Turbochargers for optimised performance
The latest technology from turbochargers manufacturers is timed to meet a new era of low engine-emissions and low fuel-consumption. Mark Langdon reviews the options.
As fuel prices rise and margins for shipowners become ever tighter, slow-steaming becomes ever more popular. However, as Ralf Grosshauser, head of the MAN Diesel turbocharger business unit points out: “Reducing a vessel’s speed by decreasing engine power below its design point will mean that although lower, fuel consumption is not optimised at the new load-point. Likewise, emissions will be adversely affected. The correct procedure would be to rematch the turbocharger to the engine’s new operating profile, but this process then needs to be reversed if slow steaming is no longer desired for some reason.”
This where having a system of adjustable vanes upstream of the turbocharger turbine that allows the amount of air compressed by the compressor wheel to be flexibly varied comes in. The system, which is incorporated into the latest TCA and TCR turbochargers with VTA (variable turbine area) allows the charge air to be more precisely matched to the quantity of fuel injected, which in turn allows the combustion to be optimised at all the engine’s operating speeds and loads. VTA’s variability exactly matches the flexibility of MAN Diesel’s electronically controlled fuel-injection systems, e.g. common rail, and allows fuel consumption and related emissions to be minimised while the engine’s ability to respond to load changes is greatly improved. Significantly, VTA technology can be both supplied on new turbochargers or retrofitted on units already in the field. “With our VTA technology, the quality/efficiency of power produced by the engine can be optimised towards the amount of power needed by the ship,” explains Grosshauser. “Ensuring efficient combustion of the fuel in this way further reduces fuel consumption and ensures compliance with emission limits.”
Another method of optimising fuel consumption at both full and part load introduced by MAN Diesel & Turbo is the turbocharger cut-out retrofit solution. A turbocharger cut-out with swing gate valves permits a frequent change between full and part load operation without manually having to install and remove blinding plates.
“After the system for retrofit installation has been thoroughly tested and several of the biggest shipowners have placed orders, the interest and determination to install turbocharger cut-out on vessels have soared.” says Peter Rytter Jensen, manager of the retrofit department in PrimeServ Copenhagen. He continues: “The big advantage is that you have a flexible solution to save on fuel oil. A lot of customers need to be able to continue to have the possibility of running on full load, but at the same time saving fuel oil when they are slow steaming.”
ABB Turbocharging’s approach to slow-steaming is to vary the number of turbochargers in the engines’ exhaust gas stream rather than fitting new turbochargers, modifying the internal components of existing turbochargers or using techniques like variable turbine geometry to vary turbocharger air delivery characteristics.
Engineers have devised a new A100-L turbocharger configuration for slow speed engines which facilitates rapid conversion from a higher power output matched to a ship’s normal cruising speed to a lower rating matched to a slower fuel saving speed – i.e. “slow steaming”.
The concept is devised to meet the ‘dual rating’ engine specifications of a series of 10 container ship newbuildings contracted by major Singapore-based container ship operator NOL. It takes advantage of the very wide compressor maps of the A100-turbocharger to achieve this.
In detail, the ABB system adopted for the new container ships is designed to allow the 12-cylinder 98 cm. bore low speed two-stroke diesels onboard the 10 ships to achieve ratings of just over 72MW at 104rpm or just over 54MW at 97rpm. The solution comprises four A190-L turbochargers, one of which can be cut-off from the exhaust gas stream. This is achieved either via a motorised valve or a simple blanking plate. In spite of the reduced level of energy in the engine exhaust gases due to the lower engine power rating, the A190-L’s wide compressor maps allow the three turbochargers still in the exhaust gas flow to efficiently produce charge air at the pressures and volumes needed for the lower engine slow steaming rating.
“Substantially modifying the compressor map of a turbocharger can involve the exchange of many of its internal components,” notes ABB’s head of global turbocharger sales for the low speed engine segment, Arie Smits. “In the extreme case this can mean the turbine and compressor wheels as well as the nozzle ring and diffusers. In any event, the turbocharger has to be opened, dismantled and reassembled. But with the A100-L’s excellent compressor maps it was possible to achieve dual ratings with optimised fuel consumption by, essentially, only reducing the number of the turbochargers providing combustion air to the engines.”
Wärtsilä and ABB Turbo Systems have been co-operating in a joint development programme for a new and groundbreaking application of two-stage turbocharging on large diesel engines. Advanced engine technology, together with two-stage turbocharging, offers significant advantages in fuel consumption and engine emissions.
Wärtsilä has been focusing on developing advanced engine technology, which will allow the highest possible performance to be achieved when used in conjunction with the turbocharger whilst at the same time providing a cost-effective commercial solution. ABB Turbo Systems is delivering the turbocharging technology with defined performance in terms of airflow, pressure ratios and efficiency.
While originally designed for power plant applications, there are plans for the technology to be used in the marine sector at a later date. “The main focus area was to have an engine for a power plant and the next logical step, which is what we are looking into now, is where we will implement this in the marine industry,” Mikael Troberg, director of Wärtsilä testing & performance, told The Motorship.
In the new engine design, two turbochargers are arranged in series to generate increased air pressure, airflow and a superior turbocharging effect. This results in an efficiency rating of up to 76%, which is said to be extremely high. The increased air pressure, combined with the advanced engine technology, improves the engine output and power density by up to 10% while, at the same time, reducing both fuel consumption and CO2 emissions.
Further emissions reduction can be achieved with additional engine systems or by the use of exhaust gas after-treatment. A precise combination of fuel consumption levels and reductions in CO2 and NOx emissions can be selected through detailed systems configuration. Intelligent engine control allows optimum operation of the advanced engine design over the whole load range, and a significant reduction in NOx emissions can be reached. At high altitudes, two-stage turbocharging technology maintains the engine’s operational performance by compensating for the reduced air density.
The significant reductions in fuel consumption and emissions are the result of extensive joint testing of the two-stage turbocharging system on the Wärtsilä engine. The tests have taken place at Wärtsilä’s test facility in Vaasa, Finland, and the targets for the development programme have been successfully met. Wärtsilä and ABB Turbo Systems are planning to initiate a major pilot project with a customer in the near future. “We haven't been able to sign any contracts around that yet but we have several projects ongoing,” says Troberg. He explains: “There is a small challenge with this current economic situation but the power plant market is picking up again, so we are optimistic.”
Another driving factor in this development work has been to significantly lower lifecycle costs, and this target has been met. So too has the goal of lowering NOx emissions. The market potential for this technology in power plants is believed to be big. Similarly, the advanced technology for combustion control developed by Wärtsilä will be beneficial to the shipping market, when the market requirements are in place. Here, in addition to costs, the compactness and cost effective design are considered to be of importance. “No decisions have been reached yet as we are in the research phase where we are evaluating the technology and the benefits of it,” Troberg told The Motorship. “And based on that we need to make a decision when and what we want to introduce because it is not just a matter of turbocharging: we may also need a full system change including the valve mechanism and the fuel injection system and so on. So we need to develop a whole total package.”
“The conceptual design for the new power plant engine began three years ago,” says Troberg. “Testing of the concept, and verification of the technology, has been successfully carried out using four-stroke Wärtsilä 20 and Wärtsilä 32 engines. We see this technology as being a key for the next generation of Wärtsilä emissions-friendly engines. The technology has been developed for the four-stroke portfolio, in both marine and power plant applications. The next logical step is the two-stroke engine family for large vessels, typically as single engine installations.”
Troberg told The Motorship: “We haven't made any decisions there of course but we are looking into something happening within five years. However, with the legislation coming into force in 2016, there is the need for a step change in engine technology in order to cope with the reduced NOx emissions required.”
He explains: “With two stage turbocharging we need air which we are regulating with the valves so we need some kind of variable valve mechanism in order to adjust the opening of the inlet and exhaust valves. We also need to have a flexible fuel system as well - we need to be able to regulate the timing and hopefully, the injection pressures.”
Two stage testing
ABB Turbocharging recently carried out type testing on a Power2 system in the presence of the classification societies on a hot gas test rig at its technical center in Baden, Switzerland passing a major milestone on its way to gaining type approval for use on four-stroke, medium speed marine diesel engines.
The type test of Power2 took place in early March 2011. It consisted of running a two stage turbocharging system consisting of a high and low pressure turbocharger connected in tandem on the hot gas test rig for one hour at its maximum operating speed and maximum operating temperature. Subsequently the turbochargers were dismantled and their components inspected.
The tests and component inspection were witnessed by representatives from Lloyd`s Register (LR), Germanischer Lloyd (GL), American Bureau of Shipping (ABS), Bureau Veritas (BV), Registro Italiano Navale (RINa), Russian Register (RS), Polish Register (PRS), Nippon Kaiji Kyokai (NK), Korean Register (KR), Det Norske Veritas (DNV) und China Classification Society (CCS), in addition to a representative of the Japanese Government Ministry for Land, Infrastructure and Transport. (JG/MLIT).
On four-stroke diesels it has been demonstrated that the turbocharger pressure ratios produced by ABB Turbocharging’s Power2 technology are high enough to achieve Miller cycles capable of giving very high double digit NOx reduction percentages. Simulations for Power2’s potential made by ABB Turbocharging’s development engineers using advance software indicate that, for example, the 80 % reduction in NOx values vis-à-vis IMO Tier 1 required under IMO Tier 3 for ships operating in designated emissions control areas (ECAs), is a prospect.
ABB Turbocharging also recently completed final testing of the A190-L turbocharger for two-stroke low speed diesels, measuring a peak efficiency of 75.8%. “The 75.8% figure is all the more impressive because it was measured on a turbocharger featuring a higher pressure ratio than the model that achieved the previous record figure,” notes Urs Gribi, vice president of engineering at the Baden, Switzerland headquarters of ABB Turbocharging.
Small bore turbocharger
MAN Diesel & Turbo recently completed testing of its TCA44 in Augsburg. Weighing in at 1,970kg, the TCA44 completes MAN’s TCA turbocharger range for two-stroke engines at its lower end. Stefan Mayr, project engineer responsible for the TCA44, said: “This new turbocharger has been specifically designed to meet the needs of small-bore, two-stroke engines and provides optimised turbocharging solutions for engines that exceed the capacity limits of the smaller TCR22 turbocharger.”
Until now, the TCA55 with a small turbine and compressor wheel has been used for such engines. Though thermodynamically feasible, the TCA55 suffered by its larger size and weight, which was a decisive disadvantage on the cost side. In contrast, the TCA44 is some 40% smaller and lighter. With the predicted development of the Chinese short-sea and coastal operation and the growing demand for small-bore, two-stroke engines, the TCA44 is coming to the market just in time to enhance these applications. Demand for the TCA44 has been strong and MAN Diesel & Turbo reports that serial deliveries will start in the second half of 2011.
The TCA44 follows the principal design philosophy of the TCA series, but with some major innovations that facilitate production, installation and maintenance, including:
- 45 steel turbine blades with new airfoil profile;
- the fixation of turbine blades by one retaining ring instead of 45 individual fixing-plates;
- oil supply and discharge integrated in turbocharger feet as integral part of the bearing casing;
- a new compressor-wheel profile;
- a simplified IRC design; and
- a modified silencer.
Stefan Mayr told The Motorship that these innovations offer the following benefits:-
- Fixation of turbine blade: new developed retaining ring system; easy installation; and shorter maintenance time needed.
- Modified silencer: robust design for highest durability
- Oil supply / discharge: new design; flexible connection (flanges on both sides); and wide installation space for optimised lube oil supply
Some of these innovations, such as the turbine blade fixation, will gradually be introduced to the other frame-sizes of the TCA family.
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