TECHNOLOGUE Putting a chill on diesel con­sump­tion with liquid ni­tro­gen

Motor Trend (USA) - - Contents - Frank Markus TECHNOLOGUE

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—such as 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 mashup 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. This 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, fuel is in­jected, and it 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. 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 high­tem­per­a­ture (1,100 de­grees F) hea­tex­changer 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 in com­par­i­son to es­tab­lish­ing a na­tion­wide hy­dro­gen or Bev-charg­ing in­fra­struc­ture. n

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