Comparison of Emission characterization between Open-W and Omega Piston of a CAT3410 engine

Aim:

Here we are more concerned about the comparison of the emissions between Open-W and Omega pistons. Here we are comparing with below

1. Nox
2. Soot
3. UHC(Unburnt Hydrocarbons)

Performing Transient Simulation. Steps involved in running Simulation

1. Introduction
2. Geometry Creation
3. Boundary flagging
4. Diagnosing the geometry
5. Case Setup
6. Solve
7. Post-processing the results

Step 1

Introduction

Spray Modelling

For Gaseous sprays, converge uses the eulerian solver. Set the species mass fraction and mass flow rate at the inflow boundary

For liquid sprays, converge uses the lagrangian solvers to model discrete parcels and the eulerian solver to model the continuous fluid domain. Heat, momentum and mass transfer occur between the discrete and continuous phases via source terms in the transport equations

Parcels

Converge introduce parcels (collection of drops) into the domain at the injector. Each parcel represents a group of identical drops(i.e. same radius, velocity, temperature, etc). Converge solves for radius, velocity, etc on a per parcel basis rather than a per-drop basis(because the drops in a parcel are identical

The parcels statistically represent the entire spray field

• Parcels undergo several physical processes
• Primary breakup
• Secondary Break up
• Drop Drag
• Collision and Coalescence
• Turbulent dispersion
• Evaporation

Parcel Distribution

For solid cone sprays, converge assume the injection is conical. While setting up spray modeling we need to choose one of the parcel distribution mentioned below

SGM Sub-Grid Modeling

Converge Uses sub-grid models for the spray processes that occur on length scales that are too small to be resolved

Combustion Modeling

Combustion facilitates te energy transfer in the engine. Converge contains a detailed chemistry solver and simplified combustion chamber

SAGE detailed chemistry solver

• Most predicted an accurate way to model combustion
• Accurate model ignition and laminar flame propagation

Simplified Combustion Models

• Generally less computationally expensive and predictive than sage
• May provide acceptable results for specific applications

SAGE

The SAGE detailed chemistry solver uses local conditions to calculate reaction rates based on the principles of chemical kinetics. In our study, we have 42 species and 168 reactions involved in combustion chemistry.

After running the simulation the first thing that need to be looked after is P-V or TS profiles curves to understand how combustion took place

Step 2

Geometry

Step 3

Boundary flagging

Step 4

Diagnosing the geometry

We will just run the diagnosis for a set of important problems which are going to affect the solution

After running the diagnosis we see that no errors are present let us proceed to the case setup

Step 5

Case Setup

Case setup is the same for both the bowl profiles

Emission Modelling

In order to capture the emission data accurately, we are enabling

Grid Control

Base Grid: dx=dy=dz=0.04m

AMR

output/Post-processing

The time interval for writing 3D output data file:- 2 deg.

The time interval for writing text output:- 0.1 deg.

The time interval for writing restarting output:- 5 deg

Timing Map

Step 6

Solve

Then we need to export the input files to a folder to start running a simulation.then by using Cygwin we are directing to the input file and simulation is stared by using an executable file and simulation ran by using two processors. After successfully running the simulation we can post-process the results and check the general outcomes whether simulation ran properly and are they making any sense.

Step 7

Post-Processing the results

NOx:

Theory

Graph A: NOx variation based on temperature

Graph B: NOx variation based on O2 %

• NOx emissions do not form in significant amounts until flame temperatures reach 2800 F or 1537°C. Once that threshold is passed, however, any further rise in temperature causes a rapid increase in the rate of NOx formation (A).
• NOx production is the highest (B) at fuel-to-air combustion ratios of 5–7% O2 (25–45% excess air). Lower excess air levels starve the reaction for oxygen, and higher excess air levels drive down the flame temperature, slowing the rate of reaction.
• It is impossible to reduce Nox unless pure oxygen is supplied to the chamber because air contains 78% nitrogen and 20.5% of oxygen and rest are other gases
• NOx reduction in the area of most concern today. Thermally produced NOx is the largest contributor to these types of emissions.
• Thermal NOx is produced during the combustion process when nitrogen and oxygen are present at elevated temperatures. The two elements combine to form NO or NO₂.
• NOx is generated by many combustion processes other than boiler operation. It combines with other pollutants in the atmosphere and creates O₃, a substance known as ground-level ozone.

Nox Graph

• Actually the explanation given in the above Mean Temperature is right here we can see that the NOx generated in Omega Piston is quite high compared to Open-W piston. So by this, we can conclude that the higher the internal temperature higher would be the Nox Generation. So we need to concentrate on temperature to control the NOx emissions but thinking of keeping low temperature leads to other emissions
• Other than engine modification by using existing techniques like Catalyst converters such as LNT and SCR (with supplementary reductant Diesel Exhaust Fluids (DEF))we can control Nox further in both Open-W and Omega Pistons

Why NOx is greater in Omega Piston?

To know that Let us look into Mean temperature Profile for both the piston type

Mean Temperature Plots:

• If we look into the temperature graph we can say that the peak means the combustion chamber temperature of Open-W Piston is 1464 K compares to 1768 K Omega piston.
• As I mentioned in the theory as the temperature rises the Nox content also rises and the main thing if the temperature rise is greater than 1537K then the Nox would rise significantly.
• We can see that 1768 is the mean peak temperature that has occurred in Omega Piston so we can conclude that Nox has risen significantly in Omega Piston. But this can not conclude which piston is better and more efficient. Because there are DEF(Diesel Exhaust Fluids) to control NOx emission

Why temperature is less in Open-W Piston?

Let us look into vapourization of fuel

Gaseous Fluid:

If we look clearly we can see that more fuel is getting vaporized in Omega Piston than the Open-W. Because of this more temperature is generated in Omega Piston. So we need to know why the fuel is not readily getting vapourised.

Why fuel is not getting vapourised for that?

Let us look into the piston animations

As we can see the angle of the injector of open_W most of the fuel is on the piston wall so it takes time to get vapourised so changing the angle of the injector will increase the temperature. Because of the low temperature, there will be less Nox but at the same time other emissions will increase so we need to maintain a moderate temperature.

NOx in Omega type piston is greater than Open-W Piston

SOOT

• Soot is an environmental contaminant. It is the result of the incomplete combustion of hydrocarbons. Soot is the common name by which we refer to “particulate matter”(PM2.5) or Particle Pollution is a complex mixture of tiny particles.
• it is so small that it can only be detected with an electron microscope. Engines that burn diesel can spew a large amount of pollution containing soot particles.
• These particles have an impact on health and climate in some of the world's most sensitive regions.
• Engines that burn diesel can spew a large amount of pollution containing soot particles. These particles have an impact on health and climate in some of the world's most sensitive regions.
• So in order to control Soot either change needs to be done in the engine for complete combustion or adding filters such s Diesel Particulate Filter (DPF) it is going to remove any particular matter or soot.

To distinguish between complete and incomplete combustion let us take an example

Fuel: Propane

So the main issue that causing incomplete combustion is a lean mixture of oxygen.

Because in partial combustion, there is not enough oxygen to fully transform the fuel into carbon dioxide and water, there is no oxygen to go into the CO² compounds, meaning that the product is just C - soot

Unburnt Hydro Carbons:

• When unburnt fuel is emitted from a combustor, the emission is caused by fuel "avoiding" the flame zones. For example, in Piston engines, some of the fuel-air mixture "hides" from the flame in the crevices provided by the piston rings grooves.
• Further, some regions of the combustion chamber may have a very weak flame, that is, they have either very fuel-lean or very fuel-rich conditions and consequently they have a low combustion temperature.
• These regions will cause intermediate species such as formaldehyde and alkenes to be emitted. Sometimes the term "products of incomplete combustion," or PICs, is used to describe such species.

ii)Comparing the IMEP and power values graphically

Power: The power of a combustion engine is a function of the volume of fuel and air combusted. Thus, the size of the cylinder, the number of cylinders, and the engine speed determine the amount of power the engine generates. By boosting the engine’s intake of air using a blower or compressor – called supercharging – the power output of the engine can be increased. A commonly used supercharger is a turbocharger, which uses a small turbine in the exhaust gas path to extract energy for driving a centrifugal compressor.

IMEP: Indicated mean effective pressure is the uniform pressure that would be required throughout the power stroke of an engine to do the same amount of work as is done by the varying pressures that are obtained during the stroke Or we can say it as the average pressure obtained in the combustion chamber during the entire working cycle.

The calculation for finding the power that has produced in omega and Open-W type pistons is given below

After calculating the time the gross work obtained from the engine performance calculator is used to calculate power.

Based on the power generated we can say that Omega Piston is more efficient than Open W piston. But based on IMEP we cannot come to the conclusion that which type of piston is more efficient because mechanical efficiency depends on Mean effective pressure

Mechanical Efficiency =BMEP/IMEP

So greater the IMEP lesser the Mechanical Efficiency here we know that IMEP of omega piston is greater than IMEP of Open-W Pistor but we don't have the Brake mean effective pressure (BMEP) for both piston so it is difficult to say which would be more efficient based on the currently available data

So from this data, we can interpret that Open Piston is doing well in controlling Nox and CO2 emissions but it is poor in the performance of the engine, soot, and CO emissions. In the case of Omega Piston, the engine performance is good compared to Open-W it is performing well in almost all the areas. Other than modifying the engine parameters there is external equipment connected to the exhaust to control the emissions. Let us do a final analysis to find which is better piston by calculating mass-consumed for extra 60J for every cycle

The data of maximum Heat generated is taken from thermo.dat to compare how efficiently the fuel has burnt in CC in Open-W type and Omega type of Pistons

So by this calculation, we can say that the fuel is wasted or incomplete combustion took place in Open-W so that the reason Soot and CO emission are quite high compared to Soot and CO emission in Omega Piston.

PV curve Profile

From this curve profile, we can say that area under the curve represents the work done by the engine. So the work done in Omega type Piston is greater than the work done in Open-W type piston

Conclusion

After going through all the comparison that is done between Open-w and Omega Piston I can conclude that the Omega Piston's overall performance is good compared to Open-W in terms of Power, IMEP, Fuel consumption, CO and Soot Emissions. Open W is only Performing well in Nox and Co2 emissions