Class 2 MEO examinations - Knowledge base - Q5. …akramanujan.org/wp-content/uploads/2018/10/EKM-AU… · Web viewEfficiency is increased by having the same effective compression

AUGUST 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. Fatigue is one of the main causes of crankshaft failure. (a) Indicate on a sketch (he most likely location of a fatigue crack- (b) Explain how a fatigue failure is identified. (c) Describe how a fatigue crack may be initiated (d) Describe with the aid of sketches. the methods used to inhibit fatigue cracks.

Answer:- Repeat question.

Q2. 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. Q3. With regards to

modern diesel engine raising the Life Cycle Value (LCV). describe the importance of following. a) Low Sac Volume of Fuel Injection Valve b) Fuel Valve opening Pressure regulation c) Contamination of combustion Chamber and impact on LCV d) Contamination of lube oil and impact on LCV

Answer:- LCV or life cycle value is a measure of the length of service of a component or its endurance level. The LCV is reduced by non congenial conditions prevailing in the operation of the component which causes the endurance level to fall.

(a)In the case of the sac volume of the fuel injector, the sac volume is the volume of fuel oil remaining in the fuel injector and the delivery pipe from the fuel pump after end of injection is constantly held in the space. The volume remaining in the delivery pipe is usually more than the volume of oil remaining in the fuel injector and the difference is mainly dependent on the length of the delivery pipe.. The oil contains sulphur varying from 3 to 4 percent and when at high temperature it is likely to oxidise and cause corrosion of the pipe and the ful injector sac volume surfaces. Hence a low sac volume should result in a better LCV , as the relationship is inverse proportion.

(b) The fuel valve opening pressure is high and depends on the viscosity of the oil at the moment of atomisation. The rise of pressure in the fuel pipe as well as the injector is almost instantaneous depending on the speed of the fuel pump operation . This results in pressure pulses at regular intervals of time dependent on the RPM of the engine.This leads to regular repeated volumetric strain which induces repeated hoop stress and tensile stress on the fuel delivery pipe leading to fatigue conditions resulting in breaking or cracking of the fuel pipes. The fuel injectors are also subjected to fatigue.

(c)The combustion chamber consisting of the cylinder head flame plate and the piston crown is in close contact with

the combustion products , especially sulphur di oxide and water vapour leading to acid formation and hence corrosion of these surfaces. At times due to improper penetration flame impingement occurs on the piston crown as will as the flame plate indicated by surface cracks and heavy pitting

(d) Fuel dilution in the lub-oil, especially medium speed engines causes oxidation of the lub-oil and reduction in the TBN value of the oil and this leads to corrosion of crank pins and journals. This corrosion also affects bearing metal as well as the liner surfaces and piston rings.

Q4. With reference to Modern Diesel Engine describe the features of High pressure Miller cycle and discuss the following. a) Reduction in Air Temperature due to Miller Cycle b) Recove1Y of Pressure in the Combustion chamber during the Miller cycle c) Effect of Miller Cycle on specific fuel consumption and NOX emission. d) The impact on various parameters during low load operation using Miller Cycle

Answer;- A traditional reciprocating internal combustion engine uses four strokes, of which two can be considered high-power: the compression stroke (high power flow from crankshaft to the charge) and power stroke (high power flow from the combustion gases to crankshaft).In the Miller cycle, the intake valve is left open longer than it would be in an Otto-cycle engine. In effect, the compression stroke is two discrete cycles: the initial portion when the intake valve is open and final portion when the intake valve is closed. This two-stage intake stroke creates the so-called "fifth" stroke that the Miller cycle introduces. As the piston initially moves upwards in what is traditionally the compression stroke, the charge is partially expelled back out through the still-open intake valve. Typically, this loss of charge air would result in a

loss of power. However, in the Miller cycle, this is compensated for by the use of a supercharger. The supercharger typically will need to be of the positive-displacement (Roots or screw) type due to its ability to produce boost at relatively low engine speeds. Otherwise, low-rpm power will suffer.In the Miller-cycle engine, the piston begins to compress thel-air only after the intake valve closes; and the intake valve closes after the piston has travelled a certain distance above its bottom-most position: around 20 to 30% of the total piston travel of this upward stroke. So in the Miller cycle engine, the piston actually compresses the air only during the latter 70% to 80% of the compression stroke. During the initial part of the compression stroke, the piston pushes part of the air through the still-open intake valve, and back into the intake manifold.

(a) The partially compressed air mixes with the air from the supercharger whis is cooled to the original emperature before compression.Hence thew temperature of the air at end of compression is slightly lower than the temperature of the engine working on the diesel cycle. Or dual cycle.

(b) The recovery of pressure in the combustion chamber before ignition is caused by the addition of mass of air at a muich higher pressure supplied by the supercharger

(c)  the Miller cycle yields an additional benefit. The intake air is first compressed by the supercharger and then cooled by an intercooler. This lower intake charge temperature, combined with the lower compression of the intake stroke, yields a lower final charge temperature than would be obtained by simply increasing the compression of the piston. This allows ignition timing to be advanced beyond what is normally allowed before the onset of detonation, thus increasing the overall efficiency still further. An

additional advantage of the lower final charge temperature is that the emission of NOx in diesel engines is decreased, which is an important design parameter in large diesel engines on board ships and power plants. Efficiency is increased by having the same effective compression ratio and a larger expansion ratio. This allows more work to be extracted from the expanding gases as they are expanded to almost atmospheric pressure. In an ordinary diesel engine at the end of the expansion stroke of a wide open throttle cycle, the gases are at around five atmospheres when the exhaust valve opens. Because the stroke is limited to that of the compression, still some work could be extracted from the gas. Delaying the closing of the intake valve in the Miller cycle in effect shortens the compression stroke compared to the expansion stroke. This allows the gases to be expanded to atmospheric pressure, increasing the efficiency of the cycle.(d) The major advantage of the cycle is that the expansion ratio is greater than the compression ratio. By intercooling after the external supercharging, an opportunity exists to reduce NOx emissions for diesel, engine. However, multiple tradeoffs on boosting system efficiency and friction (due to the larger displacement) need to be balanced for every application.

Q5. With regards to modern 4-stroke diesel engine explain the following. a) The function of protection ring installed on the upper part of liner. b) The moderation in fuel injection drive system compared to conventional 4-stroke engine. c) Staggering of layout for multi hole nozzles. d) Effect of swirl and squish during the combustion process and how swirl and squish is generated Answer:- (A) As the piston reciprocates in the cylinder, any carbon build up forming on the top of the piston will be removed by the anti-polishing ring. This ensures a good lubricant film on the liner surface, reducing wear and giving a more consistent lub oil consumption.

(B)The modern four stroke engine especially the high speed engine has the fuel pumps for all the units in one modular block with a common rack so that the quantity of fuel injected into each unit is same as the setting of the helical groove connecting the suction port is same for all the units.

The module has its own camshaft set according to the firing order of the engine and is coupled to its drive. During major overhauls , the modular block is dismantled , its worn plunger sets are replaced with new sets and the delivery from each plunger is metered for the full load setting to ensure that all units are supplied equal quantities of oil.

(c)

(c) Turbulence contributes to the dispersion of fuel and the micro mixing of fuel and air respectively. As such, they greatly influence the diesel engine performance. The flow processes in the engine cylinder are turbulent. In turbulent flows, the rates of transfer and mixing are several times greater than the rates due to molecular diffusion. This turbulent diffusion results from the local fluctuations in the floe field. It leads to increased rates of momentum and heat and mass transfer, and is essential to the satisfactory operation of spark Ignition and Diesel engines.The sgaggering of of multihole injectors is done to improve the turbulence so that combustion is optimised.

(d)Swirl

Swirl is defined as the organized rotation of the charge about the cylinder axis. It is created by brining the intake flow into the cylinder with an initial angular momentum. Swirl is generated during the intake process in DI disel engines by the intake port and subsequently by combustion chamber geometry during the compression stroke. The swirl intensity increases the tangential component of the velocity of air inside the cylinder, Which aids in the mixing of fuel and air, and significantly affects the combustion and emission characteristics of disel engines.

Swirl ratio = Angular velocity of a rotating flow (w) / Crank shaft angular rotational speed (2∏N)

Suction swirl

During suction, air is admitted into the engine cylinder in a tangential direction. The entering air is deflected by the cylinder wall. Air thereby assumes a rotary motion i.e. swirl about the cylinder axis. This swirl is called suction swirl. Helical pots produce swirl upstream of the valve and directed ports have it downstream. In diesel engines, tangential entry of air is effected by one of the following methods:

1. By masking a portion of the inlet valve.2. By angling the inlet port in the desired direction.3. By proving a lip in the inlet port, over one side of the inlet

valve.

Compression swirl

The combustion chamber cavity tends to modify the swirl as the piston approaches the Top Dead Centre (TDS)

Position during the compression process. As the piston approaches TDC the rotating air is forced into the piston bowl. The rotational force is magnified by the reduced diameter of the piston bowl. Thin, deep bowls have a higher swirl rate.

Squish

The squish motion of air is brought about by a recess in the piston crown. At the end compression stroke, the piston is brought to within a very small distance from the cylinder head. This fact causes a flow of air from the periphery of the cylinder to its center and into the recess in the piston crown. This radial inward movement of air is called squish by Ricardo. The combustion recess, into which the air mass is squeezed in, is located either in the piston crown or in the cylinder head. The former arrangement is preferred and is widely used. In this case, heat losses from the compressed air will be lesser. This is because the piston crown is not cooled to that extent as the

cylinder head which is cooled by the coolant. The figure 3.2 shows squish motion during compression.

Q6. Describe the starting and reversing system of an electronically controlled diesel engine and compare with engine having CAM SHAFT and explain following. a) Reduction in air consumption during engine starting. b) improved performance during astern starting and crash astern. Answer:-

The air start system is similar to that on a  conventional engine except there is no need for a mechanically driven distributor to open the air start valves at the correct time. Instead of a camshaft driven, reversing air start distributor, each air start valve is opened at the correct time by the engine computers sending a signal to a solenoid controlled nc (normally closed) valve. The timing of the air start valves will vary depending on the number of cylinders, but they will be open for a long enough period to allow overlap, so that a valve opens before the previous valve closes, allowing starting from any position of rest. The nominal opening can be considered as 0° (ie TDC) and closing at 110° ATDC. The reversing action is coordinated with the com[putor responding to the reverse rotation of the electronic timing cycle

 The computer knows when to send the signal because it is receiving information as to the crankshaft position from the angle encoders which measure crankshaft position and RPM. When the engine has reached firing speed the computers shut off the air and introduce the fuel.

When compared with the cam shaft operated engine,for the (a)Reduction in air consumption during starting and (b) improved performance during crash stop and crash astern:

(a) During starting the compressed air is only needed for starting the engine.In the reflex engine there is no need for control air for starting . Hence the starting air consumption is reduced.

(b) The action of the electronically controlled valves are much faster than the mechanically controlled valves because of the higher speed of electronic response. Hence the response being faster the action will be faster and crash stop will occur at a lower distance as compared to the distance covered by the conventional cam shaft operated engine.

    Q7. With regards to modern diesel engines revolution pick up sensor. discuss with suitable diagram the following. a) functioning

of revolution pick up sensor. b) adjustment of pick up sensor. c) adjustment of rotary encoder. d) adjustment of pulse angle offset. Q8. Explain the working principle of variable turbine inlet turbocharger(VTI). With regards to VTI turbocharger discuss following. a) how is the exhaust gas inlet variation to the turbine achieved b) advantage of VTI during low load operation. c) With the aid of diagram show the mechanism to alter VTI.

Answer:-

In order to draw the highest efficiency from marine engines, they must be run at the highest rated RPM. However, with several norms, traffic rules, fuel efficiency factor etc., marine engines cannot always be operated at high speed. As the RPM of the engine reduces, the air supply from the turbocharger also reduces, starving the combustion space of air and resulting in improper combustion and increased fuel consumption. To counter this problem, auxiliary blowers are fitted. These blowers come into action at particular low loads.

(A)Variable Geometry Turbocharger or Variable Turbine Angle (VTA) Turbocharger is the solution for supplying sufficient scavenge air to marine engine’s cylinder at all load ranges.At reduced load the mass flow of exhaust is reduced. If the nozzle vane angle is suitably altered the nozzle vanes are adjusted for the massflow and reduced pressufrre of the exhaust gas. This results in a balanced operation of the turbocharger for the low load condition.

VGT or VTA is a turbocharger, wherein movable vanes are replaced for conventional fixed vanes, which can change angles to control the exhaust flow on the turbine blades. This helps the engine control to balance the volume of air with the fuel along the entire engine load range.The VGT or VTA consists of nozzle rings equipped with adjustable angle changing vanes. Each vane is connected to control a ring using a lever, which reduces the thermal hysteresis and increases the positional accuracy.

The vane position or the angle is adjusted by the control ring connected to the electric positional motor via reduction gear.The control system governs the action of the microprocessor controlled positional motor. The position or angle of vanes is thus altered after comparing the feedback signals of – air pressure after the blower, and exhaust gas temperature before and after the turbocharger.

(B) Advantages of VTA

Works on the entire load range of the engine Eliminates the need of auxiliary blower Reduces the fuel consumption Reduces exhaust smoke emission and hence air pollution Lower’s CO2 , Nox, and Sox emission Reduces soot and carbon deposits on combustion and exhaust space Reduces fouling of engine parts Improves efficiency of the engine Economical in operation

(C)

Q9. With regards to connecting rod ovality of 4-stroke diesel engine explain following. a) importance of connecting rod ovality. b) method of measuring the connecting rod ovality. c) discuss the impact of ovality if it increases beyond the maximum allowable limit. d) method of ascertaining the elongation of connecting rod bolts

Answer:-