Engine Design

Last Updated: Apr 2012

What is it?

There is increasing interest in ship engine technology, specifically improvements in the combustion process, energy efficiency, emissions reduction methods, heat recovery systems, and power conversion technologies. Engine technology is developing quickly, and the industry is now also looking at the future of dual fuel engines. Alternative fuels are also being proposed for new engines; these are covered in a separate article.

How it works

The International Maritime Organization (IMO) notes in its greenhouse gas (GHG) and emissions report that on average 43% of the energy from bunker fuel, when combustion takes place in a ship's engine, ends up generating propulsion thrust. The rest is lost to exhaust (27%) and to heat (30%). Efficiency ratings can be higher, however, with engine maker Wärtsilä, for example, noting that it achieves 50% efficiency in some of its engine models.

Efficiency increases have been achieved in new engine designs over the years by increasing the cylinder pressure, raising the compression ratio, reducing the fuel injection period, optimising the valve timing, and improving the combustion process. Wärtsilä, however, notes that the pace of improvement in efficiency has fallen off somewhat in recent years owing to the increasing restrictions placed on nitrogen oxides (NOx) emissions.

New engines are also being specifically designed to meet lower NOx emissions requirements. According to the IMO, existing engines can also be upgraded to meet NOx standards and at the same time make improvements to fuel consumption and power output. Such upgrades typically involve increasing the compression ratio, improved fuel pumps and injector nozzles, and other additional parts.

The IMO prefers these sorts of complete engine upgrades but also notes that a more limited modification, termed 'de-rating', can be used to increase efficiency and reduce NOx output at the same time. This involves reducing power output and restoring peak firing pressure through increasing the compression ratio. The analysis from the IMO suggests that a 4.3% efficiency gain is possible. In one case study, engine maker MAN B&W found that 'de-rating' achieved a 2.9% decrease in fuel consumption.

Another proposal is to install an engine energy-recovery system. This uses exhaust gas from the main engine to drive two turbines, one from heat and the other from the exhaust gas, that could provide electrical power either for the ship's systems or for another engine to drive the propeller shaft.

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