SEPTEMBER 2018

EXAMINATION OF MARINE ENGINEER OFFICER Function: Marine Engineering at Management LevelMARINE ENGINEERING KNOWLEDGE (MOTOR)TIME ALLOWED - 3 HOURS

Instructions: - 1. Answer SIX questions only. 2. All Questions carry equal marks3. Neatness in handwriting and clarity in expression carries weightage 4. Illustration of an Answer with clear sketches /diagram carries weightage 5. All unused pages of the answer script must be cancelled out by two lines (X) across the page 6. Write the full question before attempting to write the answer to same

Q1. With respect to control air supply system for Main Engine Control: A. Define the essential conditions, which must be satisfied by the air supply for a pneumatic control system; B. Sketch a control air supply arrangement and give a reasoned explanation for positioning of dryers and filters

Answer:- Repeat question.Refer to Q 4 EKM FEB 2018.

Q2. Describe the phenomena of vibration in marine diesel engine. Explain the terms: (a) Transverse Vibration (b) Torsional Vibration (c) Resonance (d) The role of vibration dampers.

Answer:- The periodic firing or forcing torque is the resultant vector sum of the firing impacts in all the cylinders. The shaft system comprising of the individual rotors representing the corresponding reciprocating and rotational masses attached to the individual cranks,the fly wheel and the propeller with its mass of entrapped water, connected by the resilient shaft system, forms the resilient torsional vibrating system.

The shaft system is continuously vibrating in the torsional mode at any given RPM about the mean equilibrium position, even as the shaft is rotating. This can be sensed by the flicker or oscillation of the tachometer needle at any steady speed.

If the frequency of the natural first- order torsional vibration mode of the shaft system is say 270 VPM , then when the engine RPM is 45, a resonant condition persists at this speed, since the exciting frequency of the engine firing is the same as the natural first order frequency of the shaft system (6*45=270). At this speed the tachometer needle flickers heavily. Prolonged running of the engine at this RPM can lead to fatigue defects occurring on the shaft system. This speed is called the critical speed and the range extending from -5 RPM to +5 RPM of the critical speed is called the critical range or barred range and as per the classification society’s regulations , owners should place a caution notice at the engine operation station

Indicating the barred range. This speed range must be passed over quickly.

The second order critical or the 2-node mode of vibration may occur at a range from 80% of service speed and also upto MCR . The maximum angular amplitudes at this critical speed is verified at all antinode locations. If the angular amplitudes observed are higher than the permissible amplitudes , the classification society will recommend the fitting of a suitable detuner. The detuner alters the shaft characteristics and the resonant condition is shifted to a higher RPM, which the engine will not reach and hence safe.

AXIAL VIBRATIONS BY VARIABLE PROPELLER THRUST:-

Axial vibrations are excited in the crank shaft from the variable axial thrust created by the propeller. Axial vibration will create a reaction force in the thrust bearing which can be considered as an excitation source for the rest of the ship.Excitation due to propeller working in the non-uniform wake field will be transmitted to the hull through the shaft system as forces and movement or through the water as pressure fluctuation acting on the hull surface. The excitation can be reduced by identifying wake field and propellerdesign.TRANSVERSE VIBRATIONS caused by engine excitation.:-

Transverse vibrations of adjoining sub –structures close to the engine frame and transmitted by the bracings and foundation bolts cause the primary and secondary unbalanced couples to be transmitted as secondary excitation . Examples of secondary excitation sources from such structures would be transverse vibration of the engine structure, Engine room bulkheads and remote structures such as Radar masts

(a)Transverse vibrations:- It is mode otr direction of vibration which is across the plane of the vibrating body or component . An example is illustrated.

(b) Torsional vibration:- It is a mode or direction of vibration in the twisting direction of the component under torsional alternative force. Example given as under.

(c) Resonance:- Resonance is merging of the forcing amplitude and the amplitude of natural vibration of a mass-elastic restraining system when the following conditions are fulfilled:

* when their frequencies are equal

* when their amplitude vectors are in phase.

In this condition the two amplitudes are added to give a higher amplitude of vibrating to the vibrating mass.

(d) The role of vibration dampers.:- Vibration dampers are fitted to components subject to forced vibration to reduce their amplitude of vibration . The damper acts as a damping force against the amplitude and so reduces the amplitude of vibration. Q3. Sketch and describe the arrangement of a main engine camshaft chain. Describe the repair procedure following fracture of one chain link during operation of the engine. Give possible reasons for the failure and explain how the chain is set initially at the correct degree of tension.

Answer:- Repeat question.Refer to Q1 EKM FEB 2018.

Q4. With suitable block and line diagrams, describes the starting and reversing systems of a two-stroke diesel engine. Explain the importance of running direction interlock provided on Diesel Engines.

(a) Answer:- Starting :- The control air selection cock is to be put in the position selected ie either Bridge control or Engine control room.. The air starting is accomplished by putting the engine control lever to air start position. The main air auto start valve opens and allows main starting air to flow to each air starting valve on cylinder head through a common air manifold. The starting air also flows through a branch to the air distributor which connects to each starting valve at top to actuate the valve. The opening of Air start valve is approximately 140degrees to provide for a good overlap so that starting is accomplished in any position of the crankshaft. When the engine picks up speed the engine control lever is moved to fuel position and the engine starts on fuel. When control moves to fuel position the air to air starting manifold is shut by the main auto air start valve. This is a safety arrangement on the main air start valve which will not allow air to flow to the air start valves when the main engine is working on fuel.

Reversing the direction of rotation is done by moving the reversing lever to the required direction. Air enters the Reversing servomotor cylinder, actuating the piston connected to the fuel pump roller and air distributor roller actuating links.. the respective rollers are moved to the correct position for the direction of rotation. The sketch shown as under explains the action.

The running direction interlock is important because it prevents reversal occurring before thew engine comes to a complete

Stop. This lock therefore prevents any damage to the camshaft and attached components if reversal is forced before engine comes to a halt.

Q5. Describe the procedure of overhauling two stroke engine stuffing box, without removing piston. All safety precaution to be mentioned. Sketch and show all parts of stuffing box.

Answer:- The stuffing box is made of two parts joined by a flanged connection and this makes it possible to dismantle the box and remove the internal sealing and scraping elements as well as the garter springs for examination and renewal

The procedure of dismantling and the safety precautions are as follows;

· give atleast a 20 minute interval to open the crankcase doors of the unit IF ENGINE HAS JUST STOPPED.

· Afterhaving taken the necessary precautioins of entering an enclosed space , have the crankcase space cleaned of oil by swiping the surfaces. This precaution is necessary to prevent anybody slipping inside and injuring himself.

· ensure all the correct tools are handy available.

· The crank is to be taken to TDC. fix the support clamp in halves on the piston rod atleast 30cms below its present fitted position.

· The holding nuts of the gland box are opened out and the entire box pulled down till it rests on the support clamps.

· The flanged box casing is opened and taken off. The elements can be dismantled by opening the garter springs. care is needed in keeping the element sets separate so thart they can be fitted in the same way. all parts dismantled and examined. The defective elements, especially the packing elements, are considered worn out not fit for further use if the central lead lining becomes flush or levelled with the brass or steel element body. Such elements are to be renewed. The garter springs if found to be elongated should also be renewed . The scrapper elements if worn out will have their knife edges blunted. They have lost their effectiveness and should be renewed. The drain hole in the gland box should be cleared.

Q6. Discuss the significance of cylinder lubrication in two stroke diesel engines considering the impact of Annex VI of Marpol 73/78. Explain: (a) Two level cylinder lubrication incorporated on few diesel engines. (b) The effect of over and under lubrication on engines.

Answer:-As per Reg 14. 4 .3 ships operating in the emission control areas are required to use fuel oil containing sulphur less than 0.1 percent sulphur(m/m) for combustion in the main engines and boilers.

(a) This condition makes it necessary for ships to have two sets of storage, settling and day service tanks for storing and using the normal 3.5 percent sulphur and the 0.1 percent sulphur fuel oil.

When operating on fuel oil with 0.1 percent sulphur , it becomes necessary to change the grade of the cylinder lub oil to suit the low sulphur fuel oil. The cylinder oil suitable for the normal 3.5 percent sulphur in fuel oil has a higher alkaline content which if used with the low sulphur fuel oil will lead to deterioration of the liner material by alkaline brittleness or caustic embrittlement. Hence a lower grade TBN cylinder oil should be used for the engine when operating on low sulphur fuel oil. This therefore makes it necessary to have separate storage and service tanks for the low grade cylinder oil.

In addition to this problem, the fuel pumps need to be set to suit the low grade sulphur in fuel oil. The reason is that the low grade sulphur in fuel oil is made by blending a refined D/O by adding residual oil in the correct proportion to arrive at the correct 0.1 percent sulphur. This oil will have a higher cetane value and that must be determined to get the correct delay period and the pumps have to be suitably retarded to get the correct combustion setting.

(b) cylinder lubrication is essential for reducing the friction between the piston rings and the liner so that the relative motion between the sliding surfaces is resistant- free, and to some extent the heat generated by friction is reduced. This is the fundamental principle of lubrication. The type of lubrication is boundary lubrication which is appropriate for this motion because of the low mean piston speed and the moderate radial pressure existing between piston rings and cylinder liner.

Over lubrication reslts in excess oil depositing on the liner surface causing the oil to burn and leading to excess carbon deposition , which collects in piston ring grooves , resists the free movement of piston rings in their grooves and causing them to break.

Whereas under lubrication causes dryness of the liner surface resulting in scruffing and clover leafing at locations on the liner where oil has not spread. It also results in excess wear of the liner.

Q7. With reference to piston rings; A. Analyze the causes of breakage; B. How maintenance and engine operation can minimize breakage; C. Explain the possible consequences with respect to performance and safety of operating the engine with broken or severely worn rings.

Answer:- Refer to Q6 EKM JAN 2017

Q8. Fatigue is one of the main causes of crankshaft failure. A. Sketch and Indicate the most likely location of a fatigue crack; B. How a fatigue failure is identified; C. Describe initiation of a fatigue crack; D. Sketch and Describe the methods used to inhibit fatigue cracks.

Answer:- Repeat question repeated over two times.

Q9. With reference to mechanical/hydraulic governors: A. Why flyweights are driven at a higher rotational speed than the engine; B. How dead band effects are reduced ; C. How hunting is reduced ; D. How the output torque is increased.

Answer:- Repeat question repeated over two times