Been thinking too about whether you'd be likely to suffer inlet valve damage if the timing belt were to break - or skip a number of teeth - while the engine is running. Although I have never worked on a Multiair - or Twinair - I find the concept very interesting and have read a number of technical articles and other "stuff" about them. In simple terms I think I understand that the camshaft lobe drives a pump plunger which pressurizes engine oil. The valve has a plunger sitting on top of it and the two are connected by an oilway. If there was nothing else involved then, as the camshaft lobe depressed the pumping plunger, oil would be pumped to the plunger over the valve which would cause it to depress - and thus open - the valve. The "clever" bit is that there is a solenoid controlled relief valve which can allow oil to bleed from this system. So, for instance, if the solenoid valve is closed then the valve will behave just as a normal valve will - which is to say it will start to open as the lobe depresses the plunger and close again when the lobe clears the plunger. On the other hand if the solenoid is activated and kept open the cam will still depress it's plunger but the oil will be ejected through the "spill" and can't develop any pressure so the plunger over the valve can't act on the valve so it will stay closed. The clever bit comes in how the electronics modulate the opening and closing of the solenoid valve. In effect giving the ability to provide almost infinitely - between limits - valve timing. Very clever.
However, we are interested in whether some, or at least one, of the valves could remain open if the belt breaks while the engine is running. Well, if any particular valve was in the part of it's cycle where the solenoid was closed and the cam plunger had pressurized the oil so opening the valve then if the cam stops rotating at that moment the valve must stay open? I presume there will be a certain amount of leak back in the system so the valve, if left like this for long enough, will slowly find it's way back onto it's seat but this will be far too slow to avoid the fast moving piston from contacting it. The repercussions of the valve hammering back on the oil in the multiair unit - which seems to be well known to be a bit "fragile"? - would also be most undesirable too I think. So, all things considered, much better to spend a bit of money on timeously renewing the belt than risk a lot of money if the belt breaks?
