Turbocharger arrangement for part-load efficiency
A new turbocharging arrangement from ABB Turbocharging is set to offer greater flexibility and lower fuel consumption for vessels with two-stroke main propulsion operating at part loads.
ABB Turbocharging has developed a sequential turbocharging system that increases fuel efficiency for two-stroke engines running at part-loads, while maintaining the flexibility to load up to full engine output immediately. The company claims that its system offers the potential for fuel savings of up to three percent, depending on load profile.
The Flexible integrated Turbocharging System for Two-Stroke Engines (FiTS2) was developed in close collaboration with low-speed two-stroke engine designer, Winterthur Gas & Diesel (WinGD), which has developed a special tuning for the system.
The previous decade of slow steaming has resulted, at times, in engine operation outside of original design parameters, leading to increased maintenance and repair costs. Typical fuel bills for such vessels have been cited by management consultant McKinsey & Co as “the largest cost item for shipping lines” and “often exceeding 40% of all costs”, despite current lower oil prices.
“From an operator’s or charterer’s perspective, FiTS2 efficiency benefits will enable major cost reductions,” explains Joachim Bremer, head of product line low-speed, ABB Turbocharging. “The pay back of initial costs can be less than two years, making the FiTS2 solution very attractive.”
Considering the typical current annual fuel bill for very large crude oil carriers, at around US$3-4 million, FiTS2 has potential to provide fuel cost savings of up to US$100,000 compared to conventional turbocharging systems. Big savings are also possible for other vessel types: for container vessels the reduction can amount to US$1 million over 10 years, the company calculates.
Dominik Schneiter, vice president research & development, WinGD adds: “In our long-lasting strategic cooperation with ABB, WinGD has developed its own engines with features such as optimized two-stroke Miller tuning, high boost pressure ratios and part load optimised waste gate applications. With the new FiTS2, ship operators can now further benefit considerably from lower auxiliary power consumption, improved response time, and lower fuel consumption of the main engine, improving the CO2 footprint of their ships at low and mid-load conditions. In today’s changing market conditions such flexibility is a valuable asset to our customers.”
The system consists of sequential turbochargers of different sizes and specifications, with wide compressor maps. In sequential turbocharging, two or more turbochargers operate in sequence to
complement each other’s air delivery at different engine loads. The turbochargers are controlled by
a specially developed FiTS2 tuning software, combined with dedicated, flow optimised valves, which can operate under full load.
Bremer explains: “Typically the exhaust gas from the engine feeds into two turbochargers of the same size, half of the gas to one and the other. The problem is that when the engine is running on low load there is not much exhaust gas going to the turbochargers, which means they will charge relatively slow, so there is not much charging pressure going to the engine. That is not optimal for combustion.
“We blank off one charger with valves, with another valve so that we don’t have compressed air flowing in to the compressor side, so all exhaust gas goes through one turbocharger. That doubles the gas through that one turbocharger, meaning it turns faster, compresses more air and delivers a higher charging ratio.”
To optimise engine efficiency via improved turbocharging in low and part load, the engine uses only one turbocharger during low-load operation, whereas at higher loads (typically above 50-60% of engine output) two turbochargers are operating simultaneously. Very large engines with FiTS2 will run with two turbochargers in lower loads and with all three turbochargers for higher load operation.
Cut out of one turbocharger for lower engine loads leads to higher scavenging air pressure, increasing the compression ratio and firing pressure optimized by special tuning of FiTS2. The result is enhanced engine efficiency and lower specific fuel consumption while remaining compliant with emission regulations. NOx emissions from engines fitted with FiTS2 will also remain within IMO Tier II limits. Additional abatement technologies like SCR or EGR can be used to comply with IMO Tier III NOx emissions and will not greatly affect the system, still delivering fuel saving benefits.
“Looking at the bigger picture, FiTS2 also has the potential to be a very useful factor in helping vessel
designers achieve the progressive improvements in actual vessel operation efficiency – and hence lower greenhouse gas emissions – required under the International Maritime Organization’s Energy Efficiency Operational Indicator (EEOI) and European Union MRV regulation,” notes Bremer.
Specially designed cut-off valves for the FiTS2 system are flow optimised and integrated with the turbocharger casings, ensuring a compact and lean design. The valves can be operated rapidly and automatically under load, without interrupting operation of the engine up to full load. The flow optimised valves will be controlled by a system specifically designed by ABB. It is connected to the overall engine control system and activates the system’s pneumatically driven valves at prescribed load points.
Robust design and very wide compressor maps of ABB’s A100-Land A200-L turbocharger series allow switching under load without surging and there are no requirements for additional by-pass valves or pipework, making FiTS2 a uniquely simpler and relatively low-cost solution compared to any alternative.
An additional benefit comes from the possibility to switch off auxiliary blowers, used to start the engine and to supplement charge air at low engine outputs, at 25% engine load instead of around 35% load. Bremer notes that for some modern ship engines running at low loads, these large, electrically powered systems run almost constantly, contrary to their intended short-term operation. Turning them off earlier enables additional savings via reduced power consumption and lower blower maintenance costs due to significantly fewer operating hours.
The concept of sequential turbocharging is already present in the automotive market, and the phenomenon of slow steaming is now a decade old. This begs the question, why has such a relatively simple turbocharging solution only just become available? For Rolf Stiefel, vice president sales and marketing, WinGD, the development of modern engine controls is one important enabler.
Stiefel says: “You need electronic-controlled engines because you have to modify the whole tuning of the engines: engine control system, injection pressure and timing, exhaust valve opening and closing – all needs to be adapted to the available charge air pressure. You can’t do this with a mechanical engine.”
The concept has already been tested on a WinGD 6X72 at Diesel United and will be available for all the engine developers X-generation engines. It is particularly suited to larger engines of 600mm cylinder bore or greater, and engines with more than six cylinders. The system is not applicable for dual-fuel engines, which require a very different charging concept. FITS2 is more suited to newbuilds than retrofit projects given the expense of recertifying an existing engine and updating its technical file.
Bremer notes that the company optimises the point at which the switch is made from one to two turbochargers a case by case basis. The size of the turbochargers also depends on where in the engine loading the switch is made. These considerations will vary according to vessel type, operating profile and owners’ requirements.
With environmental pressures increasing and financial constraints still significant, slow steaming will be a feature of the shipping industry for the foreseeable future. With sequential turbocharging operators can drive for even greater fuel efficiencies.
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