SubscribeIn this article, Cummins will detail the emissions reduction solutions and provide an understanding of the implications of each one on vehicle installations.
Whilst the primary focus for the Tier 3/Stage IIIA emission rules has been focused on nitrogen oxides (NOx) reduction, the final Tier 4/Stage IV standards will drive lower particulate matter (PM) and even lower NOx. The move from Tier 3/Stage IIIA to Tier 4/Stage IV represents a further 90% reduction in PM emissions and 50% reduction in NOx emissions. This will require some form of exhaust aftertreatment to be incorporated in the vehicle.
There are many ways to meet the coming regulations and given the range of engines to which the regulations apply, it’s critical to match the right solution with the right application. The solutions range from controlling the by-products of combustion in-cylinder to recirculation of exhaust gas back to the combustion chamber with cooled exhaust gas recirculation (EGR), and various forms of after treatment such as lean-NOx catalysts and active particulate filters.
The availability of low sulphur fuels becomes important when looking at after treatment solutions. Most off-highway fuel today contains sulphur up to the 3000 – 5000 parts per million (PPM) ranges. Sulphur levels above 500 PPM can be problematic for some NOx reduction solutions such as cooled egr and after treatment methods such as NOx absorbing catalysts. Such low sulphur fuels will not be available until 2007 for off-highway markets.
NOx Reduction
Cooled Exhaust Gas Re-circulation (EGR)
Cooled EGR may be a NOx-reduction technology candidate for Tier 4/Stage IV. However, due to the potential higher EGR rates necessary for the lower NOx level at Tier 4/Stage IV, this technology solution may drive large engine system and installation considerations.
A few of these considerations for the use of Cooled EGR in off-highway applications are the potential for more frequent oil changes, the need for larger fans and as low sulphur fuels that are not readily available for off-highway today.
In on-highway applications ram air is available, abundant, and helps to cool an engine with the additional heat rejection requirements of cooled egr. This lack of ram air becomes somewhat problematic for the cooled egr approach utilized on off-highway applications like excavators. Without ram air, larger fans and radiators are required and with them come parasitic power loss. The larger the fan, the more power is taken from the engine for cooling as opposed to performing its mission.
Selective Catalytic Reduction (SCR)
Selective Catalytic Reduction (SCR) systems use a chemical called urea, which is injected into the exhaust stream. Once in the system it converts to ammonia, and reacts with NOx over a catalyst to form harmless bi-products - nitrogen gas and water. Urea is a benign substance that is generally made from natural gas and widely used in industry and agriculture. Cummins will introduce the SCR systems for truck and bus applications in Europe in October 2005 to meet the Euro 4 standards.
The urea-SCR system basically consists of three elements
·Catalyst –The catalyst is mounted in the exhaust stream. It can be similar in outward appearance to a muffler, but depending on NOx reduction required could be marginally larger. It contains chemical compounds which, in the presence of ammonia, help transform nitrogen oxides into harmless chemicals.
·Urea –Urea quality and concentration in aqueous solution are important and must be controlled and distributed properly. Urea is carried on board the equipment as a water solution in a storage tank with a typical capacity of 5% of the diesel tank. The storage tank is sized to minimize operator filling, but within packaging and weight constraints of the equipment.
·Urea injection and control system –A sophisticated injection system and controls (including NOx and urea quality sensors) are required to deliver a precise amount of urea under all environmental conditions. The injection of urea has to be carefully controlled so that the availability of ammonia is closely matched to the amount of NOx being produced by the engine in real time.
NOx Adsorbers
The NOx Adsorber Catalyst (NAC) is a new technology developed in the late 1990s. The NOx Adsorber Catalyst uses a combination of base metal oxide and precious metal coatings to effect control of NOx. The base metal component (for example, barium oxide) reacts with NOx to form barium nitrate – effectively storing the NOx on the surface of the catalyst.
When the available storage sites are occupied, the catalyst is operated briefly under fuel-rich, low-oxygen exhaust gas conditions. This releases the NOx from the base metal storage sites and allows it to be converted over the precious metal components to nitrogen gas and water vapour.
Diesel engines normally operate with an excess ratio of air-to-fuel – “lean” operation.
Under lean operating conditions it is extremely difficult to control nitrogen oxides (NOx) with a catalyst because of the excess of oxygen in the exhaust stream. Under lean operating conditions, the NOx is simply stored in the catalyst. Regeneration is required to release and convert the NOx to nitrogen gas. Regeneration of the NAC requires elimination of all excess oxygen in the exhaust gas for a short period of time.
This can be accomplished by operating the engine rich, or by operating the exhaust flow through the catalyst rich by injecting fuel prior to the NAC. Either way, the engine and catalyst must be controlled as a system to determine exactly when regeneration is needed, and to control the exhaust parameters during regeneration itself.
Sulphur poses challenges for NOx adsorbers. In addition to storing NOx, the NAC will also store sulphur, which reduces the capacity to store NOx. Although fuel sulphur levels are being reduced in 2010 to 15 ppm, sulphur at any level requires the engine design to provide for a periodic de-sulphation process – a process to remove sulphur from the catalyst. This is similar to the NOx regeneration process, but at higher temperatures.Obviously, additional regeneration events will also have a negative impact on fuel economy.
Lean-NOx Catalysts
A lean-NOx catalyst uses unburned hydrocarbons to reduce NOx over a catalyst. The catalyst may contain precious metals such as platinum or other materials such as zeolite. The successful operation of a lean-NOx catalyst requires continuous injection of fuel upstream of the catalyst. The NOx conversion efficiency depends on many factors – but typical values are 10%-25% in use over practical duty cycles.
Both catalyst metals have temperature range limitations that keep the conversion efficiencies lower than desired in typical operation. Lean-NOx catalysts do not have adequate NOx reduction capability for Tier 4 applications. However, lean-NOx catalysts are often an excellent option for retrofits. They are relatively easy to install and integrate with existing engine and equipment systems.
NOx-Reduction Summary
In summary, key challenges for all NOx aftertreatment technologies (Cooled EGR, SCR,
NOx adsorber [NAC] and lean-NOx catalyst) include designing and developing integrated systems to:
- Be reliable and durable in all environmental conditions and applications.
- Minimize packaging and weight.
- Control emissions over the life of the product.
- Minimize maintenance.
- Be affordable in both initial price and operational costs.
PM Reduction
At Tier 4/Stage IIIB emissions standards, particulate emissions are 90% lower than the
Tier 3/Stage IIIA standards. While previous reductions in particulate matter emissions have been achieved through engine combustion improvements, the stringent Tier 4/Stage IIIB particulate standards require particulate aftertreatment. The active diesel particulate filter (DPF) is the only current technical option for meeting the Tier 4/Stage IIIB PM emissions standards. It is assumed that all engine manufacturers will use this technology.
Active Diesel Particulate Filters (DPF)
In order to reach Tier 4/Stage IIIB particulate standards on all applications and duty cycles, “active” diesel particulate filters are needed. Filtration of exhaust gas to remove soot particles is accomplished using porous ceramic media of cordierite or silicon carbide. A typical filter consists ofan array of small channels that the exhaust gas flows through.
Adjacent channels are plugged at opposite ends, forcing the exhaust gas to flow through the porous wall, capturing the soot particles on the surface and inside poresof the media. Soot accumulates in the filter, and when sufficient heat is present a “regeneration” event occurs, oxidizing the soot and cleaning the filter.
The challenge of particulate filter design is to enable reliable and consistent regeneration, so that soot is removed in all types of duty cycles. The use of this “active” method involves monitoring the particulate filter backpressure and regeneration events and managing the temperature entering the filter.
There are several methods to control or raise the exhaust temperature to “actively” manage the DPF. The most promising methods for an active integrated system forTier 4/Stage IIIB are management of the engine combustion process in combination with an additional oxidation catalyst.
This will allow regeneration to take place under low-ambient/low-load conditions when exhaust temperatures are low and during normal operation as well. EPA requirements state that every engine has to achieve the required reduction in every operating condition, which is why active controls are needed.
DPF Challenges
Maintenance may be required on diesel particulate filters. Metals in lubricant additives will become ash and collect in the filter as oil is consumed and particulate matter is burned off through regeneration. If this is the case, the ash must be cleaned from the filter or plugging could occur. Fleetguard Emission Solutions has recently introduced its first commercial cleaning system to the field for the automotive retrofit market. However, Cummins long term goal is to avoid this maintenance altogether.
Cummins is currently working with oil manufacturers on the development of low-ash oils and to understand how different additive components may behave differently with regard to filter plugging. If maintenance of the diesel particulate filter is required, we anticipate that it will be at relatively high-hour intervals.
System Integration
Cummins remains focused on providing outstanding customer value, while meeting the toughest emissions standards. Our Research and Development effort is the result of a partnership between Cummins subsidiaries, such as Holset and Fleetguard, key suppliers and customers. In addition, Cummins is deeply involved with our OEM partners and key suppliers in the development and real-world testing of prototype equipment. This test equipment is being put into service years in advance of the dates when regulations take effect.
Cummins is positioned to provide turnkey solutions with advanced base engines, controls, and packaging of the aftertreatment in the muffler. Cummins is the only engine manufacturer with wholly owned subsidiaries providing technology for air-handling (Holset) and aftertreatment systems (Fleetguard Emission Solutions).
The Cummins portfolio of products includes all of the key emissions technologies, allowing them to meet global engine emissions requirements. Cummins evaluates customer needs and market conditions in order to provide the optimum products with the appropriate technologies for each market in which it operates.
Cummins has invested significantly in research and engineering efforts to ensure that the proven products in operation today and at Tier 3/Stage IIIA are the base platforms for Tier 4/Stage IIIB. In 2011, Cummins will use an active particulate filter to achieve the Tier 4/Stage IIIB 90% reduction in particulate matter.