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.