Introduction to Engine After-treatment using GT-Power

INTRODUCTION TO ENGINE AFTER-TREATMENT:

To maintain compliance with different emission norms, after-treatment techniques are used. In BS6, the allowed levels of emissions are 0.46 g/kWh of NOx, 0.01 g/kWh of PM for heavy vehicles. The main emissions for SI and CI engines are shown below along wiht the different after treatment system.

The threeway catcon (TWC) is one combined system which maintains check for all emissions for gasoline engines. But this is not the case for Diesel engines. There is a need of 3 separate systems which keeps in check the different emissions. TWC cannot be used in Diesel engines as, engine is operated in lean condition compared to Gasoline with is operated near the shoichiometric condition.

Strategies involved in reducing Tailpipe Emissions:

There are two main strategies to reduce tailpipe emissions. These methods are common to both gasoline and diesel engines.

1. In-Cylinder strategy:
   
   1. Bowl/ Injection strategy to optimize the combustion patterns to reduce emissions
   2. Exhaust Gas Recirculation to reduce NOx
   3. Downsizing/ Downspeeding to reduce the amount of emissions
   4. Component design optimization
2. After-treatment technology:
   1. SCR
   2. DOC
   3. DPF
   4. TWC

Examples of Aftertreatment devices in GT-Power:

1) Diesel Oxidation Catalyst (DOC)

2) Diesel Particulate Filter (DPF)

3) Gasoline Particulate Filter (GPF)

4) Lean NOx Trap (LNT)

5) Selective Catalyst Reduction (SCR)

6) Three-way Catalytic Convertor (TWC)

7) Urea Decomposition:

Important Chemical reactions for TWC and SCR from example files in GT:

Example from TWC: Ramanathan and Sharma

CO+0.5O2 ⇒">⇒⇒ CO2
C3H6+4.5O2 ⇒">⇒⇒ 3CO2+3H2O
C3H8+5O2 ⇒">⇒⇒ 3CO2+4H2O
H2+0.5O2 ⇒">⇒⇒ H2O
CO+NO ⇒">⇒⇒ CO2+0.5N2
C3H6+9NO ⇒">⇒⇒ 3CO2+3H2O+4.5N2
H2+NO ⇒">⇒⇒ H2O+0.5N2
CO+H2O ⇒">⇒⇒ CO2+H2
C3H6+3H2O ⇒">⇒⇒ 3CO+6H2
2Ce2O3+O2 ⇒">⇒⇒ 4CeO2
Ce2O3+NO ⇒">⇒⇒ 2CeO2+0.5N2
CO+2CeO2 ⇒">⇒⇒ Ce2O3+CO2
C3H6+12CeO2 ⇒">⇒⇒ 6Ce2O3+3CO+3H2O
C3H8+14CeO2 ⇒">⇒⇒ 7Ce2O3+3CO+4H2O
H2+2CeO2 ⇒">⇒⇒ Ce2O3+H2O

Example from SCR: Fe & Zeolite

Z + NH3 ⇒">⇒⇒ ZNH3
ZNH3 ⇒">⇒⇒ Z + NH3
4ZNH3 + 3O2 ⇒">⇒⇒ 2N2 + 6H2O + 4Z
4ZNH3 + 4NO + O2 ⇒">⇒⇒ 4N2 + 6H2O + 4Z
NO + 0.5O2 ⇒">⇒⇒ NO2
4ZNH3 + 2NO + 2NO2 ⇒">⇒⇒ 4N2 + 6H2O + 4Z
8ZNH3 + 6NO2 ⇒">⇒⇒ 7N2 + 12H2O + 8Z

Tutorial in GT-Power: Modelling of a DOC Aftertreatment system: based on Sampara and Bisset:

In modern Diesel After-treatment Systems, DOC (Diesel Oxidation Catalyst) is an important aftertreatment device used to convert carbon monoxide into carbon dioxide and water. In this tutorial, the DOC system based on surface reactions as reported in "Hydrocarbon storage modeling for diesel oxidation catalysts", Chaitanya S. Sampara, Edward J. Bissett, and Dennis Assanis, Chemical Engineering Science, 63 pp. 5179-5192 (2008).

Any aftertreatment model will consist of a general flow layout, a surface reaction template and a parameter monitor (Conversion efficiency, Error, etc). The complete circuit of a DOC system is shown below. In the flow circuit, the geometry of the system is defined. The surface reactions will contain the surface reactions relevant to the system and its mechanism. And the monitoring device will record the concentration of each species before and after the treatment process.

Important Components/ Templates in Aftertreatment Circuit:

1) Catalyst Brick

The catalyst brick is the component where surface reactions occur. Usually at low temperatures, adsoption process takes place and at higher temperature, desorption takes place. Since, these are surface phenomena, we need to define the surface geometry. The image of the catalyst brick is shown below. The frontal area consists of substrate walls on which the catalysts are embedded. These substrate walls may contain additional washcoat layers to model diffusion as well. In this project, washcoat layers are neglected.

The main and thermal specifications set are shown below. Parameters such as frontal area, length, channel type, etc are set. The thermal object containing the thermal conductivity of the material is also set.

2) Surface Reactions:

In surface reactions template, the species involved, reactions, solver type and other related parameters are defined. initially, the species involved are set followed by the turnover number which defined as the number of moles reacted per moles of active sites per second. Theoretically, if a mechanism is well calibrated to one catalyst using turnover rates, then the reaction parameters could be applied with reasonable confidence to a different sized catalyst with the same washcoat, and the user would only need to accurately enter the active site density of the new catalyst. The species involved are shown below.

Next, the solver has to be defined. The Advanced Adaptive solver is the recommended solver option for most storage catalysts using surface reactions. This solver is the most robust while at the same time superior and faster at handling stiff kinetics.

Next, the Site Element folder is navigated to and elements are in the washcoat and/or substrate material, and the number of active sites that can form coverages needs to be defined. For this model, this catalyst contains platinum group metals (PGM) and Zeolite, and the amount of loading is 0.3 and 21.4 moles of active sites per cubic meter of reactor volume.

Next, coverages folder needs to be defined. Coverages contains information for different coverages like initial coverage and atomic composition of each coverage. For this model, sites may either be vacant or occupied by a HC species called DF1. For the vacant site, the coverage name is represented by “Z”, which contains the Site Element zeolite. The Initial Coverage is 1, meaning all active sites are initially vacant at the start of simulation. The Number of Site Element Atoms is 1, meaning a vacant site is represented by a single zeolite atom, and the remaining atoms (carbon, hydrogen, oxygen, nitrogen, and sulfur) are 0. Since the Initial Coverage of vacant sites is 1, the Initial Coverage of occupied sites is 0.

Next, the surface reactions involved must be entered. Usually, this data is available from the engine supplier. In this project, this data is ontained from the reference paper followed in the tutorial. Reaction rates in the SurfaceReactions">SurfaceReactionsSurfaceReactions template are defined based on the Arrhenius equation shown below. The list of reactions involved is put up as well.

3) Monitoring Parameter:

In this model, a conversion efficiency monitor is plaed. The circuit of the same is shown below. The main function of this module is to calculate the conversion efficiency of every species based on the formula max(0,100*((inlet_conc-outlet_conc)/(max(1e-10,inlet_conc)))). The convertor takes in inputs of concentration from the inlet and outlet of the catalyst brick and other parameters such as Inlet gas temperature to yeild a final conversion percenatge based on the above formula.

CASE-SETUP:

Two cases are run based on temperature input to the model. The first case has a constant inlet mass flow rate, linear temperature variation and steady Inlet concentration. The second case runs on the first 200 seconds of FTP-75 Cycle. The FTP-75 (Federal Test Procedure) has been used for emission certification and fuel economy testing of light-duty vehicles in the United States.

The entire FTP-75 cycle consists of the following segments:

1. Cold start transient phase (ambient temperature 20-30°C), 0-505 s,
2. Stabilized phase, 506-1372 s,
3. Hot soak (min 540 s, max 660 s),
4. Hot start transient phase, 0-505 s.

Out of which, the data for the first 200 seconds has been pre-loaded in second case of the tutorial. The case setup is shown below:

RESULTS:

1) Conversion Efficiency Plot for Linear Variation of Temperature:

2) Conversion Efficiency Plot for first 200 seconds of FTP-75 cycle:

INFERENCE:

1. In case 1, the combined effect of the linear temperature ramp along withe the constant inlet mass flow rate of gases, leads to a cold start and hence the low conversion efficiency of the DOC system. Once the light-off temperature is reached, there is a surge in the conversion efficiency. Light-off temperature is the minimum temperature at which the catalytic reactions are initiated.
2. In case 2, due to fluctuation in temperature as well as the mass flow rate in the FTP-75 cycle, the transience effects of the conversion efficiency is captured. The overall principle and trend remains the same; Conversion efficiency surges past the light-off temperature.

CONCLUSION:

After-treatment techniques were explored at the basic level for both SI and CI engines and different after-treatment devices were cited in GT-Power. Finally a DOC device was modelled and run with in two cases: linear increase in temperature and first 200 seconds of FTP-75 cycle. It was observed that, the conversion efficiency increased drastically past the light-off temperature.