If there is one development in marine engines that has contributed to improved efficiency more than any other it must surely be the ship turbocharger. It is hard to imagine how restricted early marine engines would have been when it is considered that a modern engine without a functioning ship turbocharger would be limited to about 25-30% of its potential power output.
All internal combustion engines need a steady supply of air for ignition to take place inside the cylinder. Situated low down inside a ship which is relatively slow moving means that air can be in short supply. In the very early days of motorships a variety of means were employed to deliver air to the engine.
Collectively known as superchargers; pumps and blowers with an independent power supply or belt or mechanical take offs from the engine itself were all employed. All of these means can be considered parasitic in that they reduce the energy available for propulsion or other requirements of the ship.
The ship’s turbocharger by contrast uses the waste energy of the engine’s exhaust stream to move the air needed. It does this by the simple method of a rotor contained in a turbine housing through which the exhaust gasses pass. On the same shaft as the rotor but in a separate chamber, a compressor rotor draws in air and forces it through a cooler where it is made denser and ultimately moved into the combustion chamber.
In 1905, seven years before the first diesel engine was installed in a ship, Swiss engineer Alfred Büchi had patented the ship turbocharger but it would be ten years before the first prototype was constructed and a further eight before the first operational turbocharger was installed on a marine engine.
The history of the ship turbocharger
The first ship turbocharger allowed a power increase of over 42% on the engine it was fitted to and was built by the Swiss engineering concern Brown Boveri the predecessor to ABB which is a major player in the modern turbocharger sector.
By the 1930s, turbocharging was a common feature on ships but only for four-stroke engines. The gas flows of a two-stroke engine are more complex than four strokes and because of this and the limitations of early turbocharger design and construction, it would be another twenty years or more before two-stroke turbocharging became possible. In the 1950s turbocharging was not employed on all motorships and steam was still being used on a high proportion of ships although diesel power had by then passed the 50% mark.
By the 1970s steam had all but disappeared except in a few specialist ship types and turbocharging on diesel engines was the norm. With a much larger user base, development of turbochargers accelerated and improvements have been made on a regular basis.
Ship turbocharger benefits
All internal combustion engines gain efficiency when turbocharged so because the shipping industry has adopted turbocharging on virtually all the engines in use, it could be argued that it is pioneering efficiency measures rather than lagging behind as it is often accused of doing.
It is not difficult to understand why turbocharging was so enthusiastically embraced by shipping. The 42% power increase of the first turbochargers used would have been inducement enough on its own. Even though fuel costs in the 1920s were less a drain on operating finances than they are today, they have never been an insignificant expense. Even though modern diesel engines are able to use a little more than half of the energy contained in the fuel, the turbocharger along with other improvements means that they are four times more efficient than early engines.
More than a third of the energy in the fuel is lost through the heat of the exhaust. A turbocharger allows a high proportion of that to be recovered for performing useful work. Even more is recoverable by heat exchangers and other means including power turbine in a waste heat recovery system.
A more efficient engine means that for the same power requirement, a smaller engine is possible. Alternatively, the same size engine can allow for a faster or larger ship with more earning capacity. If these were benefits that were only being developed today, it would mean that the EEDI requirements of phases two and three would be ridiculously easy to meet. Unfortunately, that is not the case and the improvements that are available are much smaller increments than in previous generations.
Ship turbocharger principals
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 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 and there also 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 but is the choice of the engine builder. 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.
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 diesel’s 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 requiring multiple turbochargers.
The demands of environmental regulations on modern engines with regard to both emissions and efficiency is spurring development of turbochargers. The result is that new concepts such as variable turbine geometry and two-stage turbocharging are areas that have been the focus of considerable R&D investment in recent years. Some of the results of that development are already in use on some newer ships but as yet are far from mainstream. As they become more common, engineers will need to become familiar with their potential and their maintenance.
Being more complex systems will mean that special attention will need to be given to servicing and spare part procurement – at least in the early stages before third party specialists come up to speed.