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Medium speed engines. Vibration

The vibrations set up in a diesel engine are most complex, as both the magnitude and direction of the forces creating the vibration vary throughout one revolution. A mathematical approach is required, but the results of vibration can easily be understood by the watchkeeping engineer and should always be aware of the potential problems that continued vibration can bring. There are not only different magnitudes of vibration, but also great variations in the frequency of the vibration.

Medium speed engines. Cooling system

To keep the materials of the engine within the limits of their thermal and mechanical strengths it is necessary to regulate the heat build up in them. This is achieved by circulating coolants at the correct rate around and through the various components within the engine. Most of these systems can readily be traced out by the ship’s engineer, although the bore cooling passage ways may only become apparent when the engine is overhauled.

Medium speed engines. Air receivers and air bottles

The term ‘bottle’ is often used to cover both types of structure. Both have to be manufactured to specific requirements. On board maintenance amounts to regular surveys of the inside and mountings. The latter will include, at least, relief valve(s), pressure gauge connection, inlet and outlet valves, manhole, or inspection doors and drains. The drain should be situated at the lowest point of the bottle in such a way that accumulations of oil, water and solids can be blown out.

Medium speed engines. Air compressors and receivers

Air compressors, for starting air systems, are invariably of the reciprocating type. Although of slightly loss volume through-put than rotary, the reciprocating unit is more easily capable of developing the pressures required for starting air systems. Compression should, for maximum efficiency, follow the isothermal law, but in practice it is more closely aligned to the adiabatic curve, with the result that the delivery temperature is somewhat higher than is really desirable.

Medium speed engines. Scavenge fires

For a scavenge fire to occur there must be the three sides of the fire triangle: air, fuel and a source of ignition. The removal of any one of these would not only extinguish fire but prevent it occurring in the first place. It is impossible to prevent air flow through the scavenge spaces as scavenging implies air flow. However, fuel should never be present in the scavenge spaces so a clean scavenge space can never ignite. Ignition itself could occur were there to be any blow past the piston or were the piston to begin to seize in the liner. It may even be possible for the piston rod gland to overheat to the point where it could cause ignition.

Medium speed engines. Crankcase explosions

Crankcase explosions can cause serious damage to engine room equipment, but more important is the hazard to the engine room personnel. It is necessary therefore for the engineer to completely understand the process leading to the propagation of conditions favourable to an explosion. The engineer can then maintain his engine so that those conditions should not occur.

Medium speed engines. Starting air

There are several methods of starting a diesel engine, including manual, electrical and mechanical devices. The techniques used on a particular engine depend largely on its size, design and service requirements. Small diesels, such as those employed in lifeboats etc., may well be hand, or perhaps electrically, started. In main propulsion engines, or even diesel generators, such methods are unable to supply the substantial torque required to overcome the inertia of the large masses involved. These engines usually employ a system using the energy stored in compressed air.

Medium speed engines. Valves

Exhaust valves come in for a great deal of abuse as the engine is running, particularly if there are traces of vanadium or sulphur in the fuel. Many exotic, corrosive-resistant materials have been developed and employed on valve seats to combat this problem. Rotation of the valves has also proved to be a successful method of prolonging valve life, by maintaining the valve seating area at a more uniform temperature. A valve which is not rotated tends to heat up in one area more than another due to the directional flow of exhaust gas and air during the gas exchange period.

Medium speed engines. Piston and piston rings

Pistons may be cast iron or, on higher speed engines, aluminium alloy. The use of alloy pistons reduces weight and therefore bearing loading, and the loading on the cylinder walls. Allowance must be made for the larger expansion ratio of the aluminium alloys, and so large piston to cylinder clearances (cold) are adopted. As the engine warms up and expansion of a piston occurs this clearance reduces. For this reason, care should be taken to avoid overheating such a piston. Aluminium also suffers from carbon build up with possible burn out when heated above 3000C.

Medium speed engines. Connecting rods

Connecting rods in medium speed engines are usually of the ‘marine type’. The bottom end is separate from the palm of the connecting rod, thereby allowing the fitting of compression plates (these in turn control the compression ratio and compression pressure). These connecting rod palm may be of reduced width to allow its withdrawal through the cylinder as the piston is lifted.

Medium speed engines. Crankshafts and main bearings

Crankshafts may be under slung to protect the bedplates from firing stresses. The crankshaft is supported underneath the crankcase framework by bearing housings bolted up into the frame. There is therefore no real need for a bedplate, and frequently a simple sheet metal sump is sufficient. This means that the frame provides the required longitudinal and transverse stiffness of the engine and the omission of the bedplate reduces the overall weight of the engine.

Guides

The firing (and compression) forces are transmitted through the crosshead into the connecting rod so that, apart from TDC and BDC, a turning moment is developed at the crankshaft level. There will be a transverse reaction to these forces at the crosshead level, and this is taken by the guide shoes (slippers) onto the guides. A simple triangle of forces shows this reaction for one point alone in the cycle, for as the cylinder pressure changes so too does the loading on the piston.

Crossheads

Crosshead bearings are very difficult to lubricate and they run under the most arduous of conditions.
Throughout  the full cycle, 2-stroke crossheads are subjected to a vertical downward loading that is never reversed, whereas in the 4-stroke engine, the induction stroke reverses the loading on the gudgeon pin. The magnitude of the load varies throughout the stroke, being a maximum around TDC and gradually reducing as the cylinder pressures drop during the expansion stroke.

Governors

Considered by many merely as a means of speed regulation, the governor is, in fact, a very refined component, which in its most developed form is able to load limit, load share, load sense, regulate rates of acceleration. Not all these features, however, are available or even required in any one unit. A governor is usually made to suit the service demands on the engine to which it is to be fitted. It may provide single speed running conditions irrespective of load changes (isochronous), or be able to respond to increases in load so that acceleration is regulated to a level compatible with the effective and safe running of the engine. 

Piston rings

Piston rings are the engine components which undergo the moat arduous of service conditions. They are subjected to great heat during combustion and then substantially cooled as they pass over the scavenge ports. The net effect is quite considerable thermal stressing of the rings. The rings are also subjected to gas pressure. The forces generated by these fluctuating pressures vary in both magnitude and direction. At the top of the stroke the combustion chamber pressure rises to its maximum and forces the rings onto the lower faces of their grooves. In so doing the gas gains access to the back of the ring and pushes it hard against the liner wall.

Indicator diagrams

An indicator diagram traces out the pressure and volume relationships in the cylinder of an engine on rectangular axes, and it can be used to estimate the work done by the engine per cycle. The indicator must move a vertical distance proportional to the pressure in the cylinder and the drum, as it rotates, provides a horizontal motion proportional to the change in cylinder volume.

Load diagram

All engine builders provide diagrams for their particular models, from which various running conditions can be determined. The diagrams are assembled from information from engine tests taken under controlled conditions. The diagram for one engine of a range is then used as a standard for all other engines in that range.
Unfortunately there is no standardised format for these diagrams and the engineer must familiarise himself with the chart prepared for the engine with which he is working.

Surging

Surging (variously known as coughing, barking etc.) is a vibration of audible level emanating from the compressor end of the rotating element. The compressor, depending upon its speed at any particular time, can only discharge up to a given pressure. If for any reason the pressure in the scavenge space is equal to or higher than this discharge pressure, air will attempt to flow back through the rotating impeller. In essence this is like a centrifugal pump attempting to pump against a closed valve, but with the air compressors the back flow of air throws the rotating element into a vibration which produces the so called barking noise. 

Turbochargers

Labyrinth seals are fitted just inboard of the bearings to seal the shaft against air leakage. To assist this seal and to help cool the shaft, particularly at the turbine end, the labyrinth is supplied with pressure air bled from the compressor discharge volume.

Turbochargers. Sleeve type, white metal lined

These bearings provide, through their greater length, a stabilising influence on shaft alignment and longitudinal vibration. They tend to be adopted in the larger turbocharger, the thrust being taken by a face machined to provide the requisite oil wedges, similar to those formed in the classic tilting pad thrust block. In fact some models do adopt tilting pad thrust blocks. The clearance in these must be set so as not to interfere with the rotor to casing clearance mentioned above. Sleeve type bearings are usually supplied with oil from an external feed.

Turbochargers. Ball or roller bearings

The bearings may take one of two forms; ball/roller or sleeve type. 
Ball races will be fitted at the compressor end to locate the shaft, and thereby fix clearance between the casing and the blades of the compressor impeller (the most critical of clearances where performance is concerned, as any increase in clearance there would result in a rapid fall off in compressor performance, and too small a clearance would result in rotor to casing contact).

Turbochargers. Structure

In general, the modern turbocharger serves a system known as ‘constant pressure charging’, and one, or at most two, turbochargers per engine are all that is required. The more complicated ‘pulse charging’ system used on earlier engines and on some lower powered medium speed engines often needed three or more turbochargers. In the constant pressure system the pressure in the exhaust manifold leading to the turbocharger is virtually steady. That is, the pulses of energy that occur as the exhaust is released  from the cylinder are absorbed in the large volume exhaust manifold so that, at the turbocharger, almost steady flow conditions exist. The pulse system used these pulses to improve the output of the turbocharger, but the system was extremely complicated and best suited to an engine with cylinder numbers that were multiples of three. 

Scavenging

Scavenging is only applicable to 2-stroke engines, and is the process of clearing from the cylinder any remaining products of combustion from the previous cycle. Air, at a low pressure, is introduced into the cylinder through scavenge ports which are opened shortly after the opening of the exhaust valve. The prior opening of the exhaust valve allows the exhaust gases to expand out of the cylinder, reducing the pressure in the cylinder to well below that of the scavenge air.

Fuel pumps and injectors

Over the years there have been many forms of fuel pump and fuel injection systems. Present trends have settled, almost without exception, one the ‘jerk pump’ method, and by far the greatest proportion use pumps with the well known helix form of fuel regulation.

Fuel

The quality of fuels provided for ships today has deteriorated, so combustion process have to be regulated and monitored with ever increasing attention. As fuel qualities have deteriorated it is to be expected that they will include more and more elements that are either non-combustible or so difficult to ignite and burn that they form no useful part of the combustion process. In fact, they frequently deposit out as harmful substances where fuel pump and cylinder liner wear is concerned.

Cylinder covers

The design of a cylinder cover is very complex, particularly where a large valve operates through its centre. In some of the earlier loop scavenged engines, cylinder covers were simpler components, with a central fuel injector and then pockets for starting air, indicator cocks ect. In those cases the head was often cast iron with some form of steel backing ring to absorb the bending forces created as the head was tightened down. In later designs the head was a solid steel forging of immense strength, the cooling passages of which were formed by the bore cooling process described above.

Cylinder liners

Cylinder liners are, almost without exception, cast components which at first sight appear to be cylindrical units of no great complexity. However, even though their shape is simple the materials from which they are made are not quite so basic. For many years a good quality cast iron was used in their production. More recently, the worsening quality of fuels has given rise to greater wear rates. This has led to improvements in the liner material quality, to resist wear and to provide the liner with as long a life as possible. In alloying any material there is such a quantum leap in costs that no-operator owner must consider the benefit worth while before he buys in such components. 


Deflections

Deflections are readings obtained from between the webs of individual crank throws as the crankshaft is rotated. Standard procedure is to fit a dial gauge between the webs, usually as close to the shaft circumference as possible (at opposite side to throw), and set to zero when crank throw is as close to BDC as possible. Turning the crankshaft slowly and taking a reading at every 90 deg thereafter will provide top and bottom readings indicating the state of the shaft alignment in the vertical plane, and port and starboard readings indicating the state of alignment in horizontal plane.

Crankshafts

At some time, and in varying degress, the crankshaft is exposed to all forms of mechanical stressing. On the larger engines the crankshafts, for many years, been manufactured by forging, from a single billet, the combined ‘webs’ and ‘bottom end’ comprising one ‘throw’. These were then assembled into the composite structure of the crankshaft by ‘shrink fitting’ the relevant main bearings journals between each throw. These shrink fits, in the region of 1/600 the shaft diameter, used to be achieved by heating up the web and then entering the pin when the required expansion had taken place. However, with this method there was the possibility that slight products of oxidation, created by the heating, might become trapped in the interface such that the integrity of the grip was marginally reduced. To avoid this, liquid nitrogen or similar cooling agents have recently been adopted to cool the pin sufficiently to be entered into the web.

Bedplates

Bedplates are the “foundation” of the engine, without the support of which the shaft alignment in particular, and engine structure as a whole, would inevitably be lost.

Preparation of the engine systems and components

Be sure to make it a rule to check the readiness for every starting of the engine according to the following procedures. More careful checking of the readiness is required for the starting initially after installation or overhaul or a period of standstill.

Starting the engine and checks after starting

Engine start should be done by authorized personnel only. Except starting after short period of ‘Stop’ with normal engine condition, the engine should be started manually by ‘Local’ control system (‘Local’ mode) and the operator should be ready to stop the engine immediately, if needed during checking the condition of engine running. Be sure to check the surroundings of the engine in case of ‘Remote Start’.

Engine load-up

1. Load Up with Cold Engine

It is required to warm up the engine by loading up gradually upto about 50% load for a few minutes with MDO only.
The cylinder cooling water temperature should be minimum 60 ℃ to load-up to 100 % load.

Engine normal operation

Operating with constant load provides better service results continuously. Avoid abrupt load or speed change as possible. Regular checks and measures mentioned should be carried out during normal operation, which will contribute to earlier detection of any abnormality. The most important actions required during normal operation are as follows;

Engine stopping

The engine provides various stop mechanisms, which cut off fuel supply and stop firing fundamentally.

1. Normal Stop Procedure

  • Change over heavy fuel to marine diesel oil, if necessary for maintenance or long-term standstill.
    For the case of normal operation on heavy fuel, it is recommended to stop on heavy fuel oil, if only heavy fuel oil can be circulated continuously until next start. Otherwise, the fuel should be changed over to marine diesel oil to avoid clogging of the fuel system due to cooled down of heavy fuel oil. 

Engine Standstill

1. Stand-by Engine

In order to be ready for imminent normal service, all the requirements of the preparations for starting should be fulfilled together with further requirements as follows;

  • Keep fuel oil and cooling water circulating and pre-lubricating continuously.
  • Keep the engine in warm condition similar to normal operating condition by circulating high temperature cooling water of the other engine or cooling water heating system. Otherwise, warming up of the engine should be required before entering into normal service.

Running-in Engine after Renewal of Sliding Parts

After renewal or repair of piston rings or cylinder liners or bearings, the new sliding parts need to fit to mating parts to avoid any abnormal wear. Therefore, the engine should have running in operation step by step as follows:

Long Term Low Load Operation

Operating the engine with the load of below 20% may cause incomplete combustion and result fouling of combustion chamber as well as air and exhaust gas flow passages. Therefore, long term low load operation should be avoided as possible. However, if it is inevitable to operate the engine at lower than 20% load for a long time continuously, following measures should be carried out to minimize contamination of the engine inside.

Emergency Operation

In case of very unavoidable emergency situation such as the last solution, the engine may be operated with abnormal condition as follows with some restrictions to minimize hazard to engine

Engine Starting Failure

1. Engine is not under ‘START READY’ condition

  • Check whether starting tried after ‘START READY’ lamp ‘ON’.
  • Prepare for starting and reset.
  • Check function of control system and governor.
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