Nanosecond pulsed power ignition systems for reliable supersonic combustion

Introduction

Attention and effort to the research and development of hypersonic aircraft, which can fly at several times the speed of sound, has significantly increased in the past few years. One fictional portrayal of real-world pedigree (Lockheed Martin’s Advanced Development Program’s unmanned SR-72 announced in 2017) even played a significant role in the 2022 $1B+ summer blockbuster Top Gun: Maverick. Engines designed for hypersonic flight have revolutionary applications in national security as advanced hypersonic weapons, in space exploration as reusable stages for access to low Earth orbit, and in commercial aviation as fast, long-range methods for air transportation of passengers around the globe.

One major challenge for hypersonic flight is flameouts. A flameout is the loss of propulsion due to the extinction of the flame in the engine’s combustor, which can occur for many reasons including fuel starvation, excessive altitude, severe precipitation, or incredibly low ambient temperatures. Early jet engines were prone to flameout following disturbances of inlet airflow, or sudden thrust lever movements, resulting in incorrect air-fuel ratios in the combustion chamber. Modern engines are more robust, and are often digitally controlled, allowing for significantly more effective control of all engine parameters to prevent flameouts and even initiate an automatic restart if a flameout occurs.

However, in hypersonics, the relight challenges are increased due to the incredibly high speed of the air flowing through the combustor, flying at higher altitudes, and the reduced air resistance needed to achieve maximum airspeed.

The faster we go, the harder it is to ignite

According to NASA, once a flameout has occurred, it is critical to relight the engine as quickly as possible. However, the conditions that caused the flameout are the same conditions that will make a relight difficult. The first step is to descend to lower altitudes more favorable to relight. This can be a problem depending on the mission. To attempt relight, ignition kernels—pockets of high-energy flow and free radicals—are introduced into the combustion chamber by firing the engine ignitor. The kernel develops into a flame front, and eventually reaches flame stabilization or extinction, depending on the kernel’s initial state and the turbulent flow evolution. The strong turbulent flow, combined with the reliability of getting the ignition spark to discharge make the engine relight challenging. 

These relighting problems are further compounded in advanced engines like scramjets. A scramjet (supersonic combustion ramjet) is a variant of a ramjet airbreathing jet engine, in which combustion takes place in supersonic airflow. The airflow is compressed dynamically through an intake system that does not require rotating elements, and the fuel and oxidizer are burnt under supersonic velocity conditions in the combustor. However, at such high speeds, mixing and combustion processes cannot easily fit within the combustor length because the total residence time available for reactants to burn is typically a fraction of a millisecond with supersonic flow throughout. The fuel, which is injected in the combustor through a separate port, needs to be mixed at the molecular level with the oxygen present in the ingested airflow for combustion chemical reactions to occur. Sufficient residence time must therefore be allotted for large-scale turbulent structures in shear layers to grow and cascade into smaller eddies that trigger microscale mixing between reactants. Attenuation of this growth rate occurs at supersonic speeds due to compressibility effects that slow down the required mixing. Eventually, the fuel and oxidizer burn upon mixing in a chemical sequence. 

In short, the challenge of supersonic combustion in scramjets is as difficult as lighting a match in a cyclone.

Science Direct says reliable ignition is key to the efficient and safe operation of combustion systems. Observations of ignition kernels, which occur after ignition, but prior to the development of a freely propagating flame, can provide insight into the phenomena that determine the success of an ignition attempt. Nanosecond Pulsed Power ignition systems are shown to be more effective in increasing ignition probability and kernel growth rate with high energy efficiency as compared to traditional spark discharges. With this approach, energy is deposited only during the breakdown phase of the discharge, resulting in more efficient use of energy, along with different kinetic pathways for induction chemistry to proceed. 

Nanosecond Pulsed Power Ignition Systems

The nanosecond pulsed power ignition system is being developed by Torrance based Transient Plasma Systems (TPS), a spin off from the University of Southern California. TPS was formed in response to a critical need in the Department of Defense (DoD) for a reliable domestic supplier of custom, high-voltage, high-current, low-average power high-repetition rate pulsed power systems. TPS entered the commercial market in 2009 with products designed for laboratory applications and basic science research for customers in the Department of Defense and the aerospace industry. TPS has utilized venture capital to develop its ignition system technology in response to demand in commercial sectors.

As the name suggests, the Nanosecond Pulsed Power Ignition System produces fast rising (0-5 ns rise time) high voltage, short duration (10-15 ns) electric pulses as shown in Fig. 1 below. These nanosecond pulses can transfer energy very efficiently to a fuel air mixture producing a richly ionized air-fuel mixture.

© Transient Plasma Systems | https://www.transientplasmasystems.com/

As shown in Fig. 2, the pulses can be repeated at different frequencies (Pulse Repetition Rates (PRF)) ranging from a few Hz to 100s of kHz. Additionally, the pulses can be delivered in bursts.

The nanosecond pulsed power ignition system has three major pathways to impact the combustion.

(1) Enhanced Chemistry. This is the essence of nanosecond pulses. Fast-rising pulses are inherently more efficient in ionizing gases. Electrons energized by the nanosecond pulses collide with the gas producing, chemically reactive species which catalyze the combustion process, enhancing ignition and stabilizing combustion.

(2) Volumetric Impact. Electrode geometry can impact the volume of air fuel mixture that can be ignited. Spatial distribution of plasma can enable a single streamer discharge to impact a large volume. Fig. 3 shows an electrode designed around the surface of the inside of a combustor, allowing plasma generation in a large area simultaneously.

© Transient Plasma Systems |

(3) Software closed-loop control.

Sophisticated closed-loop control techniques enable further control of the quantity of energy delivered enabling a range of outcomes depending on the situation. For example, experiments conducted using nanosecond pulsed power ignition systems have demonstrated that probability of ignition can be significantly increased by using high pulse repetition rates. Similarly, by deploying a deliberate time interval between bursts of pulses, the growth in flame kernel can be enhanced. Fig. 4 below shows different ignition outcomes based on different ways of delivering the same amount of energy. The figure shows flame kernels created by using nanosecond pulsed power ignition and a combustible mixture. When the pulses are delivered extremely quickly vs at a slow rate, a strong ignition event is more likely. However, if the energy is delivered at the “Goldilocks” rate for the flow, strong ignition and a large flame kernel are possible, which would be best in a challenging environment for ignition.

© J.K. Leftkowitz, T. Ombrello| Combustion and Flame 180 (2017)
136-147 Schlieren images of ignition kernel development
after the initial discharge pulse for a single pulse and for
a 10-pulse burst for three different inter-pulse time conditions.
These conditions represent the fully coupled (τ = 3.4 × 10⁻⁶ s),
partially coupled (τ = 2 × 10⁻⁴ s), and decoupled (τ = 1 × 10⁻³ s)
regimes of inter-pulse coupling, which have distinct characteristics
with regard to ignition probability and flame kernel growth rate.

Nanosecond pulsed power ignition systems have been demonstrated in laboratory to ignite or enhance supersonic (Mach 3.0) air fuel mixtures. They are ideally suited for igniting supersonic air fuel mixtures because of the speed of the chemical reactions induced by the presence of plasma.

Not just Top Gun cool, its green too

Besides serving as a reliable ignition source in supersonic aerospace applications, nanosecond pulsed power ignition systems are also capable of igniting highly dilute air fuel mixtures. Using highly dilute air fuel mixtures in gasoline/natural gas engines improves fuel efficiency while reducing CO₂ and NOx emissions. Comparatively, conventional ignition systems struggle to ignite highly dilute air fuel mixtures.

The nanosecond pulsed power ignition system has attracted the attention of automotive companies looking to meet carbon emission regulations set by regulatory authorities, such as the EPA.

Technology & the expertise

Designing and manufacturing high-voltage, nanosecond pulsed power generators requires highly specialized know-how and skills that are not readily accessible. Generating high-voltage, fast-rising, short-duration pulses creates several challenges including EMI (electromagnetic interference) which can affect electronic devices in the immediate environment, impacting cost and efficiency. There is also a key component known as the drift step recovery diode (DSRD) which is a semiconductor junction diode with the ability to generate extremely short pulses. Designing and fabricating such DSRDs for noticeably short-rise times and getting them to be work reliably is a very esoteric capability that is available only with a few entities such as TPS.

Conclusion

Solving the challenges related to hypersonic flight has become more relevant than ever, most prevalent is the ignition and combustion of an engine moving through the air at more than five times the speed of sound. Nanosecond pulsed discharges offer a potential attractive solution, due to the simplicity of the system, the effectiveness of nanosecond discharges at producing plasma and the speed at which plasma chemistry impacts the combustion process.

Transient Plasma Systems