Design Features and Operating Mechanisms of MDE
Design of Marine Machinery
In the basic design of Marine Machinery for main propulsion, the following points to be considered.
What type of Main propulsion Engine do we need? That depends on a number of factors, Cost of the engine and its delivery schedule; to be able to match the ship construction work in progress; designed machinery space available on the ship; the overall plant efficiency; specific fuel oil consumption; compliance to current and near future environmental restrictions for emissions; operating costs including spares and their ready availability; and issues involving important components like Turbocharger, Governor, the auxiliary engines and so on.
In many cases, the shipowners experience with a type of engine also makes a big difference; if a shipowner has deep experience with a particular slow-speed engine, it is unlikely that he will go for another. The company tends to build upon the skills of its workers and would feel uncomfortable introducing a different engine, e.g., just to save 0.5% of fuel economy. That is the reason the engine makers always try to introduce a new version of an engine at a discounted pricing until, the engine type has proven its worth on board. It took no less than 5 years for the super-long-stroke engines to gain its acceptance.
So, if new Technology is to be used, question will be, are they proven? And, are the current Maine Engineers capable of managing these engines or, would they require extensive additional training? These are the dilemmas of a shipowner.
Based on these points and mainly on the propulsion characteristics, either a diesel engine, with the latest available proven technological improvements or a COGES system (as the one seem to be a current preference) -- Again, initial cost plays a major role, One needs to pay more for the latest technologies. An alternator coupled with a Turbocharger which uses exhaust gas as a normal turbocharger, but also takes care of the complete electrical load, when the main engine is running; this combination was costing less, when it was introduced in the market; for the same engine system: the cost is more than doubled after it has been a success in the first few ships.
Since it is very important that the Engine and the propeller are correctly matched, their selection often goes in tandem. We can decide on the diesel engine and the propeller type- fixed pitch or variable pitch based on the availability of space where the machinery layout plays an important part. But the more popular choice today, for the larger ships demanding higher power , is to go for long stroke slow speed engine. Even if the vessel is not placed on a liner run and must tramp as demanded by the market. The winner here is the lower fuel consumption and ability to manipulate speed within a reasonable range with optimum efficiency. Whereas, if a LNG carrier is built, on a contract covering the life cycle period of the engine; and, with assured loading and discharge ports on fixed voyages; that commercial assurance, will certainly play a major role in selection of the main propulsion machinery, where new innovation can be supported with lesser commercial risks.
Improvement in Design
Engine manufacturers have been busy with a number of design improvements over the last decade; some of those are contribution from,
- Sophisticated methods of numerical analysis permit accurate predictions of the thermodynamic performance of the engine, the scavenging process, stresses, strains and deformations under both mechanical and thermal loads, heat transfer and temperature distributions in engine components and the vibration modes of complex structures.
- Mechanical design concepts such as bore cooling of the combustion chamber components, thereby achieving improved mechanical and thermal loads capacity with good reliability.
- New measurement and monitoring techniques on running engines for stresses, flow patterns, temperatures, providing more in-depth analysis and thus better design optimization.
- Improved turbocharger performance, which led to higher working cycle efficiency and enabled a higher specific engine output.
- Wear and corrosion of key components are kept within limits in spite of higher combustion temperature and pressure such as in exhaust valves, cylinder liners by improved materials, and better understanding of tribological problems with improved lubrication
- Electronic Fuel Injection and removal of Camshaft
- Electronic Governor with finer degree of control and ability to maintain engine RPM
- Common rail fuel injection with removal of fuel pumps from individual units
- Hydraulically driven Exhaust valves that operates electronically from a central engine computer
- Multiple fuel injectors in units improving the combustion efficiency
- Turbocharger re-circulation with better efficiency and stability at lower load operations
- Improved surveillance and monitoring of all hotspots and efficiency points, and many more.....
Ships Engines and propulsions: 2 and 4 stroke
- For the progressively larger powers and approaching the mega-ships, the continuation of slow rotating large propellers and directly coupled long stroke diesel remains a majority choice.
- The stem apertures of vessels have been redesigned to accommodate larger diameter propellers. The Hydrodynamic improvements have also been possible by adding all sorts of propulsion aids. e.g. ducts, fins, vane wheels, bulbs, asymmetric stems, flow straightening nozzles, propeller boss cap fins. etc.The present state of engine development appears to favor the two-stroke cycle for the large, slow-running, direct coupled engines and the four-stroke cycle engine for medium speed and high speed engines used for generators and indirect propeller drives.
High bmep increases the stresses on nearly all the engine components, with a consequently greater rate of cylinder wear, whilst the alternative of higher speed, valve flutter may become a serious problem.
But the 2 stroke engines have many more advantages,
- Each out-stroke being a working stroke gives more even turning for the same number of cranks, consequently a lighter flywheel may be employed.
- The operation of reversing the direction of rotation is simplified since there are not so many cylinder head valves to contend with.
- Fewer moving parts and lower maintenance
- Lower specific fuel consumption
- No gear loss
- Simplicity in construction
- Longer life time, Higher reliability
- Low lube oil consumption
- Better ability to bum low quality fuel oil.
The 4-stroke engine have good volumetric efficiency, Good combustion characteristics, Positive exhaust scavenging and a less arduous lubrication requirements due to the presence of an idle stroke which allows spread of lubricating oil through bearings and around the liner. The thermal and mechanical efficiency are slightly better than those of two-stroke engine. In this type of engines only half the quantity of the heat generated in the cylinders has to be dealt with a given time, so that efficient lubrication of the piston and cooling of the cylinder is more easily accomplished.
The followings are also advantages of using four stroke engines:
- Lower initial cost for equivalent power
- Ease of installation
- Lower weight per power output
- Saving in weight and engine room length
- Increase cargo capacity
- Free choice of propeller speed through gearing
- Suitable for electrical power take
In conclusion it can be summarized, that from a purely efficiency and usefulness point, both 2 stroke and 4 stroke engines have made their presence felt within the maritime sector. In that larger vessels have been using slow speed 2 stroke, whereas the smaller vessels made use of the 4-stroke engines, which due to their higher speed could generate higher power and did not need a lot of space. Thus, the choice has been largely driven by commercial suitability.
The current trend toward newer plant designs with combined cycles and better Waste Heat Recovery and also with electric propulsion initiatives, the medium speed engines could become a strong contender. The push for burning better fuel and gas could also tip the balance toward more application of the 4-cycle and even higher theoretical efficiency in gas burning via Otto cycle is on vogue.
Wärtsilä and MAN- B&W are the foremost license providers across the globe; in 2015, they accounted for more than 95% of the global low speed engine market share and over 75% of medium speed engine market share. At present, Japan, China, and South Korea collectively account for over 80% of the regional marine diesel engine market size..Global marine diesel engine market size was valued at around USD 6 billion in 2015 and is anticipated to exceed USD 8 billion by 2023, with growth rate at over 4% from 2016 to 2023.
Industry Analysis
Marine Propulsion Engine Market size was over USD 9 billion in 2015, and is anticipated to grow at 4.3% CAGR from 2016 to 2024.
