Emission characterization on a CAT3410 engine

Objective: To run simulations for two types of pistons and characterize the emissions (Soot, Nox, and UHC)
                 1. Omega piston / Re-entrant piston
                 2. W-type piston / Open W piston

To do:
1. Create cut-plan animations showing Soot, Nox, and UHC.
2. Compare the IMEP and power values graphically.

Given: Open W-piston with 360 geometry.

The sector of complete 360 geometry with different bowl profile is as:

Theory:

                                 Diesel Engine (Compression Ignition Engine)

In compression-ignition engines, air alone is inducted into the cylinder. The fuel (in most applications a light fuel oil, though heated residual fuel is used in marine and power-generation applications) is injected directly into the engine cylinder just before the combustion process is required to start. Load control is achieved by varying the amount of fuel injected each cycle; the airflow at a given engine speed is essentially unchanged. There are a great variety of CI engine designs in use in a wide range of applications-automobile, truck, locomotive, marine, power generation.

`***`Naturally aspirated engines where atmospheric air is inducted, turbocharged engines where the inlet air is compressed by an exhaust-driven turbine-compressor combination, and supercharged engines where the air is compressed by a mechanically driven pump or blower are common.
`***` Turbocharging and supercharging increase engine output by increasing the air mass flow per unit displaced volume, thereby allowing an increase in fuel flow. These methods are used, usually in larger engines, to reduce engine size and weight for given power output.

`***` The compression ratio of the diesel engine is much higher than typical SI engine values and is in the range 12 to 24, depending on the type of diesel engine and whether the engine is naturally aspirated or turbocharged.

Working of CI engine:

Diesel Engine Emissions:

Levels of emissions of oxides of nitrogen (nitric oxide, NO, and nitrogen dioxide, NO2, usually grouped together as NOx,), carbon monoxide (CO), unburned hydrocarbons (HC), and particulates are important engine operating characteristics.

`***`The NOx emissions increase with load because of the increase in combustion temperature and this increase in combustion temperature is why the hydrocarbon emissions fall. Similarly, retarding the injection timing in all cases leads to lower combustion temperatures. This lowers the NOx emissions but increases hydrocarbon emissions.

`***` Internal combustion engines produce soot as a result of incomplete fuel combustion. The fuel and air mixture in diesel engines typically do not mix as thoroughly as they do in gasoline engines. This creates fuel-dense pockets that produce soot when ignited. While the majority of soot easily escapes through the exhaust, some get past the piston rings and end up in the oil.

Recently developed Emission norms BSVI to regulate the output of air pollutants from IC engines and SI engines equipment, including motor vehicles is as:

Case Setup:

Make Engine Sector surface: This tool helps us to save computational time by simulating an axisymmetric sector instead of a complete engine. Make engine sector surface tool to simplify the sector creation process. Engine sectors created with this utility will be free of defects and symmetric with respect to the XZ plane.

Path:

`\"Tool \"->\" Make engine sector surface\"`

Sector Angle:

It is calculated as Assume we have 8 nozzles inside the injector

`\" Sector Angle \"= 360^0/\"Nos. of Nozzles\"`

`i.e. 45^0`

Before processing for case setup we make sure that the geometry that we have is free from any kind of surface errors.

Parcel Simulation:

In this case, we are going to use C7H16 (n-Heptane) as our fuel spray. 

Boundary flagging and Region initialization: 

Region Initialisation:

Spray Modeling:

Parcel Distribution - Cluster parcel near cone center
Spray wall interaction - Rebound/Slide

Injector Configuration:
Rate shape - We use the Injected Species/Rate Shape tab to designate the parcel species to be injected and the proportional velocity (rate-shape) of the injection over the duration of the injection.

Time/Temp/Mass/Size:

Nozzle Configuration:

Combustion Modeling:

SAGE model we are going to use.

`***` We are going to looking for NOx and Hiroy Soot after combustion. 
Fuel species name: C7H16 (n-Heptane).

Turbulence Modeling: RANS based RNG k-epsilon.

Grid Control: 

Base grid:

AMR: Velocity and Temperature based AMR is applied to REGION-0

Velocity:

Temperature:

Fixed Embedding:

Post variable selection:

List of mass fraction we are going to look after:

Results:

`***\" Emissions Characterisation:\"`

Owing to stringent pollution norms, most of the researchers have been exploring for better engine designs with minimum emissions. The two most important concerns in the diesel-fuelled CI engine are NOx and soot emissions.
The improvement of air and fuel mixture will improve combustion engine performance. There are many ways to improve the air-fuel mixture inside a cylinder, and changing piston bowl geometry is one of them.
The combustion of air and fuel mixture and emission formation in diesel engines show a very close relationship with piston bowl geometry.

Temperature inside the combustion chamber:

Temperature variation animation inside combustion chamber using both Open W type and Omega type bowl profile:

Omega type bowl profile:

Open W type bowl profile:

From the graph itself, we can say that the temperature inside the combustion chamber is high for OMEGA type piston than that of OPEN_W_type piston.
In Diesel Engine, as we know the NOx formation is highly active at a higher temperature which is being analyzed in below NOx formation plot.

NOx formation animation:

The values  will be as:

Here, we can see a significant rise in temperature after the combustion of the fuel-air mixture inside the combustion chamber gives rise to NOx formation. On the other hand, this rise in temperature is why Unburned Hydrocarbon (HC) falls as shown below.

The Direct Injection diesel engine has lower NOx emissions than the In-Direct Injection diesel engine because the lower compression ratio gives lower in-cylinder temperatures.

Soot formation:

Internal combustion engines produce soot as a result of incomplete fuel combustion. Ideally, complete combustion in a cylinder would only produce carbon dioxide and water, but no engine is completely efficient.
Because of the way that fuel is injected and ignited, soot formation occurs more commonly in diesel than in gasoline engines. Unlike gasoline engines where the fuel/air mixture is ignited with a spark, fuel-air entering the diesel cylinder ignites spontaneously from the high pressure in the combustion chamber. The fuel and air mixture in diesel engines typically do not mix as thoroughly as they do in gasoline engines. This creates fuel-dense pockets that produce soot when ignited. While the majority of soot easily escapes through the exhaust, some get past the piston rings and end up in the oil.

Soot formation comparison:

Here, homogeneity of air-fuel mixture plays an important role as complete burning of fuel is expected during the combustion process that\'s where equivalence ratio comes into play.

In reality, internal combustion engines do not work exactly with ideal AFR, but with values close to it. Therefore we’ll have an ideal and an actual air-fuel AFR. 

Equivalence Ratio: The ratio between the actual air-fuel ratio (AFR_actual) and the ideal/stoichiometric air-fuel ratio (AFR_ideal) is called equivalence air-fuel ratio or lambda (λ).

`lambda=(AFR)_(actual)/(AFR)_(ideal)`

Depending on the value of lambda, the engine is told to work with lean, stoichiometric or rich air-fuel mixture.

Animation of the equivalence ratio of both Omega and Open W type piston bowl profile:

OMEGA bowl profile:

Open W bowl profile:

From the above animation, we can see that the Omega type piston bowl profile has the ability to maintain air-fuel properly during the combustion phase than that of the Open W type piston profile. That is the reason, Omega type bowl profile is emitting less SOOT (Unburned fuel particle).

As we can see in animation fuel spray in open W type bowl profile directly impinging on the surface of profile because of that distribution of fuel is uneven unlike in Omega type bowl profile fuel spray has very good recirculation inside chamber causing increasing in turbulence kinetic energy which leads to proper fuel-air mixing and complete combustion.

CO and CO2 emission:

CO formation animation:

CO2 formation animation:

From all the above animations, we can see that Open W type is producing and emitting more CO and CO2 after the combustion process.

`***performance Parameter:

Heat released by combustion inside cylinder:

The heat released by Omega type bowl profile is considerably higher than that of the Open W type bowl profile causing an increase in combustion efficiency.

Pressure:

Mass fraction burned comparison:

We can see that Omega type bowl profile is very much efficient in terms of mass fraction burned. In lesser time, this omega profile is able to combust 90% of fuel fraction although there is little ignition delay comparatively W type bowl profile.

PV Diagram

CONCLUSION:

1. From the emission standpoint, the results of the Omega bowl profile of the piston are dismaying since it emits comparatively higher NOx due to high temperature inside the combustion chamber but due to complete combustion of fuel-air mixture SOOT, CO, CO2 emissions are lesser than W type piston.

2. The area enclosed as shown in the PV diagram, Omega type bowl profile has a more enclosed area with lesser peak pressure because of that it is more efficient than that of Open W type bowl profile.