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Different driving scenarios produce different challenges in respect of reducing journey-specific pollutant emission, as shown in Figure 1 using the example of the temperature level in the exhaust gas aftertreatment system. The following requirements can be derived: Open image in new window Figure 1 Vehicle speed profiles and exhaust gas temperature curves in the SCRF for journey profiles under review (© IAV) NOx conversion at low temperatures in low-load cycles NOx engine-out emission control at high loads [2] large spread of space velocity in the SCR catalyst [3, 4] constantly high NOx conversion rate at rising temperatures in the SCR on filter (SCRF) [3, 4]. While controlling emissions from high-load cycles is more of a dimensioning matter, controlling them in the low- temperature range is always more complex [3, 4]. At extremely low average driving speeds (down to stop-and-go in congestion situations), exhaust gas aftertreatment temperature can fall below light-off. If this is immediately followed by rapid acceleration, the high levels of engine-out emissions are not treated adequately. To avoid such situations, the exhaust gas aftertreatment system must be kept at a constant temperature. This requires the application of additional exhaust gas temperature management measures like calibration-based measures, strategies for variable valve timing (VVT), hybridisation and exhaust gas aftertreatment (EAT) positioning: With regard to the CO2 advantages, which must be maintained compared to the gasoline engine, typical calibration-based measures in relevant heating modes (e.g. retarding the start of injection, throttling, post-injection) are detrimental to fuel consumption and therefore not ideal [3, 4]. Regarding VVT strategies discussion focuses in particular on secondary exhaust valve lift and cylinder shutoff as these strategies can be implemented more or less without increases in consumption. Moderate temperature advantages can be achieved and HC and CO emissions reduced during low-load operation [5]. Hybridisation can have an adverse effect on exhaust gas temperature as the average power output in the driving cycle is reduced by the higher efficiency achieved in the powertrain. Measures, such as avoiding trailing throttle and low torque levels as well as active load control, are capable of providing significant temperature and emission-related advantages. Depending on the hybrid powertrain's topology, the engine operating point could be decoupled completely from the driving situation [1]. Avoiding temperature losses as well as minimising the light-off time is a primary criterion in designing the powertrain. Particularly in hybridised concepts, this produces degrees of freedom in positioning the turbocharging (TC) and EAT components. Positioning EAT upstream of the TC provides the optimum in relation to temperature losses and light-off behaviour, but also with regard to the efficiency of any heating and operating strategy measures. |
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