Updated 22 Oct 2019
In recent years it would seem that most technology advances have taken place on two-stroke engines. In part this is due to the fact that marine two-strokes are very specialised engines with few if any counterparts in other industries and rapid change has been needed to ensure that the most popular engine type for medium to large vessels can remain viable in an increasingly regulated environment.
With at least 25 different manufacturers plus a number of licensees, the four-stroke market is much more competitive than the two-stroke market. That could make survival for some makers difficult, but four-stroke engines are also used much more extensively for non-marine applications. The same engine that is used in a marinised version on a ship or smaller vessel might be found in a power station, for powering a train or industrial plant, on a truck or bus and many more applications beside.
That diversity helps in engine development and it is often in the four-stroke sector that innovations such as electronic valve timing, common rail, Miller timing and variable and two-stage turbocharging take place. Usually those developments migrate into the marine sector from land-based uses.
In terms of vessel numbers, four-stroke engines are much more prominent than two-strokes, but the ships concerned are, with a few notable exceptions such as multi-engined cruise ships, much smaller and include craft such as tugs, workboats and similar.
Four-strokes are higher speed engines encompassing both the medium- and high-speed types and as such are not suited to being directly connected to the propeller so require a gearbox, thus complicating the power transmission. Alternatively, the engines can be linked to generators providing their power through electric rather than mechanical means.
They are frequently installed in multiples on a ship in both configurations although in many ships, a single four-stroke will be the main engine. Because of the more situations that four-strokes are employed in, the number of the various models produced is generally higher than for two-strokes. This can mean faster development and series production of the engines. Four-strokes also provide the majority of dual-fuel engine types having been the first types to offer this additional flexibility.
Medium-speed four-strokes are able to run on all oil fuel types from HFO to MGO. In smaller ships, the use of HFO is less common, mostly because of the need for separate tanks for different fuel types and for the extra fuel treatment needed for HFO. Adding LNG as a fuel type obviously requires even more additional equipment for fuel storage and handling. With the 2020 MARPOL global sulphur cap now imminent, compliance among ships with four-stroke engines may present less of an expense since many such ships already run on the more expensive distillate fuels. For meeting the most stringent Tier III NOx requirements, four-strokes can employ Miller timing, SCR or EGR, which are described in greater detail in the section on two-stroke engines.
Where the normal choice would once have been mechanical drive or diesel-electric, today many more hybrid drive systems are being used and experimented with. There are for example combined diesel and diesel-electric drives, combined diesel and gas turbine drives, permanent magnet drives and systems that store excess power in batteries for use when power demand increases. It is the latter type that are now most commonly being referred to when the term ‘hybrid’ is used.
Power take-in and power take-out systems are most often built around a medium-speed diesel. Generally speaking, it is accepted that four-strokes as a type are marginally less efficient than two-strokes. They also tend to be squarer with bore and stroke much closer in dimension than the two-strokes where long and ultra-long strokes are the norm.
Four-strokes for propulsion purposes come in many sizes with the larger bore sizes from 400mm and up generally revving slower at up to 500rpm than the smaller sizes, which have speeds between 1,600rpm and 3,000rpm. Between the two are the intermediate bore sizes such as Caterpillar’s very popular 3500 series models with bores of 170mm and through to the 320mm-plus bore engines produced by the likes of MaK, Wärtsilä, MAN Energy Solutions and Rolls-Royce among others. Although under the same ownership, there is some overlap in the Caterpillar and MaK lines with the C280’s 280mm bore being larger than MaK’s smallest M20 engine.
The high revolutions of four-stroke engines mean they are too fast for direct mechanical drive and so will require a gearbox for mechanical drive or must be connected to a generator in a diesel-electric set-up. Although the large-bore four-strokes may come in six-cylinder in-line variants that mirror typical two-stroke configurations, most have many more cylinders and vee models to give more power output but with only a small additional length. For example, the MAN 48/60CR engine range has four in-line models with the smallest being a six-cylinder version producing 7,200kW and the largest a nine-cylinder version producing 10,800kW. The same range has 12-, 14-, 16- and 18-cylinder vee variants with power outputs of 14,400kW through to 21,600kW. The length difference between the in-line and their respective vee versions is in the region of 1.6 and 2.0m.
Multiple engine systems
With the notable exception of a small number of twin-propeller tankers and container vessels, most two-stroke engines are installed as the sole prime mover on the vessel. By contrast, on ships with highly variable power demands, such as a cruise ship or offshore vessel, it would be quite common to find four, six or occasionally more four-stroke engines installed.
Because all engines have an optimum load at which they operate most efficiently, operating below this will increase fuel consumption. In such cases, the power required will be provided by an appropriate number of engines operating at near to optimum speed with perhaps another engine operating at low load as a spinning reserve.
A multiple engine arrangement also means that failure of a single engine will rarely have disastrous consequences. The power arrangements on multiple-engined vessels will normally mean that engines of different outputs are available. This can be achieved by having engines of the same type but with different cylinder numbers or larger bore engines supplemented by smaller bore types.
The modular design of engines and common parts across a range mean that ships can benefit from carrying lower numbers of spares. Even when vessels are mechanically driven, often an owner will choose to have the same basic engine for propulsion and smaller version of the same type as a genset for electrical power.
Diesel electric systems
Unlike the mechanical power that is delivered directly to a propeller or through a gearbox, electric power produced by four-stroke gensets needs to be managed to allow for safe and efficient distribution to all the consuming systems. Opting for a diesel-electric propulsion system does mean that electric motors must be used to power any propulsors.
This has advantages and disadvantages. Unlike the mechanical drive systems, the cables from the electric distribution system to the electric motors can be routed in any direction without any problems and with far less space needed than for mechanical drives. This can allow engines and motors to be isolated from each other and permit power to be generated from any genset on the ship. The most obvious disadvantages are that electric motors are not cheap and when installed they are an additional point of potential failure.
Recent developments in electric distribution have focussed on the benefits of direct current (DC) rather than alternating current (AC) systems.
Another major advantage of diesel-electric is the potential for integration of energy storage sources such as batteries. The energy storage sources reduce the transient loads on the diesel engines and give much better system dynamic response times. Also, emission-free propulsion can be realised when running on batteries especially when they are topped up using energy from solar panels, recovered from waste heat onboard or even when charged in port using a local grid. The footprint of such a propulsion plant is up to 30% less compared with a classic diesel-electric propulsion plant.
Latest new models
In such a competitive field, regular releases of new models are essential to keep pace although it can be two or three years following the launch of a new engine before orders begin to be placed. Most of the new engine types announced over the last five years have been models that are available in gas burning versions as well as oil burning. That is not seen as marking the end of oil-powered ships as the sales figures show that the majority of orders are for single fuel oil variants.
At SMM in 2014 and Nor-Shipping in 2015, two new engines were unveiled. These were the Rolls-Royce Bergen B33:45 and the Wärtsilä 31 respectively. The bore sizes of the two engines placed them in one of the most competitive segments of the four-stroke market where they will be competing against models from MaK and MAN Energy Solutions. Rolls-Royce is almost unique in not offering a dual-fuel engine preferring instead to produce its engines in either pure diesel or pure spark-ignited gas variants; the B33:45 is no exception but the Wärtsilä 31 is offered in three versions: diesel, dual-fuel (DF) and spark-ignited gas (SG). The latter version for marine used was announced only in 2019.
The Rolls-Royce engine was described at the time as having world-class fuel efficiency and offering 600kW per cylinder in a compact engine design. In response to customer enquiries, Rolls-Royce focused on five main areas when designing the engine – achieving the lowest fuel consumption and emissions; highest power per cylinder in this engine class; increased power within the same footprint, and potential for fewer cylinders with lower weight and cost; a compact modular design and a base engine suitable for liquid or gas fuel; and dynamic and extended service intervals.
The Bergen B33:45 runs at 450-750rpm as a marine propulsion engine on propeller law, or 720/750rpm for 60/50Hz generator set drive. In-line engines were the first to be produced, with 6, 7, 8 and 9 cylinders spanning a power range from 3,600kW to 5,400kW. At Nor-Shipping in 2017, Rolls-Royce announced a V12 version and said more vee engines will follow. The V12 version allows for 20% more power over the B32:40 predecessor while maintaining the same footprint.
Power and fuel consumption figures are much boasted about by manufacturers but there is often very little between the engines on paper and in practice the running parameters selected by the operators will likely affect the claimed figures to some extent. The 600kW per cylinder of the B33:45 is the same as for MAN Energy Solutions’ 32/44CR but the SFOC at 100% is said to be 176g/kWh for the Bergen and 174g/kWh for the MAN engine. Wärtsilä has outdone both in the claims for its new 31 type, quoting 610kW per cylinder and a SFOC of just 170.6g/kWh for the diesel version. The Wärtsilä 31 is available in 8- to 16-cylinder configurations and has a power output ranging from 4.2 to 9.8 MW, at 720 and 750 rpm.
Most makers have opted for a modular design for new engines as a means of both reducing production costs and facilitating maintenance. On the 31 series engine, Wärtsilä has taken this to a new level by shifting from single parts to exchange units. For example, the powerpack unit now consists of a connecting rod, piston, cylinder liner and cylinder head with related pipes combined in one single exchange unit. Standardisation also makes conversion of the engine from one version to another a simple matter of swapping components without the need for machining.
In September 2017, MAN Energy Solutions unveiled a new engine range based on its earlier 48/60CR. Compared to its predecessor, the new 45/60CR engine achieves a sizeable increase in power per cylinder as well as a substantial SFOC reduction. Featuring the engine maker’s proven common-rail technology, the engine is fully geared to its next generation of innovative ECOMAP function. The 45/60CR, along with its brand-new safety and control system, is designed for operation in combination with MAN Energy Solutions’ SCR system to fully comply with the IMO Tier III emission regulations.
At 1,300kW per cylinder and with a SFOC of 166g/kWh, the new engine outperforms rivals in the segments that the engine is targeted at. It achieves this partly by running at 600rpm rather than the 500-514rpm of competing models. MAN Energy Solutions plans to introduce a dual-fuel version at a future date.