Slow, medium and high speed diesel engines

In the name of Allah who is the most beneficient and merciful

The term medium refers to diesel engines that operate with in the approximate speed range 250 to 800 revolutions per minute. High speed is usually 100 rpm and above, while slow speed diesel engines have 100 to 120 rpm. Each has their various advantages and disadvantages for various duties on board ship.

The slow speed two stroke cycle diesel is used for main propulsion units since it can be directly coupled to the propeller and shafting. It provides high power, can burn low grade fuels and has high thermal efficiency. The cylinders and crankcase are isolated, which reduces contamination and permits the use of specialized lubricating oil in cylinders and bearings. The use of two stroke cycle usually means there are no inlet and exhaust valves. This reduces maintenance and simplifies engine construction.

The medium speed diesel engines are generally to operate on the four stroke cycle and are trunk piston construction. These egines provide a better power to weight ratio and power to size ratio and there is also a lower initial cost for equivalent power.

The higher speed how ever requires the use of gearbox and flexible coupling for main propulsion use. These engine may be constructed in either “In-line” or “vee configuration” (shape). Vee types engines are constructed with two banks of cylinders arranged at an angle using a common crankcase and bed plate with considerable saving in size and weight. The most common method for connecting two pistons to each throw of the shaft is the side arrangement with two bottom end bearings.

Various makes of medium speed engines

Various make of diesel engines are as under

MAN B&W L58 ENIGNES

Engine of this type have cylinders bore of 580mm and 640mm, stroke operating at 400rpm. This is larger medium speed four stroke diesel engines; these are in six and nine cylinders. All are in line and operate on heavy oil and are designated ease in maintenance with extanded periods between overhauls.

SULZER ZA  40S ENIGNES

The engine has cylinders bore of 400mm and a stroke of 500mm and normal speed of 510 rpm. This engine produced in many cylinder numbers, ranging from six to nine for “In-line” and twelve to eighteen for “vee form” engines.

SEMT PIELSTICK PC26 ENGINES

It is one of the wide ranges of medium speed engines produced by the company for marine and land applications and available as “In-line” or “Vee configuration”. The cylinder bore is 400mm, with a stroke of 400mm and speed of 520 rpm. Cylinder numbers are from six to nine for “In-line” and from ten to eighteen for “Vee” versions. These are four stroke engines.

WARTSILA VASA 46 ENGINES

Wartsila R-46 engines are medium speed four stroke engines having trunk piston. The engine has cylinder bore of 460mm, a stroke of 580mm and designated speeds of 450, 500 or 514 rpm . it is available with between four and nine cylinders “In line” or with between twelve and sixteen cylinders in “Vee form”.

Fuel system of this engines uses twin or pilot injection. This gives easy starting and smooth efficient combustion of low quality fuel over the full ranges of engine power.

ENGINE DESIGN

The principle design parameters for a medium speed diesel engines are:

1. high power weight ratio
2. simple, strong, compact and space saving.
3. high reliability
4. able to maintain the fact that components are smaller and lighter than those for slow speed diesels makes for easier handling.
5. easily capable of adoption to un-manned operation
6. low fuel and lubricating oil consumption
7. high thermal efficiency
8. low cast and simple to install

TYPE OF ENGINE

Either 2 or 4 stroke cycle single acting turbo charged with inline or vee cylinder configuration. The main choice is certainly at present, for the four stroke engine due to the following reasons.

1. they are capable of operating satisfactory on same heavy oils which are used for slow speed two stroke engines.
2. effective scavenging.
3. higher mean piston speed which will give greater power.
4. engine can operate effectively with turbocharger out of commission, this would present a considerable problem with some two stroke engine of the medium speed type.
5. turbocharger power and size can be reduced.
6. it is also claimed that the fuel consumption would be reduced.
7. to reduce inertia forces use is made of aluminum alloy skirted piston or complete aluminum alloy pistons. Inertia forces must be taken into account for bearing loads. Important in trunk piston engines(which are the majority of medium and high speed diesels) where the guide surfaces is the cylinder liner, the smaller the side thrust the less  the friction and wear.

The following are the data’s of medium speed diesel engine currently in use:

RPM 600, cylinder bore 400mm, 4stroke, turbocharged upto 18 cylinders, developing approx: 700KW per cylinder, break mean effective pressure 23 bar, maximum cylinder pressure 160 bars, at approx: 600 rpm.

MAINTENANCE OF DIESEL ENGINES

The reliability and safe working of marine diesel engine depends upon the technical staff and in particular upon sea going engineers.

Planned maintenance of marine machinery can prolong its life. Engine builders, pump makers, in short all the makers of the equipment in ship provide instruction about the regular care and maintenance to be given to their equipment, with information and knowing the continuous survey requirement, the chief engineer plans the voyages maintenance work.

On board ship every engineer is delegated with the responsibility for the efficient running and maintenance of various items of machinery. Duties and responsibilities are clearly defined and are not to be left any uncertainity.

GENERAL MAINTENANCE INSTRUCTION

1.      trained and qualified personnel must operate diesel engines only. Unauthorized people must not allowed to operate the engine

2.      a logbook should be kept for each engine, in which all important data and work concerning the engine (eg number of running hours each day, actual fuel consumption, intervals between oil changes, pressure and temperature of engine and general performance remarks about machinery) are written .this provide useful information when irregularities or trouble occur.

3.      the safety regulations and instruction concerning safety measures to be observed during operation, maintenance and repair work.

4.      all essential parts of engine should be overhauled and cleaned up at regular intervals as per makers’ maintenance schedule. In order to keep the engine in good condition and constant operational readiness.

5.      A part from others factor the frequency of overhauls of various engine parts is determined by the load at which the engine is normally run and by the grade of fuel and lubricating oil used.

6.      overhaul jobs should be carried out only with the special devices and tools normally supplied with engine. The use of inappropriate tools will cause loss of time and damage to engine parts.

7.      all parts overhauls during maintenance work to be checked for correct functioning before they are put into operation again particularly pipes must be pressure tested for possible leaks.

8.      after the completion of the overhauls or maintenance work the engine should be cleaned with cleaning cloths (not by cotton rags or wool). Petrol should not be used for cleaning in closed rooms because of danger of explosion.

9.      for putting the engine into operation after overhauling or maintenance work, turn the engine over by hand or with the turning gear for checking that all moving parts are moving freely.

10.  to fill empty starting air bottle ( for starting engine) no other gases are to be used than air or carbon dioxide, even in emergency. In particular, oxygen or hydrogen must not be used, as this would expose the personnel and the engine  to very danger.

11.  engine must always have a reverse output hence it is advisable in continuous operation to limit the output to 85 to 90% of the rated output.

12.  if the engine had to be stopped on account of running gear components or bearing running hot the engine should be allowed to cool off for at least 10 minutes before opening the crankcase doors for only repair work or checking.

13.  if the carbon dioxide ( CO2) had been used in scavenging space or in any other compartment of the engine room for fire extinguishing, it must be thoroughly vented before going to work inside it.

DEVELOPMENT OF MEDIUM SPEED DIESEL ENGINES

The trend of the medium speed engine is towards higher power outputs per cylinder, with high  reliability, when operating on cheaper high viscosity fuels, manufacturers to improve the combustion process are carrying out much development work. This work focuses on the timing and duration of fuel injection to achieve reliable combustion and manufacturer are now testing engines operating with firing pressure in excess of 210 bar.

• Cylinder bore 580mm
• Stroke 600mm
• Speed 450rpm
• Power per cylinder 1250KW

The development in the medium speed engine has been such that it is now a serious competitor for applications, which were once only the domain of  slow speed 2 stroke engines. If the advantages and salient features of the medium speed diesel engines are examined then it will be appreciated that why this engine is dominating for certain vessels. Advantages and salient feature of medium speed diesel engine are as under.

1. compact and space saving
2. reduced weight of components
3. more cargo space available on ships having medium speed engines
4. higher power weight ratio of the engine a greater weight of cargo can be carried.
5. as using a reduction gear a useful marriage between ideal engine speed and ideal propellers speed can be achieved
6. modern tendency is to utilized unidirectional medium speed geared diesel engine coupled to either a reverse reduction gear or controllable pitch propeller which will give advantages of:
• less starting torque-reduced number of starts
• engine can be tested at full speed with the vessel alongside
• improved maneuverability engine can stop with in short time

Written By: Zeeshan Ahmed