Piston bowl swap analysis on a CAT-3410 engine

Objective

To simulate and analyze a CAT-3410 engine with two different piston bowl profiles to capture the characteristic differences caused by piston bowl design in the engine performance and emission.

This analysis will provide insights on spray parcel ejection, turbulence, combustion, fuel utilization, engine properties variation at various stages of the power generation process, and more importantly characterization of emission. It will also provide information on the power and torque produced by the simulated engine to provide a sense of complete engine simulation. Simulation results will be utilized to analyze which piston bowl profile performs more efficiently.

Parameter Guide

1. Introduction

Converge Studio is the flagship model from Convergent Science which is widely used for CFD application. With its advanced physical models and high-performance computing, it provides an easy to interact GUI and focuses on applications in IC engines, gas turbines, exhaust treatment, and general flow.

In this project, Converge Studio was used to analyze two simulations of a CAT-3410 engine - one with open-w bowl profile while the other with baseline omega bowl profile. Simulations were executed under certain parameters, which are elaborated systematically in the Parameter Guide.

Also, in this project, we will see the application of spray modeling, combustion modeling, and spark modeling to replicate accurate functioning of the engine.

The flow of information in this report will be as follows

1. Abstract
2. Geometry
3. Case Setup
   a. Preprocessing
   b. Solver
   c. Postprocessing
4. Comparative Analysis
   a. Emission Characterization
   b. Performance Evaluation
5. Animation
6. Inference
7. Reference

1. Abstract

The utilization of internal combustion engines has deeply penetrated into our day-to-day life due to its relatively inexpensive and reliable source of power for innumerable applications ranging from large-scale industries to the transportation sector to domestic uses. This has been the story for about a century.

Now, there are basically two types of engines - Spark-Ignited (SI) engines and Combustion-Ignited (CI) engines. CI engines are also known as Diesel engines as the main fuel used in CI engines is Diesel. Evidently, Diesel engines have proved to have higher thermal efficiency than any other engines. It is mainly used in serving heavy-duty and light-duty vehicles. Swirl interaction with compression plays a vital role in Diesel engines, as it can affect the rate of fuel-air mixing. It induces squish flow which increases turbulence in the combustion chamber promoting mixing. Even though the piston geometry has a negligible effect during the intake stroke, it has a significant effect as the piston moves towards the Top Dead Center. Depending on the piston profile, the air swirl produced can result in better atomization, better mixing of fuel and air and hence better combustion [1]. To understand this better, a simulation was executed using two piston profile - open-w piston and omega piston. Upon simulation, a comparative study was performed to understand which profile would be more efficient to use than the other.

2. Geometry

3. Case Setup

a. Preprocessing

In this section, we basically set up the necessary operating parameters for our geometry. Upon transfer of the geometry, boundary conditions were defined on the respective faces of the geometry namely Piston, Cylinder Wall, Cylinder head, Front and Back face.

With the completion of the above process, the case was set up with the necessary parameters as mentioned in the Parameter Guide.

b. Solver

Once the setup was done, the files were exported to an allocated folder for processing. Processing was carried out using Cygwin, a DOS environment which extracts the information from the exported files to process the simulation and create an output folder, which was then used to create log files to perform graphical visualization of the simulation for analysis.

c. Postprocessing

The output files from Cygwin was then used to process a log file consisting of all the information of the simulation which will be required to produce a visual for the analysis. Converge Post-Processing 3D window helps to perform this task. However, both compatible Cygwin and Windows Command Prompt can also aid in performing the same task in the DOS environment. The advantage of using Converge Post-Processing 3D is that it provides a more interactive and easier environment to perform the task of unfurling and combining the information. Once the post-processing was performed, a group of ParaView compatible files was generated which was used to produce the graphical visualization. ParaView is a graphical user interface used to produce graphical outputs to the simulation performed on Cygwin using the data from Converge Case Setup. Upon simulation of the engine, the mesh, temperature, pressure, fuel injection and emission profiles were studied which are graphically represented in the Animation Section.

Mesh Profile

Initial Timestep

Timestep = 70

Timestep = 80

Timestep = 100

It can be seen that Converge Studio has successfully manipulated the mesh size in the regions of interest, based on the parameters provided.

Timing Map

An overview of the complete engine process can be visualized in Fig. (a)

Fig. (a)

4. Comparative Analysis

The outputs from Cygwin were used to characterize the emission and evaluate engine performance using Converge Line Plotting window.

4.a. Emission Characterization

Emission Characterization of Open-W Piston Bowl Profile and Baseline Omega Piston Bowl Profile

4.a.1. Comparison of Emission Characterization between Open-W Piston Bowl Profile and Baseline Omega Piston Bowl Profile

Soot Comparison

The above plot shows the difference in the level of soot emission from the open-W piston bowl profile and baseline omega bowl profile calculated using the Hiroyasu Model. It can be seen that the open-w piston produces more soot particles than omega piston which can be attributed to the higher spray-wall impingement in case of the open-w piston as compared to that of the omega piston. At the end of the cycle, the soot has been burnt in the case of the omega piston while is exhausted in the environment by the open-w piston profile.

It can also be attributed to Turbulent Kinetic Energy (TKE). The intensity of turbulence immensely influences the gas motion and fuel vapor mixing with air in the combustion chamber. Even though the quantification of the effect of turbulence is difficult, it has been noticed that soot emission decreases with increased TKE levels inside the combustion chamber [3]. To understand it better, TKE comparison was carried out between the two piston profiles.

From the plot, it can be seen that the omega piston operates with higher TKE as compared to the open-w piston. Hence, it can be inferred that due to higher TKE generated in the omega piston the engine-out soot emission is lower while due to relatively lower TKE in case of the open-w piston, the engine-out soot emission is comparatively higher.

Carbon Monoxide (CO) Comparison

In this emission comparison, the omega piston bowl profile can be seen to have higher CO emission than the open-w profile right at the combustion phase, but soon evaporates to zero once the combustion is complete while such is not the case with the open-w piston. In case of the open-w piston, the CO remains in the cylinder and is eventually exhausted out to the environment. This can be attributed to incomplete combustion in case of the open-w piston while in case of the omega piston it seems to fully utilize the input mixture.

Incomplete combustion can be imputed to spray penetration length. With longer spray penetration a higher degree of atomization can be achieved thereby improving the homogenous mixing, and hence fully utilizing the air charge in the combustion chamber. This promotes evaporation in a very short time and hence promoting higher combustion efficiency. Furthermore, with longer spray penetration, the diameter of the fuel particle changes at a slow rate or in other words, the enlargement of the fuel particle takes place at a farther spot as compared to shorter spray penetration length. With the enlargement of fuel particle at a farther spot rather than at a shorter distance from the injection, the injection delay period decreases thereby contributing to better mixing of the fuel and the air and hence causing low smoke level and CO emission [4]. To analyze this, a comparison was drawn between the two piston profile with respect to the spray penetration length.

From the above plot, it can be seen that the spray penetration length for the omega piston setup is longer than that of the open-w piston. We have established that longer spray penetration leads to better fuel and air mixing which leads to better combustion and hence lower engine-out CO emission. Thus, it is evident that the omega piston profile will produce lower engine-out CO emission as compared to the open-w piston.

Hydro-Carbon (HC) Comparison

Here, the open-w piston bowl profile can be seen to have higher HC emission than the baseline omega profile. Again, this is caused due to incomplete combustion as we have learned earlier. With this plot, it is evident that the open-w piston takes longer to evaporate the after combustion gaseous content as compared to omega piston. Even though it can be seen that by the end of the cycle, the hydrocarbon content has become zero in both the piston profiles, with developing analysis, it seems the open-w piston is unable to utilize the complete fuel during combustion.

Nitrogen Oxide (NOx) Comparison

When compared between the open-w piston and baseline omega profile, NOx is way higher in the case of the latter. To make the explanation easier, the comparison of the mean operating temperature between the open-w piston and baseline omega profile has been presented below.

So from the mean temperature plot, it can be seen that the omega piston profile operates at a much higher temperature than the open-w piston profile. Now, at elevated temperature, nitrogen and oxygen combine to form nitrogen oxide. Hence, it can be attributed that due to higher operating temperature, the omega piston will produce higher NOx content than the open-w piston.

4.b. Performance Evaluation

5.b.1. Comparison of the engine performance between Open-W Piston Bowl Profile and Baseline Omega Piston Bowl Profile

Engine Performance

Abbreviation used in the table

IMEP: Indicated Mean Effective Pressure

CA10: Crank angle at which 10% of the fuel is consumed.

CA50: Crank angle at which 50% of the fuel is consumed.

CA90: Crank angle at which 90% of the fuel is consumed.

PFP: Peak Firing Pressure

PFP CA: Crank angle at peak firing pressure

HRR: Heat Release Rate

HRR peak CA: Crank Angle at peak heat release rate.

Power and torque of the engine

While the power of an engine represents the maximum power an engine can put out [5], the torque of an engine represents the amount of turning power the engine can produce which helps in the rotation of the wheels in a vehicle.

We know, `Power = \frac{Wo rk}{Time}`

and, `To rque = \frac{60\ times\ Power}{2\pi\ times\ RPM\ }`

4.b.1.1. Open-W Piston Bowl Profile

Power

RPM = 1600

Work = 3028.53 Nm

From the engine performance table, Duration = 270.156 deg.

Therefore, time per duration = `= \frac{60}{360\ times\ 1600} times 270.156 = 0.02814125 s/(deg.)`

From the calculated time, `Power = \frac{3028.53}{0.02814125\ }=\ 107.62\ KW`

For Torque,

or, `To rque\ =\ \frac{60\ times\ 107.62\ times\ 10^3}{2\pi\ times\ 1600}`

or, `To rque\ =\ 642.64\ Nm`

4.b.1.2. Baseline Omega Piston Bowl Profile

Power

RPM = 1600

Work = 3409.28 Nm

From the engine performance table, Duration = 270.171 deg.

Therefore, time per duration = `= \frac{60}{360\ times\ 1600} times 270.171 = 0.02814281 s/(deg.)`

From the calculated time, `Power = \frac{3409.28}{0.02814281\ }=\ 121.14\ KW`

For Torque,

or, `To rque\ =\ \frac{60\ times\ 121.14\ times\ 10^3}{2\pi\ times\ 1600}`

or, `To rque\ =\ 723.38\ Nm`

5. Animation

6. Inference

[a] From the emission characterization,

Soot: Omega piston profile performs better.

CO: Omega piston profile performs better.

HC: Omega piston profile performs better.

NOx: Open-w piston profile performs better.

HC and CO tend to decrease with increasing operating temperature while NOx tends to increase in the same condition. HC and CO are comparatively easy to address with after-exhaust treatment systems such as Diesel Oxidation Catalysts (DOC) and Diesel Particulate Filters (DPF) whereas NOx is relatively more difficult to address and requires the treatment from the Selective Catalytic Reduction (SCR) catalysts. NOx poses more issues to be addressed as it is stable, unlike other emissive products.

[b] From the performance evaluation, the omega piston profile performs better. It can be further corroborated using the comparison plot provided below.

We know the area under the PV curve represents the work done and hence, it can be seen that the work done by the omega piston is more than the open-w piston.

[c] For the general study, between the open-w piston profile and baseline omega piston profile, the latter performs more efficiently but in the industry, several other factors are required to make the judgment call while finalizing the piston design that shall be used for the engine. For instance, the engine market one focuses on is not necessarily interested in higher performance but rather desires low NOx level, in which case, open-w piston profile will be clearly preferred upon omega piston profile. Also, if the government regulates for an allowable NOx emission to be lower than 4e-06 kg/cycle than clearly omega piston profile will not be utilized when moving forward with the design finalization. Similarly, there can be several instances where the omega piston profile will be preferred over the open-w piston profile. In a nutshell, there are numerous factors critically analyzed before choosing or finalizing a piston design for an engine.

7. Reference

[1] M. Peeraiah, M. Chandra Sekhar Reddy, A. Venugopal; Enhancement of Engine Performance and Reduction of Emissions by changing Piston Geometry (2015)

[2] Which gas in more harmful?; https://www.researchgate.net/post/Which_gas_in_more_harmful

[3] Henrik W. R. Dembinski; In-cylinder Flow Characterisation of Heavy-Duty Diesel Engines Using Combustion Image Velocimetry (2013)

[4] International Journal of Engineering and Innovative Technology (IJEIT), Volume 2, Issue 4, October 2012