posted on 2011-02-09, 11:24authored byNabil S.M. Salama
The thesis is concerned with the study of the
characteristics of the turbulont flow field inside the
combustion chambers of spark ignition engines.
A comprehensive literature survey has been undertaken
of the problem of cyclic variation in spark ignition engines,
and the role of turbulence on flame propagation.
The experimental methods used involve a detailed
I
application of hot wire anemometry, signal processing and
computer techniques to the statistical analysis of the
problem of cyclic variation in gas velocity and turbulence
structure (intensity, scale, power spectrum and eddy
diffusivity), at the spark plug location of motored spark
ignition engines.
Two engines have been used in the investigation, a
Rolls-Royce V-SL and a Ford 2 litre V4. Extensive test programs
were c arried out on both engines at a range of engine speeds
and throttle settings, and a range of turbulence promoting
devices have been investigated with regard to their suitability
for promoting small scale turbulence during the ignition
period.
The variation of flow field characteristics with depth
inside a wedge combustion chamber of the Rolls-Royce engine
has been investigated. Cylinder-to-Cylinder variation in
turbulence characteristics has also been investigated for the
Rolls-Royce engine. The spectral composition of turbulent eddies inside
a wedge and a heron combustion chamber was invetstigated
over a wide range of engine speeds. It was found that
increasing engine speed resulted in increases of both the
fluctuating velocity components and the high frequency
content of the eddies. Both effects are believed to be
responsible for increases in turbulent flame speed in engines
with increases in engine speed.
Comparisons between the results obtained in the
present work and the reported data of other investigations
on turbulence measurements in spark ignition engines and
closed vessels, show close agreement.
A direct correlation has been established between the
characteristics of the turbulence field in bn-g-ilnes and other
isotropic flow fields. In particular, a correlation is found
to exist between the eddy diffusivity as obtained from the
anemometer measurements in an engine and the eddy diffusivity
data for highly turbulent pipe flow obtained by other workers.
These findings establish a relatively straightforward method
for obtaining quantitative information on turbulence
characteristics from mean velocity measurements on an engine,
which require simple equipment.
A theoretical model of combustion variation has been
developed, based on the assumption that cyclic variation
originates during the growth period of the initial flame
kernel. This process was related to variations in eddy
diffusivities of the small scale turbulence for different cycles. The model equations are chocked by making use of
the established correlation between eddy diffusivity and gas
mean velocity and using the reported experimental data of
other workers, which shows very good agreement between the
predictions of the model and the measured values.
History
School
Mechanical, Electrical and Manufacturing Engineering