diesel engine

diesel engine, an internal combustion engine, which means that the fuel is burned inside the engine cylinders, as opposed to a steam engine where the fuel is burned externally in a boiler for steam propulsion. A diesel engine may operate on a two-stroke cycle or a four-stroke cycle. A two-stroke engine gives one power stroke for every two strokes (up or down) of the piston or for every one revolution of the crankshaft. A four-stroke engine gives one power stroke for every four strokes of the piston or for every two revolutions of the crankshaft. This means that for the same size and rotational speed a two-stroke engine is likely to develop twice as much power as a four-stroke engine. The engine has a number of cylinders, each with a piston which attaches to the crankshaft by means of a connecting rod.

Marine diesel engines for ships are of the crosshead type or the trunk piston type. The crosshead type is similar to a reciprocating steam engine, in that it is tall, and runs at relatively low speed. It is connected directly to the propeller shaft, which means the engine speed is changed to change the ship's speed, and it is stopped and reversed to make the ship go astern. It almost always operates on the two-stroke cycle, and is used to power container ships, tankers, and bulk carriers because there is plenty of space to fit such an engine into them. The trunk piston type of engine is similar to an outsize car engine. Most of these are not connected directly to the propeller shaft but via a gearbox, though some are used to drive an electric generator for electric propulsion. Unlike the crosshead-type engine, it is uncommon for them to be reversible and, as the engine's speed is constant, a controllable pitch propeller is used to change the ship's speed and direction. This type of engine is not as tall as a crosshead type which makes them ideal for ferries and cruise ships where low engine-room height means additional car or passenger decks can be fitted. Medium-speed trunk piston engines can also be used to drive electrical generators for purposes other than the ship's propulsion.

Diesel engines were introduced to ship propulsion in the early years of the 20th century. The first installations were for river and harbour craft, but as they became more reliable their use for marine propulsion gradually increased. The Fram, which in 1911 took Amundsen to the North Pole, had a diesel engine, and the first successful ocean-going ship powered by diesel machinery, the Danish Selandia, launched in 1912, showed that the diesel was a practical and economic alternative to coal- and oil-fired ships. By the end of the First World War (1914–18) many ships had diesel engines, although these tended to be the smaller cargo ships; and its lower fuel consumption, greater range, and the availability worldwide of bunkering stations, led to a further increase in diesel's popularity during the 1920s. One of the earliest diesel-engined passenger ships was the 18,815 gross ton Swedish American liner Gripsholm. Built in 1925, she remained in service with her original engines until 1966.

The early diesel engines burned diesel oil, similar to the diesel fuels used in modern diesel-engined road vehicles. This is a distillate fuel and much more expensive than the boiler oil used for oil-fired steamships. However, during the 1950s diesel engines were developed which could burn the residual oil used by oil-burning steamships, which is also known as fuel oil, bunker C, and heavy fuel oil. Residual oil is what remains after crude oil has been processed and the lighter, higher-value commodities, such as kerosene, gasoline, gas oil, and diesel oil, have been extracted. With the development of residual fuel oil-burning marine diesel engines in the 1950s, the last economic advantage the steam plant had over the diesel engine, using cheaper fuel, was gone; and from that time diesels gradually replaced steam engines for all forms of conventional ship propulsion.

With few exceptions, all commercial ships built from the late 1980s onwards have been fitted with diesel engines and some ships have been re-engined with diesels. For example, the liner Queen Elizabeth II had its steam turbine plant replaced in 1986/7 with nine diesel engines, and the ship is now powered by diesel electric propulsion. Not only did this reduce the fuel consumption, it reduced the number of engineers running the plant. The efficiency of diesel-powered ships is further increased by using the engine's exhaust gas to generate steam. This can be used for various heating purposes within the ship, and waste heat in the engine cooling water can be employed in evaporator plants to generate fresh water from sea water.

Apart from its economy, the diesel-engined cargo ship could also often travel twice as far as a similarly sized steamship on the fuel stored in its double-bottom tanks, and the diesel's thermal efficiency, just over 50%, means that just over half of the chemical energy in the fuel is converted into work at the engine crankshaft. This compares very favourably with the best marine steam-turbine plant which only achieves a thermal efficiency of about 30%.

However, residual fuel oil does have its drawbacks. It has a very high viscosity which, at room temperature, has the consistency of treacle. For it to be pumped and burned it must be heated, so that its viscosity is the same as diesel oil. It also creates a major pollution problem if it gets into the sea, particularly in large quantities from oil tankers that founder or run aground. It does not break down easily and if washed ashore it coats rocks and beaches with a tarlike substance which is very difficult to remove, destroying local fisheries and tourism. Such accidents raise important environmental issues as does the fact that residual oil emits chemical impurities when burnt. A large cruise or container ship may burn over of 200 tonnes per day and, if this contains 3% sulphur, over 12 tonnes of sulphuric acid can be directed into the atmosphere each day, a problem that MARPOL is addressing.

Before the introduction of nuclear power, and with the exception of K-boats, submarines used diesel engines when surfaced, or when employing a schnorkel.

Denis Griffiths