The present work aims at analysing and comparing the thermal performances of active and passive aftertreatment systems. A one-dimensional transient model has been developed in order to evaluate the heat exchange between the solid and the exhaust gas and to estimate the energy effectiveness of the apparatus. Furthermore, the effect of the engine operating conditions on the performances of emission control systems has been investigated considering standard emission test cycles. The analysis has demonstrated that the active flow control presents the higher thermal inertia and it appears more suitable to maintain the converter initial temperature level for a longer time after variations in engine load. Conversely, the traditional passive flow control is preferable when rapid “cooling” or “heating” of the solid phase is requested. Moreover, the investigation has highlighted the significant influence of the cycle time and converter length on the energetic performances of the aftertreatment apparatus.
The development and the optimisation of high efficiency aftertreatment systems are fundamental keys to meet the more and more severe regulations concerning automotive exhaust emissions [
Conversely, in the last few years, an innovative active flow control has been proposed to assure the correct thermal level for energy efficient aftertreatment operations and to reduce the addition of supplemental fuel [
Active aftertreatment system: forward (a) and backward (b) operations.
This work aims to compare the energy efficiency of aftertreatment systems with active and passive strategies. Several studies on emission control systems exist in the literature, while a few investigations refering to active flow control. The analysis is focused on the thermal behaviour of the system to compare different flow control strategies and to quantify the aftertreatment system response after sudden variations in engine load. The European Stationary emission test Cycle (ESC) has been considered and the influence of the cycle time and the aftertreatment length on the energetic performances has been investigated.
A one-dimensional single channel transient model was used to simulate the thermal exchange between the exhaust gas and the aftertreatment system. A structured monolith was analysed. The proposed numerical code simulates the thermal exchanges inside a single channel of the aftertreatment system. The basic model assumptions are the following: working fluid as ideal mixture, one-dimensional unsteady flow, and adiabatic systems towards the surroundings. The catalytic reactions are not taken into account and the results can be extended to a generic aftertreatment system with structured packed bed.
In accordance with the literature [
The monolith temperature was calculated according to the following energy balance for the solid phase:
The equations were solved with a finite difference scheme. More detail on the model and on the relative validation is reported in the literature [
The heat transfer coefficient
The geometric characteristics and the operating conditions of the monolith aftertreatment system are shown in Table
Operative conditions for the working fluid and emission control systems.
Reference converter size, |
141 mm | 141 mm | 300 mm |
Cell density, |
62 cell/cm2 | ||
Channel size, |
0.90 mm | ||
Wall thickness, |
0.35 mm | ||
Wetted surface per unit volume, |
2201 m−1 | ||
Solid phase density, |
2807 kg/m3 | ||
Solid specific heat capacity, |
800 J/kg K | ||
Exhaust flow rate, |
100 g/s | ||
Exhaust gas temperature, |
200 (700)°C | ||
Initial solid temperature, |
700 (200)°C | ||
Cycle time, |
20 s | ||
| |||
Further exhaust flow rate, |
22 ÷ 252 g/s | ||
Further exhaust gas temperature, |
194 ÷ 616°C | ||
Further initial solid temperature, |
194 ÷ 434°C | ||
Further converter length, |
400 ÷ 600 mm | ||
Further cycle time, |
30 ÷ 60 s |
First, the aftertreatment “cooling” and “heating” processes were investigated. In particular, during the cooling phase, exhaust gas enters into the system at 200°C, while the initial solid temperature is 700°C. This corresponds to a sudden decrease in the engine load after a long full load operation [
Furthermore, to evaluate the influence of the engine operating conditions on the energetic performances of active and passive emission control systems, different engine conditions have been imposed. First of all, a progressive increase in the engine speed, followed by a progressive speed decrease has been considered.
Successively, the two control strategies have been compared over a sequence of steady-state modes according to the European Stationary Cycle (ESC) [
European Stationary Cycle (ESC).
The characterisation of the thermal performances of the emission control system was achieved through the analysis of the temperature profile and the evaluation of the mean solid temperature (
Finally, the influence of the cycle time and aftertreatment geometric characteristics has been investigated.
The transient numerical model was used to predict the thermal and the energetic performances of emission control systems with active and passive flow control, respectively.
Figure
Temperature profiles along the aftertreatment system as a function of time. Cooling phase. Passive (a) and active (b) flow control operations.
The investigation highlights the great influence of operating time on the temperature distribution within the monolith. The leading part of the emission control system is almost completely cooled after 30 seconds, while at the outlet, the temperature values are close to 700°C. Furthermore, the solid appears completely cooled after 100 seconds.
The evolution of temperature profiles with the active flow control for the previous operating conditions is shown in Figure
The comparison with the results obtained with conventional unidirectional flow highlights the greater thermal retention capacity of the active emission control system. Specifically, for the passive control system, the solid phase is almost completely cooled after 100 s. Conversely, the active control maintains significantly higher temperature values in the monolith central area. An increase of about 260°C is found. When high temperatures are required, the reverse flow control determines the maintenance of suitable temperatures for the proper operation of the system, even under lean mixture conditions and low load operation, without additional fuel. Therefore, the active technique permits a significant energy savings with respect to the passive control strategy.
Figure
Mean temperature profiles as a function of operating time.
The comparison between active and passive operation was repeated considering the solid heating process in order to analyse the effect of the reversal flow control in more detail (Figure
Temperature profiles along the aftertreatment system as a function of time. Heating phase. Passive (a) and active (b) flow control operations.
The temperature profiles with the active flow control during the heating process are shown in Figure
To evaluate the influence of the engine operating conditions on the energetic performances of active and passive emission control systems, a progressive increase in the engine speed, followed by a progressive speed decrease, has been considered. Each engine regime has been maintained for 120 s as visible on Table
First steady-state test cycle.
Mode | Engine speed (rpm) | Time (min) | Exhaust temperature (°C) | Mass flow rate (kg/s) |
---|---|---|---|---|
1 | 1000 | 2 | 434 | 0.022 |
2 | 1500 | 2 | 464 | 0.055 |
3 | 2000 | 2 | 495 | 0.083 |
4 | 2500 | 2 | 525 | 0.106 |
5 | 3000 | 2 | 556 | 0.127 |
6 | 3500 | 2 | 586 | 0.143 |
7 | 4000 | 2 | 616 | 0.157 |
8 | 3500 | 2 | 586 | 0.143 |
9 | 3000 | 2 | 556 | 0.127 |
10 | 2500 | 2 | 525 | 0.106 |
11 | 2000 | 2 | 495 | 0.083 |
12 | 1500 | 2 | 464 | 0.055 |
13 | 1000 | 2 | 434 | 0.022 |
Mean temperature profiles within the emission control system are plotted in Figure
Mean temperature profiles as a function of operating time. First steady-state test cycle.
Finally, the two control strategies have been compared over a sequence of 13 steady-state modes according to the European Stationary Cycle (ESC). Table
European Stationary Cycle (ESC) test.
Mode | Engine speed (rpm) | Time (min) | Exhaust temperature (°C) | Mass flow rate (kg/s) |
---|---|---|---|---|
1 | 700 | 4 | 194 | 0.054 |
2 | 1212 | 2 | 527 | 0.151 |
3 | 1525 | 2 | 415 | 0.147 |
4 | 1525 | 2 | 466 | 0.173 |
5 | 1212 | 2 | 423 | 0.110 |
6 | 1212 | 2 | 490 | 0.127 |
7 | 1212 | 2 | 340 | 0.098 |
8 | 1525 | 2 | 513 | 0.197 |
9 | 1525 | 2 | 358 | 0.125 |
10 | 1837 | 2 | 484 | 0.252 |
11 | 1837 | 2 | 338 | 0.154 |
12 | 1837 | 2 | 436 | 0.217 |
13 | 1837 | 2 | 396 | 0.181 |
Figure
Mean temperature profiles as a function of operating time. European Stationary Cycle (ESC).
When the load decreases (odd modes) the temperature of the passive system reduces rapidly, while the thermal inertia of the active system maintains higher temperatures.
In order to investigate in more detail the influence of the flow control strategy on the aftertreatment thermal behaviour, the effect of the cycle time has been analysed (Figure
Influence of the cycle time on the mean temperature profiles. Solid line: active control; dotted line: passive control. European Stationary Cycle (ESC).
Finally, the large effect of the device length on the mean thermal level is shown in Figure
Influence of the converter length on the mean temperature profiles. Solid line: active control; dotted line: passive control. European Stationary Cycle (ESC).
The plot refers to four monolith lengths (from 300 to 600 mm): the higher the monolith lengths, the higher the thermal inertias. The passive flow control presents slight differences in the mean temperature profiles. Conversely, for the active configuration, the results highlight that the differences reduce with the operating time. As an example, by increasing the aftertreatment length from 300 to 600 mm, the mean temperature difference reaches 122°C and 37°C at 360 and 1440 s, respectively.
The analysis demonstrates that, if high temperatures are required for the proper functioning of the aftertreatment system, the active control is useful at low load operating condition with the solid at high temperature. Conversely, the passive control system is recommended during the warm-up phase and/or to accelerate the cooling or the heating process.
Depending on the engine load and the requested converter thermal level, the coupled operation of active and passive flow represents the solution apt to guarantee the highest energy efficiencies.
A one-dimensional transient model has been developed in order to analyse and compare the energetic performances of passive and active emission control systems. Specifically, the numerical model enables the calculation of the heat exchange and the temperature profiles of solid and exhaust gas.
The effect of the engine load on the system behaviour has been analysed and the influence of the cycle time and monolith length on the energy efficiency of the system has been investigated. To this purpose, two sequences of steady-state modes have been imposed. The first operation is based on a progressive increase in the engine speed, followed by a progressive engine speed decrease. The European Stationary Cycle (ESC) for heavy-duty diesel engines has been also considered.
The comparison between active and passive flow control showed the greatest thermal inertia of reverse operation. When the load increases, the temperature of the active system upsurges slightly, while the low thermal inertia of the passive system determines higher temperatures. Conversely, a decrease in the engine load produces higher temperatures for the reversed flow configuration.
The active control appears more suitable to maintain the solid initial temperature level for a longer time after sudden variations in engine load, even under lean mixture conditions and low load operation. The traditional unidirectional operation is preferable when rapid cooling or heating of the aftertreatment system is demanded.
Furthermore, the numerical investigation demonstrated that the energy performances of the active control approach those of the passive control system as the cycle time increases.
Finally, the significant effect of the converter length on the thermal system behaviour has been analysed: the higher the monolith lengths, the higher the thermal inertias. In particular, for the active flow control the differences in temperature profiles as a function of the device length reduce with the operating time.
The authors would like to thank AVL List GmbH for the provision of the AVL Boost software.