MARCH 2018

MARINE ENGINEERING KNOWLEDGE (MOTOR) TIME ALLOWED - 3 HOURS

Instructions 1. Answer SIX questions only. THREE

Questions from each section 2. All questions carry equal marks.

3. Neatness in handwriting and clarity in expression carries weightage

4. Illustration of an answer with

clear sketches / diagrams carries weightage

Q1. With reference to a particular make of main propulsion unit, describe how the engine is reversed manually and discuss the problems involved and the safety precautions which would be required if the control were operated remote from the machinery space.

Answer:- The engine is reversed manually in the MAN B&W engine by an actuating lever connected to a screwed spindle which can be connected to the actuating link for emergency use and manually operated from the local maneovering station. The system is shown i in the sketch given under .

(b)When the control is operated remote from the local emergency control station say either the Bridge or Engine control room, the reversing interlock which is activated when engine starts to turn can only be unlocked when engine stops. In this condition the reversing lever can be moved to the desired direction. This protective device is provided to safeguard the camshaft and the cams mounted on the camshaft. This also provides safety to the engine components if reversal is accomplished whilst turning has not stopped . In extreme cases it can cause slippage of journal web shrink fit.

PLAN AT XX

Q2. Discuss the various types of indicator diagrams and discuss the relevance of this method of determining engine performance. B. Suggest how engine performance may be assessed other than by taking indicator cards.

Answer:- The various types of indicator diagrams taken using a mechanical indicator are;

(1) Power card.(2)Draw card(3) Compression card (4) Light spring diagram.

Of the above four cards normally the power card and the draw card are recorded , whereas the compression card and light spring card are taken only if there is a suspected problem with compression and or scavenging as well as exhaust operations.

The draw card is super imposed on the power card and its speciality is that it distinguishes clearly and distinctly the peak pressure from the compression pressure., which is normally not clearly distinguished in a power card. The four cards are shown in the sketch given under.

The area within the power card indicates the work done in one cycle or one revolution in a two stroke engine.

The power produced in each unit is determined as follows and is termed the indicated power :

(1)The area of the diagram is determined by a planimeter or by Simpson’s rules and is given in mm2 . This area when divided by the length of the

ATMOSPHERIC LINE

PEAK PRESSURECOMPRESSION PRESSURE

diagram in mm gives the mean effective pressure stated in mm.

(2) The mean height of the diagram in mm( area of diagram /.length of diagram in mm) multiplied by the spring constant given in KN per M2 per mm of the mean height of the diagram. This figure is indicated BY P KN/M2

(3)The length of stroke of the engine is found from the engine manual and given in meters.

(4) The area of the bore of each cylinder is given in the manual as diameter, from which the area of the bore is calculated. And given as A meter2.

(5) N is the calculated mean RPM determined from the difference of counter readings taken at end of experiment and the commencement of experiment and divided by the time interval in minutes. The time interval is arbitrarily chosen as one hour or one and half hours .

(6) The power developed is given as P*L*A*N *2 π KNM per sec or KW 60

The total power developed is given as the sum of the individual powers developed in each unit.

The specific fuel oil consumption is s calculated as follows:

The fuel oil meter counter reading is taken at commencement of experiment and end of experiment. The difference between these readings is the quantity of oil consumed in the time interval chosen. This quantity is indicated in litres as given in the consumption meter. The quantity in litres is converted into kilograms by multiplying the quantity of oil consumed during the chosen interval by the sp gravity of the oil corresponding to the temperature of the oil noted during the experiment.

The S.F.O.C is now found by multiplying the total power developed by time interval in hours and dividing the product by quantity of oil consumed in the same time interval. as grams per KW hour.

(a) In Aux Engines which run at medium speeds between 700 and 1000 RPM, a mechanical indicator drum actuator will not work at that speed since the drum spring return speed will be less than the engine rotation speed. Hence the indicator is used as a pressure recording device and pressure bar diagrams are recorded for each cylinder taken on the same card for easy comparison. The pressures are recorded for two conditions;

(a)With fuel cut out for the unit, which gives the compression pressure.

(b) With fuel on for the same unit, which gives the peak pressure.

This card is mainly taken for comparing the individual peak and compression pressures and for making suitable adjustments on fuel settings of each fuel pump so that the pressures in all units are the same. Which indicates that the power developed in each unit is almost the same. And the engine is performing satisfactorily Satisfactory performance is also judged from exhaust temperatures. If a cylinder is overloaded it will exhibit high exhaust temperature. If a unit is not developing sufficient power it will be noticed by a low exhaust temperature..

The load measurement or power developed is the KW reading . The S.F.O.C. calculated is based on electrical load.

Note+-

Given below is the sketch of a mechanical indicator instrument. This need not be sketched in the exam for this question. It is for understanding the instrument,

Q3. With reference to large starting air receivers: A. Explain where corrosion is likely to occur and state why it occurs in these regions: B. State the how the incidence of corrosion in air receivers might be minimized; C. If serious corrosion is detected in a starting air receiver and that receiver must be used explain how you, as Second Engineer, would determine the maximum pressure to which the receiver should be subjected; D. State the further action a Chief engineer must take upon discovering such air receiver corrosion.

Answer:- The air within a starting air receiver contains air with some percentage of moisture or water vapour. Water vapour enters the compressor along with air from atmosphere , and although drained from intercoolers ,some water vapour finds its way into the receiver, ,condenses and collects near the drains.. The drains are situated in small depressions or wells where the drained water accumulates.. Corrosion mostly occurs in these locations and is caused by air and moisture.

(b)Corrosion conditions might be minimised by internal examination once every year , cleaning and scraping the corroded area and then coating the affected area with a suitable anti corrosive polymer paint.. sufficient time should be given for the paint to dry, before the air receiver is closed and put into service.

(c) If an area shows excessive signs of corrosion which would be noticed by heavy pitting in these areas, the minimum thickness in these areas should be gauged using a portable digital U.S. thickness gauge. The maximum pressure the receiver should be subjected to is by applying the Cylindrical Pressure vessel formula reduced to the condition as needed ,given by :- P0 / P1 = T0 / T1

Where P0 is the design or working pressure and P1 is the new set pressure.. T0 is the design thickness and given in the drawing. T1 is the thickness found out.

In deciding the minimum thickness many readings must be taken in the affected area and the least thickness applied.

(a) The further action will be to take assistance of a workshop specialised in boiler repair work. The area affected should be cleaned thoroughly and using compatible electrodes as recommended by Class surveyor. The area to be built up by welding should be pre heated to 1000 c whilst welding is on. The welding should be carried out by a Class approved welder as recommended and witnessed by Class surveyor.. After repairs the area should be ground smooth and painted with approved polymer paint before the receiver is put in service. The receiver can now be hydraulically pressure tested to the working pressure before put into normal use. .

Q4. Describe how a crank pin bearing of a 2-stroke main propulsion engine is opened up for inspection B. Which half of the

bearing is subject to greater wear? C. Which are the various causes of wear down of the bearing.

Answer:- Precautions :- (1) At least 20 minutes must elapse before crank case doors are opened ,if the engine is working.

(2) Obtain propeller clearance from deck department till completion of the job.

(3) Carry out safety check list for C/C entry.

(4) Thoroughly clean clean crank case free of oil, so that no one slips inside whilst working.

Preparation :- (1) Attach lifting gear at correct locations to dismantle bearing half and to remove it out of crank case.

(2) Ensure hydraulic opening tools are in good order.

Procedure :- Before dismantling the bearing , the bearing clearance is checked with crank at bottom centre using long feeler gauge.

(2) Take crank to top centre,attach hydraulic unfastening device and slacken the holding nuts. Support the lower half bearing on the attached chain blocks and lower bearing to bottom of crank case. Transfer the lower half bearing outside crank case using a chain block attached to support outside the crank case. Refer to sketch FIG:1

(3) Fit Guide bars on both guides to prevent the cross head from lowering whilst turning the crank.

(4) Wrap a sling around the bottom of con-rod and attach to a chain block supported on side of crank case.. Refer to fig (2)

Turn enginein the direction of the attached sling, while pulling the sling using the chain block till the top half bearing clears the crank pin.Ensure top half of bearing is locked tight on con-rod palm so that the tpo half will not fall off.

(5) The crank can now be turned to bottom so that the top half of bearing is fully exposed . Refer to fig (3). If needed it can be removed from con-rod by dismantling the lock plates..

FIG: 1

FIG:2

Q5. A. State, with reasons, THREE properties required of crankcase oil which is to be used for a trunk piston main engine. B. Explain how a representative sample of crankcase oil would be obtained from a trunk piston engine. C. Briefly describe the action

to be taken if the crankcase oil charge cannot immediately be replaced and analysis shows – (i) Water is present; (ii) Alkalinity has fallen; (iii) Viscosity has changed appreciably; (iv) Carbon content has increased.

Answer:- (A) The three important properties a crank case oil for trunk piston engines are:

(1) A high viscosity at ambient temperatures.(2)A high TBN value to prevent acidic corrosion.(3) Good dispersant quality ie impurities should not be

dissolved in the oil so that they are easily separated or filtered.(B) the procedure of removing a representative sample is as follows: * The smplae is to be removed from the discharge side of the lub-oil pump from a drain cock fitted on the level section of the pipe. * take out about two buckets of oil and empty them in the sump, after which the sample should bre taken in the clean bottle with proper tight lid provided by the lub-oil supplier and the labrl filled with correct information as asked for.

(c) Water is present :- Continue purifying thr oil till water is completely removed. The purification must continue even when the engine is not running. Alkalinty has fallen :- Top up with fresh oil till alkalinity level is in safe level Viscosity has changed appreciably:- If it has reduced it might be caused by fuel dilution. Trace the source of fuel oil leakage and repair suitably. If viscosity has reduced by emulsification with water, the oil will be transferred to a settling tank ,heated for over 18 hours , till separation between oil and water has occurred. This can be found by testing the discharge from settling tank. If water is draining out ,

it means the separation is more or less completed. The oil water mixture must be continuously purified till water is completely removed.

Increase of carbon content :- This is caused by high temperature of the return oil, which means the oil needs to be further cooled .In trunk piston engines accumulation of carbon occurs in the crank case caused by slack piston and scrapper rings in any of the units. Such units should be opened up and after verification defective rings must be renewed. The oil has tobe continuously purified and filtered. If needed new filter cartridges must be used if the system has throw away type filter cartridges.. The crank case oil temperature must be maintained lower

Q6. Evaluate the influence of the following factors upon cylinder and piston ring wear rates; A. Position of rings in relation to piston crown; B. Spread and proximity of coolant passages from liner wall; C. Flow rate and specific heat of coolant; D. Chromium plating of ring faces.

Answer:- (a) Piston ring scuffing is aggravated by high temperatures and liner surface finish. The gas side liner surface temperature at upper end of top ring travel is to be kept below 2000 c, if scuffing has to be avoided. This limiting factor decides the distance between piston crown and first groove. Generally high surface temperatures increase wear rate by scuffing. (b) The quantity of heat transfer for given period of time is the rate of heat transfer . If this quantity of heat transfer rate is denoted by Q, then; Q varies directly as the temperature difference ie difference of temperature between the liner wall and temperature at outer wall of liner, and inversely as the thickness of the liner wall. The temperature gradient is given as the ratio of temperature difference with wall thickness. Thermal stress is the internal

resistance of the material grain structure to resist the free expansion caused by addition of heat, and thermal stress varies directly as temperature gradient. The proximity of cooling passages from liner surface is therefore based on two factors.: (1) The required temperature gradient to give a suitable limiting thermal stress (2)The mechanical strength to overcome the high pressure inside the cylinder. Hence a suitable thickness is selected to suit the selected temperature gradient, and give adequate strength to overcome the high pressure.

(c)The flow rate signifies the rate of heat transfer, whereas the specific heat of coolant specifies the heat carrying capacity for unit mass of coolant flow. Hence both factors are important in the choice of the coolant..The flow rate is decided by the cooling pump capacity and its delivery pressure

(d) Chromium plating of ring faces is carried out by electro plating process and is still a widely used coating and provides the best compromise for ring and liner wear. It increases surface hardness and thereby reduces wear

Q7. With reference to main engine turbocharger; A. Explain why cleanliness throughout the turbochargers system is critical to engine performance? B. Describe in-service cleaning procedure for gas and air sides of a turbocharger indicating safety precautions to be observed?

Answer:- (A) The fuel oil being burnt in a marine diesel engine used for propulsion as well as electrical power generation, burns blended residual fuel which contains a small percentage of sulphur , now restricted to 3.5 percent by mass in accordance with MARPOL Annexe 6 , Tier 2. This sulphur burns to sulphur-di-oxide and combines with water vapour formed by burning of

hydrogen in the fuel . This reaction forms a weak sulphuric acid and deposits at lower temperatures on the turbine blades and nozzle blades causing corrosion. Apart frpm this aspect the residual fuel oil contains catalytic fines consisting of sodium and vanadium compounds which burn and form eutectic compounds of sodium and vanadium which has a low melting point (5300 c ) which causes hot corrosion of T/C components.

(B) When water washing the air side ie the compressor side the following safeguards must be observed.:

(1) Use pure distilled water for washing.

(2) The turbine speed should be lowered sufficiently by reducing engine speed or load.

(3) The water flow should be controlled and drained out from the air cooler

(c)The disadvantages of water washing the gas side is corrosion occurring on the components of the turbine and nozzle. Hence dry washing of gas side is now recommended . The material used is dry kernel shells of small nuts such as almonds and pistachio.

Q8. State the reasons for the progressive slackness of an engine camshaft drive chain in service; B. State the effect chain stretch has on engine timing and performance; C. Describe how correct timing is restored when chain stretch becomes excessive.

Answer:- (A) The reason for stretch is the cumulative effect of individual link stretch along with wear of the pins and the sprocket teeth

(b) The effect of chain drive stretch causes elongation and slackness of the chain. This results in chain vibration and non uniform motion of the camshaft caused by jerky movement of the

cam shaft wheel which is driven with alternating slackness and tightness. The stretch causes a slight change in timings of fuel pumps as w ell as the exhaust valves.. The jerky motion causes impact of chain links against chain guides.

(c)The chain stretch condition is normally checked every 4000 hours of working of the engine. The slackness is measured by measuring the sag on the slack side of the chain. It should be between half link length and one link length. The method of measuring the sag is shown in the sketch illustrated below.

The chain is tightened by slackening the checknuts of the jockey wheel and turning the jack screw to tighten the chain to the required tightness as stated above. When tightening the checknuts the spring compression should be checked and maintained to the same length as indicated by the length etched on the chain frame

(a) The effect of tightening the chain leads to a small shift of the camwheel, thereby altering the timing slightly. This effect has to be corrected and that procedure is shown in the next sketch

The tolerance allowed in shift is within 2 degrees. This operation is repeated for the other half of the camshaft with the same reference of number one unit at TDC.

Q9. With reference to large fabricated bed plates explain: A. With reason, why longitudinal strength and rigidity is important in spite of the contributions made by ship’s structure. B. With sketches show how the combustions loads imposed on piston and cylinder heads are transmitted to and absorbed by bed plates.

Answer:- Repeat question. Refer to Q6 EKM JAN 2018