Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 1
Mikael Jonsson
Stratified scavenging in two-stroke engines using OpenFOAM
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 2
Acknowledgements
Mikael Jonsson
I would like to thank
• Associate Professor Hakan Nilsson at the Division of Fluid Dynamics at the De-
partment of Applied Mechanics.
• Erik Linden and Johan Spang at Husqvarna AB.
• Tommaso Lucchini at Politecnico di Milano
• Mikael Bergman at Husqvarna AB.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 3
Outline
Mikael Jonsson
• Aim and purpose.
• Approach.
• The two-stroke engine and the stratified charged two-stroke engine.
• CFD.
• Mesh operations.
• Boundary conditions.
• Results.
• Future work.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 4
Aim and purpose
Mikael Jonsson
• Evaluate the scavenging performance of the Husqvarna H576X engine.
• Evaluate the use of OpenFOAM for two-stroke engine simulations.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 5
Approach
Mikael Jonsson
• Determine interesting information.
• Limit the problem.
• Method to find the information.
• Possibilities in OpenFOAM.
• Set up the case.
• Extract the results.
• Validation.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 6
The two-stroke engine
Mikael Jonsson
• The conventional two-stroke engine.
• The cycle of operation.
• Advantages and disadvantages.
• Trapping efficiency.
ηtr =Mass of delivered air fuel mixture retained
Mass of delivered air fuel mixture
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 7
Expansion stroke
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 8
Compression stroke
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 9
The stratified charged two-stroke engine
Mikael Jonsson
• Reduction of emissions and a lowered fuel consumption.
• The cycle of operation.
• Trapping efficiency.
ηtr,fuel =Mass of delivered fuel retained
Mass of delivered fuel
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 10
Expansion stroke
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 11
Compression stroke
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 12
The H576X geometry
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 13
The H576X geometry description
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 14
Geometry modifications
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 15
CFD and the Finite Volume Method
Mikael Jonsson
• The Reynolds-Averaged Navier-Stokes equations.
• Turbulence modelling using the standard k − ε turbulence model.
• Wall functions.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 16
Fluids
Mikael Jonsson
• Approximation with air as the only fluid.
• No combustion, the combustion is modelled.
• Air fuel mixture for trapping efficiency
• Passive scalar transport
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 17
The passive scalar
Mikael Jonsson
• Adding a passive scalar, φ to represent the air fuel mixture.
• Convection dominated flow.
• The governing equations independent of the passive scalar.
• Volume concentration of φ in the cell.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 18
The passive scalar, continued
Mikael Jonsson
• The passive scalar transport equation.
∂(ρφ)∂t
+ div(ρUφ) = 0
• The cell volume passive scalar concentration.
0 ≤ φ ≤ 1
• A compressible flow requires the mass to be calculated
mfuel = ρ ∗ φ ∗ Vcell
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 19
Mesh operations
Mikael Jonsson
• The solver must manage some mesh operations.
• The cylinder volume is being compressed and expanded, layerAdditionRemoval.
• The cylinder and the ports must interact, slidingInterface.
• The twoStrokeEngine-library handle these features.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 20
Layer addition
Mikael Jonsson
• Morphing to a certain limit.
• Layer addition.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 21
Layer removal
Mikael Jonsson
• Compression to a certain limit.
• Layer removal.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 22
Sliding interface
Mikael Jonsson
• Sliding interfaces.
• Ports, cylinder and piston pockets.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 23
Sliding interface, continued
Mikael Jonsson
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 24
Boundary conditions, inlets and outlets
Mikael Jonsson
• Scavenging channel inlets.
• Exhaust outlet.
• Stratifying channel inlets.
• Flow both into and out of the domain.
• Adaptive B.C. for temperature, T , velocity ,U, turbulent kinetic energy, k, dissi-
pation of turbulent kinetic energy, ε, and the passive scalar, φ.
• Flow into the domain, Dirichlet B.C.
• Flow out of the domain, homogenous Neumann B.C.
∂φ∂xn
= 0
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 25
Boundary conditions, inlets and outlets, continued
Mikael Jonsson
• Temperature set according to results from 1-d simulations.
• Pressure set according to results from 1-d simulations.
• Turbulent properties, k and ε are set to small values.
• The passive scalar is set to a 100% concentration in the scavenging channel inlet.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 26
Boundary conditions, inlets and outlets, continued
Mikael Jonsson
• Isolated walls. Homogenous Neumann for temperature and pressure and the
passive scalar
• No slip for velocity
• Wall functions used for k and ε
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 27
Combustion modelling
Mikael Jonsson
• No actual combustion. Modelled combustion.
• Cylinder pressure and temperature set before exhaust port opens.
• Pressure from 1-d simulations.
• Temperature from 1-d simulations exhaust channel.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 28
Results, scavenging
Mikael Jonsson
• Trapping efficiency about 93%.
• Solution is periodic within four revolutions.
• Trapping efficiency from experiments 92%.
360 CAD 720 CAD 1080 CAD 1440 CAD
99.34 % 93.14 % 93.61 % 93.77 %
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 29
Results, validation
Mikael Jonsson
• Comparison in massflow over scavenging channel inlet with 1-d simulations.
-0.035
-0.03
-0.025
-0.02
-0.015
-0.01
-0.005
0
0.005
0.01
0.015
0.02
0 100 200 300 400 500 600 700 800
Mas
sflo
w [k
g/s]
Time [CAD]
1-d dataOpenFOAM
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 30
Results, validation, continued
Mikael Jonsson
• Comparison in massflow over stratifying channel inlet with 1-d simulations.
-0.014
-0.012
-0.01
-0.008
-0.006
-0.004
-0.002
0
0.002
0.004
0.006
0.008
0 100 200 300 400 500 600 700 800
Mas
sflo
w [k
g/s]
Time [CAD]
1-d dataOpenFOAM
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 31
Results, validation, continued
Mikael Jonsson
• Comparison in massflow over exhaust outlet with 1-d simulations.
-0.01
0
0.01
0.02
0.03
0.04
0.05
0.06
0 100 200 300 400 500 600 700 800
Mas
sflo
w [k
g/s]
Time [CAD]
1-d dataOpenFOAM
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 32
Results, validation, continued
Mikael Jonsson
• Comparison in massflow over scavenging channel inlet with Fluent simulations.
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0 100 200 300 400 500 600 700 800
Mas
sflo
w [k
g/s]
Time [CAD]
OpenFOAMFluent
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 33
Results, validation, continued
Mikael Jonsson
• Comparison in massflow over stratifying inlet with Fluent simulations.
-0.014
-0.012
-0.01
-0.008
-0.006
-0.004
-0.002
0
0.002
0.004
0.006
0.008
0 100 200 300 400 500 600 700 800
Mas
sflo
w [k
g/s]
Time [CAD]
OpenFOAMFluent
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 34
Results, validation, continued
Mikael Jonsson
• Comparison in massflow over exhaust outlet with Fluent simulations.
-0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 100 200 300 400 500 600 700 800
Mas
sflo
w [k
g/s]
Time [CAD]
OpenFOAMFluent
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 35
Conclusions, OpenFOAM
Mikael Jonsson
• Useful for two-stroke engine simulations.
• Approximately 24h per revolution with checkMesh on 3 CPU:s. Periodic behav-
ior within four revolutions.
• Results in correlation with commercial CFD-code and 1-d simulations.
Hakan Nilsson, Chalmers / Applied Mechanics / Fluid Dynamics 36
Future work
Mikael Jonsson
• Non-modified geometry.
• Expand the geometry.
• The mesh impact on the solution.
• Add more species and combustion.
• Heat transfer with walls.