Flame propagation and autoignition in a high pressure optical engine

”Downsizing” engines with a turbo-charger is considered a promising way to realize the
reduction of CO2 emissions and the improvement of fuel efficiency. Understanding high
pressure engine combustion and knock is a prerequisite of developing any ”Downsizing”
Spark Ignition (SI) engine. Nevertheless, the lack or inconsistent of experimental
data about dynamic behaviours of premixed flame and autoignition at elevated pressure
hinder further research. The aims of this study are developing an optical experimental
boosted spark-ignition engine, and applying advanced diagnostic tools for investigation
of flame propagation and autoignition.
In this study, the optical engine LUPOE (Leeds University Ported Optical Engine)
was employed, which was supercharged using electronically controlled exhaust valves.
The controlled exhaust valves can increase the back pressure and extend the inlet boosting
time, to raise the initial pressure without changing the inlet flow rate. This new experimental
boosting configuration enables the intake mass flow rate and the initial pressure
to be independently varied. New engine control and data acquisition systems also were
developed to fulfill the requirements of the high pressure experiments.
This new boosting method has further been deployed to investigate the influence of
a highly boosted initial environment (inlet pressure was up to 2.5 bar) on the flame development.
These studies have been conducted at almost the same conditions of turbulence
intensity. The turbulence intensities, and the integral length scales, were measured by using
two dimensional Particle Image Velocimetry (PIV). The turbulent flame development
was recorded with high speed CH* chemiluminescence. In addition to the image analysis,
”reverse” thermodynamic analysis was applied to derive the in-cylinder charge state
and mass burning rate. The results show that an inlet pressure rise from 1.6 bar to 2.0 bar
decreases the flame burning velocity weakly. However, it has different effects upon the
flame acceleration at the early stage, and flame deceleration when the flame approaches
the side walls. Burning velocity still shows a slight raise with the pressure increasing at
the ”fully developed” stage. The structure of the flame at high pressure and its response
to pressure effects also were investigated. A laser sheet visualization technique was applied,
and a new image processing algorithm was developed to derive the detailed cross
section flame front topology. Wrinkle and curvature of the flame front were characterized
to compare the flame shapes under different boosted initial pressures. ”Self-similar”
properties of flames were evaluated with mean progress variables. The results show that
the initial pressure has only a slight effect on the flame structure. Flames at high pressure
have the same ”self-similar” properties as that observed at low pressure.
Further analysis and modelling of turbulent combustion requires information on
the laminar flame speed. In order to gain the iso-octane laminar flame speed at high igni-tion temperatures and pressures up to 600 K and 15 bar, the LUPOE engine was operated
at extremely low engine speeds, i.e. at an engine speed of 100 rpm. A turbulent-free
condition was attained and confirmed by PIV measurement, the flame speeds in enginerelevant
conditions were collected. By comparing these data with the laminar burning
velocities from the correlations calculation and chemical mechanisms simulation, the
measured burning velocities could be twice faster than that of unstretched and stable
flame. This is possibly caused by flame surface wrinkling, induced by hydrodynamic
instabilities at high pressure.
Finally, knock characteristics were examined in the strongly boosted SI engine. Images
of different knock development processes provide a detailed understanding of the
pressure oscillation in relation to in-cylinder phenomena. It was found that the extreme
knock events, observed during the strongly charged operation, occurred at lower pressures,
and larger mass fractions burned compared with knock at the normally aspirated
operation. The gas dynamics of autoignition, and flame-autoignition interaction played
an important role for the pressure oscillations. The reaction front initiated by the autoignition
events propagated at velocities much lower than the speed of sound at the
extreme knock onset.