BLACKBOURN

POPPET-HEAD STUFF

Unique Cars - - CONTENTS - ROB BLACKBOURN

ONE OF THE mates re­cently cir­cu­lated a link to video-footage of a re­stored B29 bomber called ‘Fifi’ tak­ing to the air. If you’re into se­ri­ous petrol-pow­ered ma­chines it was fab­u­lous stuff. Sur­pris­ingly for a bloke wed­ded to en­gines (Fifi’s four Wright 18-cylin­der ra­di­als were su­per im­pres­sive), my take­away info-nugget was that this air­craft had some com­puter-con­trolled sys­tems. Way back in WWII!

No sur­prise though, that like all pe­riod elec­tronic-equip­ment the com­put­ers re­lied on vac­uum-tube valves. Re­mem­ber valves? In­vented in the ear­ly1900s they were re­placed pro­gres­sively by solid-state com­po­nents from the mid1960s. While pon­der­ing the elec­tronic valve’s demise it oc­curred to me that an­other valve with an­cient ori­gins has hung in there con­vinc­ingly – the con­ven­tional en­ginevalve (AKA the poppet valve). Its sur­vival was achieved de­spite French au­to­mo­tive en­gi­neer Charles Faroux’s con­fi­dent as­ser­tion in 1910 that ‘our good old en­gine with…poppet valves is on its death bed.’

They didn’t have it all their own way though. Sleeve valves that had pro­vided solid com­pe­ti­tion in the early stages of in­ter­nal-com­bus­tion en­gine de­vel­op­ment took un­til the 1930s to fi­nally give up the ghost. Had two-stroke en­gines, or sub­se­quently Wankel ro­taries en­joyed wider suc­cess, it might have been a dif­fer­ent story al­to­gether for four-stroke, poppet-valve en­gines.

While con­ven­tional valves are still on the job, the re­sem­blance be­tween to­day’s valves and their early an­ces­tors is su­per­fi­cial, lim­ited to the ba­sic shape. The prim­i­tive­ness of early en­gi­neer­ing and met­al­lurg y left valves, par­tic­u­larly the ex­haust valves, vul­ner­a­ble to the me­chan­i­cal stress of high-fre­quency im­pacts with valve seats and the ther­mal stress from hot ex­haust gas. I can re­call read­ing in the yel­low­ing pages of an early Mor­ris Cow­ley owner’s man­ual that en­gines should be de-coked and valves freshly lapped-in at 1000mile (1600km) in­ter­vals. Pic­ture the intrepid in­ter­state trav­eller pulled over be­side the Pa­cific High­way, the bon­net of his Mor­ris raised, fer­ret­ing around in the boot for the tin of lap­ping paste…

Since then we’ve ob­vi­ously seen valve-de­sign evolve dra­mat­i­cally in­clud­ing the use of im­proved al­loys like stain­less steel, in­conel and ti­ta­nium. Then there’s the sex y stuff like ex­haust valves fea­tur­ing sodium-filled hol­low stems to aid heat dis­per­sion. As well we’ve seen multi-valve set-ups of smaller lighter valves, that give im­proved com­bus­tion and more con­trol­lable high-rpm op­er­a­tion, largely dis­plac­ing tra­di­tional two-valve de­signs. As a re­sult the trusty poppet valve is still clat­ter­ing away hap­pily in a com­bus­tion-cham­ber near you.

Clearly much has also changed in valve ac­tu­a­tion sys­tems. While we still need to drive the camshaft at half crank­shaft speed, the drive method has of­ten swapped be­tween gears and chains over the decades, more re­cently favour­ing toothed tim­ing-belts. Camshaft lo­ca­tion and twin- ver­sus sin­gle-camshafts have been a move­able feast as well. The game-changer though has been the in­tro­duc­tion of vari­able valve-tim­ing (V VT). Re­mem­ber when your new race cam gave you heaps of top-end boost but made your mo­tor idle like a splut­ter­ing pig? Not now – it’s the best of both worlds with V VT.

Now comes amaz­ing news from Tech-head web­site New

At­las (search ‘dig­i­tal valves’). A Bri­tish out­fit, Cam­con Au­to­mo­tive, is de­vel­op­ing a sys­tem that op­er­ates valves in­di­vid­u­ally us­ing dig­i­tally con­trolled elec­tric ac­tu­a­tors. In­di­vid­u­ally vari­able tim­ing, lift, du­ra­tion, or even whether a par­tic­u­lar valve op­er­ates at all on a given stroke, is part of its gob­s­mack­ing schtick. Start­ing can be made eas­ier by de­com­press­ing some cylin­ders. A brief per­for­mance boost could come from switch­ing to two-stroke mode. High­way cruis­ing could use a su­pere­co­nom­i­cal 12-stroke mode, with in­di­vid­ual cylin­ders tak­ing rest breaks in turn.

With­out want­ing to put the mock­ers on Cam­con’s project, play­ing around with a valve’s be­hav­iour means you need to know pre­cisely where its pis­ton is at each given mo­ment. So the sys­tem re­lies cru­cially on the per­for­mance of its crank-an­gle sen­sor. Whoa! Now they’ve not been ex­actly 100 per cent re­li­able, have they?

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