How to check the Tappet clearance in marine diesel engine and it importance

Taking tappet clearance (also known as valve clearance) in a marine diesel engine is a critical routine maintenance task that ensures proper operation of the engine's valve mechanism. Here’s a detailed guide suitable for engine room practice and oral exam preparation. 

What is Tappet Clearance?

Tappet clearance is the gap between the rocker arm and the valve stem (or cam follower and camshaft) when the valve is fully closed. 

It compensates for the thermal expansion of engine parts and ensures valves close completely.

Tools Required

• Feeler gauge set 
• Spanner set
• Torque wrench (for locking nuts)
• Screwdriver (if adjusting screw type)
• Manufacturer's manual (for correct clearance values)

When to Take Tappet Clearance

• After a major overhaul or DECARB
• At routine intervals (every 2,000–3,000 hours or as per PMS)
• When abnormal sound or loss of compression is observed

 Step-by-Step Procedure

Always on a cold engine unless specified otherwise

 1. Prepare the Engine

• Ensure the engine is shut down and properly cooled.
• Open indicator cocks for safety.
• Turn off starting air system and tag out the engine.

 2. Find Top Dead Centre (TDC)

• Rotate the engine manually using turning gear.
• Set the piston of the unit you're checking to TDC on compression stroke.

Confirm by:

• Valve overlap of the adjacent unit
• Exhaust and inlet valves both closed
• Confirm by firing order and indication on the flywheel

3. Check Clearance

• Insert the correct feeler gauge between
• Rocker arm and valve stem (for 4-stroke engines)

4. Move the gauge:

• Should slide in with slight drag.
• Too tight = Low clearance
• Too loose = High clearance

5. Adjust if Needed ( Depends on engine -Refer Maker instructions)

• Loosen lock nut on the adjustment screw.
• Turn screw clockwise (decrease) or counterclockwise (increase) clearance.
• Re-check with feeler gauge.
• Tighten lock nut and re-confirm.

7. Repeat for All Units

Advantages of Good Tappet Clearance

1.  Ensures Complete Valve Closure

• Prevents compression loss and blow-by.

• Avoids hot gases leaking past valves, which can cause burned valve seats.

2.  Maintains Correct Valve Timing

• Ensures valves open and close at the precise moment in the cycle.

• Supports proper air intake and exhaust, leading to efficient combustion.

3. Compensates for Thermal Expansion

• Engine components expand as they heat up.
• Correct clearance accounts for this, preventing valve sticking or misalignment.

4. Improves Cold Start Performance

• Proper clearance allows valves to seat fully even in cold conditions.
• Better sealing means higher compression and easier ignition.

5. Reduces Wear and Tear

• Prevents hammering action between rocker arm and valve stem
• Minimizes mechanical noise and stress on camshaft, pushrods, and rockers.

6. Improves Fuel Efficiency

• Correct valve operation = optimal air-fuel mixture.
• Leads to complete combustion → less unburned fuel → better SFOC (specific fuel oil consumption).

7. Prevents Valve Overheating

• A valve that doesn’t close fully can't transfer heat to the seat → leads to burnt valves.
• Proper clearance ensures valves seal properly and cool during the compression stroke.

8. Supports Engine Power Output

• Balanced compression and timing mean the engine delivers rated power without misfiring or hesitation.

Pro tips

• Follow the firing order to check each unit at its TDC compression.
• Common Clearance Values (Always Check Manual)
• Always on a cold engine unless specified otherwise
• Feel for “slight drag” — not loose or tight. This will definitely take some time and experience, so be patient and recheck your work.
• Use New Feeler gauges — old ones wear thinner with time.
• Always follow the manufacturer's instructions. If you are working on a new engine, read through the manual before the job.
• Try out the engine after the adjustment and listen for any abnormal noises

Axial play in Diesel Engine

Axial play in a big end bearing refers to the amount of movement of the connecting rod along the crankshaft axis. It's also known as end play or axial clearance. 

Excessive axial play can cause vibrations, misalignment, and potential failure, while insufficient play can lead to binding and premature wear.

What is Axial Play?

In bearings, axial play is the movement of the inner ring relative to the outer ring along the bearing axis. In the context of a big-end bearing, it's the movement of the connecting rod cap along the crankshaft axis.

Why is it important?

Axial play is a crucial factor in the performance and longevity of bearings.

Consequences of excessive play:
Vibration and Noise: Excessive axial play can cause vibrations and noise due to the connecting rod moving excessively within the big end bearing.

Misalignment: The connecting rod may not be properly aligned with the crankshaft, leading to increased stress and wear.

Potential Failure: In severe cases, excessive axial play can lead to bearing or connecting rod failure.

Consequences of insufficient play:

Binding and Friction: If there's not enough axial play, the connecting rod can bind with the crankshaft, causing excessive friction and heat.

Premature Wear: Insufficient play can lead to premature wear of both the big end bearing and the crankshaft.

Measuring Axial Play:
The axial play in a big end bearing can be measured using a dial indicator.

Proper Installation: Ensure the big end bearing is correctly installed and torqued to the manufacturer specifications.

Normal Range:
The acceptable range of axial play for big-end bearings varies depending on the engine and application.

Sparks from main engine exhaust

Engineer on duty sometimes get dreaded calls from the bridge reporting sparks from the funnel which could lead to panic.

Reasons for sparks from funnel

1. Slow streaming and frequent maneuvering could have built unburnt soot in the Exhaust Gas economizer.
2. Lack of maintenance in Exhaust Gas Boiler-Infrequent/delayed water washing.
3. Fault fuel injector, improper combustion, or incorrect governor setting
4. Indication for the start of an economizer Fire or a scavenge fire.
5. Water in the service/ settling fuel  tank or maintaining the wrong viscosity as per FO lab analysis
6. Improper FO treatment onboard or wrong FO purifier setting
7. Dirty turbocharger air inlet/ fouled air cooler 
8. Improper cylinder oil lubrication feed rate.
9. Dirty scavenge space or choke drains

Prevention

1. Frequent soot blow and water washing of EGB exhaust gas boiler.
2. Check the integrity of funnel wire mesh with frequent inspection.
3. Maintain soot-blowing equipment in good working condition.
4. Carry out proper maintenance on the main engine as per PMS.
5. Fuel oil purification, storage, and transfer should be carried out as per the FO lab analysis report recommendation.

Pro Tips

• Start boiler circulation pump after EGB water washing. Collect/remove soot at the bottom of EGB , this may have to be done multiple times if time permits before departure. 
• At times sparks do come from the engine exhaust right after EGB water washing so avoid water washing at port.
• In oil tanker do not carry out soot blow during cargo operation, venting, and tank cleaning operations. Always inform the bridge prior to soot blowing.
• Remove any combustible material around the poop deck or around the vicinity of the funnel and follow good housekeeping practices. 
• If sparking occurs keep a crew on standby around the poop deck for fire watch and keep fire fighting equipment on satanby until the problem is resolved.

As always prevention is better than cure so carry out proper maintenance as per PMS, have good watch-keeping and housekeeping practices. Share and guide the crew with personal experiences, and good practices.

Troubleshooting abnormal exhaust temperature in marine engines

It may sound gross, just as doctor advise to pay attention to your poop as bowel movements are a pretty good indicator of your digestive health. The same goes for the exhaust temperatures of the engine. It is one of the most important parameters to observe during rounds and especially before putting the vessel to UMS mode. Any abnormal deviation is a sure indication of any abnormalities in the system, best practice would be to investigate and rectify the issue before putting the engine in use.

The exhaust temperature of the different units may be slightly varying due to design factors, distance from the turbocharger inlet, exhaust grouping, design of inlet air manifold, etc.

The acceptable difference between cylinder may be less than 40deg cel or the value specified by the manufacturer.

Generally, there are two types of abnormality that may occur

1. All cylinders abnormal.
2. Any-one or some cylinders abnormal.

Note always check thermometer are not faulty and the indicator valve is not clogged

High exhaust temperature in all units

1. Increase in engine load due to hull fouling, bad weather, damage to propeller blades,,etc.
2. Improper fuel qualities, fuel viscosity decrease in fuel temperature.
3. Fouling of scavenging air side due to oil mist, clogged air coolers.
4. Fouling of exhaust air passage, due to build up in turbocharger nozzles and increase in exhaust gas backpressure restricting the efficient removal of exhaust gas.
5. Inefficient blower due to clogged suction filters.
6. Leakage of scavenging air between the blower seal leading to inefficient scavenging.
7. Fire in scavenge space and under piston area. 
8. Exhaust valve timing problem.
9. Problems in the governor.

High exhaust temperature in any one unit/some units

1. Excessive large fuel injection due to improper fuel rack setting or fuel rack stuck in a position.
2. Defective fuel injection valve due to a defective nozzle or improper opening pressure.
3. Leaks in the exhaust valve due to damage in the exhaust valve seat due to wire drawing effect.
4. Improper tappet clearance.
5. Piston blow past leading to a decrease in peak pressure, hence improper combustion which may lead to higher exhaust temperatures.

Safeties 

Some engine may /may not have the following safties depending on the manufacturer

1. Alarm for hight exhaust gas deviation 
2. Slow down for high exhaust temperature

In an emergency situation-cancel slow down can be activated to bypass the slowdown

Microbial contamination of fuels/lube oil

It is the degradation of fuel oil/lube oil that takes place when bacterial, yeast or molds thrive when a favorable condition occurs in the tank

 Water may enter the storage system either through condensation, rainwater penetration, absorption of air or another means when things go wrong. The presence of water along with phosphorous, nitrogen, carbon encourages microbe growth on the interface between oil and water especially on the tank walls

Dangers

• They require water to grow in the beginning but later they can self sustain themself at 20-10 deg celsius in stagnant condition.
• They multiply at a very rapid rate ie. double the size and divide into two every half an hour.
• Once the aerobic bacteria have consumed the dissolved oxygen the sulfate-reducing bacteria is activated
• They attack metals and forms hydrogen sulfide, resulting in corrosion of steel
• Properties of fuel oil /lube oil are effected enhancing corrosion and reducing load carrying properties
• Microbial degradation is normally seen in distillate fuel and not in residual fuel

Indication

• Rotten egg smell
• Sliminess  of oil, especially visible on the painted surface 
• Increase in water content and acid content
• Filter chocking more often.
• Poor heat exchanger performance.
• Black staining of white metal and bearing surface

Minimizing the risk of contamination

• Purchase Bunkers reliable sources to known specifications
• Crankcase content to be weekly monitored and within limits
• Regular inspection of tanks, crankcases storage tanks
• Regular circulation of oil to avoid stagnation 
• lube oil temperature at the purifier is to be at least 75deg cels as Bactria perish above 70 deg cels
• Recirculation and purification of the crankcase oil to be carried out even at long port stays
• Inspection of sludge from purifier or cocked filters to find an indication of any degradation
• Ensure all tanks, vents, heating coils are well maintained, in good condition and leak-free
• Always follow recommended methods when refilling tanks, transfers, and bunkering to avoid the accidental introduction of contamination
• Periodically drain the tank of water and other sediments
• If storing fuel in multiple tanks, employ a rotation system to use the oldest fuel first(first in, first out)
• Filter fuel every time it is moved – this is considered to be ‘best practice’ as fuel is at most risk from contamination when it is being transferred
• Ensure all appropriate crew and engineers are trained in good fuel housekeeping. Processes and procedures should be in place to minimize the possibility of accidental fuel contamination.
• Support the on-tank polishing unit with a fuel additive and biocide to maintain fuel stability and minimize microbial growth

Treatment

• Use of biocide and fungicide
• LO purifier operation at a temperature above 75deg cels but not exceed the suppliers limit as it may lead to oxidation of the oil
• Manual cleaning of sumps, filters pipelines
• Replenishment of the sump oil

Reason why temperature at the exhaust manifold is higher than of the exit of the Exhaust Valve.

When as an operator, we observe the exhaust temperature is generally higher on the common exhaust manifold when compared to the temperature right after the exhaust valve. A practical example can be that  no:1 exhaust valve temperature maybe 380 deg cel and the common exhaust manifold temperature might be 420deg cel, clear 40 deg increase from the exhaust valve.

The common perception is that the exhaust manifold is operating at a higher pressure hence a higher temperature in comparison to the temperature after exhaust valve is following gay lussac's law which makes up part of the ideal gas might just be partially right or a myth.Gay-Lussac's law states that "the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant."

What really happens.

The temperature sensor after the exhaust valve goes through three very different situations:

1. When the exhaust valve open: hot gas is flowing past the sensor with very high kinetic velocities or flow.
2. Exhaust and inlet valves both open: a mixture of hot gases and relatively cool air is flowing past the sensor.
3. Exhaust valve closed: the sensor is immersed in a relatively cool “scavenging pocket”.

Basically, a cycle or pulses of very high temperature combined with high velocities and  then to a relatively cooler temperature .

The temperature sensor after the exhaust valve measures the average of the rapid temperature fluctuation and this value is seen by the operator which in turn is lower than the exhaust manifold temperature.

Hence, as the exhaust manifold is exposed to a constant uniformly hot gas, it has a higher temperature reading in comparison to the temperature reading right after the exhaust valve.

Maximum combustion pressure (P-max) in generator engine

Maximum combustion pressure and exhaust temperature are a very important indicator for knowing the engine combustion conditions and the changes with elapse of time

A decrease in engine performance will be indicated by the following

• Reduction in maximum combustion pressure 
• Abnormal Increase in exhaust Temperatur 

Maximum combustion pressure(Pmax)

• Pmax of an engine is determined by the load fuel quality, fuel temperature, and viscosity, air intake temperature, fuel pump timing.
• Measurement of Pmax should be carried out at least ones for every 250 running hrs or within the recommended time interval on the manufacturer's instruction.

Precaution

• Pmax should be measured after warming up of the engine and when the load is stable
• The measured value can change due to the fluctuation of load, in that case, perform  the measurements again
• Wear proper PPE as handling very hot objects

Measuring Pmax

1. Open indicator valve and close after releasing after 1 or 2 times this confirms that indicator valve is not chocked
2. Install the Pmax indicator on the valve and secure it by turning the lock valve
3. Close the exhaust valve of the Pmax indicator.
4. Open the indicator valve.
5. Check the reading of the Pmax indicator.
6. Close the indicator valve.
7. Open the exhaust valve and then confirm the Pmax pressure to be zero.
8. Remove the indicator after loosening the lock nut.

Ideal limits

Ideal Pmax difference between cylinder should be within 0.6mpa or less or whatever is specified in the manual

Reason for lower Pmax

• Leaky exhaust valve (due to wear down, incorrect lapping, etc).
• Exhaust valve opening too early, incorrect tappet clearance.
• Leak through piston ring (Broken, worn out, wrong piston ring clearances, etc).
• Worn cylinder liner.
• Worn piston crown.  
• Low boost air pressure or temperature.
• Poor fuel quality or wrong fuel pump timing. 

Reason for higher Pmax

• Exhaust valve opening too late due to incorrect tappet clearance.
• Overload of the engine. 
• Wrong fuel pump timing.

TIE RODS on marine engine

FUNCTION

To hold 3 major engine components, that is cylinder block,A-frame and the crankcase in compression and transmit the firing load to the bed plate.

Provide for fatigue strength.
Aligns the running gear.
Helps reduce the bending stress to be transferred the tranverse girder

How bending moment of tie rods are reduced

As bending moment = force * distance of tie rods from centre line of crankshaft.

So, tie rods should be as close as possible to centreline of crankshaft

What are the Causes of Tie-Rod Breaking

• Tie rods are not properly tightened.

• The material and threading of tie rod are under rated and not properly machined.

• Aging of tie rod leading to failure or breakage.

• Tie rods bolts are over tightened by hydraulic pressure crossing its elasticity limit.

• Engine is over loaded or peak pressures of the cylinders are very high.

• Previous fretting of engine mating surface.

• Foundation bolts have become loose or chocks are damaged leading to transmission of vibration in to tie rods.

• Scavenge fire loosen the rods as they pass from the scavenge space and the heat leads to the expansion of the rods.

EFFECT OF BROKEN TIE ROD

1) Excess vibration

2) fretting of the mating surface

3) crankshaft deflection

4) unequal wear down of the main bearing surface

5) foundation bolts or chocks might get loose

Number of tie rod for engine up to the MC series

no of tie rod =2n+2

Number of tie rod for engine  MCC ME series

no of tie rod=4n

where n is the number of units

Spark Erosion

Technically, when two current carrying dissimilar metals are in contact, a sparks travels at the point of contact which erodes the small metal by making a cavity.

In a Vessel, different metals are used to building propeller, hull, bedplate, crankshaft, bearing etc. The current from the cathodic protection system is generally present in these parts, which eventually creates the perfect situation for spark erosion

In short -Spark erosion occurs if electrical potential in the crankshaft discharges through main or thrust bearing

EFFECTS OF SPARK EROSION

When the propeller is at rest, the stern tube, propeller shaft and bearings are in contact with each other. Similarly main engine bearing and journal are in contact with each other, maintaining continuity of the circuit. When the ship is running, due to the rotation of the propeller and lubricating oil film the shaft becomes partially electrical insulated. It may also happen on the tail shaft using non metallic bearing which acts as an insulation.

The propeller at the aft is a large area of exposed metal which attracts protective cathodic current which produces an arc while discharging from the lubricating film. This results in spark erosion of bearings, which can lead to worse situation if lube oil is contaminated with sea water.

If this effects continue for a considerable amount of time, it may lead to overheating of Main engine bearings caused by improper lubrication resulted by cavities from spark erosion. It may also lead to formation of oil mist, emergency shutdown of the engine or in extreme cases crank case explosion.

Why take your time to speed up or down...

Slowing down...

 So when slowing down one must consider the simple fact that such a large amount of weight needs a bit of time to react to speed adjustments. Many efforts in design and maintenance are made to make sure the response time of the engine matches the load as quickly as possible - considering the laws of physics.

Others things to consider when slowing down; the Propulsion Electric Motor (PEM) are connected to main electrical system of the ship, which means that along with lights and propulsion, are connected computers and other sensitive electronic systems. These systems do not respond well to changes in frequency / voltage and that is why numerous safety devices exist.

When abruptly slowing down on the propulsion motor telegraphs, as if downshifting in a "sport car", the load on the PEM is rapidly reduced. This in turns reduces the load on the generator, which is sensed by the engine's governor, reducing the fuel needed to turn the engine at the steady RPM needed. Slight performance imperfection in any of the many components of the system may slow that response, and the result would be a governor delivering too much fuel for a load that is no longer there, therefore speeding up the engine.

This slight surge may be enough to activate the various safeties designed into the system such as over voltage, frequency fluctuations and reverse power. The reverse power safety system detects when other engine(s) are driven by the faster engine, which will trip the circuit breaker, this generally causes an overload on the remaining generators, tripping them as well, resulting in, of course, a total black out.

So the moral of this little story is "take your time slowing down", although the bridge is far removed from the sounds and feels of the propulsion system, please consider the mass of steel and iron that needs to adjust. Avoid the temptation of slowing rapidly from 140 shaft rpm to 0 in one fast swoop. Even in an emergency, take a "minute" and prevent a bigger problem, like a black out, and a possibility of frying anything electronic.

If a speed change can be planned, it is always best for operational needs to give the Engine Room a ten minutes advance warning. That time allows the engineer and motormen to adjust water production, or steam consumption to match the engine load, assuring a constant service to all steam users.    

Speeding Up...

Speeding up needs the same considerations as slowing down, although the results may not be as dramatic as a black out. A lack of care when speeding up could be very damaging to the company in government imposed fines and sanctions, due to excessive visible pollution. Not to mention the real worries, thermal load of the numerous engine components.

Much more time is required when speeding up to full speed on the PEM; in particular, the upper range of load. Generally, the engines will easily adjust to loads up to 50% therefore usually not an issue during a "Stand By" condition. The problem of overloading the engines usually arises after "Stand By", where people just want to "get on with it!" when in fact its the most critical time for loading the engines.

There is no real rule of thumb of how many minutes it takes to reach full load in a safe and considerate manner, but the telegraph operator should expect 1/2 hour after "Stand By", before safely reaching full load on the engines.

The best guide for the average person is smoke from the stack. Anytime smoke is visible, it is an indication of overload, because fuel is not being burnt properly for whatever reason; "cold steel", reaction time of temperature devices, speed of turbo-charger, etc. If speeding up and a thick black smoke is noticed from the stack, the step taken was too much, reduce a little and wait until the smoke is not visible in order to take the next step.

Remember that the anxiousness to get up to full speed is usually the highness, when you need to take the most time speeding up, towards the 80-90% load on the engines. In this power range, a 5 shaft rpm increase may take 10 minutes to achieve without overloading the engines. 

Turbocharger surging

It is a phenomena of irregular pulsation due to a change in mass flow with respect to pressure ratio,at this breakdown the pressure pulsation is relieved backward to the compressor.

Sudden pressure changes due to chocking or throttling effect causes a breakdown of mass flow.A back flow of air now takes place from scavenge manifold at higher pressure o the turbocharger compressor side at a lower pressure. The reverse flow tend to drive the turbocharger in the opposite direction and partly stall it. This is repeated until air demand is increases and stable condition is achieved

IN SHORT-Air flows in reverse direction in a surge due to higher pressure at the scavenge manifold than the turbo charger compressor

Symptoms

• gulping air sound  at compressor intake
• violent pressure fluctuation in scavenge pressure
• howling/banging noise

Causes

• Fouling of air intake filter
• fouling of compressor turbine wheel,nozzle ring,EGE
• Sudden load change during maneuvering /overloading/crash astern condition
• Rough Weather
• fuel starvation,faulty fuel pump,fuel injector, severely wrong pump timing

Action

• Dirty air intake to be replaced or cleaned
• Regular cleaning of turbocharger (air and exhaust side)
• proper matching of turbocharger to the engine with respect to the compressor map characteristics

Leaky delivery valve in fuel pump

If the fuel pump barrel and plunger are worn out, there will be leakage between them, this will mean that after the spill Port is closed it will take longer for the pump to reach sufficient pressure to open the fuel injection valve, delaying injection.It also means that the total quantity of fuel delivered will be less than it should be for any given fuel rack setting.

The consequences are that fuel injection will start late, even if the spill timing is correct, and the maximum amount of fuel that can be injected will be less.

Symptoms

• Symptoms will be higher exhaust temperatures in relation to the load, and reduced maximum power output. 
• The severity of these symptoms will be dependant on the viscosity, and therefore the temperature of the fuel, because the viscosity &
• The density of the fuel, together with the clearance between the Barrel & Plunger will determine the mass of fuel "Lost" from the fuel pump.

The best solution 

• Would obviously be to change the Barrels and plungers as soon as possible.
• It is common for Engines that normally run on HFO to have problems after changing over to MDO because the reduced viscosity of the fuel causes such a large increase in leakage that the engine will no longer start.

Design factor

• Most modern fuel pumps are fitted with 2 spring loaded non return valves, the larger one, with the greater spring force opens to allow fuel to flow to the injector and closes at the end of the delivery stroke, the other opens at the end of the delivery stroke and releases the fuel pressure in the H.P. fuel pipe and injector, to prevent " dribbling" from the injector.

 On some engines the smaller valve is actually mounted inside the larger one. It should be fairly easy to test if these are opening at the correct pressure, and closing tightly, unfortunatly they are often sealed units (especially on Medium speed engines) that can not be dismantled and lapped in.

•  If the Non Return valves are leaking this can reduce the volume of fuel delivered IF air is drawn in from leaking discharge pipe connections, or combustion gas leaks back through the injector during the pump suction stroke.

INJECTION VS IGNITION LAG

Ignition lag

• It is a time delay between start of injection and star of ignition. 
• Factor affecting this delay are a rise in scavenge air or cooling water temperature, retarded fuel injection timing,ignition quality, the cetane number of fuel low load, load speed.

The duration of this period is set as a definite period of time, irrespective as to how fast the engine turns, and that period depends upon the chemical structure of the fuel. Basically, the lag period depends upon the number upon the number of molecular bonds which must be broken in order to release atoms of hydrogen and carbon from the fuel molecule. The longer and more complex the molecular chain, the greater will the amount of heat energy required to release the atoms and the longer will be the amount of heat energy required to release the atoms and the longer will be the ignition lag period. Because modern residual fuels result from complex blends of crude oil of many different types, they are complex structures and the ignition quality may be very variable between nominally the same grade of fuel. 

Formerly the cetane number was used to define ignition quality but cetane is a single element fuel and relating this to the complex nature of residual fuels is not realistic. The general term ignition quality is now used.
Ignition lag is the preparation period of the fuel within the cylinder for spontaneous ignition and beginning of combustion. The physical and chemical processes occurring during this period are characterised by weak ABSORPTION and liberation of heat. Thus there is little if any deviation from the compression curve. The length of the lag period depends on the fuels ignition quality and nothing else. The higher the ignition quality, the shorter will be the lag period, and the lower the ignition quality, the longer the lag period.

INJECTION LAG

• It is the time delay between the closing of spill port /valve of the fuel pump and the opening of the fuel valve. 
• It depends on the pressure rise of the fuel pump and the pressure in the injection line there is a delay due to compression of the fuel and expansion of the pipework.

Although liquids are often classed as being incompressible, they can be compressed to some extent at the pressures involved. Pipework will expand at these pressures and a certain amount of oil must be delivered in order to take account of these factors.

 Pump timing can be adjusted to take account of this because the amount remains the same at all engine speeds. When oil pressure reaches a high enough value the injector needle will lift and injection commences

HOT VS COLD CORROSION

Hot corrosion(vanadium and sodium)

• Hot corrosion is basically vanadium corrosion. Vanadium is the undesirable impurity in the fuel which is naturally occurring in marine fuel in soluble form
• When both vanadium and sodium are present in Na: Va of 1:3 ratio vanadium pentoxide is formed, a hard component whose melting point is 630deg.c.
• This component eats into the metal surface, leaving the surface exposes to corrosion.

Practical ways to restrict

• Vanadium cannot be separated by onboard centrifuges or purifier
• By using special fuel additive like ash limiters
• By maintaining exhaust temperature below a melting point less than 400 deg cels
• Special Sterlite coating on exhaust valve seats
• Regular inspection of the exhaust manifold and systems
• Vanadium can be restricted by limiting the content in the fuel by checking the BDN and confirming with the help of lab analysis

What is cold corrosion

• Sulphur is another element found in marine fuel
• Cold corrosion is the abnormal corrosion that occurs, when there is a drop in engine temperature(due to low loads) and temperature fall below the dew point 120-160 deg celsius and sulphur products sulphur trioxide (So3)reacts with condense water to form H2So4
• Sulphuric acid forms on the liner walls in an engine cylinder and corrodes the liner surface.

Effects of cold corrosion

• This abnormal corrosion then creates excessive wear of the liner material.
• Sticking of piston ring and piston grooves
• Decrease in engine life
• Cold corrosion is at its most serious in the newer engine designs. for part-load or low-load operation (also known  ‘slow steaming.
•  Some modified engines become mildly corrosive whereas others may be more seriously affected.

Practical ways to restrict

• Increasing scavenge temperature
• Using modern lubrication methods like alpha lubricator system and pulse lubricating system
• Using appropriate cylinder oil (TBN number)
• Scape down analysis of the scavenge space oil and employing condition monitoring system onboard to in TBN and iron wear
• Other modifications may include; turbocharger cut-out, variable turbocharger nozzle rings fitted, exhaust gas by-pass valve fitted, and engine tuning changes. 

After 2020 Global sulphur cap

• the IMO has decided that the global fuel sulphur limit of 0.50% will enter into force in 2020.
• This requirement is in addition to the 0.10% sulphur limit in the North American, US Caribbean, North Sea and Baltic Sulphur Emission Control Areas (SECAs). 
• Vessels that have exhaust gas cleaning systems installed will be allowed to continue using high-sulphur fuel oil (HSFO).

SCUFFING (microseizure)

This type of failure - caused by the local breakdown of oil film as the surfaces slide over each other during mating and disengaging 

With oil film breakdown, very high tempo are generated and welding of local high spots occurs Similar to that occurring with microseizure). These are then torn apart.
It is most prevalent at the tips and the root were relative sliding is at its greatest .

Scuffing is definitely due to failure of the oil film to carry the load, either because the operating conditions are abnormally severe, or because of incorrect oil selection.

Two stroke engine starting

•The direct admission of high pressure air, into the cylinders of the engine for starting, is used for larger engines, where the large masses and inertia do not make external cranking practicable.

•  Each unit has a separate starting valve, and there is a period of overlap, so that even if anyone unit starting valve should not operate, the engine can still be started.

Draw back of comp.air starting

•large quantity of compressed air is injected into each unit, and if the fuel injector were to leak, there is a chance of premature explosion, leading to excessive pressure rise and possible engine damage.

•To take care of this, cylinder relief valve are fitted.

•There need to be a minimum number of units, ( 4), to enable the engine to be started from any position.

•Moisture in the starting air will enter the unit, which could create corrosion problems

 Comp.air starting

•Starting air can be supplied from TDC till the opening of the exhaust valve / exhaust ports, in theory.

•In practice, the valve is timed to open a little before TDC, since there is always a period of 'lag', between the opening of the valve, and the actual action of the air, which must fill the cylinder first, before its turning effect is felt on the piston

•Inverse cams are preferred. It becomes easier to take the rollers off the cam,

•once the engine has picked up speed and there is no possibility of stuck pilot valves or broken springs causing any cylinder air starting valve to remain in the open position.

 comp.air starting/safety devices

•. Non-return valve in the Starting Air line, closest to the Main engine.

•. Relief valve in each Cylinder head of the Main Engine.

•. Means to prevent Starting Air admission to a unit which is already firing.

Main engine interlocks

Interlocks are provided so that the engine can be started or reversed only when certain conditions have been fulfilled. When there is remote control of engines, it is essential to have interlocks.

This reduces the possibility of engine damage and any hazards to the operating personnel.

1.  Turning gear Interlock. This device prevents the engine from being started if the Turning gear is engaged.
2.  Running Direction Interlock. This prevents the fuel from being supplied if the running direction of the engine does not match the Telegraph.
3.  Starting Air Distributor in the end position. This prevents starting from taking place if the shifting of the Distributor has not been completed.
4. Main Lube. oil pressure, Piston cooling pressure, Jacket water pressure, and important parameters must be above the required minimum.
5.  Auxiliary Blower Interlock. The Auxiliary Blower is provided in the case of Constant pressure turbocharging.
6.  Air Spring pressure Interlock. In case of the present generation of engines using Exhaust valves shut by Air Springs, the Air Spring pressure must always be maintained, else the exhaust valve may not close 

A smaller generator engine might have a tommy bar for manual turning interlock, the engine would not start if the Tommy bar is removed from its stoved position by a limit switch

MAIN ENGINE D.O. TO H.F.O. CHANGING PROCEDUR

PREPARATION

1. IT IS TO BE CONFIRMED THAT FO SERVICE TANK, FO TEMP IS ABOUT 75⁰ C.
2. OPEN THE STEAM TRACING FOR M/E F.O.  LINES. 
3. M/E FO HEATER AND VISCOCITY CONTROLLER TO BE PREPARED FOR WORKING.

PROCEDURE.

1. M/E LOAD SHOULD BE AROUND 75% OF THE NORMAL OUTPUT (i.e. AROUND 64%).
2. OPEN VISCO CONTROLLER STEAM INLET VALVE  AND OUTLET VALVE .
3. M/E HEATER INLET AND OUTLET STEAM VALVE..
4. BY OPERATING THE VISCOCITY CONTROLLER MANUALLY IN THE ENGINE CONTROL ROOM, D.O. IN THE SYSTEM SOUL BE HEATED TO 60 – 80⁰C, GRADUALLY AT THE RATE 2⁰C PER MINUTE.
5. IT IS TO BE CONFIRMED THAT THE D.O. TEMP IN THE SYSTEM IS 60 – 80⁰C.
6. OPEN THE H.F.O VALVE , AND SHUT D.O. INLET VALVE .
7. AFTER THE HFO IN THE SYSTEM IS TO BE HEATED UP TO THE PREDETERMINED VISCOCITY 10 TO 14 Cst AT RATE 2⁰C PER Min. BY CONTROLLING VISCOCITY MANUALLY FROM THE E.C.R.
8. ONCE THE REQUIRED VISCOCITY OF HFO ATTAINED – 14 Cst, CHANGE THE VISCO CONTROLL TO AUTO MODE (VISCO MODE).