Two-Stroke Diesels

Updated 22 Oct 2019


Large marine engines are impressive examples of the engineer’s art and have improved in thermal efficiency over time to today’s level which is above 50%. Most wasted energy is in the form of heat, some of which is recovered through the use of heat exchangers to provide hot water and domestic, cargo and fuel heating as necessary. It is possible for more to be recovered through waste heat recovery systems that can power small turbines to produce electricity removing the need to run an auxiliary engine.

The lost energy may seem excessive, but it is on a par with other methods of producing energy from combustion of fuels and a diesel engine is as efficient as current fuel cell technology in practice, even if considerably below the theoretical efficiency of such systems.

Two-strokes are the engines of choice for almost all large ships and, with the exception of a relatively small number of recent newbuildings, run on heavy fuel oil. Those that do not use oil are either of the dual-fuel type or are running on ethane or methanol. Two-strokes are used as prime movers only and ships equipped with them will also have at least one medium- or highspeed engine operating as a genset.

After 2020, when the MARPOL Annex VI global cap on sulphur in fuels is reduced to 0.5%, only ships equipped with SOx scrubbers will be permitted to run on any fuel that does not meet the requirements. It will in fact be an offence for ships that do not have a scrubber to have non-compliant fuel on board other than as cargo except when compliant fuel is unavailable.

After 2020 most ships will have to run on distillate fuel or one of the new fuel types being developed that may retain most of the properties of HFO but without the typical sulphur levels associated with it at the present.

Two-strokes are the operators’ engine of choice because, at the engine’s optimum point, their speed is most commonly around 90-110rpm which by coincidence is the preferred speed for propellers, so direct connection without a gearbox is the usual configuration.

The choice of engine model will depend upon the chosen power output selected by the owner and with so many overlapping options available, several factors will come into play including experience with different engine types and the benefit of similar engines across a fleet. The latter point ensures crew experience and the advantage of a reduced spare part stock.

The power available for the largest engines of the type can be astonishing: more than 6,000kW per cylinder. However, the race for power of the early 2000s has given way to a more conservative approach and many of these most powerful engines look destined to be supplied in de-rated versions.

Three names dominate this sector for main engines: MAN Energy Solutions, Winterthur Gas & Diesel (WinGD) – which has acquired the two-stroke business formerly owned by Wärtsilä – and Japan Engine Corporation (J-ENG), which marks another change of ownership after Mitsubishi Heavy Industries spun off its engine business which integrated with the former licensee Kobe Diesel. MAN Energy Solutions has an almost 90% market share, WinGD around 9% and J-ENG the small remaining balance.

There are two variants of a two-stroke diesel engine: the trunk type or the crosshead type. Trunk engines have a shorter stroke than a crosshead and have the piston connected to the crankshaft by a simple connecting rod. They use a common lubricating oil for all aspects of the engine and the oil splashes up to lubricate the liner. Trunk two-strokes are rarely used these days as prime movers.

Crosshead engines have much longer strokes. In these engines, a diaphragm plate separates the crankcase from the cylinder liner space and the piston has a long rod passing through the plate using a stuffing box that separates the upper cylinder lubricant from the system oil. The piston rod is connected at the crosshead to the connecting rod attached to the crankshaft. These are the engines that power the vast majority of bulk carriers, container ships, PCTCs, container ships and general cargo vessels. There are also a significant number of LNG carriers that have two-stroke diesel engines.

Development of two-stroke engines accelerated around 20 years ago when it became clear that the IMO would limit NOx output from engines. It was also a period of rising fuel prices when shipowners began demanding more efficient engines.

Camshaft-less engine development

For most of the history of the internal combustion engine, mechanical control of valves has been by way of a camshaft. In October 1998, the first electronically-controlled intelligent engine – a MAN B&W 6L60ME – was installed heralding the gradual demise of the camshaft-controlled two-stroke. That process is still continuing and now less than one in ten of all two-strokes are camshaft models.

The electronic control that replaced the camshaft allows for more flexibility in valve timing, permitting improved flexibility in power output and reduced environmental impact. As can be expected, development of electronic control has not ceased and improvements to valves and openings are regularly made.

MAN Energy Solutions has two main types of electronic control in regular use. On the newer ME-C engines, the electronic control includes flexible control of fuel injection timing and actuation of exhaust valves, starting valves and cylinder lubrication whereas on earlier type ME-B engines, which are still favoured by some owners and remain in production, the injection timing is electronically-controlled but actuation of the exhaust valves is camshaft-operated, but with electronically-controlled variable closing timing.

Simultaneous developments were also taking place in the fuel injection systems with individual cylinder injector pumps giving way to common rail systems where a single high pressure pump and common feed were employed. On some large multi-cylinder engines, a segmented common rail system is employed rather than a single unit to eliminate stresses in materials and pressure fluctuations.

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