High speed engines bring emissions and cost benefits
Demands for higher efficiency, lower emissions and fuel restrictions are prompting designers to look at high speed diesel-based prime mover options, by Dag Pike.
The role of high speed diesels in the marine world is expanding. Such engines offer the prospect of increased efficiency and reduced cost compared with their heavy-duty and higher-cost counterparts. Increasing legislation regarding the operation of marine diesels that is demanding increased efficiency, reduced emissions and the use of specific fuels is prompting the move towards the use of high speed diesels.
Vessel designers are starting to appreciate the benefits that can accrue from using high speed engines rather than their heavier lower-speed counterparts. Naval architects are developing innovative propulsion solutions where the high speed diesel forms part of a fully integrated propulsion system.
Part of this move towards using high speed engines comes from ECA and local port requirements in many parts of the world, under which low sulphur fuels must be used. This can take away any cost advantage of using lower-speed engines running on cheaper heavy fuel oils. This is particularly applicable to applications such as offshore supply vessels that operate mainly in regions where the high-sulphur heavy fuels are not permitted.
Once the commitment has been made to make the vessel operate on low sulphur distillate fuels then doors can be opened to use a variety of alternative propulsion solutions that were not viable when the machinery was restricted to low speed diesels because of the cost benefits of using heavy fuel oil. Take cost benefits of heavy fuel out of the equation and there is a much more level playing field between the different engine types. An example of this can be seen in the use of MTU’s Ironman series of high speed diesels that have been developed specifically to meet the requirements of tugs and inland waterways craft, previously the exclusive domain of medium and low speed engines.
High speed diesels cover a wide range of power outputs from the smallest engines of perhaps 10bhp, up to the mighty top-range V20 8000 MTU diesels that can produce over 12,000bhp. It could be easy to lump all these together as high speed diesels but there are significant differences between the large and small engines in the high speed ranges. Gone are the days when diesel engines were developed exclusively for marine use, and the smaller high speed diesels up to around 300bhp are today based mainly on car units such as those from the Volvo and VW ranges. These are highly developed engines that are mass produced in their thousands and so the base price of these engines is low compared with the power than they produce.
Above this size, and up to 500bhp to 600bhp, marine diesels are largely based on truck engines that are also mass produced and relatively cheap per horsepower. They may not be produced in quite the same quantities are the car units but the numbers are sufficient to justify mass production. For marine use these engines are then modified with the addition of marine inter-coolers and cooling and exhaust systems. Today these engines almost invariably employ common rail injection systems and electronic control and this enables the output and the operating parameters to be easily adapted to the requirements of the marine market.
Above this size and up to perhaps 2,500bhp to 3,000bhp the engines used in the marine sector are largely based on engines used for mining, military and rail use. Again the qualities involved are sufficient to allow the engines to be built with mass production methods, which serve to keep the price down while allowing for the engines to be highly developed and tuned to meet emission controls and other regulatory requirements. Once you get above this size the market for each engine type becomes a lot smaller and is largely focussed on the power generation market and marine use.
These larger sizes of engine have to be virtually hand built although of course many of the individual components of the engines are mass produced. By using engines of different configurations within a given engine design the manufacturers can meet a wide range of power requirements within one basic engine design. So a particular engine design may embrace engines of six cylinders in an in-line format followed by engines of V8, V12, V16 and V20 format that could cover power outputs from say 4,000bhp to 12,000bhp.
So from the ship and boat designer’s point of view there is a huge range of engines and options to choose from, but the conservative approach still favours a twin engine layout and on most cargo ships the single large engine option remains most favoured. These options of single and twin engines will probably be based around low or medium speed engines which are traditionally used in these installations. However some enterprising designers are looking at other options, and taking a lead from large motor yachts. In this sector designers have started to appreciate the benefits of a multi-engine installation using high speed diesel engines.
In one installation, in a fast 45 knot motor yacht of 35m in length, a comparison was made between using three MTU V-16 engines from the 2000 Series or two engines from the MTU 4000 range. Both installations would offer the required 7,000bhp, but the weight of the three 2000 series engines was some 9 tonnes less that that of two 4000 series engines, which would give a significant boost in performance. The three 2000 series engines could sit side by side, and this would significantly reduce the space requirements, gaining another 2m of valuable accommodation space. A clinching argument for the three-engine installation came in the form of the cost savings with these three engines coming in at about €0.5 million cheaper than two 4000 series engines.
When looking at the application of these smaller high speed diesels the propulsion systems available can play an important part. The yacht example above used Arneson surface drives, which offer a very efficient system for high speed operations. An alternative would be waterjets, and these can be configured for lower-speed applications, such as several Gulf of Mexico supply boats. There have been some attempts to use high speed diesels in displacement vessels such as coasters and fishing vessels where two or more engines are used to drive a single propeller, but the complication of a sophisticated gearbox can negate any economic advantages of smaller high speed diesels. This may change as gearbox technology advances, and this could form a useful study as the cost differential between high and low speed diesels widens and the need to operate on low sulphur fuels expands.
Emissions are the other factor that is focussing the minds of the engine designers; here the high speed diesel does appear to have an advantage. The quest for obtaining more and more power out of smaller engines demands greater efficiency and that in turn can lead to reduced emissions. Most high speed diesels on the market today can, or will be able to, meet the tier 2 emission requirements by selective tuning and internal changes to the engine. When it comes to tier 3 levels of emission control then it does seem that in most cases external features will have to be added to the exhaust system, such as filters and catalytic converters. There is a lot of research going on to see what will be required to meet these low emission levels and it seems likely that the need for external requirements will be considerably less than at first thought.
Halyard Marine, which specialises in marine exhaust systems, has introduced the PureGen system. This exhaust unit is claimed to remove 99% of all particulate matter from the exhaust. It is aimed at marine generator sets powered by high speed diesels, where there is particular concern because gensets are often used within harbour limits where emission control tends to be tighter.
Emission control on generators will be the focus of considerable attention in the development of tier 3 standards, and the main problem here is the variable loading. One of the difficulties with any form of emission control is that it can be a challenge to provide effective limitation under variable loads. There are two approaches with generators, one to vary the speed of the generator in accordance with the demand ,and the other to maintain a constant speed as the demand fluctuates. With modern sophisticated control systems both of these techniques can be viable but the jury is still out as to which is the more effective and fuel-efficient system.
It is the same with hybrid propulsion systems. They can certainly reduce emissions in harbour areas by allowing vessels to operate on electric power alone for short periods, but that electric power has to be generated somewhere - and that will lead to emissions. Also the conversion of electric power creates losses, so overall a hybrid system will be less efficient that direct diesel drive. There are some small high speed diesels where a motor generator is incorporated between the engine and the gearbox to facilitate hybrid propulsion.
The high speed diesel sector is seeing some structural changes as the major players seek to consolidate their positions. Rolls-Royce and Daimler have successfully bid to take over Tognum, the MTU parent company, which will allow Rolls-Royce’s Bergen Diesel subsidiary to offer a full range of high speed and medium speed propulsion systems, in direct competition with Caterpillar. High speed diesels are the missing factor in the Rolls-Royce portfolio, and in terms of performance MTU is recognised as the main player. Cummins is investing heavily in new production facilities but that is mainly to meet demand from the truck sector. Its marine side, in the smaller engine sizes, has been hived off into Cummins Mercruiser Diesels where the link with Mercruiser allows the combination of engines and drives to be offered. It appear that VW-based marine diesels with be joining this group. The larger engines, based on industrial and mining engines, have proved popular for tugs and offshore support ships in some markets.
Fiat has a developing interest in the marine sector through its newly formed FPT subsidiary, and the French engine manufacturer Badouin is expanding its offerings with higher-performance versions of its mainly fishing vessel and workboat units. Caterpillar remains a strong force in the high speed sector, with the Cat high speed engines matched by the lower speed units from the MaK range to offer one of the widest marine ranges available.
The marine sector of the high speed diesel market is going through a process of evolution and development to produce more efficient and emission free engines but there is one development that promises a revolution. Concerned that so much energy is wasted through heat lost through the exhaust and the cooling system, British company Ultramo is seeking to reduce some of these losses with a new approach to engine design. The Ultramo concept would see the very hot exhaust gases passed through a heat exchanger to recover otherwise lost heat there, and to operate the cylinders at a much higher temperature, so that far less heat is lost into the cooling system. Ultramo’s prediction see engine efficiency rising to 70% from the current 40 % and the company has received recent grants to enable a prototype engine to be built. “Our theories are based on sound thermodynamic principles and are an original solution to the process of converting fuel into power”, says Chrissi Wilkins, MD of Ultramo.
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