Week 11: Project 2 - Emission characterization on a CAT3410 engine
Aim: Simulating closed-cycle analysis for diesel engines with different bowl geometries.
Objective:
Simulate combustion in an isolated chamber for two different piston bowl geometries.
Compare the results from both the pistons and find out which among them is more suitable.
Introduction:
CI 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.
Geometries:
W-piston:
Omega piston:
CASE setup:
Application type – Crank angle based IC engine
Physical parameters-
Materials:
Gas simulation: Same as that of no hydro simulation (providing therm.dat file for including all the reaction species).
Parcel simulation:
Add pre-defined liquids we will be using iso-octane as our fuel select it, all the properties of fuel at a different temperature will be included (i.e. viscosity, surface tension, density, etc.).
Reaction mechanism:
Add mech.dat file which consists of mechanisms for our fuel and validate.
Species:
Under parcel, we should add our fuel species (DIESEL2).
Simulation parameters:
Run parameters:
Solver: Transient
Temporal type: crank angle based engine simulation
Mode: Full hydrodynamic
Gas flow solver: compressible
Boundary conditions:
Piston:
The front face of the wedge:
Backface of the wedge:
Cylinder wall:
Cylinder head:
Regions and initialization:
Cylinder region:
Spray modelling:
General:
Drop evaporation:
Parcel distribution: Distribute parcels evenly throughout the cone.
Turbulent dispersion: O’Rourke model
Evaporation modelling: Frossling model
Evaporation source: Source specified species
Sources species: C7H16
Temperature discretization is not required since it will take more computational time.
Penetration:
Liquid fuel mass fraction for LPL: 0.95
Bin-size: 0.001
Fuel vapour mass fraction for VPL: 0.001
Collision/Breakup/Drag:
Collision model: NTC collision
Collision outcomes: Post-collision outcomes
Tick use collision mesh for realistic animation.
Drop drag model: dynamic drag model.
Wall-interaction:
Spray-wall interaction:
Model: Wall film
Formation of film on the surface of the liner on which splash of fuel parcel occurs.
Injector:
Add injector and click edit.
Injected species:
DIESEL2, and we can add a profile to specify when the fuel injection should start and end.
Models:
Kelvin Helmholtz model (KH)
Rayleigh Taylor model (RT)
Discharge coefficient model.
We have a set of recommended values for some fuels we can use for setting the above models for injection modelling
Time/Temp/Tke/Eps/Mass/size:
Total injected fuel mass (kg):
Given,
RPM = 1600
Total fuel injected(kg/s) = 3.602*e-04
Time taken per cycle:
t = 2*60/N (Since for each cycle there is 2 complete rotation)
t = 2*60/1600
t = 0.075s
Time taken for 1cycle (720 deg) is 0.04s then time taken per deg = 0.075/720
= 1.041*e-04
Fuel mass per cycle(kg) = fuel flow rate (kg/s) * time taken per cycle
= 2.70167*e-05
So the value for Total injected mass (kg) = 2.70167*e-05
Nozzles:
Combustion modelling:
General:
Grid Control:
Base grid: 0.004m
Adaptive mesh refinement:
Fixed embedding:
Emission modelling:
PLOTS:
Pressure plot:
The pressure for W-piston is slightly more since the clearance volume made by the w-piston is less compared to the omega piston hence the air is compressed more by the w-piston than the omega.
Clearance volume for w-piston is 0.00014 m3, and for omega piston is 0.00016
Temperature:
The temperature for the omega piston is much higher compared to the w-piston from which we can say the ignition delay for the omega piston will be small and the fuel will be burnt properly the only demerit is the NOx emissions increases with an increase in temperature.
The greater temperature for the omega piston is because of its shape and depth which increases the spray penetration length and the liquid spray of fuel is more in contact with the hot air of the cylinder before hitting the piston. also, there may be the reason for swirl formation which results in a good mixture of air a fuel hence when burnt produces greater temperature.
Heat release rate:
The heat released by w-piston is generally slow which can be seen from the plot and at the end, for the same mass of fuel we see that the heat release rate for w-piston is '67J' less compared to omega piston.
The heat release rate for W-piston and omega piston is found to be,
HRR:
W-piston = 7212J
Omega piston = 7279J
Difference in heat release rate = 7279 – 7212 = 67J
We can find the mass of fuel corresponding to 67J of energy released from the fuel.
The mass of fuel in kg used per cycle is = 2,7015*10-5 kg
The maximum amount of energy it produces = 7279J
So mass of fuel lost for 67J of energy loss is = (2.7015*10-5 *67) / 7279
= 2.4866*10-7kg
This corresponds to a very small amount of fuel but it's just for 1cycle, during operation of the engine it’s crankshaft undergoes many revolutions, in our engine, the RPM is 1600 that means 1600 revolution per minute, for every 2 revolutions we complete 1 cycle hence no of the cycle is 800cycles per minute if we convert it to hours we get 48000cycles.
For 1hr the amount of fuel that’s lost by w-piston = 48000cycle * 2.4866*10-7 (kg/cycle)
= 11.9 g
As rpm and no of hours increase, this value becomes significant but we should also compare other parameters such as emissions, maximum pressure, power, and temperature before finalizing any piston geometry.
Soot:
The soot is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons since our w-piston has a lower temperature the fuel is not fully burnt which results in the formation of soot.
Fuel gaseous mass:
The gaseous mass of fuel in the case of omega piston is more since it has a greater penetration length compared to w-piston where fuel impingement takes place.
CO emission:
The CO emissions are greater for w-piston compared to the omega because of the lower temperature it has.
HC emission:
The fuel is unburnt in the case of a w-piston because of low temperature which results in more HC(hydrocarbon) emissions.
NOx emission:
NOx occurs at higher temperatures which is the reason omega piston has higher NOx emissions.
Videos:
W-piston:
Fuel injection:
Temperature variation:
Pressure variation:
Omega piston:
Fuel injection:
Temperature variation:
Performance parameters:
W-piston:
Work done(Gross) = 3028.53 N.m
Time taken for combustion process in seconds = 0.028s
P = Work done/ time taken
= 3028.53/ 0.028
= 108161.78 N.m/s * (1 Kw/1000 N.m/s)
= 108.161 Kw
To calculate torque we can use power equation.
T = (P*60*1000)/2*π*N
= (108.161*60*1000)/2* π*1600
= 645N.m
CAD(Crank angle degrees for which 10,50,90 % of the fuel is burnt):
CAD10 – 0.70348
CAD50 – 17.8347
CAD90 – 53.0275
Omega-piston:
Work done(Gross) = 3409.28 N.m
Time taken for combustion process in seconds = 0.028s
P = Work done/ time taken
= 3409.28/ 0.028
= 121760 N.m/s * (1 Kw/1000 N.m/s)
= 121.760 Kw
To calculate torque we can use power equation.
T = (P*60*1000)/2*π*N
= (121.760*60*1000)/2* π*1600
= 727.07N.m
CAD(Crank angle degrees for which 10,50,90 % of the fuel is burnt):
CAD10 – 1.00833
CAD50 – 12.5019
CAD90 – 27.5282
Conclusion:
Power: w-piston = 108.161 kW
omega piston = 121.760 kW
Torque: w-piston = 645N.m
omega piston = 727.07N.m
Emissions:
NOx:
w-piston: Lower
omega piston: Higher
CO:
w-piston: Higher
omega piston: Lower
HC:
w-piston: Higher
omega piston: Lower
Peak temperature: w-piston: 1760K
omega piston : 1440K
HRR: w-piston: 2279J
omega piston: 2212J
Overall the omega piston is better in every aspect compared to w-piston so omega piston is more suitable since it increases the overall efficiency of the engine, also by comparing CAD value we can see that the flame propagates faster in omega piston geometry compared to w-piston which explains that the fuel is burnt rapidly and more amount of energy is released in it.
The only sad side of using omega piston is NOx emissions since we can further improvise our geometry by increasing/decreasing the depth and making some design changes such that NOx emissions are reduced.