Heavy Duty Exhaust Systems for Low NOx Application: Thermal Management and Control

The currently discussed NOx limits reductions for commercial vehicles in Europe, as well as the recently published CARB regulation in the USA, will require a new approach for exhaust gas aftertreatment in order to keep greenhouse gas and CO2 emissions within the limits.In order to reach the required high conversion rates for nitric oxides in the SCR system it is essential to operate the exhaust aftertreatment in the optimal temperature window. Load points with low exhaust gas temperatures and especially cold starts will require (active) heating.Increasing the exhaust temperature by “engine measures” cannot reach the high efficiency rates of energy-release compared to direct combustion of fuel in the exhaust system. A typical Particulate Filter regeneration in Heavy Duty applications is supported by fuel, which is injected into the exhaust system and combusted over the oxidation catalyst (DOC). For this the DOC must be within an appropriate temperature window which is not ensured in all engine load points. Utilizing electrical energy, the temperature of the Oxidation Catalyst (= electrically heated catalyst) can be increased, even in cold start, to the required level. This insures a safe combustion of the injected fuel with highly efficient heat release. With this setup it is possible to heat up (and keep warm) the exhaust system independently of engine operation. Thus, NOx-conversion can start earlier and operate with higher efficiencies. With this configuration it is realistic to operate the engine a longer period of time, in more fuel-efficient load points, with no penalty for exhaust heating.During low load engine operation, the temperature of the exhaust system can get very low, so that NOx-conversion-efficiency is reduced. The exhaust system needs to be kept at operation temperature. Pure electrical heating might be sufficient for many low load points.The measurement of tailpipe emissions using on-board exhaust sensors is widely discussed as one further solution to ensure the emission compliance over the vehicle lifetime and in real driving conditions. The information about the tailpipe emissions allows to adjust the control of the exhaust system to maintain low pollutant emissions. Additionally, the continuous measurement of tailpipe emissions helps to detect damaged or tampered systems quickly not only in the certification cycle but also in real driving conditions, thus extending the diagnostic coverage to a broad range of operating conditions. A typical example for exhaust sensors is the NOx sensor, which is already installed in all modern Diesel engines to control the SCR and measures the tailpipe NOx emissions continuously.In the first section of this paper the different exhaust configurations and the heating strategies are described. The impact on the CO2 emissions is analyzed in different driving cycles (FTP, low load cycle). In the second section, the measurements performance of NOx sensors in the exhaust system is investigated. The investigation is based on the study about the different heating strategies. Therefore, the performance can be examined at various emission levels and operating conditions. The second section is concluded by a Monte Carlo Analysis, which addresses the influence of component tolerance on the system accuracy.