Turbochargers - Diesel engines’ biggest boost
Although shipowners are often accused of ignoring demands for increased efficiency unless obliged to do so by regulation, the reality is that this is very far from the truth. There is probably no better example of shipowners’ willingness to embrace fuel saving technology than the development of marine turbochargers.
Turbocharging technology is covered in greater detail in the ShipInsight Turbocharger but since almost all modern marine engines rely on them to provide the needed quantity of air for efficient combustion, it is appropriate to cover the basics here.
For combustion to take place in the traditional marine engine, three things are necessary – fuel, heat and oxygen. The fuel is always a hydrocarbon whether it be oil, bio-fuels, LNG or any of the other alternative fuels now in use; the heat can come from the compression in a Diesel engine or a spark and the oxygen must be brought into the engine as air.
It is not impossible for a normally-aspirated engine to run in a ship but, without forced air provided by a blower or turbocharger, combustion would be restricted and the efficiency of the engine compromised.
Early diesels were built without turbochargers and were far less efficient than their modern counterparts. Turbochargers first appeared in the marine sphere in 1923 on the Hansestadt Danzig and the Preussen, boosting the ships’ twin MAN-built Diesel engines from 1,750bhp to 2,500bhp – an increase in output of almost 43%. Initially turbocharging was employed for four-stroke Diesel engines but, in 1934, the development of turbocharging for two-stroke engines was taken up and in 1952 the first marine application was made when the 18,000dwt tanker Dorthe Maersk entered service. The ship was fitted with a single two-stroke, 6-cylinder B&W main engine. Its two VTR-630 turbochargers improved the power output from original 5,530bhp to 8,000bhp.
A turbocharger in its simplest form is in principal nothing more than a compressor and consists of two connected sets of rotating vanes in separate housings. One (the turbine) is driven by the exhaust gases from the engine and it rotates the compressor which draws ambient air from outside and forces it into the combustion chamber. Without sufficient air, the combustion process would not allow all of the fuel to be burnt causing black smoke in the exhaust and poor efficiency performance. The black smoke that is often seen coming from a ship’s funnel immediately after the main engines are started is a consequence of the turbocharger not immediately cutting in.
Efficiency gains and space and weight savings
Today it would be almost unthinkable for a marine Diesel engine not to be fitted with at least one turbocharger and for very large low-speed engines a triple turbocharger set up is not uncommon. The increased efficiency of modern turbochargers is now above the 70% mark and their importance is underlined by the fact that although a turbocharger represents only a small fraction of the cost of an engine it is responsible for between 60% and 75% of its power output.
That efficiency translates into a number of benefits. For example, the size of the engine needed to produce a particular power output can be much smaller saving weight and space. Alternatively, the same size of engine installed in a ship will mean greater power increasing its speed and or carrying capacity. Many engine makers also produce turbochargers and supply engines ready fitted but there are also a small number of independent manufacturers such as ABB, KBB and Napier whose products can usually be specified as options when purchasing engines.
Although turbochargers are familiar items of equipment, there have been several recent developments aimed both at further increasing engine efficiency and reducing emissions. On most engines, turbochargers operate best within a defined engine load range. Outside of that problems can arise.
When slow steaming or operating continuously at low loads, it is necessary to reduce the turbocharging effect. One way of doing this is to either reduce the number of turbochargers fitted or to fit a turbocharger cut-out device. This was a recommended option when slow steaming strategies were adopted after 2008.
A better way is the development of variable turbocharging in which the vanes of the turbine can be manipulated so as to reduce or increase the turbine speed. Depending upon the maker concerned, the concept is referred to as VTG (variable turbine geometry) or VTA (variable turbine area). When the vanes are rotated so as to lie in the direction of the exhaust flow,
they present very little resistance and so reduce the turbocharging effect. Turning the vanes so that they present more of their surface to the flow will cause the turbine to spin faster at speeds of several thousand rpm and increase turbocharging.
The most interesting of recent developments is the potential of two-stage turbocharging to meet NOx Code requirements and to improve overall fuel efficiency. In a two-stage turbocharger, the exhaust gas is first fed to a high-pressure turbine and then continues to a second low pressure turbine before being exhausted. At the compressor end, air is first drawn in through the low-pressure side where it is compressed and cooled before reaching the high-pressure side where it is further compressed before entering the engine. Under low
load conditions when less air is needed, the high-pressure side of the compressor is by-passed.