Emission characterization on a CAT3410 engine

Objective:

• To characterize and compare emissions between Omega and Open W Pistons.
• To Create animations showing Soot, NOx and UHC
• To compare other performance parameters between Omega and Open W Pistons.

Engine sector surface generation and Geometry set up:

• In this project, piston profile is used to generate the geometry in converge.
• Following engine data is considered for building the profile:
  • Engine bore: 137.16 mm
  • Stroke: 165.1 mm
  • Connecting Rod length: 263 mm
  • compression ratio: 17.5
• Piston bowl profile data for both Omega and open w pistons are as shown below.

Figure: Omega Piston bowl profile

Figure: open W piston bowl profile

• using the above piston profile data and engine data, Engine sector profiles are generated as shown below.
• The sector angle considered is 45 deg which corresponds to 8 nozzle holes.

Figure: Generating piston profile from Make surface option.

Figure: omega bowl piston Figure: open w piston

Figure: Top view of the piston profile (sector angle 45 deg)

Case Set-up:

• It is a IC engine appication and following are the engine details
  • Bore: 0.13716 m
  • Stroke: 0.1651 m
  • Connecting rod length: 0.263 m
  • speed: 1600 rpm.
• Materials:
  • Parcel simulation: Diesel 2 is defined as the liquid.
  • Species:
    • Diesel 2 is defined as the Parcel
    • HIROY_SOOT and NOx are defined as passives.
• Simulation parameters:
  • This is a Full Hydrodynamic Simulation.
  • Simulation Time parameters:
    • Start time: -147 deg.
    • End time: 135 deg.
    • initial time step: 5e-07 sec
    • minimum time step: 1e-08 sec
    • Maximum time step: 2.5e-05 sec
• Boundary conditions:

Figure: Bounday Flagging

• • Piston:
    • Boundary type: Wall
    • Wall motion type: translating.
    • Temperature of 553 K.
  • Front face:
    • Periodic boundary condition.
    • Periodic type: Stationary.
    • Periodic shape: Sector with angle of 45 deg.
    • Match face: back face.
  • Cylinder wall:
    • Wall bundary type with temperature of 443 K.
  • Cylinder head:
    • Wall boundary type with temperature of 523 K.
• Regions and initializations: Incylinder region
  • Incylinder region with temperature of 355 K
  • Pressure of 197000 Pa.
  • Following are the species and Passives defined.

• Physical Models:
  • Spray modelling:
    • Evaporation model: Frossling model.
    • Parcel distribution: Cluster parcels near cone centre.
    • Injector is defined and Diesel 2 is the parcel species.
    • Kelvin helmoltz and Rayleigh-Taylor Model are selected.
    • Nozzle discharge coefficient: 0.7
    • Start of Injection: -9 deg
    • End of Injection: 21 deg
    • Injected mass: 2.70167e-05 kg
    • Injected Liquid temperature: 341 K.
    • Nozzle diameter is 0.000259 m with spray cone angle of 9 deg.
  • Combustion modelling: SAGE combustion model is defined.
  • Turbulence model: RNG k-epsilon.
• Base grid size is 0.0014 m.
• Velocity and Temperature AMR are defined for the Incylinder region with Max embed level of 2. The details are shown in the figure below.

• Fixed embedding is used for Nozzle, piston and cylinder head.
• After the case set up, Simulation was run and the results are post processed.

Mesh movement:

• Mesh movement along with AMR for both Omega and Open W pistons are shown in the animations below.

Results:

• Cylinder pressure:

Figure: Cylinder pressure

• Cylinder pressure comparison of both the Piston are as shown in the figure.
• Cylinder pressure vs Crank angle data is one of the important parameters that affects the Engine performance and emissions.
• Also, this data can be used to obtain information related to combustion progress and more importantly calculation of heat release rate.
• From the above graph, Both the pistons have almost same peak cylinder (open W piston cylinder pressure is slightly higher compared to omega).
  • peak cylinder pressures are
    • Open W piston: 11.53 Mpa @ 5.89 deg CA.
    • Omega Piston: 11.25 Mpa @ 12.16 deg CA.

• Mean Cylinder Temperature:

• omega piston has slightly higher mean cylinder temperature compared to open W piston.
• Higher temperatures lead to NOx formation.

• Heat release rate:

• Heat release rate is defined as the rate at which the chemical energy of the fuel is released by the combustion process.
• It can be calculated from the cylinder pressure vs crank angle data.
• From the above figure, Rate of heat release of Omega is higher compared to open W and hence the Mean cylinder temperature of Omega piston is higher.
• This in turn also indicates that the combustion efficiency of the omega piston is higher compared to open W piston.

• Gamma: Ratio of Specific heats

• Value of gamma, which is defined as the ratio of specific heats keeps on varying through the cycle due to variation in pressure and temperatures.

• NOx emissions:

• NO and NO2 are together termed as NOx.
• NOx emissions is one of the harmful emissions from diesel engines and a lot of research is being done in order to reduced these emssions.
• There are various sources of NOx like Thermal NOx, chemical NOx, Fuel NOx, Prompt NOx etc.
• One of the important mechanisms that givern the formation of NOx is Zeldovich mechanism. The reactions with reaction constants are shown in the image below.

• From the above graph, NOx emissions of Omega piston is higher compared to Open W piston due to higher Incyinder temperatures.
• The animations of NOx emissions in both the pistons are as shown below

• SOOT Emissions:

• Soot is combustion generated carbonaceous substance on which organic compunds have absorbed formed in the cylinder due to incomplete combustion.
• lubricating oil also contribute to Soot emissions.
• Diesel engines produce high amount of soot compared to gasoline engines due to high carbon content in the diesel fuel. Also, due to high soot emissions in diesel engines, they are always operated in lean mode (Higher lambda).
• In the above graph, we can see that SOOT emissions of Open W piston is slightly higher compared to Omega.
• Also as stated above, the combustion efficiency is low in open W piston and hence leads to higher soot emissions in Open W piston.

Figure: Example of Soot formation in omega bowl piston due to incomplete combustion at 2 time steps

• Above 2 figures show the soot formation due to incomplete combustion of an Omega bowl piston.
• At the tip of the spray we can observe higher concentrations of Soot, soon after the fuel injection. This is because at the spray tip or near the walls, temperature will be lower and the oxygen content will be less leading to incomplete combustion.
• Below animation provies clear picture of the Soot formation in both the pistons.
• From the animation, we can see that in Open W piston the fuel spray is directly impinging on the piston wall without proper atomization. This leads to incomplete combustion and hence more soot formation.
• In Omega bowl, the atomization is good compared to open W piston.
• in the below animations, we can see that the Soot formation is highest at a distance far from injector, where the temperature and oxygen availability for combustion is lower also near the injector tip where the fuel concentration is very high (low lambda).

Swirl Ratio:

• Swirl ratio is defined by the movement of fresh charge (e.g. air) around the cylinder axis.
• Swirl ratio of both the pistons is shown in the above figure.
• Higher the swirl ratio, higher is the turbulence in the cylinder and hence better mixing rate.
• Above graph shows that the swirl ratio of omega bowl piston is higher compared to open W piston and hence better is the combustion efficiency of Omega bowl piston.

• Carbon Dioxide (CO2) emissions:

• CO2 emissions are Green House Gas emissions that is gaining lot of importance for reduction.
• CO2 is a result of complete combustion.
• CO2 emissions of Omega chamber are slightly higher due to more complete combustion.

• Hydrocarbon (HC) emissions:

• Hydrocarbon emissions of open W piston is higher compared to omega.
• HC emissions increase due to incomplete combustion.
• HC emissions are also formed near the fuel rich regions like injector tips, valve recess etc.
• In the above soot animations, we observed that there is direct wall impingement in Open W piston leading to higher HC emissions.

• Carbon Monoxide emissions:

• Carbon Monoxide emissions are due to incomplete combustion.
• They are formed in fuel rich regions due to shortage of oxygen.
• Total CO2 emissions of open W piston is higher compared to Omega bowl due to poor atomization of fuel spray and incomplete combustion.

• Engine Performance calculator:

• Some of the combustion parameters and engine performance values calculated from Engine performance calculator are shown in the figure below.

• • Omega Bowl:

• • open W piston:

Power and IMEP:

• Power = Work done / Time
• Time per degree = 60 /(360 * Engine Speed) = (60/(360 * 1600) = 1.04 * 10^-4 sec/deg
• Omega Bowl =
  • Time for 270.17 deg = 270.17 * 1.04 * 10^-4 = 0.028
  • power = 3409.28/0.028 = 121.33 kW.
  • power of engine with Omega bowl piston is 121.33 kW.
  • IMEP of engine with Omega bowl piston is 13.96 bar.

• open W Piston:
  • Time for 270.156 deg = 270.156 * 1.04 * 10^-4 = 0.028
  • power of engine with Open W piston = 3028.53/0.028 = 108.16 kW.
  • power of engine with Open W piston is 108.16 kW.
  • IMEP of engine with Open W piston is 12.40 bar.

Conclusions:

• Simulation of 2 piston bowls namely Omega and Open W piston bowl of CAT3410 Diesel engine is performed in Converge Studio software.
• Soot emissions from open W piston is higher compared to Omega bowl due to direct wall impingement of the fuel spray leading to poor atomization.
• NOx emissions of Omega bowl piston is higher due to higher incylinder temperatures
• Combustion efficiency of Omega bowl piston is higher compared to Open W piston due to Good atomization of fuel and higher heat release rate.
• Engine with Omega piston bowl has higher power compared to open W piston bowl.
• Based on the above conclusions Omega bowl piston is a better choice for this engine.