Putting a Chill on Diesel Consumption with Liquid Nitrogen – Technologue

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Automotive futurists and savvy engineers expect battery EVs to eventually handle all our light-duty personal transportation and most of our urban/short-distance hauling needs. But they also disagree with Elon Musk that BEVs can swing long-distance heavy hauling. Faster refueling is needed, so some—like tech startup/disruptor Nikola One—are proposing fuel-cell big rigs. I remain obstinately bearish on this idea.

In the nearer-term, however, there’s the decade of work by engineering consultancy Ricardo that might be close to bearing fruit. Ricardo is developing a novel concept called CryoPower, which promises a quantum improvement in diesel-engine thermal efficiency to 60 percent—up from the typical mid-40s. (Yes, diesel, that technology you thought was dead. Not so fast.) In this case, the nomenclature refers to the engine’s use of liquid nitrogen, though obviously that is not the fuel. Rather, it serves as an amazing charge-air cooler that’s enabling an inventive mash-up of the Diesel, Miller, and Ericsson thermodynamic cycles.

Everyone knows Rudy Diesel as the compression-ignition guy. Ralph Miller’s cycle shortens the effective compression stroke relative to the expansion stroke with a blower making up the charge-air difference (see: Mazda Millenia). John Ericsson’s idea is to transfer exhaust heat to the intake charge to improve efficiency—which has not been an option for internal combustion engines because there’s typically not enough temperature difference between the exhaust and the intake air after it’s been squished in a hot, high-compression cylinder. (Remember, we want the air as cool and dense as possible before we compress it, but any heat and pressure added afterward boosts output.)

Ricardo is borrowing a page from the Scuderi Split-Cycle engine by using separate cylinders for intake/compression and combustion/exhaust, with the intake ones sweeping less volume than the combustion ones. (Thanks, Miller!) A squirt of liquid nitrogen during the compression stroke keeps the temperature roughly the same at the top of the stroke as it was at the bottom. This reduces the work required of the crankshaft, reclaiming some of the energy spent on liquefying the nitrogen.

En route to the combustion cylinders, this cool pressurized air grabs heat from the exhaust, Ericsson-style, increasing in pressure, as well. Then it enters the combustion cylinders, and fuel is injected and ignites almost instantly. Exhaust then flows out through both the heat exchanger and a turbocharger, with the compressed intake charge passing through an intercooler before entering the Cryo-compressor cylinders. Obviously, less heat escapes through the exhaust, and by maintaining a much higher coolant temperature around the combustion chambers, less waste heat escapes that way, too, yielding that jumbo efficiency percentage.

Temperature and pressure contributed by the exhaust mean less must come from the fuel, and with the injection/combustion occurring after the piston starts heading down, peak temperature and pressure in the cylinder is considerably lower than in a typical turbodiesel—which lowers NOx production. The surplus nitrogen behaves like exhaust-gas recirculation to further reduce engine-out NOx, so the overall aftertreatment needs should be no greater than in today’s diesels.

About the liquid nitrogen: It’s currently being produced by separating it from the oxygen in air, but because industrial demand for oxygen is greater than for nitrogen and because air is 78 percent nitrogen, we can consider the nitrogen separation “free.” A Ricardo study in the U.K. suggested the price of liquefying and distributing it would be 5 pence per liter ($0.26/gallon), and its usage rate is expected to be around the same order of magnitude as the diesel fuel, so trucks will carry a bit more nitrogen than diesel on board and refuel both simultaneously.

Ricardo’s initial proof-of-concept engine is an I-6 of largely conventional design with two compression and four combustion cylinders. The Inconel high-pressure (1,015 psi) and high-temperature (1,100 degrees F) heat-exchanger plumbing will add cost, as will the nitrogen storage and injection gear. But at a claimed 20 percent savings in overall fuel costs, we’re assured the payback period will be reasonable. And adding liquid nitrogen dispensing to our existing truck-stop infrastructure seems like child’s play by comparison with establishing a nationwide hydrogen infrastructure.



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