Scavenging - marine diesel engine

The process of exchanging the gases present in the cylinder after completion of expansion with a charge of uncontaminated fresh air is termed as scavenging.
The performance of marine diesel engine depends to a large extent on the efficiency of the scavenging. There are few requirement's for that which are:

1.  The exhaust valves should open early,in advance of the scavenge ports for the effective removal of the exhaust gases.
2. Scavenge ports should be closed after the process of gas exchanging is carried out.
3. Loss of fresh air escaping through the exhaust valve should be minimized.

The Scavenging process is different for two and four stroke engines. It is so because in 4 stroke engine have in between opening and closing of exhaust and scavenge valves. But for the 2 stroke engine the overlap is limited and so there is always slight mixing of fresh air with exhaust gases inside cylinder.

Different method's of scavenging: Scavenging can be of many types which can be  used in both 2 & 4 stroke engines but the basics remain the same, i,e. fresh air enter the cylinder through opening of the port/valve in downward stroke of the piston and continues till the port closes in upward stroke.These scavenging methods are based on the location,shape,size of scavenge ports and engine's exhaust arrangement.

Uniflow Scavenging: The scavenge air enters through the scavenge ports in the lower part of the cylinder liner and is expelled through centrally mounted exhaust valve. In this system charge flows in one direction and so the intermixing is minimum leading to high scavenge efficiency.

Stroke to Bore ratio: 3.5-4.2

Loop Scavenging: The exhaust ports are placed above the scavenge ports on the same side of the liner. The air flows in a loop within cylinder, expelling gas through exhaust ports.

Stroke to Bore ratio: 1.75

Cross Scavenging: In this system the air is directed upwards, passing under the cylinder cover and down on the opposite side.

Stroke to Bore ratio: 2-2.2 

Difference between uniflow and cross/loop scavenging:

UNIFLOW SCAVENGING:Uniflow scavenging with a large centrally positioned exhaust valve is used in most modern slow speed large diesel engines.

Advantages:

1. High Stroke – Bore ratio.
2. More power can be developed.
3. Design of the cylinder liner is simple due to absence of exhaust ports. Cost of liner is less.
4. Effective length of Piston stroke is considerably more.
5. Arrangement for sealing the bottom of cooling water space is simpler.
6. The increased scavenging efficiency with this type of scavenging creates greater scope for increased economy and so all modern designs are based on the design.
7. The reduced no and size of ports reduces the problem of liner lubrication, leading to less oil being consumed.

Disadvantages:

1.  Require exhaust valves, additional cost and maintenance.
2.  Require two or more fuel valves to be fitted per cylinder.

CROSS/ LOOP SCAVENGING:

Advantages:

1. Do not require exhaust valves. Simple cylinder cover design and simpler camshaft and camshaft drive design.
2. Only one centrally located fuel valve is sufficient.

Disadvantages:

1. Design of Cylinder liner is complicated because of exhaust ports hence liner is expensive.
2. Greater possibility of mixing of gases.
3. Large temperature difference between small area of scavenge and exhaust ports leading to high thermal stresses.
4. More chance of piston rings leakage.

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Exhaust Valve Explained

BASIC:

Two stroke cross-head engine have a single exhaust valve mounted in the center of cylinder head. Exhaust valves open towards inside in the cylinder,such away that the gas pressure inside cylinder ensure positive closing & helps to avoid  building up carbon on the valve seat.The valve opening and closing is controlled by cam mounted on the camshaft.

Modern 2 stroke cross-head engines work on hydraulically operated air spring  exhau.st  valves. Now a cam operate hydraulic pump instead of the push rods in earlier versions. The pump which displaces Oil from the engine LO system operate a piston in the exhaust valve which push valve to open. Here in placeof the mechanical spring there is  an “air spring”. Air at 7 bar through a non return valve goes  to the underside of the piston attach to the valve spindle. when valve open, air underneath is then compressed. When hydraulic pressure is released, the expansion of the compressed air assist in the closing of valve.

Air also taken down to guides of the exhaust valve, this not only keep it cool but also prevent exhaust valve from leaking.

The valve spindle fits with the valve rotator. The kinetic energy in the exhaust gas rotates the valve by a small degree  when it  pass. This helps to keep valve at  even temperatures & so help reducing the build-up.

DESCRIPTION:

Exhaust valve consists of a valve housing and a valve spindle. The valve housing is of cast iron and arranged for water cooling. The housing is provided with a bottom piece of steel with stellite welded onto the seat. The spindle or valve stem is made of heat resistant steel with stellite welded on to the seat. The housing is provided with spindle guide or valve guide. The exhaust valve housing is connected to the cover with studs and nuts tightened by a hydraulic jack. The exhaust valve is opened hydraulically and closed by a set of helical springs.

The exhaust valveis subjected to hot gases, and the temperature resistance of its seat and body is therefore crucial. In new designs, Nimonic (Nickel,Chromium,Cobalt)valves combined with proper seat cooling have yielded excellent service behavior and long life times. When exhaust valves are not centered in the combustion chamber, heating will not be symmetrical on the valve lid. For this reason, an automatic rotating device may be fitted causing the valve to rotate slowly, thereby avoiding localized heating.

Material Properties:

• High tensile strength and creep resistance properties.
• Resistance at high temperature properties.
• Resistance to high temperature corrosion.

 Design Characteristics: 

• Bore colling for efficient cooling system i.e as low as 327 degree Celsius.
• Valve rotational mechanism.
• Heat and corrosion resistance material used.
• Hydraulic push rod with controlled valve landing speed.
• Air spring
• Guide bush is sealed by pressurized air.

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Work Of fourth engineer on ship!

4th Engineer being the first promotion in engine department is stumbled upon by lots of responsibilities. But they have been trained to do so in their cadet ship. The only difference now is that, they have to implement all what they have learnt at that time, to present. As per experts, 4th engineer should do his or her work, and side by side, needs to learn what other engineers are doing. But this case is valid for all engineers, because of the very basic fact that, machinery are different every time (Yes basic background remains the same), thus one cannot be utmost sure about everything. Although there are handfuls of manufacturer of machinery in the world, but they too strive for making something different every time, so that, best output can be obtained from the machines.
  
Work Of fourth engineer on ship:

Pumps of centrifugal, screw, gear and reciprocating types are also under the working area of 4th engineer. Engine room of a ship is nothing or you cannot imagine it without pumps. A regular maintenance on them and also, repairing them time to time as per the running hours is quite necessary. Mostly we use centrifugal pumps, but reciprocating pumps and other positive displacement pumps are also used in places, where there need is.
The major role that 4th Engineer plays on-board is taking of bunkers. Bunker involves lot of pre-planning and him or her with chief engineer being responsible for carrying out the bunker operation properly, a thorough study of how much to take and where to take is quite necessary and imperative. During time of bunkers, he or she needs to take constant soundings, so as to have correct knowledge level of oil in the tanks.
                                                In most of the ships, fourth engineer is a part of the fire team that go first, in case of emergency. So, he or she has to make sure that, they are competent to achieve this task. He has to assist his senior's and do his watch-keeping.

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MEO class 4 Objective question papers solved.| Part1

Q- Which of the following will lead to automatic stopping of IG blower?
a. Low water level in deck water seal.
b. Oxygen content above 8%.
c. Low water level in scrubber tower.
d. High water level in scrubber tower.
Answer-A & C 

Q-Which of the following will be required to be done in order to reduce sulphur corrosion of Main Engine components
A. Increasing the atomization pressure of fuel injectors
B. Fitting exhaust valves with valve rotators
C. Running engine at or near normal sea load
D. Decreasing the F.O. purifier feed rate , running two purifier in series one acting as purifier and other as calrifier
Answer-C

Q- Viscosity of fuel oil used for ME was 145cst,new fuel comes whose viscosity was more than earlier, what should u do
a) increase temp setting
b) change setting of viscotherm
c) do nothing
d) open steam valve manually more
Answer-Critical Question(it could be anything)

Q- Main engine is running but viscosity of fuel cannot be maintained.fuel oil heater have a maximum rating of 4 bar steam pressure.steam through a pressure reducing valve is provided to the heater at 4 bar but still viscosity of fuel annot be brought down below 20 cst. as a good marine engineering practice what will you do?
a. let the engine run at same rpm n wait for the port to come to carry out the cleaning of heater
b. reduce the rpm till next port arrives n then clean the heaters
c. increase the steam pressure by adjusting the setting of steam reducing valve
d. stop the engine n clean the heaters
Answer-D

Q-When one of the M/E.F.O. heater is not in use then
A) The F.O. inlet n outlet valve should b kept shut, but steam heating valves should b kept crack open so as to keep the heater warm.
B) F.O. inlet n outlet valve should b kept shut n heating valves also should b completely shut n should b drained of any remaining F.O.
c) F.O. inlet n outlet valve should b kept crack open so that the F.O. recirculation can take place.
d) F.O. inlet n outlet valve should b kept shut while outlet valve should be left open so that
Answer-B

Q- how diesel engine governing limit
a. load limit
b. scav limit
c. torque limit
d. all
Answer-D

Q-The main causes of failure of the cylinder head of a large 2 stroke marine diesel engine are
(a) Excessive rigidity.
(b) Small core holes.
(c) Thin uncooled walls.
(d) Excessive rolling and pitching
(e) Excessive tightening of cylinder head nuts.
Answer-C and D

Q- New liner we are putt in a ring in skirt made of
A one piston ring
B bronze ring, multiple piece
C cast iron, single piece
D piston ring, multiple pieces
Answer-B(Because the main function for that ring is to if any misalinghment of piston so skirt will touch the liner so if u provide this wear ring it want allowed to touch liner and it is made up by soft material and even if wear ring will touch the liner it wont damage the liner)

Q- Sea water outlet temperature of LO cooler decreased while main engine load and sea water inlet temperature not changed this indicates
a.Cooler efficiency has increased
b.Cooler efficiency has decreased
c.Sea water outlet temp nothing to do with efficiency
d.The flow of sea water reduced
Answer-B

Q- Cold corrosion occurs in the middle part of the liner of main egine. What constructional features can be applied to it for reducing this.
A . no constructional feature can be applied only increase feed of cylinder oil or use a oil with high tbn and maintain jacket water temperature as high as possible.
B even after doing A there will be some corrosion which cannot be avoided so no constructional feature other wise.
C putting an insulation between dulinder liner outside and in the bores to avoid any corrosion.
D maintain low jacket water flow rate to avoid turbulence and corrosion
Answer-A

Q- liner crack indication
a. loss of water frm expansion tank
b. black smoke coming from expansion tank
c. jacket water fluacuating
d.all
Answer-D

Q- The machinery which is usually tested first once the ship reaches a dry dock is
A. Engine room crane
B. Engine room lift
C. Gantry crane for engine room
D. All of the above
Answer-D

Q- What is mixed lubrication?
A mixing additives to lube oil
B mixing of water with lube oil.
C where the lube oil film is partly eshtablished.
D where there is certain deformation in the mating surfaces and lube oil film is established.
Answer-C

Q- In case of modern 2-stroke, crosshead type, marine diesel engines, the cooling oil for piston crown cooling is provided:
a Through Telescopic pipes
b Through Spray nozzle provided on top of the con-rod
c Through hollow piston rod from crosshead
d Through splash cooling similar to liner lubrication
Answer-C

Q- You see white smoke coming from the ME funnel stack, this could be
a)one or more cylinders not getting enough fuel
b)too low compression pressure
c)water in fuel
d)all of above
Answer-C

Q- Primary purpose of an indicator cock
a. Remove soot exhaut gases aftr stopping the engine.
b. Take an indicator diagram aftr removing ext gases take an performance of engine.
c. Realease excess pressure
Answer-A not B(Because Primary Purpose is Blow through Before Starting an Engine)

Q- The use of ball and roller bearings in internal combustion engines is becoming more and more popular for some or all of the following
(a) The low coefficient of friction
(b) The absence of wear under favorable conditions
(c) The independence in lubrication
(d) The extreme reliability of such bearings
Answer-A

Q- how diesel engine governing limit
a. load limit
b. scav limit
c. torque limit
d. all
Answer-D

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Minimum requirements for certification of officer in charge of an engineering watch

Minimum requirements for certification of officer in charge of an engineering watch in a manned engine room or designated duty engineer in a periodically unmanned engine room ( Marine Engineer Officer Class IV)

1. Every officer in charge of an engineer watch serving on a sea- going ship powered by main propulsion machinery of 750 KW propulsion power or more shall hold an appropriate certificate of contemporary in form 8.

2. Every candidate for the certification shall -
(i) be not less than 18 years of age on the last date prescribed for receipt of application;
(ii) have completed approved education and training for a period of at least thirty months as provided in an approved training and assessment programme including on-board training documented in an approved training record book and meet the standards of competence as specified in section A-III/1 of the STCW Code;
(iii) have completed approved sea-going service for a period of not less than six months in the Engine Department of a sea -going ship powered by main propulsion machinery of 750KW propulsion power or more, closely supervised and monitored by a marine Engineer Officer Class I or an officer of the Engine Department holding certificate of competency under rule 5 and which is duly documented in an approved training examination, assessments and meets the standard of competence as specified in section A-III/1 of the STCW Code.
Source : www.mmd.gov.in

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