Diesel Engine Overview

Updated 22 Oct 2019


Internal combustion engines were first used for ships in 1912 but took more than 50 years to fully replace steam engines.

Early motors were not necessarily more efficient than steam but did allow owners to employ far fewer crew, as stokers to carry coal from the bunkers and shovel it into boilers of steam ships were replaced by simple pumps to move oil from tanks to the diesel engine. Thus, there was an immediate saving in crew numbers that will not be repeated if oil is replaced by LNG as a fuel.

Diesel engines can be either two-stroke or four-stroke with the former being low-speed engines and the latter either medium- or high-speed. Two-strokes are used for direct mechanical propulsion only, but four-strokes can be used either in mechanical propulsion systems – using a gearbox to reduce the engine speed to one more suited to driving a propeller – or in a diesel-electric configuration where the engine drives a generator to produce electricity which is then directed through cables to electric motors to drive the propeller or another type of propulsor. Either type of engine may also have a shaft generator to produce electric power when the engine is running.

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. 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.

Diesel engines can run on many types of oil fuel from heavy residuals through to light distillates but, although it is possible (and frequently necessary) to switch between fuel types, ideally an engine performs best when all parameters are matched to a single fuel type. Because of the polluting effect of engine exhaust when burning oil fuels, there has been a move to persuade owners to switch to burning LNG. This can be done either using a pure gas engine or a dual-fuel engine that is capable of running on either oil or gas fuels.

The imposition of a 0.5% m/m sulphur limit on marine fuels used outside of ECAs in 2020 is changing the driving factors in the choice of fuel type, although most analysts expect that residual fuels will still be the preferred choice of most operators. This infers that either sufficient compliant residual fuel will be available or sales of exhaust gas cleaning systems will accelerate, which has certainly been the case in the two-year period leading up to the 2020 deadline.

The development of dual-fuel engines is relatively recent but, although most major engine manufacturers now have models in their portfolios, they are still not fully accepted by the majority of shipowners and operators despite being heavily promoted. There has been an acceleration in their take-up in the last two years and there is now no major shipping sector where they are not being used.

The Diesel and Otto cycles

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 they make use of much higher pressures, in practice Diesel engines 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 and those produced by their successor WinGD 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 Energy Solutions’ 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. In 2019 MAN Energy Solutions announced that it too would be developing low-pressure engines making use of the Otto cycle.

As the 2020 deadline for the introduction of a 0.5% global cap on sulphur in fuels drew ever closer, it had been thought that the number of ships being ordered that are powered by dual-fuel or pure gas engines would dramatically take off. It is probably fair to say that this has not happened, although there has been a steady increase in the number of such ships being ordered.

Taking into account all of the ships currently afloat or on order that are capable of running on LNG at some point, including those built as ‘LNG ready’, the number falls short of 1,000 ships; 971 vessels as-at early October 2019. Against this, the number of vessels known to be fitted with or to be built or retrofitted to include an exhaust gas cleaning system or scrubber is touching the 4,000-ship mark.

In reality, that should come as no surprise as most ships afloat today are powered by an oil-burning diesel engine of some sort and conversion to gas burning would be expensive and, in many cases, unviable. After 2020 all of the oil burning ships will have to burn compliant fuel unless a scrubber is fitted.

Diesel engines come in various basic types. Direct mechanical propulsion systems are the most common, while diesel-electric, combined diesel and gas and other hybrid variants are alternative options that maybe encountered.

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