More marine engine designs are deploying two-stage turbocharging, but the concept’s potential for low-speed engines remains debated
Multiple projects over the past year have highlighted how two-stroke turbocharging has become a go-to technology for engine designers looking to improve both the emissions profile and engine performance. Going beyond the magical 6.0 compression ration by deploying two charging stages with an intercooling period, two-stage turbocharging offers a potent combination of benefits – higher brake mean effective pressure in cylinders, enabling lower fuel consumption and cylinder temperatures along with higher power density.
The Shanghai Marine Diesel Engine Research Institute (SMDERI) is among those engine designers trying to harness the miracle of compound compression as it reacts to new emission demands from the Chinese government. On 1 July 2018, the first stage of China’s new emission regulations governing marine engines used on rivers and around islands came into force. Known as China I, the regulations apply to the vast majority of vessels on the country’s domestic waterways that fall between existing domestic emission legislation on small engines (with a power output of less than 37kW per cylinder) and the engines of internationally trading vessels whose emissions are regulated by IMO.
The regulations cover carbon monoxide, unburnt hydrocarbons, NOx and particulate matter. Perhaps the most onerous levels for engines to reach are those relating to NOx. For a medium-speed engine like SMDERI’s six-cylinder 6CS21, with a total rated output of 1,320kW, China I demands a 13% reduction in NOx emissions. That will become even more onerous from 1 July 2021 when a second stage of the regulations, China II, come into force. NOx emissions for the current 6CS21 will need to be cut by nearly a third.
SMDERI turned to a two-stage turbocharging process to help the engine meet the first round of China’s new emission demands. The two-stage turbocharged engine, 6CS21TS, has 14% higher power than the 6CS21, with rated power increased to 1,500kW. The emission characteristics of the two-stage turbocharged engine are also improved. The weighted fuel consumption is reduced by 11.6 g/kWh, smoke emissions at all the operation loads are reduced and NOx emissions are reduced by 12% – satisfying not only the China I but also IMO’s Tier II NOx emission regulations.
The two-stage turbocharged engine has a much higher pressure ratio than that of the original single-stage turbocharged version, up from 5.09 to 7.15. The average exhaust gas temperature was reduced by 23°C, reducing the heat load of the diesel engine.
SMDERI also studied the influence of timing on the performance of the 6CS21TS engine. It concluded that variable valve timing technology can be used to solve low-load performance problems as well as other challenges – including insufficient air intake and higher fuel consumption – that would be caused by the use of a strong Miller cycle.
So much for China I, but how will SMDERI’s engine meet the even higher demands of China II in two years’ time. The company has conducted simulations deploying exhaust gas recirculation (EGR) on the 6CS21TS engine. It found that while the brake mean effective pressure remains constant at 2.71MPa, emissions are further reduced, to below the limit required by China II.
Although there is an inevitable trade-off between NOx reduction and fuel efficiency, the engine simulation showed good economy with a minimum fuel consumption rate of 195.1g/kWh at 75% load. SMDERI’s research has also showed that the new 6CS21TS diesel engine could make use of larger EGR rates (recirculating more than 25% of exhaust gas) to satisfy the IMO Tier III emission regulation.
The institute believes that its research so far shows that the technical route to bring the 6CS21 diesel engine up to the China II regulations is to use a high-pressure common rail fuel system, a two-stage turbocharging system, a strong Miller cycle and EGR. So far it has completed the design of the 6CS21TS with the EGR diesel engine. According to the project plan, the prototype engine is expected to be bench tested in October.
Meanwhile Hyundai Heavy Industries is also incorporating two-stage turbocharging into two additions to its four-stroke engine portfolio. For its latest dual-fuel engine, the top-of-the-power-range H54DF(V) as well as the new diesel engine H32C(V) the company has developed two-stage and single-stage turbocharged designs simultaneously.
Hyundai Heavy Industries' new 320-mm bore H32C(V) will offer a big pressure boost compared to the HiMSEN H32/40
Marine propulsion and power generation are among the core applications of the HiMSEN H54DF(V). With a bore size of 540mm and a stroke of 600mm, this is the most powerful dual-fuel engine developed in the HiMSEN range, with a power output per cylinder of 1,470kW. This means it can be applied with smaller numbers of engines and cylinders, compared to smaller engines, bringing cost benefits. A 12-cylinder prototype was started at the end of 2017.
The engine was designed with a modularised design concept. One example is the charge air cooler (CAC) module, with the two-stage turbocharger – comprising four sets of turbochargers, intercooler and after cooler – mounted on the engine in the module to save space.
The CAC module has several cooling water and lubrication oil flow paths. To optimise the air flow inside the module, the overall core flow uniformity was maximised and the pressure drop was minimised. Due to the additional weight of the secondary turbocharging system, HHI applied additional support between the CAC module and the engine block on the two-stage turbocharged version of the engine.
“Two-stage turbocharging only really offers significant benefits on four-stroke engines”
The other engine in HHI’s range to benefit from a two-stage turbocharging upgrade is the HiMSEN H32C(V), an updated version of the H32/40(V), of which more than 1,500 are already in service. The newer engine will aim to offer a power output 20% greater than the current engine due to the application of new technologies including two-stage turbocharging, dual valve timing, a double governor and electronic variable fuel injection timing. A 12-cylinder prototype is already running.
HiMSEN has already deployed the Miller cycle timing on its engines, but a more enhanced Miller cycle has been applied for the new engines, leading to lower cylinder combustion temperature than before and consequently reduced NOx levels. The Miller cycle also gives the benefit of reducing knock occurrence in gas-fuelled engines.
Because it is possible to lower the combustion chamber temperature and reduce NOx levels, injection timing can be advanced so that the peak firing pressure increases, improving engine efficiency. Compared to previous models, the H54DF(V) and H32C(V) will increase peak firing pressure by around 30%.
The enhanced Miller cycle means a shorter duration of intake valve opening, so requires higher boost pressures to get enough air into the combustion chamber. A two-stage turbocharging system is needed to take full advantage of the engine efficiency and emissions gains achievable when the Miller cycle is used.
HiMSEN electronic variable injection timing (HEVIT) is an add-on system for the injection timing control of conventional fuel injection pumps. It provides load-dependent fuel injection timing. The main component is an electromagnetically activated two-way valve unit attached on the top cover of the fuel injection pump. The valve unit either closes the pumping chamber or connects it to the fuel return line. Thus, the fuel injection is determined by the valve activation and the start of the injection cycle can be flexibly controlled depending on the crank position. Redundancy is an additional benefit – if the two-way valve malfunctions HEVIT can still function as a conventional, mechanical fuel injection pump.
If there are clear applications for two-stage turbocharging on four-stroke engines, there is still discussion about whether or not the concept could be applicable for two-stroke engines. MAN Energy Solutions has a lot of experience with two-stage turbocharging, having produced some of the first engines to feature the concept – it passed 27M service hours on two-stroke turbochargers earlier this year, and around 10-20% of that installed base is in the marine field, says vice president and head of turbocharger sales Ralph Klaunig.
Recent marine four-stroke engine launches, including MAN's 45/60CR, deploy two-stage turbocharging
For all its experience, Dr Klaunig believes that there are practical reasons why two-stage turbocharging will not be applied to low-speed engines. “Two-stage turbocharging only really offers significant benefits on four-stroke engines,” he explains. “Even if you could find a way to increase the efficiency benefit for two-stroke engines, the design changes that would be needed would be too great to make it worthwhile.”
In principle, Winterthur Gas & Diesel agrees that two-stage turbocharging, at present, is not worthwhile for two-stroke engines. But according to general manager engine performance Daniel Schäpper, it may be worth re-evaluating the technology.
“Given the demands on shipping to reduce emissions I think that we will need to look again at possibilities to drive better performance out of engines,” says Dr Schäpper. “With the upcoming changes in legal requirements [on emissions] on the one side and costs evolution both for hardware and fuel on the other, the payback time for two-stage turbocharging installations might come into a reasonable range.”
There are some potential solutions to improve the efficiency of two-stage turbocharging on larger two-stroke engines. One was studied by WinGD in 2016, using an asymmetric port device – hydraulically regulated to slide on the outer liner surface – to delay intake port opening timing. This means a later exhaust valve opening is possible without causing exhaust gas blowback into the intake receiver. The greater expansion of gases in the chamber also benefits combustion.
Although such a set-up is feasible and would offer marginal improvements in performance with the same emissions profile, there are multiple challenges, notes Dr Schäpper. First, the firing pressures required to make the concept work are higher than for today’s engines. The reduction in exhaust gas temperatures that combats NOx production could also increase cold corrosion – caused by sulphuric acid condensation – in the exhaust funnel and boiler.
The nature of the low-speed engine might also make engineering a solution difficult. “Even if the application of two-stage turbocharging on two-stroke engines may be less complex in design and operation than for four-strokes, it is challenging due to product customisation in the low-speed sector. Each engine can be ordered anywhere within the rating field. We would expect complications for turbocharger matching compared to today’s procedures.”
Financially, the benefits are not yet conclusive. The second-stage turbocharger and cooler, with the additional systems and the redesign of the engine, would substantially increase costs. This could be somewhat offset by the resulting increase in power density, which could lead to the selection of engines with fewer cylinders or a smaller bore size.
Dr Schäpper concludes: “According to our preliminary estimations, the increase in efficiency will return investment for customers in some years. The future evolution of regulations towards CO2 reduction – such as the tightening of the Energy Efficiency Design Index – may trigger additional interest on the application of the technology.”
Two-stage turbocharging offers clear advantages for four-stroke engines and could yet help two-stroke engine operators meet tightening emissions legislation. But until technical and economical obstacles are overcome, two-stroke engine designers will look to other areas to realise the efficiency gains they need.