|September 10, 2020|
Is lightning-fast plasma the key to a cleaner car engine?
|THERE ARE SOMETHING like a billion cars on the world's roads today and almost all of them are powered by internal combustion. In fact, the 150-year-old technology is at the heart of most forms of transportation, whether it's a plane, train, or boat. The engine's importance to ... well, everything, means that generations of really smart people have dedicated their lives---and untold billions of dollars---to making it better. But no matter how close it comes to perfection, the internal combustion engine will always have one major flaw: It's killing our planet.|
Most combustion engines burn fossil fuels and in the process create greenhouse gases like carbon dioxide and nitrogen oxide. In the US, transportation accounts for nearly a third of greenhouse gas emissions, despite a number of policies designed to limit its environmental impact. The internal combustion engine is a fundamentally dirty technology, but there are plenty of ways to make it cleaner. And they start with a spark---or, more accurately, a spark plug.
David Howell is the director of the Department of Energy's Vehicle Technology Office, and he spends a lot of time thinking about how to build better engines. This year, around $70 million---nearly a quarter of his office's annual budget---will be spent on combustion and fuel R&D. "We see a lot of inroads being made by battery electric vehicles, but internal combustion engines are going to be around in some form for a long time," says Howell. "And there's still a long way we can go to increase efficiencies and reduce emissions."
In combustion engines, there's a deep link between efficiency and emissions. A more efficient engine uses less fuel to accomplish the same amount of work, and less fuel means lower emissions. There are several ways to tap into these efficiency gains. For years, the Vehicle Technology Office has been focused on swapping out conventional gasoline with more environmentally friendly biofuels.
"An internal combustion engine can utilize a wide range of fuels, and some of those can be partially renewable," says Howell. But it will take a while to dethrone gasoline at the pump. These new biofuels not only have to perform as well as gasoline, they also have to be cheap. And gasoline has a big head start. "Gasoline has been around for a century, and there's been a lot of optimization in terms of its combustion properties," says Howell. So until the DOE's fancy new fuels are ready for the public, other researchers are looking for ways to put regular old gasoline to better use in engines today.
A typical automotive engine combines air and gas in a combustion chamber and then ignites the mixture with a spark plug. This century-old technology is located in the combustion chamber and mounted near the top of the engine in the cylinder head. As the piston moves toward the top chamber, compressing the fuel-air mixture, the plug creates a fleeting electric spark. The spark kicks off a molecular mosh pit that generates heat and creates greenhouse gases that are expelled from the engine as exhaust.
One way to reduce emissions is to mix more air with the fuel during combustion, which is known as a "lean burn." The idea is simple---dilute the fuel-air mixture with more air---but making it work is not. Combustion engines work best at very specific ratios of fuel to air. Deviating from that ratio can quickly render an engine's catalytic converter---an aftertreatment system designed to convert harmful gases like nitrogen oxide into more benign substances---ineffective. At a certain point, there's too much air for the engine to ignite the fuel-air mixture at all.
"If you can run extremely lean, it may have some real benefits in terms of engine efficiency," says William Northrop, who runs the Engine Laboratory at the University of Minnesota. "Car manufacturers have been trying to go lean with their engines for a very long time. But at some point you reach the limit of flammability, which is what we call the lean limit."
An engine that can approach that lean limit and still achieve combustion is what Dimitris Assanis, an expert on advanced combustion at Stony Brook University, calls the "thermodynamic holy grail." The extra air in the mixture acts like a thermal sink and absorbs some of the energy released during combustion. This lowers the combustion temperature, which is critical to boosting an engine's efficiency and reducing its emissions. But there's a problem.
"You can't ignite those air-diluted mixtures with traditional spark plugs," says Dan Singleton, the CEO and cofounder of Transient Plasma Systems. "They transfer energy too slowly." The extra air in the chamber cools the heat from the spark before it can spread enough to start the combustion reaction. Since 2009, Singleton and his colleagues at Transient Plasma Systems have been developing an ignition system that would address this challenge for lean-burn engines. It works by condensing megawatts of power into nanosecond pulses of plasma created from ionizing the air around the plug's electrodes; that's the power of six semitrucks released hundreds of times faster than the speed of lightning.
Transient Plasma's ignition system consists of a power supply that looks a bit like an internet router. It is connected to a series of plasma plugs in each cylinder of the engine. The power supply banks energy from the car's battery and releases it through the plugs in an ultrafast burst of blue plasma. It's a low-energy, low-temperature version of more energetic pulsed power systems like rail guns and the lasers that physicists use to simulate nuclear blasts.
The main difference between the plasma plug and conventional spark plugs is that it doesn't ignite a combustion reaction by transferring heat. In fact, it doesn't have enough thermal energy to even light a match. Instead, it directly bombards the air molecules with electrons to break them into more reactive elements, like atomic oxygen. This rapid infusion of non-thermal energy causes the molecules to slam together in the fuel mixture, which kicks off the combustion reaction. If a conventional spark plug is like a lighter, Singleton's plasma plug is more like a lightning bolt. And when it comes to lean-burn engines, speed is paramount.
"The basic idea in an engine is, you want everything to burn all at the same time," says Jaal Ghandhi, who leads the Engine Research Center at the University of Wisconsin. "If you can get the fuel to burn right when the piston is at the top, dead center, you'd get the best possible efficiency. That combustion event is what's important in terms of efficiency."
The idea to use low-energy pulsed power systems to achieve rapid combustion isn't exactly new. Singleton's PhD adviser, Martin Gundersen, leads the pulsed power research group at the University of Southern California and has been working on these types of ignition systems since the early 1990s. Although these early pulsed power ignition systems worked for reducing emissions, they were expensive, bulky, and not super reliable. "The technology was cool, but it wasn't there yet," says Singleton.
By the time Singleton was wrapping up his PhD, the technologies required for an affordable and reliable system had matured to the point that it finally seemed possible to bring them out of the lab. The key breakthrough was in solid-state high-voltage switches that emerged in the early 2000s. Advancements in switching technology now allow Singleton's pulsed plasma system to switch megawatts of power in nanoseconds and last for hundreds of thousands of shots. So in 2009, he founded Transient Plasma Systems with Gundersen and his colleagues at USC, Andy Kuthi and Jason Sanders, to commercialize plasma ignition.
Initially, the company focused on developing a system for aviation. Jet engines are a major contributor to global greenhouse gas emissions, but it was clear that automobiles were the real killer app. If Singleton and his team could persuade car companies to use their spark plug, he felt, it could drastically reduce vehicular emissions.
"If you want to find out what engine researchers are looking at today, it's useful to look at what aerospace researchers were looking at yesterday," says Isaac Ekoto, a principal researcher at Sandia National Laboratories' Combustion Research Facility in California. "That's where a lot of this technology comes from."
Ekoto and his colleague Magnus Sjöberg lead some of the facility's gasoline combustion labs, and in 2014 they hosted Transient Plasma Systems for the company's first major tests of its plasma plug in an automotive engine. Sjöberg and Ekoto's labs are outfitted with a custom single-cylinder engine that is designed to allow researchers to peer inside the combustion chamber during ignition using high-speed cameras and lasers that measure the combustion dynamics.
The efficiency of a combustion engine is deeply dependent on how it's being used. For example, engines tend to be least efficient at low speeds and low power, a scenario that might be encountered when driving through a city. So Sjöberg and the Transient Plasma team tested the ignition system in the engine across a range of operating modes that loosely correlated with different scenarios, like driving on the highway. During the tests at Sandia, Transient Plasma System's plug demonstrated that it could offer up to a 20 percent improvement in fuel efficiency in these driving modes, relative to the performance of a commercial combustion engine with a conventional spark plug.
Singleton says the results from the Sandia tests soon attracted the attention of the auto industry. For the past few years, Transient Plasma Systems has been working with several undisclosed car manufacturers to test the system with the companies' engines. He says he is optimistic that the first cars to use the system could be on the road within the next five years.
But lean engines might not necessarily need a plasma ignition system to get started, and Transient Plasma Systems is hardly the only company working on the problem. "Everyone is trying to extend that ignition limit and get leaner mixtures to ignite," says Assanis, the combustion expert from Stony Brook. Researchers at Purdue University, for instance, are exploring engine architectures that use "pre-chambers" with a conventional air-to-fuel ratio to start the combustion reaction, which then bleeds into a main combustion chamber full of air-diluted fuel. And companies like Mazda are pursuing engines that don't need spark plugs at all. Instead, the engine compresses the lean fuel until it spontaneously combusts.
As far as the environment is concerned, exactly how lean-burn combustion is achieved is ultimately less important than when it's achieved at scale. Like it or not, combustion engines will be around for at least the next few decades. And if there's any hope of putting the brakes on climate change, we'll need an engine that makes fuel go farther.