LATEST MARINE ENGINEERING KNOWLEDGE MMD ORAL Q & A PART-4

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(a) Explain the influence that fuel ignition quality has on engine cylinder performance, describing the adjustments which are necessary in order to compensate for changing to a fuel of different ignition quality.

(b) Explain, with the aid of sketches, how the timing of a VIT fuel injection pump is adjusted, describing the effect that this adjustment has on the actual pump in order to bring about the change in fuel injection timing.

(a)

  • Ignition quality of a fuel is a measure of relative ease by which it will ignite.
  • For distillate fuels, measured by cetane number. If cetane number is high, fuel ignite easily in engine.
  • For residual fuel, ignition quality is measured by an empirical equation, based on density and viscosity of fuel and expressed as Calculated Carbon Aromaticity Index (CCAI). CCAI rating = 800 ~ 870.
  • The higher the index, the longer the ignition delay and the more explosive initial combustion.
  • Fuel with lower ignition quality (i.e. higher CCAI) result in late ignition, burning of piston crown and liner, afterburning and loss of power.
  • Fuel with higher ignition quality (i.e.lower CCAI) cause earlier ignition, high peak pressure, excessive loading on bearings and loss of power.
  • To overcome these problems, a fuel pump with some method of varying the injection timing must be fitted.
  • MAN B&W-MC engines fit, Jerk-type fuel pump with Super-VIT mechanism
  • In order to compensate for varying fuel quality, collective adjustment is included in the system.
  • Sulzer RTA engines fit valve-type fuel pump with VIT and FQS system
  • In order to compensate for varying fuel quality, adjustment of FQS lever is possible in the system.

(b)

  • The sketch shows the principle of the VIT adjustment for a MAN B&W MC engine.
  • VIT index is operated by a servo cylinder.
  • Variable control air pressure depends on position of regulating linkage which is operated by the fuel regulating shaft.
  • Relative position of this linkage controlled by a fuel quality adjustment setting.
  • Servo cylinder output is connected to the VIT rack.
  • This rack operates a pinion which turns a threaded sleeve locate in fuel pump barrel
  • Sleeve turns, fuel pump barrel rises and falls, thus varying the position of the spill ports relative to the plunger.
  • Start of injection begins after top edge of plunger has cut off the spill port, raising barrel retard the injection timing and lowering the barrel advance the injection timing

(a) Discuss the use of variable injection timing in fuel pumps.

(b) Sketch a fuel pump of the above type.

(c) Describe how this fuel pump works.

(a)

  • VIT is load-dependent start of fuel injection control, which provides an additional influence valves timing in fuel injection pump.
  • Improves thermal efficiency and lowers fuel consumption.
  • In normal engine, designed Pmax get only at full power (load), but in V.I.T engines, injection timing automatically changes according to load and Pmax get between 85% and 100% of engine load.
  • Reduction in SFOC is about 1.5 g/kWh at 85% engine load

(b)

(c)

  • Sketch shown is V.I.T fuel injection pump of Sulzer RTA engine, timed double valves type, driven by the camshaft.
  • Each pump consists of a plunger and guide bush.
  • Roller is kept in contact with the cam by a powerful spring.
  • Fuel pump deliver is controlled by suction and spill valves.
  • Downward stroke of plunger, it draws fuel oil from suction space.
  • Suction valve is lifted by ‘suction effect of plunger’ as well as fuel supply pump’s pressure in suction space.
  • In lowest part of plunger’s downward stroke, suction valve is also mechanically lifted by push rod.
  • When upward stroke of plunger begins, as long as suction valve is not close, no fuel is delivered.
  • Only when suction valve’s push rod has lowered to point where suction valve closes, injection begins. (Start of injection)
  • As upward stroke continues, plunger delivers fuel through delivery valve until spill valve is opened by its push rod. (End of injection).
  • Relief valves is fitted for safeguard against excessive pressure
  • For lubrication purposes, driving parts of fuel pump connected to engine bearing oil system whilst plungers and push rods are lubricated with fuel oils.
  • Automatic safety cut-out device is fitted in each fuel injection pump body between suction valves.
  • Under normal running conditions, suction control valves are free to move in regular motion.
  • A fuel pump taken out of service by a mechanical cut-out lever, which lifts driving piston and roller clear of the cam.
  • This lever can also be used for priming the injection system.

(a) Describe the design incorporated in Jerk type fuel pumps to enable variable Injection Timing.

(b) Why at low load VIT is not applied?

(c) Explain why Variable fuel injection timing (VIT) is necessary for ship’s main engines.

(d) Why fuel injection system is very important for the efficient operation of the engine.

(a)

  • VIT rack operates a pinion which turns a threaded sleeve locate in fuel pump barrel
  • Sleeve turns, fuel pump barrel rises and falls, thus varying the position of the spill ports relative to the plunger.
  • Start of injection begin after top edge of plunger has cut off spill port, raising barrel retard injection timing and lowering barrel advance injection timing.

(b)

  • At low load, with slow speed engine, condition of air inside cylinder not suitable in terms of pressures and temperature,
  • Allowing advance fuel injection to accumulate fuel on internal surfaces that lead to diesel knock, local burning and after burning.

(c)

  • VIT is load-dependent start of fuel injection control, which provides an additional influence valves timing in fuel injection pump.
  • Improves thermal efficiency and lowers fuel consumption.
  • In normal engine, designed Pmax get only at full power (load), but in V.I.T engines, injection timing automatically changes according to load and Pmax get between 85% and 100% of engine load.
  • Reduction in SFOC about 1.5 g/kWh at 85% engine load

(d)

  • Efficient combustion of the fuel depends upon proper atomization, penetration and turbulence.
  • Fuel injection pump, high pressure pipe and injector need to closely matched to provide optimum fuel injection.
  • All parts are critical to performance and a slight deterioration have adverse effects on fuel combustion and operating life of engine components.
  • Fuel injection system is very important for efficient operation of engine.

(a) Describe with the aid of a sketch, a fuel injection arrangement where a single fuel pump is used to supply three fuel injectors fitted in a single cylinder cover.

(b) Explain why 3 injectors are used instead of a single injector.

(c) Explain why sheathed fuel pips are used, describing how leakage of high pressure injection pipe is detected.

(a)

  • The System shown is a fuel pump of Sulzer RTA engine.
  • High pressure fuel from fuel pump discharge is led via a double walled pipe to a distribution block close to cylinder head and then through three double walled pipes to the injectors fitted in the cylinder cover.
  • The injectors are located by dowel pins to ensure correct position of nozzle.
  • To ensure fuel is kept hot, fuel at booster pump pressure is led from the supply rail through isolation valves
  • Circulated through all three injectors and distribution block before being led back through another isolation valve to return rail.
  • A priming valve is fitted to the distribution block.

(b) 3 injectors instead of single injector

  • Modern crosshead-engines fitted with a central exhaust valve utilize uniflow scavenging.
  • Because of centrally placed exhaust valve, three injectors fitted circumferentially around the cylinder head.
  • Atomizing nozzles, which are a different design to those of the centrally placed injectors, designed to inject the fuel into air swirl without contacting the liner surface.
  • A single injector could not be used because it would result in the poor air/fuel mixing with poor combustion & localized high temperatures, although some engines utilize two injectors/cylinder.
  • Due to tangential design of the scavenge ports, charge air rotates around cylinder axis in an action known as swirl which promotes better mixing of the fuel from three injectors & the air.
  • Overall results are uniform temperature distribution, more efficient combustion, and lower exhaust valve temperatures.

(c)

  • Sheathed or double walled fuel pipes between fuel pump and infector are used as a safety measure to protect against risk of fire in the case of a high-pressure fuel pipe failure.
  • Fitted to comply with the requirement of SOLAS.
  • To be effective they must be correctly, fitted and sealed against external leakage at pump and injector.
  • A leak off is led to an alarm pot and then to the fuel oil drain tank.
  • This alarm pot can be general to all units or individually fitted to each cylinder.
  • Should the alarm operate, the engineer must identify the leaking pipe and replace as necessary.
  • In the case of individual unit alarm pot, engine control arranged to cut fuel off affected unit, reducing load on engine at same time.

Sketch a fuel pump of Sulzer engine and describe its operation and Explain

(a) Quick fuel cut off

(b) Fuel control arrangement

(c) Materials for various parts

Operation

  • Downward stroke of plunger, it draws fuel oil from the suction space.
  • Suction valve lifted by ‘suction effect of plunger’ fuel supply pump’s pressure in suction space.
  • In lowest part of plunger’s downward stroke, suction valve is mechanically lifted by push rod.
  • When plunger upward stroke, suction valve is not close, no fuel is delivered.
  • Only when suction valve’s push rod has lowered to point where suction valve closes, injection begins. (Start of injection)
  • As upward stroke continues, plunger delivers fuel through delivery valve until spill valve is opened by its push rod. (End of injection).

(a) Quick fuel cut off

While running with reduced speed, fuel pump taken out of service quickly by a mechanical cut out mechanism which lifts the driving piston and roller, clear of the cam.

Procedures – When the engine is running, quick fuel cut off can be done as follows

1. Pulls handle outwards, turn it anti-clockwise through 90˚ and release it.

2. Handle gone into long slot, fit special spanner onto the eccentric sleeve and turn 180˚.

3. Then remove the spanner.

(b) Fuel control arrangement

  • Fuel pump delivery is controlled by suction and spill valves.
  • When plunger upward stroke, suction valve is lowered onto its seat, suction valve closes, fuel is delivered.
  • When the spill valve opens, end of injection occurs.
  • The effective stroke is controlled by adjusting the suction and spill valve’s eccentric position.

(c) Materials for various parts

Fuel pump body Mild steel
Plunger, Cam Case-hardened carbon steel
Roller 3% nickel casehardened steel stainless steel
Valve and seat Stainless steel
liner (in which the plunger reciprocates) Special quality cast iron

With reference to cylinder lubrication, state EACH of the following:

(a) The indications that the correct quantity of oil is being used;

(b) The consequences of both excessive and insufficient lubricant.

(c) Why cylinder liner lubrication is important.

(d) Why top of cylinder liner is subjected maximum wear.

(a) Indication correct quantity of oil use

Following symptoms are indicated that the correct quantity of oil used for cylinder lubrication.

(1) Consumption of cylinder oil is within manufactures recommended.

(2) liner, wear down within reasonable limits.

(3) No evidence of micro seizure on liner.

(4) No evidence of pitting on liner surface due to acid attack.

(5) No evidence of excessive built up of ash on piston crown.

(6) Satisfactory movement of piston ring in grooves.

(7) Cleanliness of piston ring groove.

(8) Normal fouling of deposits at scavenge spaces and entablature.

(9) Reasonable amount of cylinder oil drain from scavenge drains.

(b) Effect of excessive lubrication;

1. Alkalinity become excessive, much more than required to neutralize any sulphuric acid in the local area.

It results serious vertical grooving of cylinder liner, and piston rings in line with the oil feed points.

2. Fouling of ring grooves and resulting ring zone deposit.

3. Consequently, lose of gas sealing effect and blow by follows

4. Effecting combustion process.

5. Leading to breakage of piston rings.

6. Fouling of exhaust system and turbocharger.

Effect of insufficient lubrication;

1. Excessive both corrosive and frictional wear

2. Overheating at local area, resulting micro seizure due to lack of boundary lubrication.

3. Major damage to piston and cylinder liner.

(c) Cylinder lubrication important

1. Cylinder liner lubrication is a critical factor for satisfactory operation of engines.

2. Piston rings and liners are heavily loaded where contact with each other.

3. Proper lubrication results in a thin film of oil, which separates these components, reducing friction.

4. Satisfactory lubrication contains an important role, in achieving much needed sealing between piston rings and liner,

5. No gas or air leaking past the rings.

6. Result in loss of compression during compression stroke, resulting in lower power.

(d) Maximum wear of top cylinder liner

1. The gas load behind the ring is maximum.

2. The cylinder is also hottest in this region.

3. Viscosity of oil film is low and breakdown under heavy load.

4. Corrosive wear occur.