NOx control measures in two-strokes
Updated 22 Oct 2019
The development of electronic engine control has allowed the two-stroke to meet the challenges posed by the NOx Code. However, the requirements of Phase III which came into effect in 2016 for new vessels operating in ECAs mean other measures are needed as well. Unlike with four-stroke engines, Miller timing is not possible on two strokes so although the electronic control can allow the Tier II requirements to be met quite easily by way of variable exhaust valve closing, two other means are employed on new vessels either alone or in combination depending upon the engine and the operating parameters need to meet the trading strategy of the ship.
Of the other means, Selective Catalytic Reduction (SCR) is a form of exhaust gas cleaning and is carried out after the engine. It can be used on both two-stroke and four-stroke engines. In this system, the exhaust gas is directed through a catalytic reactor unit usually with a vanadium catalyst where it meets an injected stream of 40% urea solution. The injection of the reductant can be done in two ways; either airless or air-assisted. High-speed engines usually have airless systems while low- to medium-speed engines use air-assisted injection to dose the exhaust stream. The resultant reaction reduces the NOx in the exhaust gas to nitrogen and water.
Applying SCR to two-stroke engines has presented several engineering problems because under normal conditions, the exhaust gas temperature after the turbocharger would be in the 230-260°C range, dependent on load and ambient conditions. These low temperatures are problematic for the SCR when HFO is employed. Thus, in order to achieve the highest possible fuel flexibility, it has been necessary to ensure that the engine produces an exhaust gas with the right temperature for the SCR system. The SCR inlet gas temperature should ideally be around 330-350°C when the engine is operated on HFO.
The alternative of Exhaust Gas Recirculation (EGR) is much more suited to NOx reduction on all engine types, especially when using low-sulphur fuels. It works by recirculating around 40% of the exhaust gas into the engine thus reducing both the temperature and the amount of oxygen in the combustion chamber. Since NOx forms when fuel is burned at high temperatures in air, the system reduces NOx formation. With an EGR system in place, there is no need for catalytic reduction.
EGR has been used very successfully in motor vehicles for some time but for two-stroke marine engines it is a relatively new technology and one that is still being developed. One of the challenges for this technology is the positive scavenging differential pressure. For this reason, an EGR blower is necessary to realise exhaust-gas recirculation.
The purpose of the blower is to raise the pressure of the cooled and cleaned exhaust gases so that recirculation through the engine inlet is possible. In this way, a reduction of combustion-temperature peaks – and a subsequent reduction in NOx formation – can be achieved. The required EGR flow varies, depending on load and ambient conditions.
MAN Energy Solutions has developed an electrical turbo blower (ETB) that plays an important role in the operation of the EGR system by providing active speed-control. It is derived from the company’s turbocharger portfolio. The desired EGR operating condition is achieved by using an electrical, high-speed motor directly coupled to the compressor wheel and driven via a frequency converter. A casing unit holds the stator of the motor and provides a supply for cooling water and lube oil for the journal bearings.
The interface between the ETB, frequency drive, instruments and control panel in the engine control room is hardwired. Since May 2015, two ETB18 prototypes have run successfully on an 82,000dwt bulk carrier equipped with an MAN B&W 6S60ME-C 8.2 Tier III engine. The first fully-commercial version of an ETB40 passed its factory acceptance test in October 2018.
Although EGR can allow two-stroke engines to meet Tier III NOx requirements, there is a problem with it in that the levels of sulphur in fuels means the exhaust gas also contains sulphurous compounds that are corrosive to the engine. To overcome this, the exhaust gas being recirculated passes through a scrubber system to remove some of the corrosive compounds. The wash water from the EGR scrubber needs to be treated before it can be disposed of.
At MEPC 73 in October 2018, IMO adopted new guidelines for EGR bleed-off water previously agreed at PPR 5. These guidelines are contained in MEPC.307(73) and apply to a marine diesel engine fitted with an EGR device having a bleed-off water discharge arrangement, for which the EIAPP Certificate was first issued on or after 1 June 2019.
The whole question of discharges of exhaust treatment systems is a topic for debate because of the use of scrubbers for cleaning SOx from exhausts and could be subject to future changes.
Efficiency improvements for two-strokes instigated due to EEDI rules
For meeting EEDI purposes, the developments to the two-stroke crosshead engine in recent years have centred around increasing the stroke, reducing rpm and matching the engine to a larger propeller designed to match the operating profile of the vessel.
For many years, two-stroke engines were generally available in two variants – short or long stroke – the difference being self-explanatory. To meet the greater efficiency requirements demanded by EEDI regulations, most engine designers agree that a longer stroke, which increases compression, is advantageous. The designers and manufacturers have responded by designing super- and ultra- long stroke variants of their engines.
As these have been adopted by customers, the short and long stroke variants are gradually disappearing from engine catalogues although they are still available if required. The new longer-stroke engines do raise some issues: engines are necessarily taller and manufacturing suitable crankshafts requires re-tooling by makers and sub-contractors.
With a larger catalogue of designs, MAN Energy Solutions engines delivered since 2016 have included models with long, super-long and ultra-long strokes while WinGD has lengthened the stroke of its main designs but does not offer the same choice of its rival.
Two-stroke crosshead engines are produced in a range of bore sizes from 35cm to 95cm. The smaller 35-45cm bore sizes are found on Handy and Handymax bulkers and similar size product tankers, the 50-60cm bores on Panamax size bulkers and tankers and the 70-80cm versions on larger bulkers and tankers. Most engines have five or six cylinders while the largest 90-95cm engines are found in 10- and 11-cylinder versions on the largest container ship types where more speed is still considered a desirable characteristic.
Today the focus of development is less on absolute power and more on the need to meet EEDI rules. Different generations of engines appear from time to time with the improvements almost always aimed at improving efficiency as well as ease of maintenance and reducing complexity and weight.
In 2016, MAN B&W – as MAN Energy Solutions was then called – introduced the Mk 10 engine which included several new developments such as a complete redesign of the valve systems, including the new FBIV (Fuel Booster Injection Valve) and TCEV (Top Controlled Exhaust Valve). This means that this engine does not have a hydraulic cylinder unit (HCU), a base plate and long high-pressure pipes between the actuators and the fuel and exhaust valves. In addition, the engine features a new bedplate design, optimised cylinder frame and a new connecting rod design. The overall benefits mean that the new version of a 90cm bore engine can deliver a 2.3% power increase for a 1g/kWh SFOC decrease and a 10% weight reduction.
WinGD’s predecessor began development of a new range of engines designated X-types in 2015 shortly before the transfer of business. Different variants have made their debuts since, with the WinGD X52 passing its factory and type-approval tests in July 2017. In 2019 the company introduced three new engine types: the X40-DF, X82-D and X82-DF. The two with DF suffixes are dual-fuel versions with the X82-D described as ‘dual-fuel ready’ meaning it can be converted to the X82-DF specification.
The X82-D is a development of the earlier X82-B engine with the cylinder distance reduced to 1,440mm from 1,505mm. The brake specific fuel consumption has also been reduced depending of the rating point and the tuning variant chosen. The X82-D engine will be available from 6 to 9 cylinders, covering a power output from 16,560kW at 58rpm to 49,500kW at 84rpm.
All cylinder configurations are possible with a one-piece crankshaft design further reducing engine length and with pistons fitted with two piston rings for reduced friction losses. The X82-D has also been designed with optimised liner design to avoid cold-corrosion occurring within the cylinder.
All WinGD X-prefix engines can also be offered with dual ratings which can be accessed via a minimum of modifications to engine and turbocharger components, enabling ship operators to readily employ a fuel-saving slow steaming mode, according to market and contract conditions.