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Motor Equipment News - - CONTENTS - By John Ox­ley. READER RE­PLY 0141214

Lexus is renowned for its in­sis­tence on high qual­ity, par­tic­u­larly in terms of build ex­cel­lence.

From the first the mar­que was es­tab­lished, as a lux­ury arm of Toy­ota, it has also con­cen­trated on pur­su­ing that ex­cel­lence in its en­gines, and sec­ond-hand Lexus V8s are much sought-after by mod­i­fiers who want great per­for­mance cou­pled with de­cent fuel econ­omy.

How­ever, un­like other en­gine mak­ers, Lexus is not known for its in­no­va­tive so­lu­tions, in­stead con­cen­trat­ing on build­ing to en­sure longer life, al­beit at the ex­pense of the out-and-out per­for­mance of its Ger­man pre­mium com­peti­tors.

Tur­bocharg­ers have been ig­nored, and spe­cific out­puts have not been par­tic­u­larly high – with the ex­cep­tion of the limited edi­tion LF-A su­per­car Lexus de­vel­oped in con­junc­tion with Yamaha.

So it comes as a big sur­prise to learn that Lexus is turn­ing all that on its head as it changes its tack, and starts a process which will see it go­ing head-to­head against Audi, BMW, MercedesBenz and Volvo in the pre­mium medium SUV mar­ket – and ul­ti­mately other seg­ments – with the main weapon in its ar­moury a two-litre tur­bocharged en­gine that not only uses lat­est turbo tech­nol­ogy, but also rad­i­cally com­bines both Atkin­son cy­cle and Otto cy­cle in one en­gine.

The new AR-FTS mo­tor will make its de­but in New Zealand in Fe­bru­ary next year in a petrol-only new model se­ries on top of the re­cently-launched Lexus NX mid-size SUV range, swap­ping fru­gal Prius-like hy­brid power for sub seven seconds 0-100km/h ac­cel­er­a­tion and huge tur­bod­iesel-like torque of 350Nm on a mas­sive plateau from 1,800rpm right through to 4,200rpm, with the max­i­mum power of 175kW pro­duced be­tween 4,800rpm and 5,600rpm.

The en­gine, pur­sued at the dogged­ness of NX project chief en­gi­neer Takeagi Kato, who con­vinced his bosses Lexus needed it for the NX and other mod­els in the Lexus lineup, has a ca­pac­ity of 1,998cc, and is per­fectly “square” for ex­cel­lent revvabil­ity, with bore and stroke of 86mm.

Sur­pris­ingly, there­fore, for a turbo en­gine, it runs with a mas­sive ten to one com­pres­sion ra­tio, and fea­tures a Lexus­de­vel­oped twin-scroll tur­bocharger with an ac­tive waste-gate valve, de­signed to give quick throt­tle re­sponse with no lag.

The main so­lu­tion to re­duc­ing turbo lag was to cast the four-into-two ex­haust man­i­fold in­te­grally with the cylin­der head, short­en­ing the gas flow to the turbo to a min­i­mum, and to have a liq­uid-cooled in­ter­cooler plus a vari­able waste gate for greater ef­fi­ciency.

Part of the abil­ity to be­ing able to uses such high com­pres­sion PLUS a pres­sure-boost­ing turbo, with­out blow­ing the en­gine to pieces, is in the ca­pa­bil­ity of the en­gine to switch over from Atkin­son cy­cle to Otto cy­cle, seam­lessly, and with­out any dis­cernible change in the en­gine note.

This elim­i­nates pump­ing losses as­so­ci­ated with purg­ing the cylin­ders of ex­haust gases and suck­ing in a new charge, while at the same time equal­is­ing in­ter­nal and ex­ter­nal pres­sures.

The Atkin­son cy­cle is used at light loads; Otto at heav­ier loads.

The orig­i­nal Atkin­son cy­cle en­gine was de­signed in the late 19th Cen­tury, and the first en­gine built in 1882, and its pur­pose was to avoid in­fring­ing patents on the Otto cy­cle en­gine.

Where it dif­fered from the Otto was in al­low­ing the in­take, com­pres­sion, power, and ex­haust strokes of the fourstroke cy­cle to oc­cur in a sin­gle turn of the crank­shaft; how­ever, it suf­fered from a lack of torque, and was quickly over­run by the Ger­man de­sign.

To­day’s so-called “Atkin­son cy­cle” en­gines don’t in fact follow the orig­i­nal de­sign, but are used to de­scribe a mod­i­fied Otto cy­cle petrol en­gine.

Ac­cord­ing to Wikipedia the mod­ern Atkin­son cy­cle de­scribes an en­gine in which the in­take valve is held open longer than nor­mal to al­low a re­verse flow of in­take air into the in­take man­i­fold.

The ef­fec­tive com­pres­sion ra­tio is re­duced (for a time the air is es­cap­ing the cylin­der freely rather than be­ing com­pressed), but the ex­pan­sion ra­tio is un­changed. This means the com­pres­sion ra­tio is smaller than the ex­pan­sion ra­tio.

Heat gained from burn­ing fuel in­creases the pres­sure, thereby forc­ing the pis­ton to move, ex­pand­ing the air vol­ume beyond the vol­ume when com­pres­sion be­gan. The goal of the mod­ern Atkin­son cy­cle is to al­low the pres­sure in the com­bus­tion cham­ber at the end of the power stroke to be equal to at­mo­spheric pres­sure, thus elim­i­nat­ing pump­ing losses, and all the avail­able en­ergy to be ob­tained from the com­bus­tion process.

For any given por­tion of air, the greater ex­pan­sion ra­tio al­lows more en­ergy to be con­verted from heat to use­ful me­chan­i­cal en­ergy mean­ing the en­gine is more ef­fi­cient.

The dis­ad­van­tage of the four-stroke Atkin­son cy­cle en­gine ver­sus the more common Otto cy­cle en­gine is re­duced power den­sity.

Due to a smaller por­tion of the com­pres­sion stroke be­ing de­voted to com­press­ing the in­take air, an Atkin­son cy­cle en­gine does not take in as much air as would a sim­i­larly de­signed and sized Otto cy­cle en­gine.

Lexus uses what it calls VVT-iW tech­nol­ogy to achieve the switch from Atkin­son to Otto, al­low­ing the tim­ing to be ad­vanced or re­tarded as re­quired, and moved from one cy­cle to the other as needed.

The new en­gine cer­tainly has the power and torque to chal­lenge some of the more-es­tab­lished play­ers in the mar­ket­place, and, with com­bined petrol con­sump­tion of 7.9L/100km on the NX 200t, should sat­isfy those who want their power and econ­omy too.

Although the NX is the first car in the Lexus range to get it, I’ve been given the nod that this rad­i­cal new en­gine will find its way into the CT range – cur­rently hy­brid only – as well as the big­ger IS.

Can’t wait to drive ‘em!

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