Euro 6c emissions limits will present a number of challenges for light duty engine emissions, both gasoline and diesel when the regulations come into force in September 2017. One significant hurdle is speeding light-off of diesel selective catalytic reduction (SCR) catalysts.
In current systems it can take up to 10 or 12 minutes for a diesel SCR cat to reach its light-off temperature. This is determined to an extent by the packaging of the exhaust aftertreatment system.
In a current Euro 6b system the diesel particulate filter (DPF) may be the first aftertreatment device downstream of the engine, with a separation of 50 to 60 cm (19.6 to 23.6 in) between the rear of the DPF and the front of the SCR cat. Under Euro 6c, the light-off time would have to be shortened to between three to four minutes. The same will apply to US Tier 3 regulations, which will require a light off time of two minutes or less. Reducing the distance between the SCR and DPF to around 10 cm (4 in) could help.
The problem then is that neither the DPF nor the SCR cat is close enough to the engine to ensure that both have reached the required working temperature in the required time interval. Tenneco’s solution, on display at the 2015 IAA Frankfurt Show, is to combine the two devices in one. In this case the DPF is treated with an SCR coating.
Double-swirl does it
“That is what we call an SDPF, an SCR coated DPF”, explains Frank Terres, Executive Director, Core Science, Advanced Engineering and Hot End Development, Tenneco Clean Air.
“If you move the SCR portion into the DPF, you also need to move the dosing point to the gap between the diesel oxidation cat (DOC) and the SDPF," noted Terres. "Normally there is 1 cm between those two substrates but now you need to increase this gap because you need to dose in this small gap. Every centimeter is a pain because you have package constraints. So you give your few centimeters, maybe less than 10cm and the whole processing of the urea additive that you inject needs to be done in this small cavity."
Processing means injection, atomization, evaporation, hydrolysis and transforming into ammonia and then distributing everything homogeneously on the SDPF–a challenge, he admits.
Tenneco has developed a solution to the problem of mixing the urea solution with the exhaust gases in such a small space. The company’s new double swirl system is able to promote the mixing even in a very compact mixing zone such as in the SDPF.
“We generate a swirl and inject the urea, then have a controlled contact of the fluid droplets with the wall, where they can evaporate,” he explained.
In addition to the shortened light-off time from a cold start, the new European Real Driving Emissions (RDE) test also presents challenges. “You need to have a system that performs under all realistic driving conditions,” Terres noted. The urea injector is positioned in a much hotter area of the catalyst system than on previous designs.
Tenneco also uses the urea supply to cool the injector by circulating the flow from the tank to the injector and back. This ensures that the urea temperature in the injector is around 40ºC (104ºF) despite the high surrounding temperature of the exhaust system.
There may be insufficient space for an in-line DOC/SDPF design. In this case some form of parallel design would be necessary. This might involve connecting the diesel oxidation catalyst directly to the turbocharger, with the SDPF in parallel to it. A step-shaped mixer offers the same functionality as the mixer in the in-line system.
New oval GPF fits tight spaces
Tenneco also launched its oval-shaped gasoline particulate filter (GPF) at Frankfurt, anticipating the EU Euro 6c regulations, which will introduce a particulate number standard for direct injection gasoline engines from 1 September 2017. Euro 6c will also introduce a new test cycle for all EU engine testing. The oval shape has been designed so that it can fit into spaces where a cylindrical shape might not fit.
“The challenge for us is that the substrates used in a GPF have a higher porosity than a diesel filter and as a result they are more fragile," said Terres. "That makes the whole canning process very challenging."
Tim Jackson, Tenneco's Executive Vice President Technology, Strategy and Business Development, explained the thinking behind the development of the GPF.
"Let’s optimize the engine for fuel economy and CO2 emissions and let’s use the affordable aftertreatment to clean up the remaining emissions," he said. "Let’s not sacrifice CO2 and fuel economy to meet criteria [emissions]. Now that all the continents are faced with CO2 reduction goals, I think that message resonates a little stronger.”
The GPF is a self-regenerating device, unlike diesel particulate filters, which need to reach a particular operating temperature for regeneration to take place. The GPF is a continuously regenerating device. Tenneco has devised two ways of packaging it. The unit can either be included in addition to the existing three-way catalyst or the three-way catalyst coating can be applied to the GPF substrate to form what Tenneco terms a four-way catalyst.
“Because of the better flow characteristics in this filter, the efficiency of the gaseous pollutants conversion is much better, so you could make the total volume smaller and reduce the amount of precious metals used," noted Frank Terres.