DIRECT INJECTION: A TICKING TIME BOMB?
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.