MOTOR & MEP ORAL QUESTIONS & ANSWERS PART-6

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1973

Q 4-stroke connecting rod Specialty.

  • Big end and big end bearings are of split type
  • Big end is in an oblique direction to reduce the width of big end, reduce the load on bolts and increase the crankpin diameter
  • Top end is having bush type bearing
  • Rectangular or I-shaped is expensive to manufacture but necessary to resist high transverse inertia whip load, gas loads and to fulfill the weight to strength requirement
  • Connecting rod is forged from Magnesium Molybdenum.
  • Edges are serrated
  • It is subjected to high compressive and low tensile bending stress as well as of axial type
  • It connects crank pin direct to gudgeon pin

Q Fuel pump timing- Sulzer- RTA

Safety precautions and preparations before going for the job.

  • Interval of checking this timing- 6000- 8000 hrs
  • Change over from HFO to DO
  • Run the engine for atleast 30 minutes on DO
  • Close the fuel pump oil inlet and outlet valve
  • Obtain immobilization
  • Arrange a meeting before doing this work
  • Read the manual and understand
  • Keep ready the Sulzer RTA setting table- load index, effective stroke, ideal stroke and flywheel readings
  • Check the spares and tools required
  • Fill the DRA form- Detailed risk assessment form
  • Set the local reversing lever on emergency local panel in ahead position and make sure that engine is in ahead direction
  • Keep the main lube oil and cross head pumps running
  • Shut the stating air system and drain the line
  • Shut starting air distributor
  • Get propeller clearance from the bridge
  • Open indicator cocks
  • Engage the turning gear
  • Set the fuel lever at local panel to position ‘8’. This will vent the cylinder in the governor
  • Note the VIT reading and then make it ZERO & lock the VIT at position Zero.

Main procedure

  • Check that the roller and cam are intact by removing the cam cover. It means that roller is on the cam profile.
  • Drain the oil from the pump by opening the drain carefully
  • Dismantle the STAGNATION PRESSURE CONTROL VALVE from the top of suction and discharge valve & take care that no parts fall down. 260 N-m torque
  • Function of SPCV- IN SULZER (SPCV is stagnation pressure control valve) used to prevent the after injection on RTA engines. If the SPCV is removed, the fuel pump is directly connected to the high pressure pipe allowing stagnation pressure to drop to a level where cavitations can occur.
  • Open the cover on all three valves and mark them properly.
  • Note the valve designation. S=suction, D=discharge & U= spill valve
  • Then remove the springs under the stagnation pressure control valve with the help of a rod
  • Then use special wrench for removing the PRESSURE BUSHES- trassi sir told that the valve seat is loose fit and not interference fit. So to prevent it from blowing out during delivery stroke, the bush holds the seat in place. Also pressure bushes (nipples) are there to regulate the pressure on the spring and the valve.
  • Then valves are taken out with the help of withdrawing device from suction, discharge and spill
  • Clean them properly with kerosene
  • Put back the suction and spill valve back without their spring
  • Put back the pressure bush and tighten them to 300 n-m torque with the help of torque wrench
  • Now check the dial gauges for their free movements
  • Tolerance for the angles is +- 0.30

Q In Sulzer engine: during engine running 3 position valve leaking, what are its indications, how will you rectify it; which all places the air is lead to from this valve

Ans– 3 Position valve is reversing shuttle valve which is operated by air by 30bar and allows crosshead lube oil pressure to pass through reversing servomotor so that the required direction of rotation can be set. Reversing shuttle valve is like a spool valve having 3 ports (1-ahead, 1-astern, 1-confirmation that reversing servomotor has reached end position. About leakage spool valve have O-rings which seal the different ports so if leakage change them with O-rings provided in the repair kit. INDICATION of this valve to be faulty is that maneuvering can’t be done from the local maneuvering panel.

If you go through the maneuvering diagram of B&W and Sulzer engines you will come to know that B&W uses 7 bar air for control purposes which are:

  • Operating the directional control valves which activate the pneumatic cylinder for reversing  and puncture v/v
  • To operate the interlocks
  • To open main air start master valve
  • Engage the pilot valve by pressing them on the distributor cam
  • Operating the safety cut out

 BUT in Sulzer 7 bar air is only and only used for safety cut out devices

Only 30bar pressure air in conjunction with lube oil pressure is used for setting the direction of rotation.

Important to know both engine builder use 30 bar pilot air to operate their starting air valve.    Reason is force = pressure*area so if we reduce the pr to say 7 bars then area has to be increased by almost 4 times to obtain the same force implication. If we use pilot air. So at 30 bar air staring valve becomes compact.

Q Significance of mean piston speed.

Ans– The mean piston speed is the average speed of the piston in a reciprocating engine. It is a function of stroke and RPM. Mean piston speed most imp significance: heat of compression required for fuel ignition. That is why dead slow is possible. The upper limit of piston speed is limited by materials, friction and power loss, relative time availability for scavenging.

Q Alpha adaptive cylinder oil control lubricator.

  • Cylinder oil injection timing is 126.50 after the BDC
  • Injection is done between the top two rings when piston moving up. By this oil is trapped in between the top two rings & then evenly spread over the liner.
  • Consist of the inlet nitrogen accumulator ( 25-30 bar ) & outlet accumulator (1.5bar )
  • Build up oil pressure 40-50 bar
  • Injection is once in every 3-4 revolution
  • Average Wear down rate of the Alpha Lubricator : 0.05 mm / 1000 Hours
  • Average Wear down rate of the Mechanical Lubricator : 0.10 mm / 1000 Hours
  • Engine performance increases
  • TBO-time between overhaul increases
  • Less liner wear
  • High pressure injection directly into the piston ring pack
  • Very Precise Injection Timing( Electronically )
  • Optimal Utilization of the Oil with minimum of loss
  • Pre-lubrication of the cylinder before start of the engine
  • Very Easy & Precise control of the feed rate
  • Consists of 1 Lubricator for below 60 bore Engines
  • 2  Lubricator for the 70- 98 bore Engines
  • ANGLE ENCODER- Installed at the Crankshaft Fore End
  • Non return valve in the injector for the cylinder oil
  • PICK UP UNIT- Fitted at the fly Wheel- Consist of two pick-up sensors
  • Angle encoder is connected to the fore end of the crankshaft, & the signals are transmitted to the computer panels via a terminal box. For engines on which the crankshaft fore end is not available for angle encoder installations, trigger ring & Tacho pickups are installed at the Flywheel.
  • The BCU-back up control trigger system comprises of 2 Tacho pickups in a box at the turning wheel, thereby transmitting the engine rpm to the BCU. The backup pickups are also connected to the MCU for the surveillance purposes
  • The load transmitter is connected to the fuel rack, thereby continuously transmitting the fuel index % to the MCU, which calculated the engine load from this information & the detected engine RPM.

For the pump station- 2 pumps, Heater, Filters, Suction Tank and Pressure/Temperature sensor. One pump is for Stand-by

Q Opposed piston engines.

  • Two PISTONS & One COMBUSTION SPACE in each cylinder.
  • The pistons are arranged in opposed positions- crown to crown with the combustion space in between.
  • When combustion takes place, the gases act against the crowns of both pistons, driving them in opposite directions. Thus, the term opposed not only signifies that the gases act in the opposite directions (with respect to pressure & piston surfaces), but also classifies piston arrangement within the cylinder.
  • Two crankshafts (upper & lower) are required for the transmission of the power. Both shafts contribute to the power output of the engine.
  • In opposed-piston engines that are common to the Navy service
  • The cylinders of the opposed-piston engines do not have valves.
  • Scavenging air ports & Exhaust ports both are opened & close by pistons.
  • Movement of the opposed pistons is such that crowns are closest together near the center of the cylinder.
  • When in this position, the pistons are not at the true piston dead centers. This is because the one of the crankshaft operates a few degrees in advance of the other. The number of degrees that a one crankshaft travels in advance of the corresponding other crankshaft is called CRANK LEAD.
  • Instead of TDC and BDC here we have ODC-Outer dead center and IDC-inner dead center.
  • Upper and lower crankshafts rotate in opposite directions.
  • With the shafts at these positions, the pistons are closest together and are sometimes referred to as being at COMBUSTION DEAD CENTER.
  • INJECTION and the burning of the fuel start during the latter part of the COMPRESSION event and extend into the POWER phase.
  • There is also an overlap of the EXHAUST and SCAVENGING periods.
  • Lower crank lead influences scavenging as well as power output.
  • Since the intake ports are open for a brief interval after the exhaust ports close, air can be forced into the cylinder at a pressure above that of the atmosphere. (In other words, the cylinder can be supercharged.) This feature results in the development of more power than would be possible if pressure were normal.
  • The amount of power delivered by the lower crankshaft varies with the engine model. In some engines, from 70 to 80 percent of the total power output is delivered by the lower crankshaft. The power available from the upper shaft is already less than that from the lower shaft power because of the lower crank lead. The power from the upper shaft is further reduced as the engine output is concerned, by the load of the engine accessories which the upper shaft generally drives.
  • Modern engines of the opposed-piston design have several advantages over the single-acting engines of comparable rating. Some of these advantages are less weight per horsepower developed, lack of the cylinder heads & valve mechanisms (and the cooling & lubricating problems associated with them), & fewer moving parts.
  • Single-acting engines have their own advantages, such as not requiring blowers, if they are of the 4-stroke cycle design. These engines are more efficient if they are supercharged with a turbocharger, which is driven by the otherwise wasted energy of the exhaust gases. Certain repairs are easier on a single-acting engine since the combustion space can be entered without the removal of an engine crankshaft & piston assembly.