Turbocharger maker prepares for next environmental initiatives
A Wärtsilä 4-stroke diesel engine with two stage turbocharging technology
Latest developments at ABB Turbocharging primarily centre around measures to ensure that engines are capable of meeting forthcoming emissions limits, whether by in-engine or external methods, or by use of alternative fuels.
All of the above can have a significant effect on the turbocharging arrangements. ABB points out that multi-stage turbocharging, regarded as future technology in large marine engine circles, is already a reality, with the first commercial application of ABB’s Power2 system on a large-bore 24-cylinder spark-ignited GE Jenbacher J624 gas engine which will operate in a stationary cogeneration plant in the Netherlands.
ABB believes that the implications for such technology in the marine propulsion and onboard power generation sectors are clear, given the interest in gas engines as prime movers for marine applications. The company adds that at the CIMAC Circle held during the SMM exhibition, spark ignited gas engines, as well as dual-fuel engines, featured strongly in the cost of emissions reduction scenarios presented by the speakers.
ABB says that its Power2 technology is capable of producing pressure ratios as high as 8 and above and it is already widely accepted as a major enabling technology of the strong Miller Cycles which can substantially reduce NOx emissions on diesel engines without adversely affecting power density and fuel efficiency. On gas engines Power2 is also seen as a Miller enabler, but the current focus is employing a high order of charge air pressures to increase power density and efficiency.
The 24 cylinder J624 with two stage turbocharging achieves a rated output of 4.4 MW compared to the 4 MW with conventional turbocharging and offers an efficiency increase of about 2% while undercutting IMO Tier 3 limits on NOx by a wide margin.
According to ABB, Power2’s potential for substantially improving gas engine performance has been demonstrated “in the iron” at a time when natural gas is increasingly being considered for marine engines, in the light of meeting restrictions on diesel gensets while in port as well as to comply with future NOx regulations in certain areas.
On 4 stroke diesels ABB says it has been demonstrated that Power2 can produce turbocharger pressure ratios high enough to achieve very strong Miller Cycles capable of achieving high, double digit NOx reduction percentages. Calculations made by ABB Turbocharging’s development engineers indicate that, for example, the 80 % reduction in NOx values required under IMO Tier 3 for ships operating in ECAs are already a realistic prospect.
The Power2 two stage turbocharging system consists of two turbochargers of different, tuned frame sizes connected in tandem on the compressor side via an intermediate air cooler – cooling the compressed air issuing from the first turbocharger means the second turbocharger needs to do less work and can be more compact.
In the early days of emissions reduction on diesel engines, a widespread measure was to reduce combustion temperatures by retarding fuel injection, thus reducing the rate of heat released from the fuel. Thus a fuel consumption penalty was incurred in the interests of lower NOx emissions.
However, while the trade-off will always be a fact of every engine developer’s life, the findings of ABB and its development partners show that using a combination of much higher turbocharging pressures, variable valve timing, advanced fuel injection technology (e.g. common rail), and electronic control, the Miller Cycle is capable of shifting this compromise between NOx emissions and specific fuel consumption values into a new, far lower range. Moreover, the higher turbocharging pressures can increase engine power density.
The Miller Cycle denotes an ingenious method of cooling an engine’s combustion air and so eliminating the high temperature peaks in the engine’s combustion chamber which are responsible for over 90 % of NOx formation. On 4-stroke engines, the cooling effect is achieved by shortening the opening period of the inlet valve. The earlier end of induction promotes expansion, and hence cooling as the air continues to expand. On 2-stroke engines, where the timing of air induction is a function of the piston passing fixed inlet ports in the cylinder wall, a similar effect can be achieved by varying the closure of the exhaust valve.
Higher turbocharging pressures are used on both 4-stroke and 2-stroke engines to compensate the shorter time for induction, allowing an equal – or even greater – mass of combustion air to be forced into the cylinder.
Lower load operation
The strong Miller Cycle described above assumes an engine operating at MCR. However, in most cases engine output is expected to vary according to the work to be done. Since at lower loads the short inlet valve timings needed for very strong Miller Cycles on 4-stroke diesels lead to poor response to load changes and increased emissions of smoke and particulates due to low combustion temperatures, a method is needed to manage the Miller Cycle. This involves varying the length of inlet valve opening to adjust the time available for combustion air induction and so allowing the strength of the Miller Cycle to be adapted to engine load and speed.
This adjustment is the function of the VCM (valve control management) system ABB Turbocharging is currently developing in cooperation with engine component specialist INA Schaeffler KG. VCM is based on INA’s UniAir system for automotive engines and allows variation of both valve timing and lift on 4-stroke diesel and gas engines in the power range above 400 kW. A prototype of the new VCM system is currently undergoing an extensive test programme. First results confirm the system’s potential for highly flexible valve timings on 4-stroke engines.
Variation in valve timing and lift is achieved by interposing a high pressure oil chamber into the engine valve train between the valve and its mechanical actuation system.
A solenoid valve varies the filling of the chamber with engine lube oil pressurised by a camshaft actuated pump. This enables both the timing of the opening and closing of the valve to be varied as well as the distance the valve opens (valve lift). The pump also feeds a brake unit above the valve to limit forces when the valve contacts its seat.
Hard faced tips
Miller cycle operation is just one of many factors that impose extreme operating profiles on large engines burning low-quality HFO. To counter the accelerated circumferential wear that has been found to be a potential problem in such applications, ABB Turbocharging has introduced axial turbine blades having hard-faced tips.
ABB says that turbocharger performance can suffer due to build-up of hard, abrasive combustion residues on and around the turbine diffuser. Wear due to contact between the deposits and the rotating turbine blades causes a loss in turbine diameter and hence an increase in exhaust gases bypassing the turbine. This reduces turbocharger and engine efficiency and results in more frequent replacement of complete sets of turbine blades, impacting heavily on service costs.
ABB has developed its own coating process, in which a hard wear-resistant layer is applied to the extremities of removable turbine blades. Nicknamed ‘dragon’s teeth’, six of the hard tipped blades need to be fitted in pairs at 120 degree intervals around the turbine wheel. At this even spacing they are able to scrape away the hard HFO fouling to clear a path for the standard blades, thus minimising contact with the abrasive residues. The 120 degree spacing of the coated blades assists rotor balance as well as ensuring a well distributed scraping effect.
One such kit of six turbine blades was fitted to a turbocharger operating on a medium speed genset as part of a package of measures designed to prolong turbine blade and diffuser lifespan, including a modified diffuser to minimise distortion during cleaning operations and improvements to the turbine washing nozzles. In this application the dragon’s teeth solution reduced wear on the standard blades to a level where only the hard tipped blades needed to be replaced during scheduled turbocharger overhaul. Diffuser maintenance was similarly reduced
As well as being an option on new turbochargers, the hard tip solution is also offered as part of ABB’s ‘hot part package’ which includes modified washing nozzles for engines operating on HFO.
The above measures are one part of ABB Turbocharging’s growing emphasis on service, which, with over 190,000 turbochargers in operation, it regards as of prime importance.
The global network of service stations and their offerings are being expanded and optimised, to take account of changes in geographical demand for turbocharger service and ensure proximity to customers, while new service offerings result from a proactive approach to addressing end user needs. The new packages target optimised levels of engine power, fuel economy, availability and - importantly in view of IMO Tier 2 - exhaust emissions, by assisting operators to keep their turbochargers in top condition at economic cost.
The new proactive offerings launched by ABB Turbocharging include CPEX (customer part exchange programme, which supplies reconditioned parts as required); MMA (managed maintenance agreement) and the turbine blade retrofit package. These are in addition to the previous service plans, such as OPAC and SIKO.
The stricter emissions limits being introduced under IMO Tier 2 will mean, according to ABB, that turbochargers will have to be maintained at peak performance levels, and the company believes that an important part of that is fitting OEM-approved replacement parts, whether new or reconditioned. Deviations in profile, and looser tolerances found in third-party spare parts will not only affect the emissions performance of the engines, they can result in higher fuel consumption, thus increases in operating costs that more than outweigh any saving from cheaper spares. And that is in addition to the safety considerations of sub-standard materials or fabrication.
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