With reference to engine performance monitoring:
(a) Explain how it is determined that a faulty indicator is reason for diagram deformation rather than an engine fault.
(b) Describe the ways in which an indicator can give a false diagram.
(c) Describe how engine performance may be assessed when the use of an indicator is not suitable.
(a)
- A typical indicator diagram from an engine in good operating condition
- Shows that curves are smooth and maximum pressure occurs just after TDC.
- Length of atmospheric line is same as length of diagram directly proportional to length of piston stroke.
- Indicator diagrams take to check cylinder and combustion condition.
- Indicator diagrams alone are insufficient for accurate assessment of engine performance.
- Usually used with exhaust temperatures, fuel rack settings and scavenge pressures.
- If a diagram indicates low power for a particular cylinder, other readings are normal, fault indicator equipment.
- If diagram is jagged throughout whole diagram, faulty indicator
- If diagram is jagged round TDC, fuel injection fault, confirmed and analyzed by taking an out of phase card.
(b) Ways in which an indicator can give a false diagram.
(1) Indicator cock chock- blown through before taking cards.
(2) Indicator cock leaking – detect by hearing.
(3) Indicator cam is out of phase with engine. Checked with compression card, compression and expansion lines coincide.
(4) Indicator piston sticky – give jagged diagram, Maintain indicator equipment
(5) Wrong spring fitted. (weak spring gives a diagram with flat top/ strong spring gives a short diagram).
(6) Indicator cord wrong length. Drum will not rotate fully during engine cycle.
(c)
- For slow speed engines, mechanical indicator is only suitable.
- Not suitable for medium or high speed engines as higher speeds cause vibration in springs and drive mechanisms.
- For medium or high speed engines, use peak pressure indicators.
- Power in the engine is related to the peak or maximum cylinder pressure
- This can be a mechanical device or electronic device with pressure transducer.
- In addition, exhaust temperatures, fuel pump rack setting, scavenge pressure and temperature taken for accurate assessment of engine performance.
- Power and out of phase diagrams taken by using Modern computer equipment for engines operating at any speed.
Explain followings with aids of engine layout diagram:
(a) Engine Layout diagram
(b) Propeller Curve
(c) Propeller design point
(d) Fouled hull
(e) Sea Margin
(f) Engine Margin
(g) Heavy propeller
(a) Engine Layout Diagram
(b) Propeller curve
Relation between power and propeller speed of engine. The curve of a fixed pitch propeller is described by propeller law.
Pb = c x n³,
Pb = engine power for propulsion n = propeller speed, c = constant
(c) Propeller design point
- Combination of power and speed obtain call ship’s propeller design point(PD). placed on the light running propeller curve (6).
(d) Fouled hull
- When ship sailed for some time, hull and propeller become fouled increase hull’s resistance
- Reduce ship speed engine delivers more power to propeller, i.e. the propeller will be loaded and heavy running (HR).
(e) Sea margin (For heavy weather)
- At same time, if weather is bad with head winds, ship’s resistance increase compared to operating in calm weather conditions.
- When determining necessary engine power, to add an extra power margin, called sea margin.
- It is normally about 15% of propeller design (PD) power.
(f) Engine margin (Operational margin)
- Add 10% ~ 15% to Sea margin, get engine margin. Corresponding point called ‘specified MCR for propulsion’ (MP).
- Power for point SP is 10% ~ 15% lower than for point MP.
- Point MP is identical to engine’s specified MCR point (M) unless a main engine driven shaft generator is installed.
- In such a case, the extra power demand of the shaft generator must also be considered.
(g) Heavy Propeller
- When determining necessary engine layout speed that considers influence of the heavy running propeller for operating at high extra ship resistance, it is recommended to choose a heavier propeller line (2).
- High ship resistance caused by bad weather, fouled hull and propeller.
(a) Explain why power card alone is insufficient for accurate assessment of engine performance.
(b) Suggestive reasons minimum information required before an accurate assessment of an engine Performance.
(c) Explain the value of draw card and light spring diagram in assessing cylinder condition.
(a) Power card alone is insufficient for accurate assessment of engine performance.
- Cannot detect – about scavenge and exhaust ports performance, beginning of any fault to prevent serious breakdown
Power card show
1. PV diagram
2. Maximum peak pressure (Pmax)
3. Calculated mean effective pressure (MEP) from PV diagram
4. Calculated Indicated horse power of the cylinder/ work done
Power card does not show condition- scavenging and exhausting, early or late ignition, compression
(b) Minimum information required to assess engine performance
1. Barometric pressure – T/C and scavenge efficiency; consequently, air and fuel ratio, to reduce specific fuel oil consumption and to increase power output.
2. Wind force, wind direction and sea condition, take performance at clam weather
3. Forward and Aft draughts – hull and propeller condition, determining of engine performance.
4. Total running hour – to identify observation time.
5. Engine rpm – for power calculation.
6. Fuel index and VIT index – to determine fuel system condition, power estimation.
7. Jacket cooling water in/out temperatures- record for engine performance
8. Maximum combustion pressure and compression pressure- to determine cylinder condition
9. Specific cylinder oil consumption, specific fuel oil consumption, record for engine perfomance
10. Exhaust temperature – to determine exhaust valve and combustion condition.
11. (a) Exhaust temperature before and after turbocharger
(b) Turbocharger rpm
(c) Differential pressure of T/C suction air filter, charge air cooler
(d) Scavenge air pressure and temperature
To determine T/C efficiency, heat load on engine, air filter condition, air cooler condition, power estimation, ratio of compression, combustion and scavenge pressure.
After taking these information, by comparing with sea trial test records, determine engine performance.
(c) Draw card
Draw card reveals the following.
1. Early /Late ignition
2. Ignition delay
3. fuel quality
4. Defective fuel pump timing
5. Fuel valve-leaky, choked, incorrect opening pressure
6. Compression loss
7. During combustion pressure change
8. After burning
Light spring diagram
- During scavenging and exhausting period, shows how pressure drop occurs.
- Pressure of air box shows a clearness of scavenging system and exhaust passage
- Fouled charge air cooler
- Chocked scavenge ports or exhaust ports
- Leaking or late inlet valve
- Early or late opening of exhaust valve. Exhaust valve burning
(a) Why fuel valve requires to be cooled.
(b) Write about anti-dribbling arrangement of fuel valve
(c) Why needle lift is kept within limit
(d) Why size of nozzle hole is important for efficient combustion
(e) Sketch hydraulically operate fuel valve
(a) Fuel valve cooling
- To maintain nozzle tip region temperature within acceptable limit, to avoid malfunction of operation mechanism.
- Insufficient cooling cause fuel oil cracking, needle valves sticking and wearing and distortion. Clogging of sprayer holes
- It can lead to carbon trumpets or petal formation on tip around sprayer holes.
- Over cooling cause insufficient flow quantity, low temperature corrosion, and too high degree of penetration.
(b) Anti dribbling arrangement
- Fuel pump, delivery valve act as a non-return valve
- To prevent total flow back of fuel from high pressure pipe.
- To provides sharp end of fuel injection.
- To withdraw a small amount of fuel from high pressure pipe as it close,
- this action is brought by a piston portion formed on delivery valve.
- The angles of the needle valve and valve seat are cut difference about 1 ° to 2° to get point contact.
- By cutting needle angel about 2° larger than seat angle (60°), to prevent weeping and dribbling.
- This arrangement ensures sharp opening and closing of the fuel valve.
(c) Fuel valve needle lift
- Fuel valve needle lift is required to be maintained within limit.
- If exceeds, spring overstressed and probable to break.
- If less, at full load fuel wire drawn and combustion will impair
- The correct fuel valve lift in all units ensures:
- equal amount of fuel injection to each cylinder to get equal power distribution, equal and constant pressure all fuel valves
- Correct lifts reduce chance of spring breakage and minimize load on pumps and pressure pipes.
(d) Size of nozzle holes
- Size of the nozzle holes’ effect good atomization, proper penetration and good turbulence with air swirl.
- Upon fuel atomizing, droplets size depends mainly on size of holes and penetration.
- Fuel droplets not damage on internal components before burning completely.
- Ratio of length and diameter of hole is about 3:1.
- An increase in length / small diameter of hole may increase penetration.
- A short in length cause indefinite and if too large diameter not allow proper atomization.
- Numbers of holes between 3 to 8 depending on type and size of engines and diameter between 0.75mm to 1.0mm.
- Some fuel valve has different sizes of hole diameter.
(e) Hydraulically operated fuel valve
(a) State why double walled fuel pipe employ for high pressure fuel line.
(b) Sketch and describe such a double walled fuel pipe.
(c) Show how high pressure pipe failure is indicated.
(a) Double walled high pressure fuel pipes
Purpose
(1) To provide proper lagging for high pressure and temperature
(2) To prevent fuel leaking into atmosphere, especially onto high temperature working parts (exhaust manifold, cylinder head, etc.) in case of pipe damage.
(3) To comply with requirement of SOLAS.
(b) Double wall fuel pipe
- Pipe has ball-shaped abutment (conical fitting) at each end
- Tightens against a receiving conical recess in the fuel pump or fuel valve or distribution block
- The thrust piece is fitted at the thread of steel pipe.
- The abutment ends properly fitted to any receiving conical recesses by the proper landing.
- This proper seating or landing by adjusting innermost adjustable nut due to Makers’ instruction.
- “O” rings fitted into the flange recesses to prevent any leakage of oil to the outside.
- To prevent Fire Hazards in E/R and Personnel injury.
- The innermost high pressure fuel pipe is surrounded by the sheathed flexible hose to prevent oil leakage in case of pipe cracks.
(c) Indication for the pipe failure
- Any possible leakage due to innermost pipe fracture or improper landing of conical recesses
- It is drain through a special drain pipe into the collecting tank which is fitted with a float alarm.
- For MAN B&W MC engine, alarm system fitted in fuel oil distributor block and operated by control compressed air.
- If leakage occurs and its pressure exceeds one bar, leaked oil force the diaphragm outward,
- Resulting in audible alarm and automatic shutting down by actuating the fuel pump rack to “O” position.
- Leakage less than 1 bar drained into drain tank through passage inside distribution block, without activating diaphragm valve.
- Some small engines have an alarm system fitted at the drain tank, activated by float switch.
(a) Sketch the common rail fuel injection system using on large marine engine.
(b) Explain the working principle of common rail fuel injection system in detail.
(c) What are the advantages of common rail fuel injection system compare to conventional types?
(a) COMMON RAIL FUEL INJECTION SYSTEM
(b) Working principle of common rail fuel injection system
- Working principle similar to large engine air starting system
- High pressure fuel already reach in common rail by high pressure fuel pump
- High pressure fuel (1000bar to 1500 bar) ready to go inside of every cylinder
- All fuel valves normally closed by solenoid, except one fuel valve, get electronic signal from computerized control unit
- Signal sent to only one unit depend firing order and load conditions
- Sensors detecting flywheel position and signal fed back to computerized control unit to get correct fuel timing
- High pressure fuel pump driven by fuel pump actuator using servo oil pressure
- Fuel pump pressure and timing variable according to engine load and controlled by computerized control unit
- Variable timing use not only fuel timing but also exhaust valve timing
- Engine more efficient and less fuel consumption
- Large power output engine, one unit has 3 fuel valves.
- Low load condition, only one fuel valve working in each stoke by sequential fuel valve control methods
(c) The Advantages of common rail fuel injection system are
1. Meet IMO emission requirements
2. Fuel Saving.
3. Engine Peak pressure can be maintained at different operation condition to improve engine performance.
4. During low load condition, complete combustion due to sequential fuel injection method.
