A key contribution to this performance must come from the exhaust gas aftertreatment system—across the entire speed and load range. 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 charged 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 (100 to 150 degrees C) before it reaches the DOC and SCRF.

IAV’s developers moved the EAT system closer to the engine—specifically, the 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. After a cold start, for example, it only takes a few seconds before the vehicle’s emissions are in the admissible range.

—Matthias Diezemann, technology scout in IAV’s Powertrain and Power Engineering Division

Topology of the variants. Click to enlarge.

Changing the position of turbocharger and EAT alone reduces NO_x emission at low load to about a fifth of the level from a conventional EU6 vehicle.

However, 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. In a hybridized powertrain, the e-motor can contribute to the torque needed. Otherwise, an electric compressor could also do this job.

—Matthias Diezemann

Whatever the case, one of the two will become necessary if an OEM goes for the “EAT upstream of turbocharger” option.

Such a solution is not the only way of moving EAT closer to the engine. In a concept study, IAV’s experts examined several topologies and compared the effects they bring about. 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. This has a positive effect on conversion in the SCRF at low exhaust gas temperatures.

—Matthias Diezemann

A further option is to position the DOC upstream and the SCRF downstream of the turbocharger.

In future, this close-coupled 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 derive from 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.

—Matthias Diezemann