engine which is rigid and strong as it has to support the whole engine whilst resisting stress forces maintaining crank shaft alignment.
Describe the function of each of the following components of a diesel engine:
c) Tie rods
c) To maintain a compression between all components through the engine, and transmit the compressive and explosive forces acting on the cylinder heads throughout the engine structure.
- Describe the function of each of the following components of a diesel engine:
d) Entablature
d) The cast iron housings of the cylinder blocks, housing the liner inserts incorporating the scavenge space and jacket water cooling passages.
- Describe the function of each of the following components of a diesel engine:
e) Holding down bolts
e) It secures the bed plate in its correctly aligned position, to maintain correct alignment between crankshaft and propeller shaft.
- Describe the function of each of the following components of a diesel engine:
f) Crankshaft
f) Converts the reciprocating motion of the piston into rotary motion to transmit to the propeller shaft.
- With reference to a small diesel engine that requires the use of a starting handle, a) State the procedure for starting
a) Check fuel and L.O. level is OK.
Check fuel in fuel tank is OK.
Check the fuel stop cock is switched on.
If the engine is water cooled, turn on seacock.
Check the gear lever is in neutral.
Set the throttle to open and switch on the cold start lever.
Set the decompression lever to the decompression position.
Swing round the starting handle building up speed until maximum speed, then push over decompression lever whilst continuing to maintain speed with the starting handle. As son as the engine starts, cut back on the throttle.
- With reference to a small diesel engine that requires the use of a starting handle, a) State the procedure for starting
b) State 8 reasons that would cause difficulty in starting
(DO B)
b) Heater plugs No fuel Blocked fuel filters Incorrect viscous of L.O. Not enough inertia build up in cranking Restricted/blocked air filters Decompression switch closed Contaminated fuel
- a) Explain the constructional difference between a slow speed 2 stroke and a medium speed 4 stroke diesel engine with respect to the connection between the piston and the crankshaft.
a) On 2 stroke engines, the connecting rod is attached to the piston rod via the crosshead bearings, which transfer much of the engine thrust to the crosshead guides in the engine frames. In 4 stroke engines, the connecting rod Is attached directly to the piston pin.
- a) Explain the constructional difference between a slow speed 2 stroke and a medium speed 4 stroke diesel engine with respect to the connection between the piston and the crankshaft. b) Describe the function of the diaphragm and stuffing box.
(DO B)
b) Its positioned between the crankcase and cylinder of a 2 stroke engine, to segregate the 2 areas. Its purpose is to prevent crankcase oil being carried into the scavenge space, and to prevent scavenge air, used cylinder oil and the products of combustion all from contaminating the crankcase oil.
- a) If the M/E control pitched propeller doesn’t respond to bridge control, describe the routine for changing to E/R control.
a) Make contact with OOW on bridge, request to take control of M/E
Ensure that the governor setting for the engine and pitch setting of the propeller in the control room matches that of the bridge.
Switch to E/R control.
If possible, lock the console in this position so it cannot be changed.
