With the awareness of the causes of climate change growing daily, super-yacht owners and engineers are under increasing pressure to adopt environmentally responsible practices, such as reducing crankcase emissions and switching to eco-friendly renewable fuels for marine applications. With currently available filtration technology, it is possible to implement these changes simply and cost-effectively, thus enabling compliance with the ever more stringent emissions regulations while significantly improving overall performance and efficiency of engines and allowing for considerable reductions in both operating costs and downtime.
Owners of vessels with diesel engines have a lot to contend with when it comes to meeting the stringent demands of today’s ever increasing raft of environmental legislation, particularly in regards to air pollution and emission levels. This affects all industries. For example, both the US Environmental Protection Agency (EPA) and the European Union (EU) have recently introduced strict new regulations, which govern the acceptable emissions levels for pollutants such as NOx, particulates, hydrocarbons, CO and CO2, having far-reaching implications for operators of vehicles and mobile equipment.
In the marine sector, this legislation is mirrored by the MARPOL MEPC 58/23 regulations, Tier II of which came into force in January, 2011, with other different stages and further tiers coming into play in coming years. As a result of these forthcoming amendments, engine system design will have to be in a state of constant development if these ever-changing emissions targets are to be met.
Engine Design and Control Changes
These vital developments can broadly be split into two areas. First, there are the engine design and control changes, which are effectively being driven by the need to modify fuel injection pressures, along with injection rate shaping and advances in combustion chamber geometries. These initial developments will help operators realise significant improvements in fuel atomisation and combustion burn efficiencies, which will help to lower raw emission outputs, while meeting the requirements of MARPOL MEPC 58/23 Tier II. Further reductions in emissions tend to come from the addition of exhaust after treatment systems such as EGR (Exhaust Gas Re-circulation) systems, SCR (Selective Catalytic Reduction) catalysts, Adblue urea injection and DPF (Diesel Particulate Filter) systems. The use of some of these systems actually reverses some of the good work performed in base engine and injection system changes. For example the use of an EGR system lowers combustion temperatures, combined with retarding the engine timing, decreasing NOx emissions but increases soot output which then needs to be caught in a DPF system- the net result of this is a reduction in overall NOx and soot particulate output (smoke), but at the expense of CO2 output and fuel efficiency.
Processing and Treatment of Engine Emissions
These changes have provided a starting point for the second area of development, which is in the processing and treatment of engine emissions. Processing systems take the form of emission after-treatment or emission reduction bolt-ons, such as exhaust gas re-circulation (EGR), diesel particulate filtration (DPF), selective catalytic reduction (SCR) and closed crankcase ventilation (CCV). All of these systems have the capability to ensure that engines achieve the requirements of Tier IV and Tier V standards.
Engine System Design Advances
Recent advances in engine system design have proven to be particularly effective and now mean that engines are capable not only of low emissions but also of increased economy, and better power and drivability. This has, however, come at a cost as modern diesel engine injection systems are now highly automated requiring total engine management control to achieve the required level of performance. As a result, injection system tolerances have become increasingly tight with injector nozzle orifices now being measured in microns, and injection pressures as high as 2,000 bar.
With tolerances this tight and pressures this high, engines become increasingly susceptible to injection system wear and corrosion even from extremely low levels of contamination. The problem is exacerbated by the fact that as injection pressures rise and NOx levels decrease, particulate and soot emissions tend to surge.
As a result we are now seeing considerable developments in high efficiency CCV, DPF and fuel filtration systems designed to remove the harmful contamination.
Fuel System Filtration
At pressures of 1,400 to 2,000 bar, water and particulate contamination can become a major hazard to efficient engine combustion. For example, the presence of even microscopic particulates can cause small changes in plunger needle and orifice dimensions or shape, that can adversely affect the profile of both fuel injection volume and atomisation. Over a prolonged period, this process will lead to pitting of the highly polished component surfaces, further reducing the overall efficiency of the system and increasing emissions beyond the permitted limits. In effect, this will make an engine non-compliant with emissions regulations. Engines are required to sustain their emissions levels throughout the life of the vessel, which means that proper maintenance of engines throughout their life is vital to remain compliant.
The solution is, however, fairly simple with the use of improved sealing materials and installation of water and particulate separation systems capable of removing at least 97% of all free water and in excess of 99% of particulate matter over five microns in size. Driven by legislation, engine manufacturers are now moving away from single stage, low grade filtration with water separation as an aftermarket accessory, towards fully integrated multi-stage filtration and water separation systems being provided as original equipment. As a result, internal injection system component cleanliness levels previously unheard of in the industry, and beyond the capabilities of the traditional aftermarket suppliers, are now a reality.
Addition of Bio-Diesel to Fuel Supplies
Another area of change has been in diesel fuel supplies. Prior to 2005 bio-diesel content in road, off road and marine fuels was almost non-existent, whereas the situation today sees much of the fuel supplied containing up to 7% bio-diesel (without notification) and as high as 30% in some areas (where notified). The use of these bio-diesels makes water separation far more difficult due to the chemistry of the fuel, while the increased dirt levels reduce the filter life expectancy.
Never before has it been so important to fit high capacity pre-filter water separators. Preferably these should be products capable of 10 micron efficiency levels in order to cope with the more difficult water removal in modern fuels, rather than the previously acceptable 30 micron versions. The reason for this is that the surface tension between a water droplet and a bio fuel rather than a traditional diesel, is far weaker, meaning that droplets tend to split up into smaller diameter droplets far more easily. This means that the filter medias have to cope with smaller water droplets and thus need to be of a tighter construction containing smaller pore sizes. While the use of positive temperature coefficient (PTC) heaters allows each filter to warm the fuel before filtering it, tackling the potential problems that can be caused by operation at low ambient temperature conditions, such as coagulation of waxy products and the overall viscosity increases or thickening which can impair fuel flow. In traditional diesels this used to start at approximately -5oC (23oF), whereas in some bio fuels this can start at temperatures as high as 15oC (59oF).
Closed Crank Case ventilation (CCV)
The control of diesel engine blow-by gases has also become a principal topic of discussion in the marine world, as well as being a core aspect in the MARPOL Tier II legislation. The focus is shifting from just simply measuring exhaust emissions to encompass all by-products from the engine, with the recent introduction of MARPOL Tier II introducing a lower than ever target of 7.7g/kWh NOx. Currently these parameters are tested by the engine manufacturer during design validation both in terms of new and worn engine conditions, and are not necessarily re- tested during the engine’s life.
In order to address these problems, piston seal designs have undergone something of an evolution and alternative fluoro-elastomer materials are increasingly being used to help minimise the escape of combustion gases into the crankcase. However, new targets are still not easy to meet, particularly as engine components begin to wear leading to a progressive increase in crankcase emissions; indeed, even a well maintained 6.0l diesel engine will eject 4 ft³ per minute of blow-by gas and soot directly to atmosphere, containing an atomised oil flow of at least five grams per hour.
The key to overcoming this problem is through the use of a CCV device, rather than venting the gases directly to atmosphere as has traditionally been the case. With this technology, crankcase blow-by is processed through ultra high efficiency filters and returned to the combustion cycle via the turbo charger, creating a closed loop that prevents environmentally harmful hydrocarbons (HC) from escaping to atmosphere.
This does mean, however, that crankcase pressure must also be controlled to avoid low pressures being induced in the engine by the turbo. This can be achieved through the use of a patented upstream pressure regulating diaphragm, or a less effective downstream vacuum limiter, enabling engineers to maintain a slightly negative crankcase pressure, which eliminates many of the oil seal leaks normally seen during engine operation. The latest high performance upstream regulators also allow crank pressure to be maintained independently of changes in the filter pressure drop that occurs during their normal working life, keeping crankcase pressures consistent at all engine speeds and loads, rather than having varying levels of regulation as found with downstream vacuum limiters.
New ISO guidelines are being developed for the testing of CCV devices, in the form of ISO 20564, with an important feature of this being the ability to measure filter performance. Although most filters can separate large, heavy particles of oil from blow-by gases, recent tests have shown that as much as 90% of diesel engine crankcase emissions are submicron in size. According to established theory and proven practice, it is the particles in the range of 0.2 to 0.4 microns that are the most difficult to separate. Thankfully, the latest generation of filters have been designed to excel when it comes to particles of this size, helping to overcome the problem.
Example of a typical diesel engine system with fitted filtration components.
This article was written by Adam Pearce, Business Development Manager, Parker Hannifin - Racor Filter Division Europe.
Click the link for further information on Parker Hannifin Racor Filter Division products for marine fuel filtration.
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