Fokion N. Egolfopoulos

Transient plasma ignition is known to improve the combustion of various fuels. Here, we compare the combustion mechanisms of carbon-free fuels containing mixtures of H 2 and O 2 via transient plasma ignition and conventional spark ignition. The transient plasma ignition is produced by five 20 ns pulses at a frequency of 1 kHz with an amplitude of 20 kV. Compared to conventional spark ignition (CSI), TPI ignited four times faster at a lean equivalence ratio (ϕ = 0.4). Optical diagnostics confirmed the presence of H α emission during TPI, providing direct evidence of atomic hydrogen radicals generated by nanosecond plasma discharges. These radicals, coupled with electrostatic flow effects, reduced ignition delay and accelerated flame propagation beyond enhancements attributable to flame wrinkling alone. Complementary kinetic modeling with Cantera and one-dimensional Lagrangian simulations revealed that plasma-generated H· radicals bypass high-temperature initiation pathways. This shortens induction times and promotes earlier OH· formation, thereby accelerating H 2 oxidation and water production under both adiabatic and non-adiabatic conditions. Together, these findings demonstrate that TPI enhances hydrogen combustion by coupling radical-driven kinetics with fluid-dynamic effects, offering a promising strategy to improve ignition reliability and efficiency in carbon-free propulsion systems.

In the work presented here, transient plasma ignition using high-voltage nanosecond pulse discharge demonstrates the ability to enhance combustion in lean methane applications. Experiments conducted in a constant-volume combustion chamber show that transient plasma has the ability to significantly accelerate early-stage flame propagation. Transient plasma ignition-initiated combustion of lean methane – air mixtures is assisted by early radical formation prior to flame initiation, representing a separate chemical-kinetic effect which allows the flame propagation to commence earlier and increase the rate of pressure rise much sooner. Complementary chemical kinetics modeling, informed by key intermediates identified through in situ plasma emission spectroscopy, revealed that transient plasma produces reactive radicals — particularly atomic hydrogen and methyl species — that reduce ignition delay and accelerate overall burn rates. Modeling also suggests that these species promote chain-branching pathways that are otherwise slow in lean methane combustion. These findings underscore the potential of transient plasma to improve the characteristics of lean methane combustion while emphasizing the critical role of plasma-generated species and mixture composition in governing ignition dynamics.

It was demonstrated that the use of nanosecond pulse transient plasma enables stable combustion of ammonia in an internal combustion engine (i.e., a modified natural gas engine) over a wide range of equivalence ratios from ϕ = 0.78 to ϕ = 1.23 Additionally, the ammonia combustion was investigated in an oxygen-rich environment to demonstrate the effectiveness of nanosecond pulse transient plasma for combustion initiation and increased rate of pressure rise (in a constant volume environment) compared to a conventional spark ignition system. Previous attempts to burn ammonia in internal combustion engines have used a dual fuel approach (i.e., diesel:ammonia, gasoline:ammonia, natural gas:ammonia) and have been limited to less than 30 % vol ammonia content. Herein, stable combustion was achieved with pure ammonia mixed with air in a single-fuel approach. The stable combustion of ammonia is enabled by two key mechanisms: 1.) ignition timing that needs to be substantially advanced relative to the top dead center of the compression stroke because of the relatively low flame speeds associated with ammonia, and 2.) multi-pulse high voltage discharge gives rise to ionic winds, multiscale turbulence and mixing, and increased flame surface area of the flame kernel. The latter mechanism is crucial, as it was not possible to achieve stable combustion using conventional spark ignition (i.e., magneto-type ignition) regardless of ignition timing advancement and equivalence ratio. The engine stability was quantified by measuring the coefficient of variation of indicated mean effective pressure (COV IMEP ). COV IMEP = 6.8 % was achieved for pure ammonia combustion with air at an equivalence ratio of ϕ = 1.23, which is very close to the industry standard of 5 %, indicating stable engine operation. The value of COV IMEP increases at lower equivalence ratios, as expected, reaching COV IMEP = 16.5 % at ϕ = 0.78 but still supporting stable engine operation. This general approach enables pure ammonia to be burned in internal combustion engines using a transient plasma ignition system that could serve as a drop-in replacement of conventional magneto-type ignition systems without requiring any further engine modifications.

IMEP ) at an engine speed of 1500 RPM) and a 4-fold reduction in NO x emissions at constant power at an engine speed of 2400 RPM. In this ultra-lean range, we also observe a 175 % increase in the maximum mechanical power produced by the engine using TPI (0.2 kW at an engine speed of 1100 RPM) by achieving more complete combustion. We compare the results of engine performance (stability, power, and NO x emissions) with those using a conventional (magneto-type) spark ignition (CSI) over a wide range of equivalence ratios ( ϕ ) from 0.2 to 0.7. The transient plasma-based enhancement arises from hydrodynamic effects, (i.e., ionic winds), which gives rise to turbulence and multi-scale mixing. This is supported by Schlieren imaging, which shows an augmented flame surface area and reduced ignition delays with TPI compared to that with CSI. This general approach enables hydrogen fuel to be burned in internal combustion engines, while maintaining low NO x emissions using a transient plasma ignition system that could serve as a drop-in replacement of conventional magneto-type ignition systems without requiring any further engine modifications.

As a society, we must find a pathway towards sustainable/clean energy and combustion. Among all zero-carbon fuels, ammonia <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{NH}_{3})$</tex> and hydrogen <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{H}_{2})$</tex> are the most promising candidates. With as much as 55% less <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{CO}_{2}$</tex> released per Joule in comparison with petroleum and diesel, natural gas (mainly <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{CH}_{4}$</tex>) is considered as a feasible “bridge” fuel. However, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{H}_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}$</tex> combustion are challenging due to the high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NO}_{\mathrm{x}}$</tex> (mainly NO and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NO}_{2}$</tex>) emissions produced from combustion of both <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{H}_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}$</tex> and the low burning rates associated with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}$</tex>. In the work presented here, we used nanosecond pulse transient plasma (NPTP) in a natural gas <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{NG})$</tex> engine to enhance the combustion of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{H}_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}$</tex>, and reduce the emissions of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NO}_{\mathrm{x}}$</tex> and greenhouse gases (GHG). To lower <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{CO}_{2}$</tex> emissions, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{H}_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}$</tex> are currently being mixed with natural gas (NG), but in relatively small ratios. With the ability to initiate a series of plasma discharges, our plasma-enhanced ignition system enables stable combustion of higher percentages of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{H}_{2}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}$</tex>. Using this approach, we find that transient plasma enables stable combustion of higher <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}:\mathrm{H}_{2}$</tex> ratios (up to 55% <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_2$</tex>) than with conventional spark ignition, which is significant considering the difficulties in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{H}_{2}$</tex> liquification, transport, and storage. Also, transient plasma enables stable combustion of higher <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{H}_{2}:\text{CH}_{4}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}:\text{CH}_{4}$</tex> ratios than with conventional spark ignition. In addition, it enables further <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{CO}_{2}$</tex> reduction (up to 58%) with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{NH}_{3}$</tex> under lean <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{CH}_{4}$</tex> conditions than conventional spark ignition. Optical spectroscopy demonstrates the formation of atomic oxygen (0) and nitrogen monohydride (NH) radicals, which are considered as highly reactive intermediates accelerating the combustion kinetics.

Abstract The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH 4 -N 2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>±</mml:mo> </mml:math> 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>±</mml:mo> </mml:math> 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation.