Engine after-treatment systems and light-off time optimisation

In the search for lower fuel consumption, engines have become leaner, have higher compression ratios and combust at higher temperatures. This results in relatively low CO and HC emissions, but high emissions of NOx. In order to counter this effect, after-treatment systems have been deployed, but cold starts remain a problem. The high NOx emissions can be reduced in several ways. Full vehicle testing is essential for obtaining the best results. OWI-Lab has the proper testing equipment that is also suitable for large off-highway vehicles.


After-treatment systems are used to counter the effect of high NOx emissions. The most common systems for reducing NOx are Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR). A Diesel Particulate Filter (DPF) is normally used for HC, CO and also particular matter.

Exhaust Gas Recirculation (EGR) redirects a portion of engine exhaust back into the engine to cool and reduce peak combustion temperatures and pressures, thereby reducing the production of NOx. EGR is commonly used by engine manufacturers as a method for complying with new engine emission control standards.

Diesel particulate filters (DPFs) are exhaust after-treatment devices that significantly reduce emissions from diesel-fuelled vehicles and equipment. DPFs typically use either porous ceramics, cordierite substrates or metallic filters to physically trap particulate matter (PM) and remove it from the exhaust stream.

Selective Catalytic Reduction (SCR) Systems inject a reductant, also known as diesel exhaust fluid (DEF), into the exhaust stream, where it reacts with a catalyst, thereby converting NOx emissions into N2 (nitrogen gas) and oxygen. The catalytic reaction requires specific temperature criteria for NOx reduction to occur. SCR systems require periodic refilling of the DEF, and the system should also ensure that the DEF does not freeze. SCR systems are commonly used in conjunction with DOC and/or DPF to reduce PM emissions. Because of new NOx standards, most 2010 and newer on-highway diesel engines come equipped with an SCR system.

These systems are essential to vehicles for passing the emission regulation tests. The regulations are continually becoming stricter and globally standardised. 

European emission standard Euro 6

Euro 6 (01.2013) for heavy duty vehicles is the first legislation to incorporate World Harmonized Test Cycles.  These tests differ from the previous European tests with respect to engine speeds and loads, and include additional cold start and hot soak tests. Together with the lowered emission limits, these new tests have resulted in increased output levels of NOx emissions.

It is now impossible for constructors to attain the earlier Euro standards using only EGR. The entire industry is now focusing on the SCR system which uses a urea mixture for reducing NOx emissions. NH3/urea SCR is a very effective and widely used technology for reducing NOx in diesel exhaust. The problem with NH3/urea mixtures is that they typically freeze around -11 °C.

As mentioned earlier, cold starts are a new facet of the test procedure. It increases NOx emissions by +/-10 % in comparison with the old test cycle. This is because catalytic converters have to be at high temperatures in order to function optimally.

This so-called light-off time can be reduced in several ways:

  • Electric heating of the system

One example is Emitec’s E-cat, which entered production 10 years ago and has been used successfully in the 12-cylinder engines fitted to the BMW 7 Series. The E-cat significantly reduces cold starting times, resulting in the pollutants in the exhaust gas being eliminated at a much earlier stage. Heated metal catalysts are supplied with between 1 and 3 kW of power. This raises the catalyst operating temperature by a crucial 20 to 30°C (or up to 100 °C in passenger cars). In SCR systems the AdBlue urea solution is injected onto the hot E-cat, thereby improving atomisation and evaporation.

 

The E-cat is able to convert into usable heat the CO2-neutral alternator energy released during deceleration. Heated catalysts are particularly useful in cars with start/stop systems, because they prevent the catalyst from cooling down and remove the need for heating during idling phases. In vehicles with braking energy recovery systems, E-cats do not affect fuel consumption, thus keeping the cost of this type of ‘active’ catalyst system within reasonable limits.

There is a downside to these systems: the electricity consumed by the systems results in extra fuel consumption. Therefore the system must be highly efficient.

  • Optimising the shape and position of components

For example, NH3/urea dosing system components could be protected from damage caused by ice expansion, either by purging the urea solution or by deploying freeze-proof designs. These designs could be optimised by thermal simulations.

Modern engines are becoming increasingly efficient. This means that more energy is converted into mechanical power, and less heat is lost through the exhaust system. This makes all the previous points more crucial with respect to light-off times.

Advantages of total system testing

Real life situations regularly indicate behaviour that deviates from what was anticipated. System testing can take place in small climate chambers for validating thermal simulations and design verification.

Full vehicle testing is the best way for acquiring data about the overall environmental suitability and operational durability of vehicles.

Testing the entire vehicle, as opposed to just the subsystems, is particularly useful. Possible problems regarding any interference arising from various subsystems can be detected. It allows subsystems from different suppliers to be evaluated together. Possible assembly faults and difficulties can be identified long before actual production is started.

Full vehicle testing - even large off-highway vehicles - is possible in large climatic chambers under controlled conditions, which also allows the systems to be calibrated. This means that the whole vehicle can also be included in the equations. The influence of components in the immediate proximity can be measured. The tests can be repeated and controlled. Environmental effects can also be tested. No real-life testing conditions can generate such useful information and insights into the systems.

The OWI-Lab climatic test chamber has a volume of 593 m³. The loading dock has a load capacity of 45 metric tons/m². Extreme temperatures can be attained, ranging from +60°C to -60°C. The chamber can go from +60 °C to -40°C within one hour. The chamber has enough space for testing objects up to 10.6 x 7 x 8 m and it can accept equipment up to 300 metric tons.