Large numbers make improvements easier
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 as 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 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 which are 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.