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.