The diesel engine needs to be even cleaner in future and reliably meet the stringent EU6 limit values in all situations. In a concept study, IAV has examined approaches that promise to do precisely this: various particularly closed-coupled exhaust gas aftertreatment layouts allow DOC and DPF/SCR to reach optimum working temperature more quickly which, even when driving in the low-load range, significantly cuts emissions.
Under the EU6 standard, diesel engines are allowed to emit up to 80 milligrams of nitrogen oxides per kilometer – not only under the legal framework in the cycle but also in respect of increasingly tighter RDE requirements: “Our aim in future is to meet the limit values in all driving situations that can occur in practice”, says Matthias Diezemann, technology scout in IAV’s Powertrain and Power Engineering division.
“This also applies to those usage scenarios that are still excluded from current RDE legislation.” On top of this, future powertrains with a diesel engine must be capable of meeting limit values that are even tighter than EU6. This means that in future the compression ignition engine can no longer afford to show any weaknesses when it is compared with a modern spark-ignition engine.
Faster to reach operating temperature
For this, a key contribution must come from the exhaust gas aftertreatment system – across the entire range from low to high load. The temperatures in the oxidation catalyst (DOC) as well as in the combined diesel particulate filter and SCR catalyst (SCRF) play a crucial part in this regard. The sooner they reach their optimum operating temperature and the better they maintain it, the quicker and more reliably emissions fall. In modern supercharged engines, however, exhaust gas aftertreatment (EAT) competes with the turbocharger for heat in the exhaust gas flow. Normally its turbine is positioned upstream of EAT so that exhaust gas temperature drops by 100 to 150 kelvins before it reaches DOC and SCRF.
This is where IAV’s developers got to work in their concept study and moved the EAT system closer to the engine. The most radical solution: turbocharger and EAT swap places. This way, exhaust gas flows through DOC and SCRF before it enters the turbine. These measures produce significant improvements: “On cold starting and while running at low load, the components of the EAT warm up far more quickly or are kept at a higher temperature”, Diezemann reports. “After a cold start, for example, it only takes a few seconds before the vehicle’s emissions are in the admissible range.” Changing the position of turbocharger and EAT alone reduces NOx emission at low load to about a fifth of the level from a conventional EU6 vehicle.
But the emission benefit also comes with a downside. To begin with, small particles from the DOC can damage the turbocharger’s turbine. Endurance tests need to show what kind of a problem this actually is. Then, after starting, the turbocharger is initially deprived of the energy that is first used to get the EAT system to its operating temperature. “Driving dynamics are inadequate when the engine is cold, which means corrective action is needed to improve it”, Diezemann explains. “In a hybridized powertrain, the e-motor can contribute to the torque needed. Otherwise, an electric compressor could also do this job.” Whatever the case, one of the two will become necessary if an OEM goes for the “EAT upstream of turbocharger” option.
Different topologies possible
However, this radical solution is not the only way of moving EAT closer to the engine. In their concept study, IAV’s experts examined several topologies and compared their effects. One of the alternatives they looked at involves leaving the EAT system in its original place and bypassing the turbocharger on cold starting until the DOC and SCRF are at operating temperature.
This gets the EAT system to operating temperature more quickly without the drawbacks of an upstream-of-turbocharger arrangement. Integrating an additional DOC into this bypass is also conceivable. “It makes HC/CO light-off faster and speeds up NO2 production after cold starting”, Diezemann says. “This has a positive effect on conversion in the SCRF at low exhaust gas temperatures.” A further option is to position the DOC upstream and the SCRF downstream of the turbocharger.
“We have a solution to offer for every OEM, whatever its vehicle portfolio”, says Diezemann, summarizing. “In future, this closecoupled setup makes it possible to operate the EAT system in diesel engines with an equally high level of efficiency as gasoline engines in all conceivable driving situations. The results of the concept study are based on measurements made on a 2.0-liter 4-cylinder diesel engine and simulations. The approach is suitable for all vehicle classes from the C-segment upwards, and could go into production relatively quickly.