Week 11: Project 2 - Emission characterization on a CAT3410 engine

 

Aim: To setup and perform the simulation of combustion process with open-w-piston and omega piston for Diesel engine.

Software used: Converge CFD

Objective:

• To run the simulation for Open W-piston and Omega piston and post process the results using PARAVIEW.
• Characterize the Emissions
• Create cut-plane animations showing Soot, NOx and UHC and compare them between omega and the Open-w pistons

Introduction:

The diesel engine, named after Rudolf Diesel, is an IC Engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to the mechanical compression (adiabatic compression); thus, the diesel engine is a so-called compression-ignition engine (CI engine). This contrasts with engines using spark plug-ignition of the air-fuel mixture, such as a petrol engine (gasoline engine) or a gas engine (using a gaseous fuel like natural gas or liquefied petroleum gas).

Diesel engines work by compressing only the air. This increases the air temperature inside the cylinder to such a high degree that atomised diesel fuel injected into the combustion chamber ignites spontaneously. With the fuel being injected into the air just before combustion, the dispersion of the fuel is uneven; this is called a heterogeneous air-fuel mixture. The torque a diesel engine produces is controlled by manipulating the air-fuel ratio (λ); instead of throttling the intake air, the diesel engine relies on altering the amount of fuel that is injected, and the air-fuel ratio is usually high.

The diesel engine has the highest thermal efficiency (engine efficiency) of any practical internal or external combustion engine due to its very high expansion ratio and inherent lean burn which enables heat dissipation by the excess air. A small efficiency loss is also avoided compared with non-direct-injection gasoline engines since unburned fuel is not present during valve overlap and therefore no fuel goes directly from the intake/injection to the exhaust. Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) can reach effective efficiencies of up to 55%.

Diesel engines may be designed as either two-stroke or four-stroke cycles. They were originally used as a more efficient replacement for stationary steam engines. Since the 1910s, they have been used in submarines and ships. Use in locomotives, trucks, heavy equipment and electricity generation plants followed later. In the 1930s, they slowly began to be used in a few automobiles.

The characteristics of a diesel engine are

• Compression ignition: Due to almost adiabatic compression, the fuel ignites without any ignition-initiating apparatus such as spark plugs.
• Mixture formation inside the combustion chamber: Air and fuel are mixed in the combustion chamber and not in the inlet manifold.
• Torque adjustment solely by mixture quality: Instead of throttling the air-fuel mixture, the amount of torque produced is set solely by the mass of injected fuel, always mixed with as much air as possible.
• Heterogeneous air-fuel mixture: The dispersion of air and fuel in the combustion chamber is uneven.
• High air ratio: Due to always running on as much air as possible and not depending on exact mixture of air and fuel, diesel engines have an air-fuel ratio leaner than stochiometric
• Diffusion flame: At combustion, oxygen first has to diffuse into the flame, rather than having oxygen and fuel already mixed before combustion, which would result in a premixed flame.
• Fuel with high ignition performance: As diesel engines solely rely on compression ignition, fuel with high ignition performance (cetane rating) is ideal for proper engine operation, fuel with a good knocking resistance (octane rating), e.g. petrol, is suboptimal for diesel engines.

In this project we have introduced two pistons such as W-piston and Omega piston to simulate using converge CFD and to find the power, torque, Engine efficiency, combustion efficiency, Emissions (SOOT, NOx and UHC) for the same engine specifications and the results were analysed.

Omega piston:

Open-W piston:

Geometry:

The below shown is the combustion chamber from which the two pistons are been extracted using the make surface profile.

Two piston profiles were extracted from the engine using the utility called make surface in order to reduce the computational time rather than taking the full engine. Make surface will automatically configure the surface geometry.

W-piston profile:

Omega piston profile:

Engine geometric parameters:

• Bore = 0.13716m
• Stroke = 0.1651m
• Connecting rod length = 0.263m
• RPM = 1600

Case setup:

Application type: Cranck based IC Engine

Materials:

Gas Simulation:

Parcel Simulation:

 Reaction mechanism:

Species:

Simulation Parameters:

Simulation time parameters:

Solver type:

Boundary conditions:

Piston:

Type : wall

Wall motion type : Translating

Surface movement : Moving

Temperature : law of wall (553K)

Front face: 

Back Face:

Matched front face boundary

Cylinder wall:

Type : wall

Wall motion type : Stationary

Surface movement : Fixed

Temperature : law of wall (433K)

Cylinder Head:

Type : wall

Wall motion type : Stationary

Surface movement : Fixed

Temperature : law of wall (523K)

Regions and Initialization:

Species:

Physical Models:

Spray Modelling:

Combustion modelling:

Emission modelling:

Turbulence Modelling: RNG k-E

Grid Control:

Adaptive mesh refinement:

Fixed Embedding:

Output files:

Results:

Meshing:

Total cell count:

Pressure plot:

The above plot shows the around -100 deg crank angle in the cylinder pressure begins to increase, this is because of beginning of compression stroke.

As the piston moves towards the TDC, the cylinder pressure keeps increasing and finally around -5 deg crank angle the combustion process begins due to which the slope of the curve increases suddenly.

The cylinder pressure peaks around 5 deg crank angle indicating that the piston has reached the TDC, high pressure generated due to combustion pushes the piston with a high force

Peak pressure in case of Omega piston is higher than the Open-W piston. 

Temperature Plot:

The temperature in the omega piston produces more heat than the open w piston because of the better combustion of air and fuel in the combustion chamber.

The temperature profiles of the combustion chamber show that when the piston moves towards the TDC. Temperature raise slowly because the air in the combustion chamber gets compressed.

When the fuel spray particles are sprayed into the combustion chamber at the high temperature and high pressure results in the burning of the mixture and thus we observed the high temperature in the chamber before the volume starts increasingly.

Volume plot:

 Density plot:

Integrated heat release rate:

Heat release rate:

Pressure:

 

Temperature:

 

Velocity:

 

 

PV Diagram:

The area under PV diagram of an engine indicates the work done by the engine. By comparing the above PV diagrams of the both pistons configuration, the area under the Omega piston PV diagram curve is indicating higher work done which can observe in the engine performance calculator.

Engine Performance parameters:

Open-W piston:

Power is defined as the ratio of doing work done per unit time.

Power = work done/time

Open W Piston:

Work done = 478.471

Rotations per minute = 1600

Rotations per second = 1600/60 = 26.667

Degree per second = 360*RPM/60 = 360*1600/60  = 9600

Time moving per degree = 1/degree per second

                                    = 1/9600

                                    = 1.0417*10e-4 sec/deg

Time for 270.286 degrees = 270.286*1.0417*10e-4

                                      = 0.028154

From Engine performance calculator

Gross work done = 478.471

Power = 478.471/0.028154

         = 16994.778

         = 16.994 KW

Torque for the Open W piston is 

T = 60*16.994*10e3/(2*3.14*1600)

 T  = 101.43 N-M

Omega piston:

Work done = 548.293

Rotations per minute = 1600

Rotations per second = 1600/60 = 26.667

Degree per second = 360*RPM/60 = 360*1600/60  = 9600

Time moving per degree = 1/degree per second

                                    = 1/9600

                                    = 1.0417*10e-4 sec/deg

Time for 270.286 degrees = 270.286*1.0417*10e-4

                                      = 0.028154

From Engine performance calculator

Gross work done = 548.2993

Power =548.293/0.028154

         = 19474.7815

         = 19.47 KW

Torque for the Open W piston is 

T = 60*19.47*10e3/(2*3.14*1600)

 T  = 116.23 N-M

The performance parameters for both the piston configuration are calculated by using the engine performance calculator

IMEP is higher in Omega piston than the open w piston and work done also higher in Omega piston.

Emissions:

Hiroy soot Emissions:

The plot shows that a significantly higher amount of soot is produced in case of Open W piston due to incomplete combustion of fuel.

Mass of soot in Omega piston = 4.81*10e-7 kg

Mass of soot in Open-W piston = 1.119*10e-6 kg

Animation link:

https://youtu.be/pkil_kqwKGA

NOx Emissions:

The above plot shows that higher amount of NOx is produced in case of Omega piston. This can be attributed to higher mean temperature in the cylinder.

Mass of NOx in Omega piston = 6.74*10e-7 kg

Mass of NOx in Open-W piston = 5.14*10e-7 kg

Unwanted Hydro Carbons:

The above plot shows that higher amount of Unburnt hydro carbons fuel is remaining in the cylinder in case of Open-W piston due to incomplete combustion of fuel.

Mass of HC in Omega Piston = 1.41*10e-7 kg

Mass of HC in Open-W piston = 8.81*10e-7 kg

Co Emissions:

The plot shows that a significantly higher amount of CO is produced in case of Open W piston due to incomplete combustion.

Mass of CO in Omega piston = 4.58*10e-6 kg

Mass of CO in Open W piston = 1.38*10e-5 kg

 

CO2 Emissions:

 The above plots show a lesser amount of CO2 is produced in case of Open W piston. 

Mass of CO2 in Omega piston is 7.63*10e-5 kg

Mass of CO2 in Open W piston is 5.98*10e-5 kg.

Conclusions:

The following conclusions can be drawn based on the comparative study between Omega piston and Open W piston:

• The performance characteristics of Omega piston are better than Open W piston. Omega piston delivers more Work/Power.
• The combustion characteristics o Omega piston is better than Open W piston. In case of Omega piston more amount of heat is released from the same amount of fuel as compared to Open W piston.
• Mean temperature in the cylinder is higher in case of Omega piston, so it produces more NOx.
• Due to incomplete combustion of fuel in case of Open W piston, it produces greater amount of soot, CO and HC but lesser amount of CO2.
• Based of all performance and Emission characteristic, Omega piston is the better choice . Open W piston is better for only if aims to reduce the NOx emissions.