Putting turbocharger technology in perspective

Malcolm Latarche
Malcolm Latarche
ShipInsight

26 July 2019


As the 2020 deadline for the IMO global sulphur cap approaches, the attention of the shipping industry is necessarily concentrated on meeting the requirements of the latest regulation aimed at reducing SOx emissions. Mostly that will be done by way of either compliant fuels or scrubbers with LNG fuelled engines being another option.

The new SOx rules are the latest, but not the last, in a long running series of emission control regulations from the IMO. Until MARPOL Annex VI was adopted and came into effect with proscriptive requirements for different components of exhaust gases, there had been no official restrictions. However, shipowners have always been pursuing their own voluntary reduction measures driven by a need for more efficient engines especially in times when fuel prices have skyrocketed.

Turbochargers have played a massive role in improving the efficiency of engines especially two-stroke low-speed engines ever since they were first applied in the mid-20th Century. When the first turbocharger was fitted to a four-stroke engine in the 1920s it resulted in a 42% increase in power output from the engine. That turbocharger was built by the Swiss engineering concern Brown Boveri the predecessor to ABB.

Two-stroke turbocharging did not become commonplace in the marine arena until the 1960s after which there were periods when ship speed was the target alternating with periods when the oil crises of various years favoured reduced speed and fuel consumption, either way it was efficiency that was at the heart of development. It was only after MARPOL Annex VI was adopted in 1997 that limits the main air pollutants contained in ships exhaust gas, including sulphur oxides (SOx) and nitrous oxides (NOx) were introduced and changed the direction of engine and turbocharger development.

Later, the IMO began a journey into greenhouse gas reduction and higher ship efficiency which has resulted in the Energy Efficiency Design Index (EEDI) rules and related matters such as voluntary Energy Efficiency Operational Index (EEOI) and SEEMPs. More recently, the IMO has set out a vision of a completely carbon-free future for shipping although it has not yet sure how this might be achieved.

The SOx reductions under Annex VI can only be achieved by burning compliant fuels or by using a scrubber to clean the exhaust gases. There is nothing that engine or turbochargers can do to control sulphur although it is likely that the performance of ships using some of the 2020 compliant fuels will be adversely affected and turbocharging improvements may be able to recover some of the lost efficiency.

Although Annex VI was adopted in 1999, it did not become effective until 2005. This introduced the Tier I NOx standards and a 2005 amendment adopted in 2008 added the Tier II and Tier III NOx standards.

The requirement for NOx reduction interrupted what had until then been a continuous drive for engine efficiency especially for two-stroke engines which powered the majority of ships including the largest ocean going vessels which were the heaviest consumers of fuel.

The change of focus to NOx emission reductions began a period of at least ten years of some of the most intense development of both engines and turbochargers. Meeting the Tier I standards which were intended to apply from 2000 was not really a problem for engine or turbocharger makers. Moving on to Tier II and Tier III there were much greater reductions in NOx emitted and this was achieved in a variety of ways.

Changes in engine timing such as the Miller cycle on four-strokes allowed the combustion chamber to be cooled more effectively and so prohibiting the formation of NOx was one means. One two-strokes, a similar effect was achieved by reduction of compression volume, increased scavenging air pressure and late closing of the exhaust valve, in some cases combined with optimisation of the fuel system.

On the turbocharger side, means of increasing the compression ratio were the main contributors. As with the engine timing methods, putting more air into the engine reduces the temperature in the combustion chamber sometimes by considerable amounts. This can be done by improvements to the turbocharger or by two-stage turbocharging.

At Tier III reduction levels much higher compression ratios are needed, especially if the ship is making use of exhaust gas recirculation as a means of aiding compliance. Accordingly increasing the compression ratio has been a goal of turbocharger manufacturers.

After the NOx changes which effectively ended in 2016 because although new areas may be added where Tier III levels must be reached there are as yet no plans for a further reductions the attention has once again switched back to efficiency improvements particularly in smaller vessels.

Large two-stroke engines are usually fitted with the largest turbocharger products where most of the development up to now has taken place. There is still work to be done for large vessels with an increasingly strict EEDI regime planned but the smaller vessels such as Handy and Handymax bulkers, car carriers, and smaller tankers and containerships cannot be overlooked.

This year at CIMAC in Vancouver, ABB presented a paper on turbocharging technology and also used the occasion to launch new turbochargers specifically developed for smaller vessel types. The new A255-L and A260-L models offer turbocharger technology with more power density with a smaller frame size. For ship owners, these new turbochargers deliver high efficiency, lower fuel consumption and emissions. They also offer lower maintenance costs, allowing for optimised total cost of ownership and can be used on low pressure dual fuel engines to optimise combustion, improving engine efficiency and allowing for the reduction of GHG emissions.

The high turbocharging efficiency provided by the new A255-L and A260-L turbochargers ensures that high air-fuel ratio is maintained in high-load operation, thus reducing the tendency for fast combustion which can adversely affect operation and emissions. The A255-L and A260-L turbochargers also facilitate the use of emission abatement technologies for NOx and SOx reduction.

Due to the significant efficiencies achieved, the new A255-L and A260-L turbochargers can reduce the energy efficiency design index (EEDI) of smaller cargo vessels in the size range of 10,000- 40,000dwt that make use of small and medium bore two-stroke engines.