The idea behind Dearman's project dated back to at least 1899, when a Danish inventor named Hans Knudsen claimed to have designed an automobile that could run on “clear, bluish” fuel—liquefied air, to be sold at a penny a gallon. Rather than spewing out a toxic mix of pollutants and greenhouse gases , it would leave a harmless trail of condensation in its wake, wafting by at the stately speed of 12 mph. Knudsen received admiring media coverage at the time, but his company went belly-up in a matter of years. Modern cynics suspect he was engaged in a Theranos-style fraud , in part because no one could figure out how he'd done it. For years, a working liquid-air engine seemed about as fanciful as a perpetual motion machine.
Still, the underlying principle was sound. Most engines rely on heat differentials. In the case of, say, a gasoline-powered car, the fuel is mixed with air, crammed into a piston chamber, and set alight, causing it to jump more than 1,000 degrees in temperature. The gas rapidly expands, propelling the piston and, in turn, the wheels. Take the same process, slide it way down the Fahrenheit scale, and you've got a liquid air engine. The nitrogen fuel starts out at 320 degrees below zero. When it enters the (much warmer) piston chamber, it boils off into gas. The change in temperature is smaller than with gasoline, so the pistons move with a little less oomph—but it's enough to get the wheels going. The real problem comes later: All that frigid fuel coursing through the engine quickly freezes it, effectively wiping out the heat differential. The air stops expanding, and the car runs out of puff.
The roadblock was clear, Dearman told me recently. He'd been pondering how to get around it since he was a teen. In a car that runs on heat, you need something to keep it cool—a radiator. In a car that runs on cold, you need the opposite. “I had an idea in my head for how to make it work, but I knew I wasn't going to get anywhere until I had some research to go on,” he said.The breakthrough came in 1999. Dearman was watching an episode of the BBC's dearly departed flagship science program, Tomorrow's World, in which the presenter visited the University of Washington to report on a rather clunky-looking converted mail truck. It had trouble with hills, and its top speed was 22 mph, but it ran on liquid nitrogen (a profligate 5 gallons per mile). Invented by Abe Hertzberg, an eccentric professor who had previously come up with a laser-powered airplane, the truck boasted one major innovation. Before the freezing-cold fuel reached the engine, it ran through a heat exchanger, a series of concentric tubes that circulated outside air around the fuel line. John Williams, who worked on the truck as a graduate student, explained that the exchanger ensured “the whole thing didn't turn into a giant ball of ice.” But it didn't tackle the fundamental problem—that the liquid nitrogen still rapidly cooled the engine, throttling its own expansion into a gas. “Our project was a proof of concept,” Williams explained. “We were reconciled to a certain degree of terribleness.”
Methane emissions burped out by cows could be cut by 99 percent if farmers changed 2 percent of the ruminants' diet to seaweed.From his sofa in the historic market town of Bishop's Stortford, Dearman immediately saw both the logic of Hertzberg's design and a way of improving on it. The answer to making sure the nitrogen continued expanding? Antifreeze. “It's obvious, but it's only obvious once you've seen it,” Dearman said. He went out into his garage, grabbed a blue plastic jug from the shelf, and started playing around with his lawn mower, hacking its engine to squirt a mixture of antifreeze and water into the piston chambers on each stroke. This brought ambient heat directly to the place it was needed most—and the engine's efficiency skyrocketed. The same trick worked on the battered Nova, bought as a guinea pig.