JUNE 2017

MARINE ENGINEERING KNOWLEDGE (MOTOR) TIME ALLOWED - 3 HOURS

Instructions 1. Answer any SIX questions. 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 5. All unused pages of answer script must be cancelled out by two lines (X) across the page.

Q1. With reference to torsional vibrations in a main propulsion installation based on medium speed engines, gearbox and controllable pitch propeller: Explain how the vibrations may be caused; State the possible effects and damage that could result; Discuss the methods employed to minimise the potential problems associate with torsional vibration; Describe how the natural vibration frequency of the system could be modified.

Answer:- Torsional vibrations exist as long as the Engine is running and this is caused by firing excitations of the Engine. The forcing frequency is given by N* RPM/2 vibs per minute for 4- stroke engines, since medium speed engines are usually 4-stroke engines. The natural frequency of torsional vibrations of the shafting system is dependant on following characteristics:

(a) Individual rotational masses of piston and part of con-rod

(b) Rotational mass of propeller and entrapped water

(c) Rotational mass of Gears in Gear Box(d) Torsional stiffness of shafting.

(a) In a non resonant condition , the amplitude of torsional vibration is negligible and becomes noticeable or sensed when it becomes partially resonant or fully resonant condition. When this happens the amplitudes of vibration become larger leading to fatigue conditions due to alternating stresses being set in the shafting. The non resonant, partial resonant , and fully resonant conditions are shown in the sketch given under

(b) Prolonged running in resonant condition (both partial and fully resonant conditions) may lead to fatigue conditions exhibited as defects

progressively starting from loosening of foundation bolts, damage to gear teeth in gear box and finally developing fatigue cracks on the shaft surface at critical locations.

(b) The methods normally employed to avoid resonant conditions, is to verify the power developed in each unit and ensure they are same for all units and as per makers data given in the operation manual.. This also includes verification of peak and compression pressures from individual draw cards, or pressure readings taken with a pressure recorder. These should also be same for all units. Suitable adjustments to fuel pumps rack controls and in some units where wear is excessive ,the piston rings and /or liners are to be renewed. The foundation bolts should be retightened when engine is not running. If these corrections are carried out the vibration levels will be within safe limits. Avoid running the engine with propeller pitch setting very close to the barred pitch range for long durations as that would lead to fatigue conditions. It should be noted that the amplitudes at critical speed are high and hence severe. Some Engine builders fit dampers in the crank shaft or cam shaft to reduce the amplitudes of torsional vibrations.

(c) The natural frequency of vibrations of the system can be altered by fitting a suitable

Detuner which will alter the frequency of natural vibration of the shaft system.

Q2. A. Why wear down in main bearings is critical to the condition of the crankshaft and propeller shaft system; B. Why total reliance is placed on frictional grip in conventional built up crankshaft; C. Why oil holes are given large fillets in crankpin and journals.

Answer:- (A) When the ship is newly delivered from the yard, the alignment of the crank shaft and the shafting which includes the propeller shaft and the intermediate shaft are in an ideal condition where by the reactions at all the main bearings are nearly the same when not running and during running the variation is cyclic and uniform. As the operating hours of the engine progresses wear occurs on the main bearing lower half bearings caused by the following:

Non uniform loads on the bearings caused by non uniform peak pressures and non uniform power developed in the units.Non -uniform wear of the bearings and journal surfaces caused by variation in the lub oil pressure at each bearing which may be caused by local conditions at the bearings caused by accumulation of dirt in oil passages and abrasive debris collecting between the rubbing surfaces. Both the above mentioned deteriorating factors lead to gradual stages of misalignment of the shafting system. This is a developing critical situation and needs regular monitoring and timely corrective action . The crank web deflections recorded at regular intervals is a

monitoring activity and when the deviation is above the permitted value the corrective action of renewing the concerned main bearing lower half is resorted to. If monitoring is not carried out conditions of fsatigue failure of the crank shaft and also of the bedplate transverse girders can easily develop.

(B) The large size crank shaft fitted on large slow speed engines can only be easily and conveniently manufactured by the assembly process. The components forming the built up crank shaft are made up of:

(a) identical webs

(b) identical crank pins.

(c) identical journals and

(d) the thrust journal with thrust collar and coupling flange.

The methods available for assembling these components at their correct angular sequence is by the following

(1)Moderately tight fitting with keys. (2)Tapered sliding fit connection with forcing tight

arrangement. (3)Bolted coupling flange arrangement. (4)shrink fit arrangement.

Of these methods mentioned the first three are cumbersome , uniform torque distribution not guaranteed and can be a source of concern for the propagation of fatigue cracks. Since this assembly is a permanent assembly for the life time of the engine, it

is worthwhile to adopt the shrink fit method which can be adjusted to give a uniform radial grip or compressive stress building up a anti slip force which can be much more than the normal torque transmitted at the service speed in moderate weather. The shrink fit allowance varies between i/570 to 1/650 of the nominal dia of the journal or pin.

(d) When an oil hole is drilled on the crank pin or journal it penetrates along the diameter of the pin or journal and the intersecting edge on the surface of the pin or journal is a square or right angled edge.

During operation at full load, the principal stress lines namely the tensile and compressive stresses acting on the surface of the pin or journal become dis continuous due to the sharp corner leading to a concentration of stress all along the edge and can become a source of development of fatigue cracks. By rounding or filleting the edge the stress lines are made to follow the curved contour of the surface thereby the concentration is dissipated. Hence filleting of the holes is done.

Q3. During the past 4 months since you have joined the ship as second engineer a number of main engine exhaust valves have suffered cracking and corrosion at the seating faces. Write a report to the superintendent covering the following points; A. An explanation detailing how the problem became evident. B. Your action upon recognising the extent and seriousness of the problem

C. Your reasoned views regarding the positive causes of the problem D. Your recommendations to avoid future incidents.

Answer:- Dear sir,

Please find attached a report on the four numbers of exhaust valves of the main engine which are having their valve seats and lids cracked during the last 4 months after I have joined the ship.

Report

(a)The problem became evident when all the four valves had their seats and lids burnt within a total of 240 hours of operation, when i got concerned and started investigating the problem . I carried out a detailed inspection of these valves after dismantling and cleaning the valves . i have observed that the cooling water spaces around the valve seats are heavily scaled .

I measured the external length of the valve cage and the depth of the water space from the same datum and observed that the residual thickness was high which allows for increasing the depth and thereby enhance the cooling of the valve seat. I have shown these facts in the sketches given under.

(b)If the heads and seats of exhaust valves become overheated they soon begin to distort and burn. To prevent this, conduction of the heat away to the surrounding media is relied upon; consequently, with large valves there are greater thickness of metal, the rate of cooling by conduction is slower and the process must be accelerated.

The cooling of exhaust valve seats becomes very necessary in engines burning high-viscosity fuel. This is due to the combination of the sodium and vanadium content in the fuel. Under some circumstances at high temperatures the sodium and vanadium content in the exhaust gases is very corrosive.

(1)Cooling space length as existing

(2) cooling space extended .

(c)It was observed that the cooling water passage up to the bottom of the valve cage can be extended by about 7mm which will enhance the cooling of the valve seat and lid. Accordingly I suggest and recommend that the valve cages cooling water spaces must be soaked by filling the spaces by a weak acid solution for a period of 24 hours so as to de-scale and de-sludge the spaces especially at the bottom. I further request you to advise the engine makers / valve makers to increase the length of the cooling water space by 7 mm.

Q4. Sketch and describe a common rail fuel system as fitted to a diesel engine. How is delivery of the fuel controlled.

Answer:- The electronic fuel injection system uses the common rail system to ensure that the injection is precise and at constant pressure. With the CRIS (common rail injection system), the high pressure pump delivers fuel to the rail which is common to all cylinders. Each injector is actuated in sequence by the ECU(Electronic control unit) as a function of the crankshaft angle..The injector opens when energised and closes when de-energised..The amount of fuel injected per cycle is determined by the time differential and the in-system pressure. The actual in-system pressure is transmitted to the control unit via a pressure sensor and the rail pressure is regulated by the ECU via the actuator of the fuel supply to the HP pump .For rapid load shedding the pressure regulator restricts the HP pump to a maximum of 1330 bar compared to the specified 1200bar

(c)The ECU has a provision similar to the rack system whereby the individual injector opening and closing times can be advanced and/ or retarded as required all within the range of milliseconds.

Q5. A. Draw an “out of phase” diagram for a large 2-stroke slow speed cycle engine. Describe in detail the part of the cycle showing the diagram.(B) Using the diagram explain the effects of burning liable and low grade fuels (when fuel injection timing is not altered). (C)Explain how good ignition and combustion can be obtained when using low grade fuel.

Answer :- (A) indicator or power card drawn by the indicator instrument when fitted on the cylinder head and the drum cord connected to the reciprocating lever giving the necessary movement to the drum. The stylus needle is controlled by the pressure in the cylinder.

To obtain the draw card or the 90 degrees out of phase card, it is usually done manually mentally counting the firing noise and synchronising the pull on the drum cord when the stylus is just touching the paper. The draw card is taken immediately after the power card on the same paper so that the data recorded is a true representation of the combustion quality i tne cylinder at that time

This draw card is a detailed view of the portion of the power card from say 100 before TDC to say 150 after TDC . this diagram distinctly shows the difference in the peak pressure and the compression pressure reached at end of the compression stroke, although their distance apart may vary which is dependent on the speed of pull of the cord. This variation is of no consequence , since we are only interested in the difference between the peak pressure and the compression pressure.

The difference between peak pressure and compression pressure corresponds and represents the delay period needed to efficiently burn the fuel so that the thermodynamic power extracted from the fuel is optimised.

Generally the delay period for refined fuels such as marine diesel oil or gas oil is less than the delay period

required for heavy fuels such as blended residual fuel or pure residual fuel.

Ignition Quality of fuel- The design of combustion chamber and rotational speed of the engine specify the fuel Quality that can be efficiently burnt in the engine cylinder. It is necessary that appropriate grade of fuel as denoted by the cetane number be injected in the cylinder. There is provision for adjustment of injection timing to suit a different grade of fuel to obtain better combustion result but this can be done only to a limited extent. A fuel with poor ignition Quality will burn with smoke and lacquer formation in the engine.

When a low grade oil is burnt in the cylinder of the engine adjusted for burning a superior grade of oil, the combustion quality obtained is considerably diminished and results in lower peak pressure as well as a power card with a smaller area( less pwer. The difference is represented in the diagram given under.

(B) When an inferior fuel is injected in the cylinder adjusted for burning superior grade fuel, the delay period

available is insufficient for obtaining the correct peak pressure which results in the lower peak pressure

The delay period is controlled by the injection timing set on the fuel pump which is the angular duration of the fuel delivery .

(C) When burning low grade fuel especially residual fuels, the oil is to be properly purified and clarified so that water ,sludge, and catalytic fines are removed resulting in a proper purified oil with a higher viscosity. The viscosity is reduced by suitable heating to result in the correct viscosity so that the atomisation of the fuel resulting in mist formation is achieved. The fuel pumps injection timings are set to correspond to the fuels cetane number which determines the delay period.

By doing these adjustments , proper combustion of the fuel is obtained

Q6. A. Sketch and describe a flywheel that would be fitted to a large marine diesel engine; B. Show in the sketches how it is fitted and secured. ; C. What is the purpose of the flywheel? Recently some engine makers have considerably reduced the size of the flywheel. Explain how this can be done.

Answer:- The fly wheel fitted on the transmission shafting system evens out the RPM within a percentage of the mean RPM expressed as + or – of the mean RPM. The fly wheel acts as an energy accumulator which supplies kinetic energy to the shafting system when the instantaneous driving torque is low and absorbs kinetic energy from the energy supplied when the instantaneous driving torque is high. In the

ship propulsion system the propeller with entrapped water is the real fly wheel , What is commonly called a flywheel is really a geared turning device to position the crank as desired for carrying out some adjustments and other maintenance work, The sketch shown below is the sketch of the flywheel showing how it is fitted on the shaft.

Modern engines are fitted with higher powered turning gear motors producing larger torques, This enables the engine to be turned at a higher speed utilising a smaller diameter fly wheel.

Q7.Explain why the residual fuels for the operation of large slow speed or of medium speed engines may be responsible for the

following problems with T/C nozzles, shrouds and blades and how in each , the problem may be minimized – A. Build-up of deposits; B. Hot corrosion; C. Erosion.

Answer:- (a) The burning of residual fuels in marine engines is now a very common practice leads to build up of carbon deposits on the components of the turbocharger. Residual fuels contain approximately 10% carbon residue M/M.. When combustion is not optimum it leads to build of carbon deposits on the components of the T/C. This occurs more prominently during slow steaming especially during long pilotage and harbour steaming.

In the burning of carbon the stages of combustion can be expressed by the following chemical equations:

(1) C+ O2 = CO2 -------Good combustion

(2) 4C+ 3O2 = 2CO+ 2CO2 -----Fair combustion

(3) 5C + 3O2 = 2CO + 2CO2 +C ---- Poor combustion

The third category , poor combustion, causes carbon build up The problem can be minimised by keeping the combustion equipment such as fuel injectors and fuel pumps in good condition and this is carried out by regularly overhauling injectors and verifying fuel settings in fuel pumps are correct.

(b) Hot corrosion :- Vanadium is the major fuel constituent which influences hot corrosion. It cannot be removed in the pre treatment process. It combines with sodium and sulphur during the combustion process to form Eutectic

compounds with melting points as low as 5300 c. Such molten compounds are very corrosive and attack the protective oxide layers of steel exposing it to corrosion. Vanadium is introduced into the residual fuel as catalytic fines in the process of refinement of crude. These fines remain in the fuel even after the refinement process. It can be removed only by running a clarifier in series with the purifier, and by running the combination with a low throughput to increase the quality of purification. This is the remedial process.

Erosion :- E rosion is caused by un burnt particles in the fuel carried with the products of combustion forming the exhaust gas striking the nozzles, shrouds and blades during its passage causing erosion of the components. The remedial measures is to increase efficiency of purification of the residual fuel. This is done by reducing throughput and using a clarifier in series with the purifier.

Q8. A. Describe the method of setting the safety valves of an exhaust gas boiler at sea. State the limits in terms of %age above maximum design working pressure for setting safety valves. B. State the formality necessary when Chief engineer sets the safety valves.

Answer:- :- (A) The valves have to be set as soon as the ship moves into full -away mode and the boiler is pressurised by the heat of the exhaust gas from main engine. The valves should be assembled without the caps and easing gear. The procedure is as follows :- (1) Keep the steam stop valve from the exhaust gas boiler shut. This will enable the steam pressure to be built up fast.

(2) One of the valves should be gagged using gagging tool (3) When the pressure comes up to the set value listen to the hissing sound from the valve as steam is escaping. If not slightly slacken the compression nut till sound is heard. (4) Change over the gagging tool to the other valve and repeat the same operation with this valve. (5) When both have been set go outside and observe the flow of steam from waste steam pipe, which must be a full flow. Come to the boiler platform and note the steam pressure on boiler. It should show a slightly higher pressure than set value. The important point is that the pressure shown on the gauge is steady and not fluctuating . It should also be understood that the set value can have an error of + or - 3%.

Refer to the boiler manual for the safety valve set pressure. This pressure is normally about 0.5 kg/cm2 lower than the auxiliary boiler working pressure. The exhaust boiler may now be connected to the steam range or connected to the steam space of the auxiliary boiler as the case may be. (B)The exhaust gas boiler safety valves cannot be set by the class surveyor when the ship is sailing, hence this activity is delegated to the chief engineer to be carried out. A documentary proof has to be provided to the class to update their records. Hence the chief engineer should send a letter to the office to be forwarded to class stating that the exhaust gas boiler safety valves have been set to the pressure as per the manual. A copy of this letter will be forwarded to the class Q9. Describe in detail the causes and conditions that; A. Create an explosive atmosphere in a crankcase; B. Initiate a primary

explosion. C. Propagate a secondary explosion; State how hazardous conditions can be avoided.

Answer:- (a) The development of conditions favourable for a crank case explosion are to be noticed in the time the engine is warmed up and during the time the engine is continuously running. The Main engine jacket cooling water is initially warmed up by circulating the Aux engine cooling water through the main engine jacket for at least six hours before controls are tested. The aux engine cooler is by-passed during this operation so that the main engine is warmed up quickly. The heat from the jacket travels downwards heating the crankcase. The air within the crankcase warms up. About one hour before control testing the main lub oil pump is started. The oil squeezing out of the bearings forms a fine spray and the droplets of oil mix thoroughly with the warm air making the mixture fit for an explosive condition. All it needs is a heat source to ignite the mixture of warm air and oil vapour. This condition is maintained constant throughout the main engine running period.

(a) (b)When the engine is running continuously, the initiation of an explosion can occur caused by an overheated bearing , overheating of the piston rod rubbing against the packing element or spark generated from chain guides in the chain casing. In trunk piston engines it can be caused by Blow- Back from a cylinder with worn piston rings.. Hence it is the hot spot or ignition source which is the third element of the FIRE triangle to start the explosion.

(a) The secondary explosion is propagated when the primary explosion has created an opening in the crank case through which air can enter the crankcase and cause an explosion which is more severe because the explosive mixture within the crankcase becomes more richer. Hazardous condition is avoided by the crank case being provided with explosion doors which prevent the occurrence of a secondary explosion by the door shutting smartly by spring action after the first explosion. Thus the secondary and severe explosion is avoided