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Q.67 What is the function of the main thrust bearing?

Ans. (b) Transmitting propeller thrust to the hull.

In case of thrust bearing, there is a thrust collar and a pad. Due to the power produced by the engine, axial thrust is produced because the power is transmitted to the propeller shaft and the load is taken by the propeller blades. This loading of the propeller blades will try to push the propeller inwards and along with that the thrust will be transmitted in the form of axial thrust to the propeller shaft and then to the crankshaft. This thrust will be transmitted to the ship side through the collar which is an integrated part of the propeller shaft or the crankshaft and the pads which are fitted on the forward side during the forward running and to the aft side during the astern running. The thrust is transmitted to the ship’s hull as this is useful which helps in moving the ship in the desired direction.

Q.68 The main function of compression ring is to

Ans. (c) seal the space between the piston and the liner.

High pressure gases due to combustion enters behind the piston rings. This pushes the piston rings in the outward direction to have a good contact between liner and the piston rings. At the same time, piston rings are also pushed downwards on the piston ring landing grooves. Thus it forms a complete sealed space above the piston rings in between the liner and the piston. Hence the force of gas behind the piston rings which forces the piston rings outwards is of prime importance. If the piston rings are seized or the grooves are not clear i.e. it is obstructed with carbon, there is a possibility the piston rings will not have proper contact with the liner and blowpast will take place which will cause breaking of piston rings and scuffing.

Q.69 The function of diesel engine piston compression ring is to

Ans. (c) to transmit heat from the piston to the cylinder liner.

As the cylinder liner is better cooled off than the piston and in order to keep the piston and the piston rings in cooled condition to have better lubrication between piston rings and the liner, it is necessary that the piston rings should be kept as cool as possible. When the piston ring is in contact with the liner, some amount of heat from the piston is transmitted to the liner via the piston rings thus keeping the piston crown and the rings in cool condition, thus promoting the lubrication between the liner and the piston rings.

Q.70 Diesel engine piston ring gaps can be straight-cut or angle-cut. In compression rings the angle cutting

Ans. (d) decreases the combustion gas leakages.

Due to the angle-cut rings and if they are left and right cut, if they are fitted alternatively, the resistance offered for the gas to flow through the gap between piston rings end i.e. butt clearance, is more. Hence less leakage takes place from the top of the piston rings to the bottom of the piston rings through the gap between the piston rings. Hence angle cutting is more preferred than straight cut. But in angle-cut rings, the joint at the cut position will be slightly weaker than the straight-cut rings.

Q.71 Modern marine diesel engines equipped with mechanical fuel injection operates on combustion space within the cycle which is

Ans. (c) combination of constant volume and constant pressure.

In the mechanical fuel injection system, when fuel is injected the piston is approaching TDC, but the fuel is not injected exactly at TDC but much earlier than TDC. During this time, the volume is changing. At the same time, when the piston is at TDC when firing takes place though the pressure is increasing but as the piston is moving downwards the volume increases and the pressure remains constant. Thus there is a combination of constant pressure and constant volume which helps to have a better thermal efficiency as compared to other fuel injection systems.

Q.72 Which of listed design is successfully used to provide the turbulence necessary for proper combustion in the diesel engine cylinder?

Ans. (c) Precombustion chamber.

The precombustion chamber is a part of the cylinder head where fuel is injected and combustion takes place in this chamber. This chamber is made up of spherical shape and because of the geometry of the chamber better turbulence is achieved. If the fuel is injected in this condition, it is burnt effectively and thus causing better combustion and thermal efficiency increases. These engines were used in earlier days but at presently they are not utilized because the geometry of the combustion space is made in such a manner including the piston top so as to have a turbulence created inside the main cylinder head rather than having precombustion chamber.

Q.73 The purpose of counter-boring the top of the cylinder liner extending to the top of point of travel of the top compression ring is to

Ans. (c) prevent wear of the liner to form ridge at the upper level of the ring crown.

Counter-boring is the word utilized to give the tapered shape to the top of the cylinder liner. Because of the tapered boring the formation of ridge is reduced or it will not take place as there is gradual change in the diameter of the liner. Hence if there is any ridge formation it will be removed and any metal will not be allowed to form a ridge. If ridge formation takes place there is a possibility that the piston rings will hit the ridge which may cause the rings to crack and the liner to break.

Q.74 The most rapid period of fuel combustion in a diesel engine cycle should begin just before the piston reaches the TDC and

Ans. (d) should be completed just after the TDC.

The maximum possible efficiency of any diesel can be achieved if the most of fuel is burnt near the TDC. In order to burn it near TDC, it is necessary that it should be injected before the TDC so that combustion mixture can be formed by the TDC. At the same time, complete combustion should be carried out before the expansion stroke begins so that no power is lost. If the combustion continues in the expansion stroke, it is called afterburning, in which there is loss of power and high exhaust temperature which gives lot of problems to the engine and hence result in loss of thermal efficiency. Taking into consideration all these things, it is necessary that the completion of the fuel burning should be shortly after the piston crosses the TDC position and should not be continued longer than this.

Q.75 Exhaust gas pyrometers are useful for

Ans. (a) detecting faulty combustion in an individual cylinder.

Pyrometers are fitted on the individual cylinders. The exhaust gas temperature is a direct indication of any fault in the combustion process. If the combustion is late, we have high exhaust temperature and low power developed. If the fuel injectors are dripping, it will be revealed by high exhaust temperature. Also, any exhaust valve is leaking it will be also indicated by high exhaust temperature. This all indicates that the exhaust gas pyrometers fitted on individual cylinders will give the complete picture of health of combustion process. Also if the fuel nozzles are choked or if the rack position is not proper or low in position, it will also indicated on the pyrometer by low exhaust temperature. Also in case of liner crack near the top dead center position, because of water ingress the exhaust temperature will drop. Hence considering all these problems which can be indicated by the use of exhaust temperatures it is necessary to installed an individual exhaust gas pyrometer on each cylinder. This can be utilized as a diagnostic tool.

Q.78 Oil control rings functions to

(a) allow proper lubrication of cylinder and compression rings.

(b) reduce the amount of lubrication oil burnt in the combustion chamber.

(c) scrap the excess oil from the liner on the down stroke.

(d) All of the above.

Ans. (d) All of the above.

Oil scrapper or control ring’s job, in case of four stroke engines, as their edges are in downward direction is to scrap down the excess oil splashed onto the liner so as to prevent the excess lubricating oil consumption and burning of the excess lubricating oil along in the combustion process. Hence excess cylinder lubrication oil is scrapped. In case the oil control rings are fitted wrongly, the lubrication oil consumption will increase and the turbocharger and the exhaust manifold will get fouled.

Q.78 The main function of the oil control rings is to

Ans. (b) reduce the amount of lubrication oil burnt in the combustion chamber.

In case of trunk type four-stroke engines the cylinder lubrication is carried out by splash lubrication. Hence large amount of excessive lubrication oil is carried to the liner. This excess lubricating oil from the crankcase which is coated onto the cylinder liner should be scrapped off so that only required amount of lubricating oil should be available for lubrication and sealing between the piston and liner. If the excess oil is not scrapped, it will get burnt in combustion space resulting in turbocharger and exhaust passage fouling and piston ring getting seized in the grooves due to excess carbon formation. In order to avoid all these problems, it is necessary to scrap the excess oil. This is done by the oil scrapper rings with sharp edge pointing downwards so that when piston comes downwards the oil will be scrapped from the liner and the scrapped oil will be dropped in the crankcase.

Q.80 What is commonly used to create turbulence in a diesel engine combustion system?

Ans. (a) Shape of the piston crown.

As the air inlet valve opens, the piston is near TDC and exhaust valve also opens during the overlap period. The air entering from the air manifold through the air inlet valve hits over the crown and drives the exhaust gas from the cylinder through the already opened exhaust valve. As the incoming air is hitting the piston crown, the shape of the piston crown very important to create the flow patterns of the air which will be ultimately useful in creating turbulence for the better combustion of the fuel. Thus the shape of the piston crown is of prime importance. This shape will form a spherical combustion space which have better combustion efficiency and better turbulence. Hence thermal efficiency will increase. In case of two-stroke engines, the liner scavenge port configuration and angle at which the scavenge ports are cut is also of importance to create the turbulence.

Q.80 How many power strokes per crankshaft revolution are there in an 8-cylinder two-stroke cycle diesel engine?

Ans. (d) 8.

As it is two-stroke engine, for every revolution there is a power stroke per cylinder. As there are 8 cylinders there will be 8 power strokes per revolution. Hence there will 8 strokes of power.

Q.81 To shutdown a diesel engine after it exceeds the set maximum speed which type of device listed below is used?

Ans. (c) Overspeed trip.

It is necessary for the engine to trip if the speed exceeds beyond the set speed as the inertia and the centrifugal forces created because of high speed of the engine will give large amount of stresses onto the moving parts which may cause the failure of the moving parts and bottom end bearing bolts. Hence to avoid these damages, it is necessary that as soon as the speed increases beyond the preset limit the fuel should be cut-off and the engine should be made to stop. This is achieved either mechanically or electrically depending upon the engine and the type of injection system fitted. Mostly it is at 10% above the normal running rpm. If the engine trips because of overspeed, the restarting of the engine is possible only after resetting the fuel lever to the start position and then starting the engine. This is provided so that continuous stopping and starting of engine is prevented in case if the engine trips and overspeeds.

Q.82 Turbulence is created in the cylinder of diesel engines to

Ans. (b) help mixing of fuel and air.

Turbulence is the swirling movement of the air due to which the air sweeps the complete volume of the liner and the combustion space. This causes the complete and efficient sweeping action thus leaving behind no exhaust gases from the previous stroke. Also because of the sweeping action, the fuel will be properly mixed with the air and complete air in the combustion chamber will be available for combustion rather than forming isolated pockets of combustion. This helps in better mixing between fuel and air and hence better combustion and hence thermal efficiency increases and fuel consumption decreases.

Q.83 To function properly oil control rings used on diesel engine must distribute sufficient oil to all cylinder parts and also

Ans. (b) prevent excessive lubrication from reaching the combustion space.

The oil control rings or scrapper rings are used in the trunk type of diesel engines so that excessive lubricating oil which is splashed on the liner walls should be removed and drained back into the crankcase so that excessive lubrication will not get carried away by the exhaust gas into the exhaust manifold and turbocharger which may otherwise get fouled because of the lubricating oil burning in the exhaust piping and the combustion space. As this oil cannot withstand high temperature, it will get burnt and form large amount of carbon deposits. These carbon deposits will cause the piston rings to seize or turbocharger efficiency to drop.

Q.84 An important design characteristics of an explosion relief valve of a diesel engine is its ability to

Ans. (a) close quickly inorder to prevent inrush of air.

Explosion relief valves or crankcase relief valves are non-return flap valves which opens against the spring pressure due to the formation of excess pressure inside the crankcase. When the pressure reaches in the region of about maximum upto 0.2bars, these valves must open and let the crankcase gases to get relieved into the engine room atmosphere. At the same time as soon as this release is carried out, the spring should close this relief valve onto the seat. In case of crankcase explosion, viz. the primary explosion and secondary explosion, if the primary pressure wave created is not allowed to relieve, it will damage the crankcase and the personnel. This primary explosion leads to the secondary explosion which takes place due to ingress of fresh air into the crankcase from the atmosphere. If the ingress of air in the crankcase is prevented after the primary explosion in such condition a major secondary explosion can be avoided. Hence inorder to relieve the excess pressure generated into the crankcase and to prevent the ingress of fresh air into the crankcase to avoid secondary explosion, the crankcase relief doors are made up of non-return type.

Q.85 In modern diesel engines which of the following is used as a support for main bearings?

Ans. (a) Bed plate.

In most of the modern engines, in case of conventional crankshaft, the crankshaft rests on the bottom half of the main bearing. The bottom half of the main bearing is resting over the saddle created in the bedplate. Thus the complete load of the engine will be transmitted via the main bearings to the bedplate and from there to the ship structure. But in case of undershrunk crankshafts, the crankshaft will not be resting over the bedplate but it is overhanged or undershrunked from the bedplate. This is useful for the easy removal of the main bearing for inspection and replacement.

Q.86 Turbulence in the cylinder of two-stroke main propulsion diesel engine is mainly created by

Ans. (d) intake port design.

In case of two stroke engines, having the scavenge ports cut in the cylinder liner, the air enters through the scavenge ports. This air is responsible for sweeping clean the cylinder liner of any remaining exhaust gases and creating turbulence in the liner and combustion chamber. The entry of air through the scavenge port is made at some angle with respect to the circumference of the cylinder liner. In such condition, turbulence can be created in the cylinder liner and combustion space. Hence to have a tangential entry into the cylinder and to have the proper turbulence created by the scavenge air entering the cylinder liner, it is necessary to cut the cylinder liner intake ports at a specific design which will help in promoting the turbulence in the cylinder of two-stroke engines. As sweeping action by the scavenge air is very important in case of two-stroke engine, the exhaust removal is carried out by the incoming air rather than the upward movement of the piston as in case of four-stroke engines.

Q.87 In internal combustion engine, which of the device listed will force the compression rings to seal the compression gases in the space above the piston?

Ans. The gas pressure acting against the back of the rings

The gas pressure due to combustion of fuel and compression of gas will find its way through the piston ring groove clearance i.e. the vertical clearance between the piston ring and the groove. This high pressure gases will act on the piston rings from behind because of which the piston rings will be forced radially outwards and it will exert a pressure onto the liner thus forming a perfect seal between ring rubbing surface and the liner. At the same time, this piston ring will be forced downwards against the piston ring groove by the gas pressure thus preventing the leakage of gas between the landing face of the piston ring and the groove. This clearance is of very importance and for this the piston ring should be free in the groove so that gas can find its way behind the piston rings to form a best possible seal. If this seal is not maintained due to any reason, the piston ring will not seal effectively against the liner which will cause the blowpast and ultimately lead to scuffing due to the wiping out of lubricating oil film between the liner and piston ring. The gap between the groove and the piston ring is maximum for the top ring and it reduces as the piston rings are fitted away from the combustion space i.e. the topmost ring will have maximum clearance and the lowermost ring will have the least clearance.

Q.89 The size of the exhaust valve opening is

Ans. (b) most critical in two-stroke cycle diesel engines.

In case of two-stroke engines the process of exhausting is carried out with the help of incoming scavenge air and not by physical movement of piston as in case of four-stroke engines. Hence in two-stroke engines the exhausting process is not effective as that in case of four-stroke engines. Hence in order to improve the efficiency of exhausting it is necessary that there is no backpressure created onto the exhausting system. In such cases, it is necessary to have a maximum exhaust opening possible so that there is no backpressure on the exhaust gas to flow. Hence the scavenging and sweeping action in the cylinder liner of the exhaust gas is carried out efficiently which ultimately improves the fuel oil combustion characteristics and also it improves the thermal efficiency of the engine.

Q.90 Provision is to be made for ventilation of an enclosed diesel engine crankcase by means of a

Ans. (d) small breather or slight suction not exceeding 25mm of water.

By ventilating the enclosed crankcase of a diesel engine, the pressure inside the crankcase can be reduced. Thus preventing the excess pressure developed which may cause the leakage in the crankcase doors and crankcase oil may finds its way out. At the same time, it will prevent the development of high concentration of oil mist in the crankcase. Thus reducing the possibility of crankcase explosion. In modern engines mostly, the crankcase external fans are not provided but crankcase weld is provided which is led to a remote plate which prevents the excessive built up of pressure inside the crankcase. This pressure should not be above 25mm of water gauge otherwise the leakage from the crankcase and the loss of oil will be very much.

Q.91 Which of the general advantages listed does electrical pyrometers have over mechanical pyrometers?

Ans. (d) It can indicate temperature at a distant point from the source of heat.

In case of electrical pyrometer which works on the principle of thermocouple and the hot and cold junction, due to the temperature difference between the hot and cold junctions, an electric current is produced in the circuit. This current is directly proportional to the temperature difference between the cold and hot junction. This electric current which is produced due to the temperature difference can be amplified by use of amplifier and can be relayed through the wire to a remote place from where it can be indicated in the form of temperature. Hence with electric pyrometer, it is possible to have a remote indication rather than in case of ordinary pyrometer. In ordinary pyrometer, it is not possible to have remote indication of temperature.

Q.92 In large slow speed diesel engines the force applied to the crosshead is

Ans. (c) against the crosshead during the power and compression stroke.

In a crosshead engine, there is a crosshead shoe with two rubbing surfaces. At the same time, there are two crosshead guides one each at port side and starboard side. During the half revolution of the engine one face of the crosshead shoe rests against one side of the crosshead guides. This will continue upto BDC when the crank turns around the dead center and the piston starts moving upwards. In such condition, the directional forces of the crosshead changes and other face of the crosshead shoe will start resting against the other crosshead guide. Hence for half the revolution one face of shoe and guides are coming in contact and for the remaining half revolution other face of the shoe and guide are coming in contact. Hence both crosshead guides and shoes are required. Sometimes they are called ahead or astern shoes and guides. Hence even the engine may be unidirectional, it is necessary to have guides and shoes at both, port and starboard sides.

Q.93 A diesel engine is driving an alternator required to run at 1800rpm. The overspeed governor is required to set within the range of

Ans. (a) 1980rpm to 2070rpm.

The overspeed governor is to be set at 10% of the rated rpm of the engine As the rated rpm of engine is 1800rpm the governor is to be set at 1980. If the rpm goes above this there is a possibility that excessive damage can be caused to the running gear of the engine as well as the alternator due to the centrifugal force which is created. At the same time, the bottom end bearings may be sheered off. Also the alternator gap between the stator and rotor. If the speed exceeds beyond particular limit because of centrifugal force, the mass of rotating poles on the rotor will try to move away from the center and reduce the air gap between the stator and rotor and the stator poles may touch the rotor and may cause severe damage. Hence it is necessary to control the speed within 1980rpm to 2070rpm.

Q.94 A crankshaft whose center of gravity coincides with its centerline is said to be

(a) statically balanced

(b) dynamically balanced

Ans. (a) statically balanced only.

In case of statically balanced crankshaft the center of gravity through which all the load will pass will coincide with the center line of the crankshaft. Hence it will not come to the standstill position at the same point again and again. In such condition, the entire weight of the crankshaft is evenly distributed but this does not mean that when it starts rotating, the forces which are created at different planes are balanced. The vectors of forces created because of rotation of crankshaft may be in different planes and they may not balanced though the crankshaft is statically balanced. For dynamic balance we have to carry out the special procedure of dynamic balancing so that the vectors created because of the rotation of masses will be balanced with respect to each other and hence there is no unbalanced vector which is affecting the crankshaft.

Q.95 & Q.96 MATHEMATICAL

Q.97 Diesel engine valve springs functions to

Ans. (c) close the valves.

Valves of a diesel engine always opens inside the cylinder and closes against the pressure inside the cylinder. So cylinder pressure acts over the valve lid which helps in keeping the valve closed. In addition to this, springs are also utilized which keeps the valve closed because of its upward pressure on the valve. These springs are of high importance to keep the valves closed onto the seat. The tension in the spring is also of high importance. If the tension in the spring is too much, the valve will be held against the seat with great pressure which will prevent the leakage but it may damage the valve seat. At the same time, for opening the valve excessive pressure will be required which will cause more amount of weardown to roller and cams. If the spring tension is not sufficient, the valve will not close properly on the seat and due to its own weight it may fall down and also cause leakage of the valve seat. Hence the spring is the most important part of valve gear and must be used for properly closing the valve on the valve seat. But in modern diesel engines, instead of springs of mechanical pressure, nowadays air springs are utilized due to various advantages it has over mechanical springs. Also it gives better rotation to the valve which will keep the valve seat clean and prevent the burning of the valve and extend the valve life.

Q.98 Built up exhaust valve is one thing in which

Ans. (a) stem and head are made of different material

In earlier days these types of valves were utilized. In these valves the valve spindle and valve head material can be selected properly depending on working conditions to which they are subjected. As the valve seat and the valve head are subjected to high temperature and corrosion they are made up of special material and coated with stelite. In case of valve stem, which is subjected to more amount of wear because of its reciprocating movement in the valve spindle guide, valve spindle has to be wear-resistant. This is achieved by providing a wear-resistant material for the valve spindle. Hence in order to achieve best result from all aspect of the valve operating condition, it is necessary to make the valve part from the material which is suitable for the working condition of that part.

Q.99 & Q100 MATHEMATICAL

Q.101 Between the periods of injection and ignition of a fuel a diesel engine crankshaft rotates through the

Ans. c delay period.

The time between the injection and ignition is called delay period. This delay period is caused because as the fuel enters the combustion space, it takes some time for it to mix up with the hot compressed air and evaporate and to form a combustible mixture. Hence it is not possible to ignite the fuel at the instant it enters the cylinder, but it takes some time and by the time this delay period is lapsed, the combustion space is filled with thorough mixture of air and fuel so that the combustion can take place efficiently. This delay period depends on the quality of the fuel. If the fuel is of inferior quality the ignition delay increases. If the fuel is of superior quality the ignition delay is reduced. If the delay period is excessively high there is a possibility of knocking as well as afterburning and loss of power.

Q.102 The cylinder liner forming the cylinder wall and the inside of the water jacket is called a

Ans. (b) wet liner

Wet liner is a liner which is cooled by water and, for cooling by water, it is necessary to form an annular space between the jacket and outside of the liner so that water can be circulated through this space. For this purpose, annular space is provided between the liner outer wall and jacket so water can be circulated. This is called wet liner.

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