Marine Engineering Learning Hub

 

Learn about the design, construction and working of an uncooled fuel injector

Introduction

Hydraulically operated uncooled diesel fuel injectors are fitted in large two stroke diesel marine engines. The main characteristic feature of these types of fuel injectors used in the main engine fuel injection system of ships is that cooling effect is not provided to the injectors, by the fuel oil. In this article, we will take a look at the construction and working of such diesel injectors. Take a look at the sketch below before you read further, in order to understand it in a better manner.

Uncooled Fuel Injector Sketch

 

Construction & Working

The design is similar for most engines and consists of a spring loaded non-return NEEDLE VALVE operated hydraulically by a fuel pressure wave from the fuel pump, to discharge fuel at high pressure through an atomizer nozzle.

A typical fuel injector consists of valve body or nozzle holder with the nozzle or atomizer being secured by a retaining nut. Needle valve is lapped into the bore of the atomizer and these must be kept as matched unit.

It has two chambers:

  1. UPPER chamber being charged with fuel oil from fuel pump sealed by needle valve.
  2. LOWER chamber or sac is sealed by mitre seat of the needle valve and has number of small atomiser holes of correct size and pattern to atomise and distribute the fuel spray into the combustion chamber.

The valve will open when the pressure from fuel pump acting in the shoulder of the needle valve overcomes the spring compression. As the needle valve lifts oil flows to the lower chamber in the atomiser.

Needle lifts rapidly allowing the fuel at higher pressure to pass through the atomiser holes into the combustion chamber. When the fuel pump cuts off pressure, the valve will close under spring compression.

Defects in Fuel Injectors and Rectification

This section lists a few of the commonly found defects in the fuel injectors and how to go about correcting the same.

Action of the needle valve must be rapid and positive with no oil leakage. The following checks should be sufficient to ensure that rapid action is retained without any leakage

  • Lapped surface must be free of damage and correctly aligned
  • Spring to be inspected for distortion
  • Atomiser holes must be clear and unworn

Defects in injector or leaking needle valve may cause :

  1. High exhaust temperature
  2. Surging of turbo charger
  3. Scavenge fire
  4. Carbon formation due to unburnt fuel
  5. Lifting of cylinder relief valve
  6. Reduction of combustion efficiency

Defects in fuel injectors while in use mainly occur due to the following reasons:

  • A possible choking due to dirt in the fuel
  • Carbon build up at the atomiser

So normally after the assembly the fuel injector is tested with a test pump to see it operating pressure and spray pattern are checked and there must be no leakages.

In the upcoming article we will take a look at cooled fuel injector used in medium and high speed engines.

 

 

Introduction

What do you do when you want to start your car or bike? Well you simply turn your ignition key and the battery of the vehicle turns the engine with the help of a starting motor and the firing process starts which continues till the engine is able to run on its own, of course this process takes very short time. But how do you turn the engine like that of a ship’s main propulsion plant? The answer lies in the use of compressed air for starting the engine, hence it is also known as the starting air.

Starting air admittance valves or air starting valves are provided in the cylinder head mountings of marine diesel engines. Their main function is to admit the starting air into the cylinder for starting the engine by air and closes when the engine picks up speed and starts running on fuel oil. In this article we will take a look at the construction and working principle behind these air admittance valves. Just take a close look at the picture given below which shows the full constructional details of the valve before proceeding to study further.

Air Starting Valve Diagram

Construction

The air starting valve is operated by pressurised air from the air bottle. Each air admittance valve is equipped with its own distributor and Ahead-Astern cam.

These cams have been adjusted for their opening times in such a manner, so as to ensure overlapping for consecutive cylinders for a certain time period, based on the firing order of the engine

Each cylinder is equipped with a starting valve fitted at the lateral side of the cylinder cover. The housing for the valve is integrally cast with the lower part of the two piece cylinder cover.

The valve chest is made out of a steel casing. The miter faced valve is carried at one end of the spindle, and the other end is provided with the guide piston.

The control piston is screwed with the extension of the valve spindle. The valve guide piston is fitted in a bush and is equipped with a number of seal rings.

A store spring keeps the valve firmly seated. A control piston with its cylinder is secured with the valve body by two studs, thus the body makes a gas tight joint with the cylinder cover.

Operational Details

The starting air valve is PNEUMATICALLY operated to admit starting air at the instant of starting.

Basically two types of air are used in the operation of the air admittance valve namely – pilot air and the actual starting air. The pilot air acts to control the entry of the main air which in turns actually starts off the engine and the procedure is described as follows.

The opening of the starting air valve depends upon the PILOT AIR which is send by the distributer; the pilot air is delivered from the control air which passes through the hydraulic interlocks and air starting handle.

The distributer valve is pressed down by the PILOT AIR when the roller stands against the inwardly depressed segments of the cam profile.

This is the starting phase of the cylinder, pilot air get passage and acts on the control piston and opens the starting air valve, thus the starting air is admitted into the cylinder.

In the upcoming articles we can see the consequence of leakage of air starting valve and starting airline explosion.

 

 

Importance of Oil Quality

We have been studying about marine diesel engines and know that lubricating oil system is an important part of the engine system. The properties of lubricating oil need to be maintained within the specified parameters, in order to the engine to operate smoothly and efficiently. In this article we will take a look at the hazards of low quality engine system oil. I am sure you know what viscosity is and what its relevance to engine running is.

Regarding the quantity of engine oil, the empirical rule states that the amount of system oil depends on the type of engine. It should be 1lit/bhp and lube oil should not circulate more the 15 times/hr.

  1. If very low quality of system lube oil is maintained in circulation, function of lubrication will be disturbed.
  2. Less amount of lube oil in circulation causes rise in temperature thus reducing the viscosity leading the failure of boundary lubrication due to decrease in oil film thickness.
  3. Insufficient time for de-aeration and it accelerates the process of oxidation of oil .Due to oxidation
  • Lubrication oil properties are lost
  • Forms sludge and high temperature sludge adhere to metal surface.
  • Formation of acids/corrosive attack.
  • Increase the viscosity of oil.

4. Increase in friction, wear, heat corrosion, contamination and noise and it reduces the engine performance to critical and in extreme cases total shut down of engine operation.

5. Also other properties of lubricating oil will be lost sooner as the additives will deteriorate faster.

Problems Associated with Lube Oil Viscosity Change

EFFECTS:

  1. Unable to form a lubrication film and losing the lubricating property.
  2. Increase in friction and wear and bearing damage.
  3. Overheating due to break down of lube oil film.
  4. Acid corrosion occurs if contaminated by high sulphur content HFO.
  5. Contamination with diesel oil reduces the flash point.

CAUSES FOR THE CHANGE:

  1. Fuel contamination – viscosity increases due to HFO and the viscosity decreases due to DIESEL OIL contamination.
  2. Contamination of insoluble like carbon from blow past.
  3. Oxidation-Increase viscosity.
  4. Contamination by other lubrication-Due to accidental topping up of wrong grade of lube oil.
  5. Less quantity of lube oil in circulation.

CORRECTIVE ACTION:

  1. Preheating of lubricating oil before purification for effective functioning of purifier.
  2. Purifier efficiency reduces contamination of insoluble and water regularly clean the sludges formed in purifier.
  3. Sufficient quantity of lube oil in circulation.
  4. Proper cooling of lube oil before sending to the engine.
  5. After circulation for the certain running period “BATCH PURIFICATION ” is done.
  6. Carry out shipboard lube oil test.

What do you mean by “BATCH PURIFICATION”:

  • Originally the term batch purification applied only to the purification of MAIN ENGINE crank case lube oil.
  • Batch purification referred to the system whereby the engine is shut down and the whole of sump lube oil charge is pumped up to the dirty lube oil tank in the upper part of the engine room.
  • The lube oil is heated in the tank and left as long as possible to settle out solids, sludges and any water.
  • It is then slowly purified in one batch, hence the name.
  • It is generally done in port where main engine is not running. While running of main engine we cannot do the batch purification because the regular continuous purification will be going on.

 

 

Ship Board Lube Oil Test

Qualitative oil test carried out in board ship do not give a complete and accurate picture of the condition of lube oil .This could be obtained in a laboratory.

However they do give good enough indication of the oil to enable the engineer to decide when the oil should be replaced or if some alteration in the cleaning procedure is considered necessary.

 

Test for Alkalinity, Depressiveness, contamination, water and viscosity are usual.

Samples of oil should be taken from the main supply line just before entry into engine since it is the condition of the oil being supplied to the engine that is of the greatest important.

 

 

 

Compared to direct diesel drives, diesel electric propulsion systems are technically and operationally superior in virtually all applications. This superiority has been a major reason for the steadily growing demand for diesel-electric main drives in marine engineering applications.

Introduction:

Electrical propulsion system offers numerous advantages for ships that are subject to specific requirements. They are rated as particularly economical, environmentally friendly and reliable, offer considerable comfort in terms of operation and control, have optimal manoeuvring and positioning properties, low vibration and noise levels, and additionally enable the best possible utilization of space owing to their reduced noise levels.

The electrical side of all systems will be based on a direct current or an alternating current motor, coupled to the ship’s propeller shaft, with the speed and direction of propeller rotation being governed by electric control of the motor itself or by the alternation of the power supply.

Layout of Diesel Electric Propulsion

The electrical propulsion arrangement for a ship is often described as a diesel-electric or turbo-electric system. It is characterized only by the type of prime mover with no reference to the type of electrical propulsion motor. When the prime mover is a diesel engine, then it is called Diesel-Electrical Propulsion. The most commonly used diesel electrical propulsion systems are not a new concept. In the past these systems were usually diesel engine driven D.C generators that supplied power to D.C motors. Their applications were generally limited to vessels that required a degree of low speed manoeuvring.

Vessels such as ferries, harbour tugs, and various other applications used diesel electrical systems for features that were not available in mechanical systems at that time like speed control and manoeuvrability. To date, electrical propulsion systems have been used mainly for specialized vessels rather than for cargo ships in general. These include dredgers, tugs, trawlers, lighthouse tenders, cable ships, ice breakers, research ships, floating cranes, and vessels for the offshore industries. Electrical-drive systems have made substantial progress in recent years.

 

 

Types of Diesel Electric Propulsion

The two systems dominating the market today are Frequency controlled A.C Motors and SCR controlled D.C Motors.

Frequency controlled A.C Motor drive system were generally more cost effective below 500 H.P and SCR controlled D.C motors systems at the higher end. The offshore drilling industries favour SCR controlled DC drives.

Modern SCR and frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue. The deep draft cruise ship industry, due to the high hotel-like power requirements, is adopting high-power diesel electrical propulsion systems in most of its new builds.

Both technologies have a proven record of efficiencies and reliability. For a direct current propulsion motor, the electrical power may be from one or more DC generators or may be form an alternator and then delivered through a rectifier as a DC supply. The power for direct current motors is limited to about 8 MW, and so AC machines are used for high outputs unless an effort is made to install DC motors in tandem. The rectification scheme can incorporate speed control and a means of reversing.

Power for AC propulsion motor is supplied obviously by an alternator; the prime movers may be a diesel engine, a gas turbine, or a boiler and steam turbine installation.

The choice of diesel electrical system as the power source for a propulsion system of a vessel has nothing to do with hydrodynamic efficiency. The propulsion system of a vessel provides thrust to move the vessel and is still chosen by the designer based on merits for the vessel’s application. Conventional propellers, controllable pitch propellers, azimuthing Z drives, transverse tunnel thrusters, and low speed water jet systems can be driven with equal effectiveness by a diesel-electrical system.

Diesel-electrical propulsion becomes viable when the installed KW for propulsion approaches or is exceeded by the KW installed for other purposes. The convenience of electric power distribution makes it possible to optimally locate the primary power source, i.e. diesel generators, exclusive of consideration as to whether it is for propulsion, thrusters, or cargo handling purposes. A large variation in propulsion power requirements, such as long periods of low speed operation or the necessity to shift power from main propulsion to thrusters for dynamic positioning purposes, can also justify diesel electric systems.

Modern turbo-charged diesel engines are efficient over a relatively narrow operating load and RPM range. They are not suitable for long period of low speed, low load, low RPM, high torque requirements for reversing large propellers. Modern generator systems with load sharing, auto-start, and load shedding features make it possible to efficiently utilize the installed horsepower of a diesel electrical system.

 

Lay Out of Diesel Electric Propulsion in Ships

Advantages of Diesel Electric Propulsion

Diesel electrical propulsion can overcome the following design problems:

When propulsive or station-keeping power requirements are a small or relatively small percentage of total power requirements, research vessels with special manoeuvring requirements, and gaming vessels where speed is inconsequential (such as a gaming vessel operating in a river).

When space and propulsion machinery limitations either exclude the use of direct diesel or adversely affect the construction costs resulting from using direct diesels:

1. Vessels with hull and struts too small to accommodate diesel engines, access, ventilation, etc.

2. Vessels with potential trim problems, such as stern wheelers, where machinery needs to be located forward to avoid trim problems.

3. Vessels that require, due to space limitations, more than one machinery space are subject to increased construction cost due to duplication of increases in system such as: engine cooling, space ventilation, control facilities, exhaust, etc.

4. Vessels that have a large variation in power consumption.

The fact that the propulsion power may be supplied by an electric motor instead of a direct driven diesel engine does not makes equipment aboard the vessel any less familiar to the operator.

The utilization of the diesel engines is transferred from direct propulsion power to generate power. This provides greater flexibility in the use of installed KW, and in some instances, reduces the number of diesel engines installed. The ability to generate only the power required to meet the needs of the duty cycle of vessels utilizing multiple generator sets reduces fuel consumption and maintenance cost. It also provides redundancy in power capacity.

Salient Features of Diesel- Electric Propulsion

1. Economic Reasons

Diesel electric propulsion is especially economical for a number of reasons:

  1. Optimal utilization of fuel for diesel engines to generate electrical power, even in partial load ranges.
  2. High efficiency across the entire speed range.
  3. Reduced maintenance costs through longer service intervals based on the optimized operating times of diesel engines with constant speed.
  4. Minimal standstill time for maintenance and service.
  5. Flexible and need-oriented use of diesel generator sets in combination power plant for drives and on-board power systems.

2. Availability

Diesel electric propulsion systems demonstrate high availability for reasons that include:

  1. Modular design with small probability of total loss of propulsion power.
  2. Sharply reduced number of moving mechanical parts.
  3. Proven technologies based on decades of operating experiences.
  4. Redundant drives with one propeller are also possible.
  5. Designs are also possible for maximum redundancy requirements.

3. Environmental Compatibility

Diesel electrical propulsion systems protect the environment because the pollution emissions of diesel engines are reduced by operating the engine at the optimal speed and load ranges.

4. Operating Convenience

Diesel electrical propulsion is very convenient for the users, because of the following:

  1. Excellent dynamic response from zero to maximum propelling speed.
  2. Short reversing time.
  3. Availability of maximum torque across the entire speed range at the propeller.
  4. Quite operation.
  5. Minimum mechanical vibrations.

5. Flexibility

  1. Flexible arrangement of components in the ship.
  2. Simplified mechanical requirements for the propeller shaft.
  3. Reduced space requirements in the shaft system.
  4. Design and engineering of propeller is independent of the drive.
  5. Flexibility in the choice of diesel engine speed.

 

 

 

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