MAZDA SKY­AC­TIV-X

A DIESEL WITH GASO­LINE

Ultimate Diesel Builder's Guide - - UDBG News -

Diesel, named af­ter Ru­dolph Diesel, is a Com­pres­sion Ig­ni­tion (CI) in­ter­nal com­bus­tion con­cept, not an en­gine. Thus a fuel is not the process it­self, so any CI en­gine can say it is a diesel.

A long sought-af­ter CI process is called the Ho­moge­nous Charge Com­pres­sion Ig­ni­tion (HCCI) process. It uses scant amounts of fuel, so is eco­nom­i­cal. How­ever, the con­di­tions that must oc­cur for HCCI oc­cur are dif­fi­cult to pro­duce, dif­fi­cult to sus­tain, and have only oc­curred in a very limited range of RPM. Sev­eral years ago we drove a test ve­hi­cle de­vel­oped by Volk­swa­gen in Va­len­cia, Spain. It worked, but ran as an HCCI en­gine only in a very nar­row (200-300 rpm) range. We know that GM has, and is, work­ing on HCCI in its Michi­gan en­gine de­vel­op­ment units.

This year Mazda joined other am­bi­tious com­pa­nies by an­nounc­ing its own ver­sion of the HCCI en­gine. Mazda alone have said they will de­liver an in­ter­nal com­bus­tion en­gine that uses the com­pres­sion ig­ni­tion cy­cle (ver­sus spark ig­ni­tion) with gaso­line fuel for 2019, the SKY­AC­TIV-X Spark Con­trolled Com­pres­sion Ig­ni­tion en­gine.

Mazda says their new Spark Con­trolled Com­pres­sion Ig­ni­tion, or SPCCI, com­bus­tion method over­comes the prob­lems oth­ers have had with HCCI and its lim­i­ta­tions in RPM range and other dif­fi­cul­ties. The com­pany says they can seam­lessly tran­si­tion be­tween Com­pres­sion Ig­ni­tion and Spark Ig­ni­tion, which is new. When com­pared to SI gas en­gines or CI diesels, HCCI en­gines burn ex­tremely lean. The prob­lem for en­gi­neers is when and how to ad­mit fuel so it will burn, as it is too lean to be ig­nited by a spark, and how to sus­tain com­bus­tion as the air-fuel mix is half, or less, that of nor­mal. In fact it is far out­side the sto­i­chio­met­ric air-fuel mix (or Lambda 1) of 14.7:1, as high as 38:1.

In an HCCI en­gine the com­bus­tion takes place af­ter top dead cen­ter, af­ter spon­ta­neous com­bus­tion ig­ni­tion. Prob­lems in­cluded a very limited HCCI RPM range and dif­fi­culty pro­duc­ing a sta­ble tran­si­tion be­tween CI and SI if a spark plug is in­cluded to fa­cil­i­tate ig­ni­tion. (Spark is still needed un­der ul­tra-low tem­per­a­tures). Mazda says it has con­quered these prob­lems.

Mazda says their SKY­AC­TIV-X mo­tor de­liv­ers lower fuel con­sump­tion than their own 1.5L gas en­gine while pro­vid­ing bet­ter driv­ing per­for­mance than their 2.5L en­gine. CO2 emis­sions were sim­i­lar to their clean 2.0L SKY­AC­TIV-D diesel (which we ex­pect to see and test later this year).

Per­haps crit­i­cal to their break­through, Mazda says it uses fuel-air mix ig­nited by the spark plug to ex­pand (what might be a lo­cally rich mix) to serve as a se­condary “air pis­ton.” This, ac­cord­ing to the com­pany, fur­ther com­presses the (lean) fuel-air mix­ture in the com­bus­tion cham­ber to cre­ate con­di­tions where com­pres­sion ig­ni­tion can oc­cur. Fur­ther, Mazda says their con­trol over ig­ni­tion tim­ing ex­pands the RPM range of HCCI to ap­prox­i­mately 6,000 RPM sim­i­lar to gas en­gines.

Are you ex­cited to see a gaso­line-fu­eled diesel? We surely are, and we look for­ward to get­ting more in­for­ma­tion from Mazda rel­a­tively soon. A Zoom-zoom diesel sure would be nice, whether it burns gas or diesel fuel! UDBG

Sin­gle cylin­der test en­gines with em­bed­ded glass view ports show con­ven­tional SI com­bus­tion, then what a su­per-lean mix looks like in the same test cylin­der. Note the third test with a fuel-air mix of 36.8:1 dis­plays ex­cel­lent com­bus­tion.

Mazda’s break­through is us­ing a spark plug “as a con­trol fac­tor to con­trol the com­pres­sion ig­ni­tion,” ac­cord­ing to Kiyoshi Fu­ji­wara, di­rec­tor with over­sight on R&D. Con­cep­tu­ally, the spher­i­cal ex­pand­ing flame front serves as a se­condary com­pres­sion source for the fuel-air mix­ture, fa­cil­i­tat­ing com­pres­sion ig­ni­tion. Mazda calls it an “air pis­ton.”

High­lighted is the “air sup­ply de­vice” for SKY­AC­TIV-X. We ex­pect the mo­tor to be based on ex­ist­ing Mazda blocks, per­haps their 2.0-liter gaso­line en­gine.

Ac­cord­ing to Mazda’s pre­lim­i­nary fig­ures, ac­cel­er­a­tion from the SKY­AC­TIV-X en­gine out­paces their SKY­AC­TIV-D en­gine, avail­able in Europe and soon North Amer­ica.

Mazda em­pha­sizes driv­ing and its plea­sures, which we ap­plaud. This graphic ex­poses both fuel econ­omy and torque. More yel­low is bet­ter.

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