| dc.description.abstract | Non-thermal or non-equilibrium plasma is an emerging field in combustion science. Stable
ignition of lean fuel-air mixtures continues to remain a challenge. The present work explores
the potential of non-thermal plasma for igniting lean mixtures using fundamental studies in a
constant-volume chamber and applied studies in an internal combustion engine. The first part
of the study focused on evaluating non-thermal plasma as an effective combustion
enhancement tool by initially conducting experiments in a constant-volume combustion
Chamber (CVCC) with optical access. A high-voltage Nanosecond Pulse Discharge (NPD)
generator that can operate at a maximum frequency of 3.5 kHz is used to generate non-thermal
plasma. The improvement in combustion is confirmed by comparing the Flame Development
Time (FDT), lean limit and flame kernel size between the conventional and NPD plasma
ignition systems. The configuration of the sparkplug for NPD plasma ignition is decided based
on the experiments carried out in the CVCC. With suitable modifications to the spark plugs,
the FDT is observed to reduce significantly. With the NPD system, a modified, conventional J
shaped spark plug (MJ) and a modified surface plug (MS) performed better compared to their
conventional counterparts. With MJ and MS spark plugs, a 12% and 15% decrease in FDT is
achieved, respectively, even with a single pulse of NPD as compared to a conventional ignition
system. Similarly, a two-fold increase in flame kernel size is observed with a single pulse of
NPD plasma ignition. The OH chemiluminescence measurements also confirm the
improvement in combustion shown in NPD plasma.
In the second part of the study, Optical Emission Spectroscopy (OES) is used to calculate the
post-discharge gas temperature. The plasma system is characterized based on the reduced
electric field (E/N) and electron temperature calculated using the BOLSIG+ software. Zero
dimensional plasma simulations are carried out using the Chemical Workbench and CHEMKIN
software to simulate NPD plasma combustion's discharge and ignition phases. The electron
impact reactions in the discharge phase are observed to generate excited nitrogen and oxygen
molecules, which in turn produce chain-carrying radicals such as O, H and OH. Among these
radicals, the O radical generated by NPD plasma affects combustion the most, as observed in
the sensitivity analysis of ignition delay and quantitative reaction path diagrams (QRPD).
These calculations are conducted to establish the initial chemical kinetics in the flame kernel,
which are then used to initiate three-dimensional simulations of the flame propagation. The
simulations successfully capture the lower FDT with NPD plasma ignition, as observed in the
CVCC.
In the third and final part of the study, the efficacy of the plasma-assisted ignition is assessed
in an internal combustion (IC) engine. Experiments are carried out in a single-cylinder Port
Fuel Injection (PFI) based four-stroke spark ignition (SI) engine. The results are very
encouraging as it is found that the engine could run leaner with the NPD plasma system,
reducing HC and CO emissions by 25.7% and 62%, respectively, in cold start tests using
gasoline fuel. Steady-state engine experiments are also carried out at two load points
corresponding to 2500 rpm and 4000 rpm engine speeds. The lean operating condition of the
engine is represented by the lambda (λ) value, which is defined as the ratio of the actual air
fuel ratio to the stoichiometric air-fuel ratio. Thus, as lambda increases beyond 1, the mixture
becomes leaner, resulting in lower fuel consumption and higher efficiency. With NPD plasma
assisted ignition, the stable operating limit of the engine could be extended from λ = 1.2 to λ =
1.4 at an engine speed of 2500 rpm and from λ = 1.25 to λ = 1.3 at 4000 rpm. The engine torque
and thermal efficiency have improved in the extended lean operation zone with NPD ignition.
Similarly, the NPD plasma ignition system at these lean operating points shows a considerable
decrease in fuel consumption, ignition delay, combustion duration, HC, and CO emissions. A
similar enhancement of lean limit is noticed with a methane-operated engine. At an engine
speed of 3000 rpm and 8% throttle, the lean limit of operation increased from λ = 1.1 to λ = 1.3
with a two-pulse NPD plasma-assisted ignition. The NPD plasma-assisted ignition could
achieve the same power output as the conventional system with 9.4% less fuel consumption.
Overall, the above results show that using an NPD plasma ignition system has great potential
to achieve higher efficiency and lower emissions under lean conditions in IC Engines,
particularly those operating on clean fuels such as natural gas and hydrogen | en_US |
