How does Selective Catalytic Reduction work?

The non-toxic and odorless reducing agent is widely used in commercial vehicle applications and has been available throughout Europe since 2004, and the USA since 2010. It is marketed in Europe under the trade name of “Ad Blue”. It consists of a 32.5 percent solution of extra pure grade of urea in de-ionized water. The amount of reducing agent added is about five to seven percent of the fuel consumption. It is stored as a second consumable fluid in a separate tank and fed to the metering device via pipelines. To ensure the high nitrogen oxide conversion rates of more than 90 percent in some cases in every operating state of the propulsion system, the electronic control system calculates the precise quantity of reducing agent needed based on key engine parameters such as operating temperature and engine speed.

Fuel consumption potential of SCR

In the combustion process inside the cylinder, in addition to the relationship between the production of nitrogen oxide and particulates, there is one between fuel consumption and nitrogen oxides. Generally speaking, high combustion temperatures lead to economical fuel consumption and low particulate levels, albeit with greater nitrogen oxide production. Since the SCR catalytic converter subsequently removes the nitrogen oxide from the exhaust gas, the development engineers can use this to configure the combustion process for extremely low fuel consumption while still remaining within the legal emission limits.

Benefits of mtu’s SCR system

mtu individually matches the SCR system to the specific engine and the application. At the same time, the drive system is optimized for low fuel consumption and a minimal space requirement for the SCR components. As far as possible, mtu uses proven SCR components from the commercial vehicle sector. Customers subsequently benefit from a tried and tested standard production solution with a long service life which is optimally adapted to the engine package. mtu drive systems are designed to be very robust in terms of changes in the operating conditions, which means that customers are very flexible in terms of how they employ their systems in a wide range of applications. Compared with other ways to reduce emissions, by using a diesel particulate filter, for example, an SCR catalytic converter does not increase backpressure in the exhaust system to the same degree. Consequently, the turbocharging system has to work against a lower backpressure and can be operated at a higher efficiency level.

System development at mtu

mtu has wide-ranging expertise in SCR systems. This has enabled the company to optimally exploit the potential of exhaust aftertreatment in combination with the engine. Using modern simulation tools, mtu matches parameters such as the exhaust gas flow through the catalytic converter precisely to the engine’s operating conditions. The results of those calculations are then used in the design of the catalytic converter casing. mtu also improves the packaging by using computer simulations. Since mtu supplies the drive and SCR system from a single source, it is able to optimally match the engine technologies such as combustion and turbocharging to the needs of exhaust gas cleaning system. This ensures, for example, that the operating temperature of the SCR system remains at an optimal level.

In the case of drive systems in the lower power range, such as Series 1000, 1100, 1300, 1500 and 1600 engines, mtu uses reliable SCR components from the commercial vehicle sector that are adapted to the specific requirements of their use in industry. mtu has also transferred this high-volume production expertise to larger engines with power outputs of up to 3,000 kW and has developed an economical modular concept for SCR metering devices and catalytic converters, with each module using two metering devices. mtu is currently advancing the development of its flexible modular concept for Series 2000 and 4000 engines: one module will completely cover Series 2000 engines, while two identical modules will be used for the Series 4000 engines. In addition to lower costs and high reliability, the benefits of the modular concept include a modest space requirement, since smaller individual modules can be better integrated into the engine package than one large unit.

mtu also assists its customers in the design of the reducing agent supply system for the SCR system. For the underfloor rail powerpack equipped with the V12 Series 1600 engine, mtu is even developing a complete SCR system that, in addition to the catalytic converter and metering system, includes a reducing agent tank, heater and piping.

Examples of SCR use in mtu drive systems

One example of particularly low emission limits is the US EPA Tier 4 final emission standard. As from 2015, it will limit the nitrogen oxide emissions of engines for gensets with power outputs exceeding 560 kW to a maximum of 0.67 g/kWh and particulate emissions to 0.03 g/kWh. mtu will comply with this nitrogen oxide standard using a SCR system.

From this year, the Tier 4 final standard applies to drive systems in the construction and industry sector with a power output of less than 560 kW. This standard limits nitrogen oxide emissions to a maximum of 0.4 g/kWh and soot particles to 0.2 g/kWh. In order to comply with these tough legal limits, mtu is using a technology package for the new Series 1000, 1100, 1300 and 1500 engines consisting of exhaust gas recirculation and SCR catalytic converter.

Summary

An SCR system can remove more than 90 percent of the nitrogen oxides from the exhaust gas in some cases. In addition, the engine can be configured for very low particulate emissions. That ensures compliance with stringent emission limits for diesel engines. At the same time, operators save on fuel costs with an SCR system, because internal engine parameters can be configured for ultra-low fuel consumption. Compliance with extremely low emission limits, however, requires a combination of internal engine optimization using exhaust gas recirculation and external optimization by means of exhaust aftertreatment with an SCR catalytic converter and, if necessary, a diesel particulate filter. mtu supplies the engine and the SCR system from a single source and can therefore ensure that the two components are ideally matched, with the key development objectives focusing primarily on low fuel consumption and low space requirement for the SCR components. mtu will be using SCR systems for genset engines with a power output exceeding 560 kW, for example, and in drive systems for construction and industrial applications below 560 kW in order to meet the very strict requirements of the US EPA Tier 4 final standard.