Total MX-5 - - INSIGHT -

Skyactiv-x uses di­rect in­jec­tion. It’s an im­por­tant part of the close­ly­con­trolled com­bus­tion needed for to­day’s, and the fu­ture’s, ul­tr­a­clean petrol en­gines. But there have been rum­blings that DI en­gines are stor­ing up trou­ble as their in­let valves coke up, per­for­mance drops off and emis­sions go hay­wire.

US au­to­mo­tive web­sites are abuzz with the prob­lem, which oc­curs be­cause, with no fuel in the in­take airstream as it passes the in­let valves, there’s noth­ing to wash off the car­bon that tries to ac­cu­mu­late on the backs of the valve heads.

That car­bon can come from oil seep­ing down worn valve guides and car­bon­is­ing as it con­tacts the hot valve head, or from a back­wash of ex­haust de­posits dur­ing the phase – ‘over­lap’ – when both ex­haust and in­let valves are open. Over­lap uses the suc­tion gen­er­ated by the de­par­ture of hot ex­haust gases to help draw in the in­take charge even be­fore the pis­ton’s down­ward move­ment on the in­duc­tion stroke has started, and is a vi­tal part of tai­lor­ing an en­gine’s power and torque curves.

Man­u­fac­tur­ers of af­fected en­gines have been al­ter­ing the valve-tim­ing map (of en­gines with vari­able valve tim­ing) in an at­tempt to re­duce the prob­lem, but this in­evitably af­fects the en­gine’s power de­liv­ery. We haven’t had re­ports of the prob­lem in the only DI MX-5S, the cur­rent ND gen­er­a­tion, but maybe the cars are so far too new to be af­fected. Or there could be a sim­pler ex­pla­na­tion: Euro­pean fu­els, it seems, are more highly re­fined than US fu­els, so there’s less claggy car­bon in the ex­haust back­wash. If your ND de­vel­ops a coke habit, please let us know.

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