After-treatment Modeling Techniques - Part 2
1.Explore -Examples provided below and write a detailed report
- Detailed SI engine+aftertreatment
- Detailed CI engine+aftertreatment
Let's start exploring and analyzing the example "SI ENGINE + AFTERTREATMENT".
This model consists of a 4-cylinder engine, naturally aspirated, port-injection, with EGR and a 3-way catalyst.
INTAKE SYSTEM
The intake system consists of different intake pipes, linked to the ambient at 1 bar and 298 Kelvin. To clean the air before entering into cylinders there is an "air-cleaner" between tow elements called "airbox in" and "airbox out". After that, the quantity of air that is allowed to pass is controlled by the throttle, and before entering into cylinders an injector injects fuel into the ducts (port fuel injection). The same amount of fuel is injected for every case, and it's 15 g/s.
CYLINDER AND ENGINE
Below that is the main tab of the cylinder part of the model, where is possible to see the main parameters used for its modeling:
The "heat transfer object" used is the classical WOSCHNI, and the "combustion object" is "SIWiebe", where into advance tab is possible to see the objects used for NOX (the Extended Zeldovich model) and CO reference.
Now, below, it's reported the cylinder geometry that defines total displacement fo the engine, bore, stroke, and compression ratio:
EXHAUST SYSTEM
The exhaust system consists of different pipes and an articulate line of after-treatment systems.
The first one that is possible to find along the exhaust line is the EGR. The quantity of gases that recirculate is controlled by the EGR controller (a valve), and it's opening it's controlled by an actuator that receives two signals: the mass flow air inside the intake system (directly from the throttle) and the mass flow EGR.
The second after-treatment system is the example model TWC_Ramanathan_and_Sharma which uses a published reaction mechanism for a passenger car TWC. A secondary circuit is used to solve the reactions using the quasi-steady (QS) flow solver, which is able to use a large master time step so that it does not restrict the computation time. The back pressure from the explicit engine circuit is passed to the QS circuit and actuated as the downstream pressure boundary, and the outlet gas temperature from the QS circuit is passed back to the explicit engine circuit and actuated as the TWC wall temperature so that the exothermic heat release is captured for acoustics. The various monitors show the TWC conversion efficiency, the exhaust lambda, the oxygen storage, the temperature upstream and downstream of the TWC, and the delta T across the TWC. Very important is the template: surface reactions, linked to the catalyst of the QS circuit, that contains all the chemical reactions (as discussed in the previous challenge: n8).
CASE SETUP
There are three cases run, at an engine speed of 3000 rpm, using different A/F ratio (rich, stoichiometric and lean condition), with throttle angle set to 45% (so it's a partial load situation).
RESULTS
Here are presented the results obtained running the all the cases of the model:
Passing from case 1, with A/F ratio of 14, to case 3, with A/f ratio of 15, is possible to see a decrease of brake power, brake torque, BSFC, and CO [ppm], but an increase of NOx, because of the major inlet air.
Below is plotted the conversion efficiency of the TWC:
CASE 1 - A/F RATIO 14
CASE 2 - A/F RATIO 14.5
CASE 3 - A/F RATIO 15
CONCLUSIONS
Looking at the diagrams, the conversion efficiency changes a lot in every case.
The richer case, so case 1, with A/F ratio of 14, generates a lot of CO and HC, and the conversion efficiency is quite low for the first two cases, and very high for the last one, so for the leaner case.
The opposite is what happens for NOx, passing from the richer to the leaner case the conversion efficiency of NOx decreases so much.
Let's exploring and analyzing the example "CI ENGINE + AFTERTREATMENT".
This model consists of a turbocharged engine, with 4-cylinders, direct injection, with a DOC, DPF, and SCR.
INTAKE SYSTEM
The intake system is very simple. The inlet air is compressed by a compressor, coupled on the same shaft of the turbine.
A wastegate controller for the turbine is used to control the mass flow exhaust gases that work into the turbine.
CYLINDER AND ENGINE
The engine is a direct-injection engine, and the quantity and the pressure of fuel injected in every cycle are defined by two different maps:
The main tab of the cylinder allows to see that the combustion object uses a NOx model: "advanced Zeldovich":
Then the cylinder geometry is defined, as reported below:
EXHAUST SYSTEM
The exhaust system, and so the after treatment line, is given by a combination of two different models: DOC_Sampara_and_Bissett and SCR_Fe_Zeolite example models along with a DPF setup for passive regeneration only, and same urea injection system used in the EngineState_Vehicle_Aftertreatment_NEDC example model.
As analyzed in the past model (SI+aftertreatment), even in this model there are two different after-treatment lines: the first one is solved with an explicit method and the second one with the QS method.
Along the line, there are three different after-treatment systems: DOC, DPF and SCR (the chemical reactions and the object part were already analyzed in the past challenge).
CASE SETUP
Into the case setup is possible to see that only one case is run, at 2000 rpm:
RESULTS
Here are presented the results:
And now the conversion efficiency of the after-treatment systems is shown:
DOC conversion efficiency is shown:
then the SOOT mass retained into DPF is shown:
finally, the SCR composition is shown:
CONCLUSIONS
Analyzing the results obtained from the simulations, is possible to visualize the work of after-treatment systems, and how they are able to reduce the pollutants.