Tech Torque

4 x 4 Australia - - Contents - FRASER STRONACH

IT’S FUNNY how old is new again. Such is the case with the straight­six (in­line) en­gine, de­signs of which are now back on the draw­ing boards af­ter years of be­ing in the wastepa­per bas­ket.

His­tor­i­cally, the straight-six en­gine was a main­stream de­sign, of­fer­ing both sim­plic­ity and low pro­duc­tion costs. How­ever, it fell from favour as it took up too much space when mounted along a chas­sis in a tra­di­tional north­south ap­pli­ca­tion.

For mod­ern east-west ap­pli­ca­tions, which is stan­dard prac­tice to­day in most cars and SUVS, a straight-six is even more dif­fi­cult to pack­age and has never been pop­u­lar.

The straight-six was ef­fec­tively killed off by the far more com­pact V6, which fits nicely side­ways in an east­west chas­sis and also works well north­south, where a shorter en­gine bay makes for more pas­sen­ger and lug­gage space on any given chas­sis length.

The V6 is ev­ery­where now, both in petrol and diesel, and is the de­fault en­gine con­fig­u­ra­tion in medium/large cars and SUVS. The only man­u­fac­turer of note to stick with the in­line-six and never ven­ture down the V6 route has been BMW, but that’s about to change.

Land Rover, along with part­ner Jaguar, has an­nounced the re­turn of both petrol and diesel straight-sixes de­signed to power a range of mod­els. The an­nounce­ment comes on the back of Mercedes-benz re­veal­ing it’s well on the way to de­vel­op­ing a straight-six diesel.

The JLR and Benz sixes are 3.0-litre in ca­pac­ity and both form part of re­spec­tive mod­u­lar de­signs that in­clude 2.0-litre fours and 1.5-litre triples.

The driv­ing force be­hind all this is cost. The en­gi­neer’s job is to make it all work, es­pe­cially in terms of pack­ag­ing a straight-six where a V6 has been the norm.

The cost ben­e­fit of a straight-six over a V6 comes from less com­plex­ity and lower man­u­fac­tur­ing costs. For each straight-six en­gine there is only one cylin­der head and ei­ther one or two camshafts to cast and ma­chine. There’s also only one cam-drive sys­tem – which th­ese days tends to be com­plex due to the wide­spread adop­tion of vari­able valve tim­ing – to man­u­fac­ture and as­sem­ble.

All that is dou­bled for a V6: two cylin­der heads, two or four cams, and two cam-drive sys­tems. A lot more cost.

From an en­gi­neer­ing point of view, the straight-six is also lighter than a V6, which brings on fur­ther ben­e­fits with per­for­mance, fuel econ­omy and han­dling – no mat­ter how in­cre­men­tal.

Ever bet­ter news comes in the form of the per­fect dy­namic bal­ance and there­fore smooth run­ning of a straight­six. In fact, the straight-six, along with the flat-six (Porsche and Subaru) and a V12 of any V-an­gle, are the only pop­u­lar en­gine de­signs that are to­tally free from un­bal­anced forces, ei­ther lin­ear or twist­ing, that cause some sort of vi­bra­tion or other dis­tur­bance.

Even the pop­u­lar V8 in its usual ‘cross-plane’ crank con­fig­u­ra­tion isn’t as smooth on pa­per as a straight-six, as there is a slight unchecked twist­ing force or rock­ing cou­ple that oc­curs at crankshaft speed. Flat-plane crank V8s, as used in rac­ing, fare even worse in terms of bal­ance, as they just repli­cate the bal­ance prob­lems of an in­line-four (see break­out).

Even worse than a cross-plane V8 is the pop­u­lar V6, even in its op­ti­mised 60-de­gree V-an­gle. The 60-de­gree V6 has two unchecked rock­ing cou­ples, one at crankshaft speed and an­other at twice crankshaft. A 90-de­gree V6, a cheap way of mak­ing a V6 by lop­ping two cylin­ders off a 90-de­gree V8, is worse again in term of bal­ance, and it gen­er­ally needs a counter-ro­tat­ing bal­ance shaft to calm things down.

If the V6 goes the way of the di­nosaur, I won’t be un­happy. With a cou­ple of ex­cep­tions, I’ve never liked them.

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