After-treatment Modeling Techniques - Part 1

1. List down for Gasoline vs Diesel

The pollutants

From the 100% in volume of exhaust gases of an internal combustion engine, the 98% consists of oxygen, carbon dioxide, and nitrogen, and the other 2% consists of all pollutants controlled by the legislation: NOx, CO, HC, SOx, PM.

The gasoline and diesel engines work in a very different way, with different lambda, different rpm, different compression ration, and more... and for this reason, they produce different pollutants.

1) Gasoline engine

As known, the gasoline engine is a spark-ignition engine, where the mixture of air and fuel can ignite thanks to a spark, few crank angle before TDC. To permit the combustion, the mixture must have an A/F ratio strictly around the unity, but in a common SI engine, this is not always possible.

The first pollutant considerable is the family of NOx. These pollutants can be classified into three different families: "prompt NOx" that indicates this pollutant is born by the oxygen and nitrogen present into intake air, that enters inside cylinders; "thermal NOx" that indicates this pollutant is born because of the high temperature during the combustion; "fuel NOx" born from the nitrogen presents into the fuel. So every time there is an excess of air into the cylinder, the NOx pollutants are the first to be generated.

During the combustion, there is a part of the fuel that doesn't take part in the process, and for this reason, there are particles of fuel into exhaust gases. These pollutants are called HC (hydrocarbons). There are different ways to obtain these pollutants. As an example, when the fuel is injected inside the cylinder, it is possible that a part of it is trapped into particular zones called "crevices". During the combustion, the flame front doesn't arrive into crevices, causing the not ignition of the fuel trapped in these zones. After that, during the exhaust phase, these pollutants are aspirated from these zones and pushed out from the cylinder.

The SI engine is characterized to work even at higher rpm, compared to the CI engine. It's important to consider that combustion time is generally always the same, in other words, the angular duration is always the same. Increasing the rpm of the engine means reducing the time that the combustion has to complete its process. For this reason, there is a percent of the fuel that doesn't finish its oxidation process, causing the pollutants called CO (monoxide carbon). There is another important reason why these pollutants born. During the combustion, there is a fast transformation of CO into CO2, so at a certain point theoretically, all the CO should be converted into CO2. But this not happens because during the expansion phase, while the piston is moving from the TDC to BTD, the temperature inside the cylinder decreases rapidly, reaching the "freezing temperature" where these transformations are totally blocked, causing the CO inside exhaust gases.

The particular matter wasn't a problem that affected the SI engine while using port fuel injection. But with the modern engines, using the direct injection, and considering a richer charge only around the sparkplug so a leaner charge inside the cylinder, the PM has become even a controlled pollutant of SI engine.

At the last, one of the most substance controlled, even if it is not directly dangerous for the humans but it's terrible for the ambient, is the carbon dioxide (CO2) that's generated by the normal process of combustion. To eliminate its formation is necessary to increase the efficiency of the engines, and so to obtain the same power with always less injected fuel.

2) Diesel engine

Totally different from the SI engine is the CI engine. The main difference is that there isn't a sparkplug in this case and the mixture is self-ignited thanks to the conditions of high temperature and pressure that are generated inside the cylinder at the end of the compression phase.

These engines are characterized to work with a big excess of air. In fact, the A/F ratio is around 19-25 and this generates a big quantity of NOx.

Generally, thank's to the excess of air the HC isn't a big problem for these engines because all the quantity of fuel takes part in the combustion process.

The biggest problem of these engines is PM, which is generated in the heart of the fuel-injected, where the amount of oxygen is very low.

2. Aftertreatment devices with catalyst materials

The main after-treatment devices with catalyst materials are different for both engines:

for SI engine is generally used a "three-way catalyst";

for CI engine is generally used a DOC (diesel oxidation catalyst) and an SCR (selective catalytic reduction).

Let's see what are the major differences and how they work.

• 3-way CAT

A 3-way CAT is generally used today for SI engines. Using this after-treatment system is possible to control the three major pollutants: HC, CO, and NOx. On the wash-coat surface, there are deposited three different noble materials: palladium, rhodium, and platinum. The catalyst is located at a specific point of the exhaust line, pointing particular attention to the temperature of the exhaust gases. In fact, the best temperature of the catalyst to work is around 500°C, and for this reason, its temperature should always stay strictly around this value. Particularly important are the first moments of the start of the engine, in which the catalyst doesn't work because of hasn't reached yet its "light-off" temperature.

When the exhaust gases enter inside the catalyst, a series of chemical reactions take part rapidly between the gases and noble materials: rhodium reacts with NOx, causing a reduction transformation, obtaining two different elements: nitrogen (N2) and oxygen (O). After that, the oxygen, in an ionic form, reacts with platinum and palladium, causing an oxidation reaction, transforming HC and CO in dioxide-carbon (CO2) and water (H2O).

One of the most important elements that are always used with a 3-wat CAT is the lambda sensor, which controls the air/fuel ratio of the gases. That is very important because the catalyst must work always with a stoichiometric mixture to permit the cited reactions.

• DOC and SCR

A CI engine, as known, generally works with a big excess of air. So the amount of NOx is bigger than a SI engine and for this reason, there are two different after-treatment devices with catalyst materials.

A DOC (diesel oxidation catalyst) promotes chemical oxidation of CO and HC as well as the organic fraction (OF) of diesel particulates. They also oxidize sulfur dioxide which is present in diesel exhaust from the combustion of sulfur-containing fuels. In modern diesel after-treatment systems, an important function of the DOC is to increase the NO₂ content of the exhaust gas, to support the performance of SCR catalysts.

After that is possible to find an SCR (selective catalytic reduction) that has the target to reduce the NOx family. It works with another substance added manually, called AD-BLUE, composed by Urea and distilled water. So, thanks to the high working temperature of the exhaust gases, the Urea (that becomes NH3 reacting with exhaust gases) and noble materials deposited on the surface of the catalyst (titanium oxides, vanadium, and tungsten) is possible to transform the NOx family into nitrogen (N2) and water (H2O).

3. Example files

Below are shown two different example files about after-treatment systems, taken by GTP examples:

CI ENGINE

SI ENGINE

4. Strategies used for reducing tailpipe emissions

Different and different strategies are used to reduce tailpipe emissions. They can be divided into:

- upstream strategies;

-in-cylinder strategies;

-downstream strategies.

Let's start with the first one.

• Upstream strategies

The upstream strategies can be summarized in:

-desulphurization and elimination of the lead into the fuel;

-homogenous charge, trying to obtain the same air/fuel ratio in every single volume of the charge, to permit regular and total combustion;

-cut-off of the fuel during the release of the accelerator pedal

and so many more...

• In-cylinder strategies

These strategies are applied at the charge inside the cylinder. They can be summarized in:

- giving the right turbulence at the inlet charge. Generally, for a CI engine, the swirl turbulence is preferred to control the delay ignition, and for a SI engine the tumble turbulence, to control the speed of combustion. In both cases is fundamental to create a homogeneous charge;

-reducing "crevices", so the places inside the cylinder where the fuel can be trapped and doesn't take part in the combustion, causing the formation of HC;

-EGR (exhaust gas recirculation), which permits to recirculate a part of the exhaust gases inside the cylinder, to decrease the maximum temperature of the combustion, reducing the formation of NOx;

- Direct injection, that permits to reduce the formation of CO2 because it allows working with a lean charge inside the cylinder. In fact, the charge is richer around the sparkplug, and leaner considering the entire cylinder.

• Downstream strategies

These are not exactly strategies, but different elements used to reduce the pollutants before exhaust gases leave the outlet system. They can be summarized in:

SI engine

- three-wat catalyst;

-GPF (gasoline particulate filter);

CI engine

- DOC (diesel oxidation catalyst);

- SCR (selective catalyst reduction);

- DPF (diesel particulate filter);

5. List down one set of chemical reactions for TWC and SCR from example files

Below is shown a list of chemical reactions for each after-treatment system defined in the challenge.

THREE-WAY CATALYST

A three-way catalyst with oxygen storage and the N2O production model is shown:

Below is possible to visualize the reactions, describe into the part: TWC_with_O2_storage:

SCR

An SCR model is shown below:

Here the reactions:

6.Explore -Example-aftertreatment-DOC-sampara & Bisset and write a detailed report

Let's analyze the following example. It's a DOC model based on a paper written by Sampara and Bisset.

It's composed of different elements, analyzed below:

- INLET: it's a particular element, totally different from the "EnvIn" used in the pas models. This GTP part permits to describe accurately the composition of the fluid:

- INLET CONE and OUTLET CONE: are simply GTP parts that describe a flow volume;

-CATALYST: that's the part modeled to be a catalyst.

In the MAIN tab is possible to describe the geometry of the catalyst, choosing: frontal area, cell density, channel shape, substrate wall thickness, and washcoat late thickness.

In the THERMAL tab is possible to define the thermal properties of the catalyst, that are fundamental for the chemical reactions:

-CONVERSION_EFF_MON: it's a very important part of the model that describes accurately the efficiency of the conversion of the chemical species.

-DOC_SAMP_AND_BISS: this part allows to describes the reactions that happen inside the catalyst, based on the paper mentioned above:

Particular important are the cases run: the first one describes the condition where flow meters inside the catalyst with a simple linear temperature ramp combined with constant inlet flow rate and composition. But the second one case uses transient inlet data from the first 200 seconds of an FTP75 drive cycle.