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Effects on Scavenging and Hydrocarbon Emissions: Altering the Combustion Chamber of a Small Two-Stroke Engine

Blennow, David LU and Grönquist, William (2026) MVKM01 20261
Department of Energy Sciences
Abstract
This thesis focuses on how changing the combustion chamber geometry in a small two-stroke engine can
improve performance. The engine used for this thesis is a Husqvarna AB chainsaw engine with Schn¨urle
porting and air-leading stratified scavenging. Four different combustion chambers were subjected to CFD
simulations, and from them two were selected for engine testing. In total, five combustion chambers were
tested in an engine testing rig. A detachable head design was used, and the different designs were compared
to the standard hemispherical combustion chambers found in production engines. The tests were performed
with wide open throttle using an electric brake to alter engine speed. Ignition timing and engine speed
sweeps were... (More)
This thesis focuses on how changing the combustion chamber geometry in a small two-stroke engine can
improve performance. The engine used for this thesis is a Husqvarna AB chainsaw engine with Schn¨urle
porting and air-leading stratified scavenging. Four different combustion chambers were subjected to CFD
simulations, and from them two were selected for engine testing. In total, five combustion chambers were
tested in an engine testing rig. A detachable head design was used, and the different designs were compared
to the standard hemispherical combustion chambers found in production engines. The tests were performed
with wide open throttle using an electric brake to alter engine speed. Ignition timing and engine speed
sweeps were performed.
The results from engine testing showed that offsetting the combustion chamber dome towards the intake
side increased the fuel trapping efficiency, and increasing the squish area showed a similar trend. An offset
oval design, with the wider part of the oval being place in the same direction as the crankshaft, consistently
showed the highest fuel trapping efficiency. This design had both an offset and an increased squish area. It
also showed the lowest hydrocarbon emissions in all tests.
Combustion performance was analyzed and no significant difference in rate of heat release was observed.
The difference was seen in the start of combustion, where the higher squish options had a later onset of
combustion. The best performer was the decreased squish ratio alternative. Although the fuel trapping
efficiency had a bigger impact on the hydrocarbon emissions and fuel efficiency, this information can be used
to further optimize future designs.
In conclusion, the offset oval performed best in total, but more optimization using the findings from this
thesis as well as validation from a solid cylinder-head is needed. (Less)
Popular Abstract
Did you know that a two-stroke engine needs less
than 5 % total efficiency to have more power per
kilogram than electric alternatives? In many ap-
plications, such as the chainsaw used in this thesis
or long distance drones, the power to weight ratio
is one of the most important things for choosing
the power source. In this thesis we looked into
how changing the combustion chamber, where the
spark-plug sits, can improve performance. We dis-
covered that having an oval shape that is offset to
the intake side showed great promise for future de-
velopment and significantly decreased emissions.
Please use this url to cite or link to this publication:
author
Blennow, David LU and Grönquist, William
supervisor
organization
course
MVKM01 20261
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Two-Stroke, Twostroke, Combustion, Scavenging, Stratified Scavenging, Loop-scavenging, Schnuerle, Combustion Chamber, Combustion Chamber Geometry, Trapping Efficiency, Combustion Efficiency, Fuel Trapping Efficiency, Hydrocarbon Emissions, Squish, Squish Ratio, In-Cylinder Flow Fuel Efficiency, Short Circuting, Husqvarna, Chainsaw engine, Small Engine Optimization
report number
ISRN: LUTMDN/TMHP-26/5687-SE
ISSN
0282-1990
language
English
id
9232902
date added to LUP
2026-06-12 11:31:30
date last changed
2026-06-12 11:31:30
@misc{9232902,
  abstract     = {{This thesis focuses on how changing the combustion chamber geometry in a small two-stroke engine can
improve performance. The engine used for this thesis is a Husqvarna AB chainsaw engine with Schn¨urle
porting and air-leading stratified scavenging. Four different combustion chambers were subjected to CFD
simulations, and from them two were selected for engine testing. In total, five combustion chambers were
tested in an engine testing rig. A detachable head design was used, and the different designs were compared
to the standard hemispherical combustion chambers found in production engines. The tests were performed
with wide open throttle using an electric brake to alter engine speed. Ignition timing and engine speed
sweeps were performed.
The results from engine testing showed that offsetting the combustion chamber dome towards the intake
side increased the fuel trapping efficiency, and increasing the squish area showed a similar trend. An offset
oval design, with the wider part of the oval being place in the same direction as the crankshaft, consistently
showed the highest fuel trapping efficiency. This design had both an offset and an increased squish area. It
also showed the lowest hydrocarbon emissions in all tests.
Combustion performance was analyzed and no significant difference in rate of heat release was observed.
The difference was seen in the start of combustion, where the higher squish options had a later onset of
combustion. The best performer was the decreased squish ratio alternative. Although the fuel trapping
efficiency had a bigger impact on the hydrocarbon emissions and fuel efficiency, this information can be used
to further optimize future designs.
In conclusion, the offset oval performed best in total, but more optimization using the findings from this
thesis as well as validation from a solid cylinder-head is needed.}},
  author       = {{Blennow, David and Grönquist, William}},
  issn         = {{0282-1990}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Effects on Scavenging and Hydrocarbon Emissions: Altering the Combustion Chamber of a Small Two-Stroke Engine}},
  year         = {{2026}},
}