GASO­LINE-FU­ELLED WIZ­ARDRY

NZ Performance Car - - Power Plant -

With the world’s au­tomak­ers and main­stream me­dia’s hard-on for uber-ef­fi­cient elec­tric ve­hi­cles (EVs) and au­ton­o­mous driv­ing these days, one could be for­given for think­ing the fu­ture of driv­ing en­joy­ment looks bleak. But don’t de­spair; not all au­tomak­ers are turn­ing their backs on the combustion en­gine and driver en­joy­ment — well, not any­time in the next 40 years, if we can believe ev­ery­thing we’re told in press re­leases from Mazda. In­stead of go­ing all volts and amps, the Japanese au­tomaker is fo­cus­ing on pushing the ef­fi­ciency of four-cylin­der in­ter­nal-combustion en­gines to new heights with­out suck­ing all the fun out of driv­ing, and, bet­ter still, its new mo­tor will be of­fered in true man­ual form. The Sky­ac­tiv four-cylin­ders are noth­ing new, hav­ing been avail­able from Mazda since 2011, but, set for re­lease in 2019 is the most advanced ver­sion so far — the Sky­ac­tiv-X. The car­maker’s aim is to to bat­tle the on­slaught of EVs with su­per-ef­fi­cient four-cylin­der tech­nol­ogy that re­tains some throt­tle-in­duced fun. It’s a first for petrol en­gines: a 16.0:1-com­pres­sion two-litre four-cylin­der of­fer­ing up some in­ter­est­ing ad­vance­ments that are truly game-chang­ing and will push the hum­ble combustion en­gine well out­side its cur­rent com­fort zone of a 14.5 air–fuel ra­tio (AFR), well into the lean zone of AFRs in the 30s (yes, the 30s) dur­ing light throt­tle ap­pli­ca­tions.

This is pos­si­ble due to the ho­mo­ge­neous-charge com­pres­sion ig­ni­tion sys­tem, which uses com­pres­sion to com­bust the mix rather than spark, just like a diesel. It’s some­thing that had never been used in a gaso­line en­gine suc­cess­fully un­til Mazda de­vel­oped its spark-con­trolled com­pres­sion ig­ni­tion, which is a hy­brid of com­pres­sion and spark ig­ni­tion that uses a small su­per­charger to con­trol the com­pres­sion ra­tio and AFR. Sim­ply re­ly­ing on the com­pres­sion to com­bust the mix is too un­con­trol­lable with petrol, as it en­ters the combustion cham­ber mixed with the air and will pre-ig­nite. Un­like a diesel, which con­trols combustion by adding the fuel di­rectly into the cham­ber at pre­cisely the right time, which is why Mazda’s sys­tem uses a com­bi­na­tion. At low rpm — that is, light throt­tle — a tiny spark is used to kick off combustion, while com­pres­sion fin­ishes the job. This al­lows a lot more air to be present in the cham­ber. At high rpm, the mix­ture is richer and switches to con­ven­tional spark ig­ni­tion. The com­bi­na­tion is said to be 30 per cent more ef­fi­cient than Mazda’s al­ready-ef­fi­cient 2013 model.

There is also some very trick, tuned-length, four-into-two-into-one, long-run­ner head­ers, which, as in any race en­gine, play the key role of scav­eng­ing ex­haust gases. Header length also helps with the torque curve, or, more im­por­tant, flat­ten­ing it out so that the fun lasts all the way from 2000 to 7000rpm. Power fig­ures on the pre-pro­duc­tion ma­chines are es­ti­mated at equal parts power and torque, with both in the 170 range. With the Sky­ac­tiv-X due out in the Mazda line-up next year, we won’t have to wait long to see how the Mazda 3s and 6s per­form in real life. Did some­one say en­durance race car?

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