Updated 22 Oct 2019
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.
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-fuel, 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 principle 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.
Principles of turbocharging
The power for a turbocharger comes from the exhaust gas stream of the engine but this can be done in one of two ways – pulse or constant pressure. In a multi-cylinder engine each exhaust stroke will result in a new injection of exhaust gas into the system. This can either be fed directly to the turbocharger or to a collecting chamber. Which method of operation is chosen is not a function of the turbocharger but is the choice of the engine builder.
When the exhaust valve is first opened and the cylinder contents expelled, it is under high pressure and, if fed directly to the turbocharger, it passes through a pipe of sufficient bore to maintain that pressure and deliver high energy to the turbocharger. As each cylinder is exhausted in turn, the turbocharger will receive regular pulses of energy. This is known as pulse turbocharging.
Each cylinder will either have a direct feed to the turbocharger or the cylinders will be grouped into sets each having its own exhaust pipe or lead and sharing a final pipe. If grouped into sets, the appropriate cylinders will be selected so that there will be no interference with the scavenging of cylinders caused by blowback of gases from one cylinder to another when one is exhausted.
Successful pulse turbocharging requires that the leads should be as short and as straight as possible and of a small bore to prevent energy dissipation. This method of operation is very responsive to changes in the engine speed and in theory provides for better scavenging. It also permits for multiple turbochargers to be used.
In a constant pressure system, there is only one lead to the turbocharger and all of the cylinders are connected to a single exhaust manifold. Being of larger diameter, some of the energy is lost through dissipation but it does mean that there is a constant pressure from the manifold to the turbocharger. This type of system is best suited to high power output engines as the energy dissipation is of less consequence.
It is a simpler system and therefore should require less maintenance. But there are other advantages: it provides a higher turbine efficiency overall and the number of turbochargers required can be reduced. Since the pipes need not be so short or straight, there is more flexibility in turbocharger placement. Constant pressure turbocharging allows for higher efficiency at normal engine speeds.
On the downside, constant pressure is less efficient at part-loads as there can be insufficient energy to run the turbocharger. The turbocharger is also less responsive to changing engine conditions because the volume of gas in the manifold will increase or reduce slower when changing engine speeds. Which method of operation is chosen is not a function of the turbocharger itself but is the choice of the engine manufacturer.
Aside from there being two methods of operation, there are two basic variations of turbochargers which relate to the direction of gas flow to the turbine rotor. In a radial flow turbocharger, the exhaust gas enters from the side and flows out along the axis of the shaft. In an axial flow turbocharger, the exhaust gas enters and leaves along the axial direction.
In general, radial flow turbochargers are more suited to smaller high-speed engines while the axial flow turbochargers are designed for more powerful low-speed engines. There is however a considerable overlap area in between where either type may be employed. For example, MAN Energy Solutions’ TCR range of radial turbochargers is quoted as being for engines from 300kW to 6,500kW whereas its TCA range of axial turbochargers are designed for engine outputs from 2,100kW to 30,000kW per turbocharger. The very large engines obviously require multiple turbochargers.
In early 2019 MAN Energy Solutions unveiled its new TCT turbocharger series to accompany and – eventually – replace the TCA generation whose design has been continuously updated since its introduction but is now some 15 years old. A factor in the development of the TCT was to reduce turbocharger dimensions to meet current market requirements. A key focus during the design phase was a reduction in manufacturing and operating costs while maintaining compliance with thermodynamic requirements.
The introduction of the TCT series means that MAN Energy Solutions’ two-stage turbocharging solution can now achieve efficiency levels of up to 80%.
Initially only the TCT40 and TCT60 types were marketed but other TCT sizes are to come on stream in 2020 and 2021.
The TCT is not only aimed at low-speed engines but is also particularly suitable for two-stage turbocharging on four-stroke engines. They are suitable for conventional fuel and dual-fuel engines ranging from 6MW up to 24MW output per turbocharger.
Also in 2019 and also in response to increasing demand for compact and efficient turbocharges, ABB Turbocharging launched the new A255-L and A260-L turbochargers that are scaled down from the older A100-L and A200-L designs. The new models are aimed at small- and medium-bore engines.
They are designed to operate on all fuel types and on engines specified for both IMO Tier II or Tier III emissions levels. The A255-L and A260-L turbochargers can be used on low-pressure dual-fuel engines to optimise combustion. The high turbocharging efficiency provided by the new 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. They also facilitate the utilisation of emission abatement technologies for NOx and SOx reduction.
ABB believes that due to the 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. These will typically represent vessels such as bulk carriers, tankers, container feeders and car carriers.
Turbocharger efficiency and engine sizing
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.
Another turbocharger development of recent years is that by Japanese engine maker Mitsubishi. The company’s hybrid MET83MAG turbocharger generator was given an initial reference in 2011 on the bulk carrier Shin Koho. The hybrid unit has been designed to meet the vessel’s entire at-sea electrical power needs by utilising the exhaust gas from the main engine not only for driving the turbocharger compressor but also for power generation. At 9,500rpm the power output of the hybrid unit is 754kW.
The system generates AC power which is converted to DC current using an insulated gate bipolar transistor and an inverter. The system can also be run in reverse using other sources of power making the generator serve as a motor that boosts the turbocharger when engine speed is low. Making use of the same technology but incorporating the variable turbine concept is the MET66MAGVTI developed in co-operation with Mitsui Zosen and Kobe Diesel. The turbocharger has gained references on a series of car carriers owned by NYK.
In 2017, Rolls-Royce acquired the exclusive rights of use for a new patented technology for electrically-assisted turbocharging from G+L innotec that is suited to engines over 450kW. The electrically-assisted charging system comprises an electric drive combined with a traditional turbocharger developed and manufactured by MTU. As a result, the turbocharger can be accelerated electrically and the charge pressure built up earlier. In operating conditions, in which the energy required for a faster charge pressure of the turbine would normally not be sufficient, it is also possible to build up with the aid of the electric drive.
The system is similar in principle to the Mitsubishi system but makes use of a permanent magnet installed upstream of the compressor wheel and an electrical winding is integrated into the casing of the compressor. With this arrangement, the air drawn in by the compressor is not obstructed and at the same time the electrical components are cooled by the air. The special feature of this arrangement is the large gap between the magnet and winding. This so-called media gap motor requires specially-designed power electronics. This ensures that there is no aerodynamic impact on the charger and that existing chargers can be adapted easily to enable them to make use of this technology.
Most large ships have engines with multiple turbochargers because of the need for sufficient air to support the combustion of the engine when running at full power.
If slow steaming is being practised, not all of the turbochargers are needed and it has become common practice to shut off one of the turbochargers on a semi-permanent basis. To make for a more flexible system, variable turbochargers have been developed and an alternative – known as sequential turbocharging – has been borrowed from other uses and developed for Diesel engine use.
Last year ABB Turbocharging launched its new FiTS2 system, which is an example of this technology. Standing for Flexible integrated Turbocharging System for two-stroke engines, this sequential turbocharging system offers significant fuel savings whilst maintaining flexibility in engine and vessel operation. Developed with key engine designers, FiTS2 is available for all two-stroke engines.
Typical engines for large tankers, bulkers and feeder container vessels with conventional turbocharging systems run with two same-type turbochargers that are always in operation in high loads and in low and part engine loads. To optimise engine efficiency via improved turbocharging in low and part load, the engine with FiTS2 runs in lower loads with only one turbocharger in operation, whereas at higher loads (typically above 50 to 60% engine load) two turbochargers are operating simultaneously. The same principle is applied for very large engines – with FiTS2, they will run with two turbochargers in lower loads and with all three turbochargers for higher load operation.
Large two-stroke engines are also often fitted with auxiliary blowers but in many cases FiTS2 will be able to support slow steaming without these running. Auxiliary blower switch-on can typically be reduced from around 35% engine load to 25%, thus saving energy.
For large container vessels, which might use three turbochargers, one unit can be adapted to shut off at lower power demands, typically below 50% load. Engine settings are optimised as part of the FiTS2 design and the system can be returned automatically and rapidly to its original full power capabilities. High load performance is therefore available combined with enhanced efficiency performance at lower loads.
FiTS2 benefits include a high potential of lower engine fuel consumption by up to 6g/kWhr when operating at part or low load, but with flexibility of operation maintained up to full engine power.