Ever since its first use in 1912, the internal combustion engine has come to dominate marine propulsion with only a very few vessels making use of any other form of motive power. Steam power in the form of turbines is still used on some new LNG carriers and it is also employed in some waste heat recovery systems where the heat from an engine and other sources is used to produce steam for electric generation.
The vast majority of internal combustion engines used by ships are diesel engines; that does not mean that they are capable of running only on diesel fuel but that they rely on the Diesel cycle for combustion of the fuel.
The early beginnings
Back in the 19th century when internal combustion engines were in the very early stages of development, two men Nikolaus Otto and Rudolph Diesel devised different means of initiating combustion of the fuel.
Otto’s method was to compress the fuel to a particular volume and to then apply a source of ignition in the form of a spark. Diesel’s method was to continue to compress the fuel until it ignited spontaneously due to the heat produced by the higher compression used. At similar pressures, the Otto engines are considered more efficient but because diesel engines make use of much higher pressures, in practice they are more efficient and consume less fuel.
Modern oil burning engines mainly rely on the Diesel cycle but dual-fuel engines need an alternative ignition source when operating in gas mode. Wärtsilä dual-fuel engines make use of the lean-burn Otto process in which gas is admitted into the air inlet channels of the individual cylinders during the intake stroke to give a lean, premixed air-gas mixture in the engine combustion chambers and ignition is obtained by injecting a small quantity of diesel oil directly into the combustion chambers as pilot fuel which ignites by compression ignition as in a conventional diesel engine.
By way of contrast, in MAN Diesel’s high-pressure ME-GI DF engines the gas is injected only after the combustion air is compressed, after which it is ignited by the pilot oil injection. The gas burns immediately after injection. As is to be expected both makers claim advantages for their systems and it is for users to judge which best suits their own purposes.
Until the mid-1930s marine diesels were invariably four-stroke and ran on distillate rather than residual fuels. Today the giant two-strokes with their better power to weight ratio are the engine of choice for most large cargo vessels. Medium speed four-stroke engines either as propulsion units or powering gensets in a diesel-electric system are used for most other ship types. High-speed diesels are rarely encountered on commercial ships except as generators but are regularly used for propulsion in small tugs, work boats and ferries.
Who owns the largest share of the market?
Three names have dominated the large two-stroke sector for main engines, MAN Diesel & Turbo, Mitsubishi and Wärtsilä but the latter has now transferred its business to a new company - Winterthur Gas & Diesel (Win GD) – initially as a joint venture with China State Shipbuilding Corporation before disposing of all its stake to its partner. Within the two-stroke sector MAN has a dominant market share and four out of five engines supplied in this sector today are MAN Diesels. Mitsubishi is considered as the smallest in terms of market share of the three companies.
Although both MAN and Win GD can build these large engines, all but a handful are manufactured by licensees with Hyundai Heavy Industries claiming the lion’s share. With support from its Chinese parent, WGD has ambitions to significantly improve its market share in the future.
In the medium-speed main engine sector, Wärtsilä has almost half of the market, MAN Diesel & Turbo around a quarter with Caterpillar (MaK) leading the rest of the field. Many more names can be found on high-speed engines which are used as propulsion mostly by smaller vessels but also as generators by all ship types. Dual-fuel engines have until recently been mostly four strokes but they have now also made initial references in the two-stroke sector and interest is growing, albeit not at the rate that some have predicted.
An era of innovation
After a pause towards the end of the last century there have been some major leaps forward in engine technology as a consequence of increasing regulation and demand from users for greater efficiency. Leading the march of developments was the introduction of the common rail and electronically controlled fuel injection in place of the conventional camshaft. Mechanically controlled engines have not been withdrawn from sale but electronic versions are now overtaking them in popularity for new orders.
The degree of additional control and the extended power range allowed by common rail and electronic injection was first promoted as improving fuel consumption, allowing different fuel oil qualities and better part- and low-load running leading to such engines being described as ‘smokeless’. At that time, slow steaming as an operating strategy was not even on the radar but electronic engines have since contributed to making it easier to initiate.
Common rail was not the only development taking place in the early 2000s. Evolution of existing engine types meant significant improvements in power/weight ratios and the re-design of the cylinder head and combustion chamber also contributed to reduced fuel consumption. Other changes included improved engineering tolerances allowing for higher cylinder pressure permitted by longer strokes and in some cases a very slightly reduced cylinder bore. Turbocharger efficiency was improved by way of narrower exhaust systems. Improvements in medium speed engines saw production turning to a more modular system with maximum commonality of parts across engines with each manufacturers range.
This period also saw the first dual fuel medium-speed engines able to run on either fuel oil or LNG making their debut. Wärtsilä was the early pioneer in this field with the engines planned for use in LNG carriers. Today most of the leading engine manufacturers have incorporated dual-fuel versions of some engines into their portfolios although Rolls-Royce has opted to go with either diesel or pure gas versions of its Bergen engine range. Further developments are underway that have seen engines capable of running on LPG, methanol and liquid gas being produced.
Most research into engine development is done by makers acting alone but collaborative research is also undertaken with the EU funded Hercules project being the prime example. The initial Hercules project was conceived in 2002 its name being derived from the full title of High–efficiency Engine R&D in Combustion with Ultra-Low Emissions for Ships.
Planned originally as a long term project of at least ten years and involving leading engine makers MAN Diesel & Turbo and Wärtsilä, the project actually ran in three consecutive phases A, B and C. The engine makers were joined in the project by a wide range of partners encompassing, academic institutions, engineering organisations, classification societies and importantly a spread of end users from the shipping and power generation industries including some of the main European shipping companies.
Positive outcomes from the project include CFD simulations of the in-cylinder combustion process, two-stage turbocharging on four-stroke engines and reduction of NOx using direct water injection and other methods, a 25% reduction in friction for piston rings and guide shoes, Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction for cutting NOx.
At the plenary session of HERCULES C (the third and final stage of the initial project) in December 2014, it was reported that as a result of the project the following concepts were either already incorporated into new engines or in the planning process for implementation in the short term.
- Optimised part load setup on DF engines
- New turbocharger application for single-stage turbocharging
- Optimisation strategies for engine components and combustion development
- EGR blowers for IMO Tier-III compliant engines
- EGR rate measurement device for IMO Tier-III compliant engines
- Just-in-Time, Water-in-Fuel mixing unit
- IMEP monitoring and control of gas engines
- Misfire diagnostics for control of gas engines
- Cylinder-individual control system (planned for serial deployment)
- \Cylinder wear sensor (planned for roll out after successful field test)
Shortly after HERCULES C began, discussions began on the next phase which it was agreed would be called Hercules-2. In 2015 the new project was officially launched led by Wärtsilä, MAN Diesel & Turbo and Winterthur Gas & Diesel The declared aims are to develop basic technologies for use in 2- and 4-stroke marine engines but the project will be taking the research of the earlier projects forward in a number of directions.
The R&D efforts will focus on four main areas. These are; the application of alternative fuels and the optimisation of fuel flexibility to facilitate seamless switching between different fuels; the development of new materials to support high-temperature component applications; the development of adaptive control methodologies to significantly improve an engine’s performance throughout its life span; and to achieve near-zero emissions via combined, integrated, after-treatment of exhaust gases.
Only a little information has been released on progress in the first year but what is available can be accessed at the project’s website.
The question of fuel flexibility is one that has come to the fore over the last ten years or so with the advent of dual fuel engines and the need for switching to low sulphur fuels in ECAs or ports where local regulations are in force. Fuel switchovers still present a problem even though they have been necessary for almost a year in the US ECAs and for several years in the EU. In most other areas switching has not been an issue.
The quest for new materials will be focused in a number of areas with castings, cylinder heads and turbocharger turbine casings coming in for special attention. With higher temperatures and higher pressures being necessary for efficiency and emissions reduction, future engines will need to be manufactured from more robust materials. While this may seem to imply a heavier engine which would be seen as a negative aspect by ship operators, the additional weight should be more than offset by higher power densities. That could result in future engines producing the same power from a lower cylinder count or additional power from an engine of comparative dimensions.
The third aim of adaptive control methodologies is perhaps one of the most complex and one that will combine electronics, mechanics and software. Electronic engine control is already established technology but the aim of Hercules-2 is to extend the capabilities of present systems to optimise performance even more. One area of research will be the development of a fully flexible lube oil injection system and the development of an advanced real time tribosystem performance monitoring system. This will hopefully overcome the issue of lube oil choice and application when switching between HFO and low-sulphur fuels.
The final objective of achieving near zero emission engines is one that will be welcomed by regulators and environmentalists but in practice zero emissions are not achievable without engine add-ons which is recognised in the work package programme. Most of the projects concern SCR technology for NOx abatement.
One aim is to develop improved methods of SCR reduction agent injections and another to investigate robust catalysts for pre-turbo SCR operations. Development of emission measurement systems for integrated after treatment technologies is another as is integration of methane abatement for gas-fuelled engines. So far, integrated systems have mainly been developed for use with four-stroke engines and another aim of the project is to migrate the technology to two-stroke engines.
As with the previous phases of the Hercules project, this latest undertaking will set targets but it has already been announced that the ambition is to go beyond the limits set by the regulatory authorities. The project will include several full-scale prototypes and shipboard demonstrators that will speed the development of commercially available products.