Wasted spark

[paulg23]

Ok. We have 3 coils. So 2 cylinders share a coil and spark at the same time. One cylinder gets the spark when it needs it, the other doesn't need it but is on it's exhaust stroke so it doesn't matter (the wasted one). What exactly is the timing of this wasted one? Where is the piston and is the inlet valve about to open? Having a bit of a 'disagreement' on an LPG forum, someone there thinks the inlet valve is already open when the wasted spark occurrs. Doesn't seem right to me.

[Turbo_Steve]

At it's most basic:

Piston A: Induction......Compression......Ignition........Ex haust.
Piston B: Ignition.........Exhaust..............Induction... ..Compression.

So, broadly speaking, it does. However, the wasted spark comes at the very very beginning of the induction cycle, so the bore should mainly still be filled with exhaust gas.

Bear in mind that there is usually a significant amount of valve overlap between exhaust and induction, so it's likely that BOTH are technically open when the wasted spark triggers.

[paulg23]

Thanks Steve - I thought you would know the answer.:thumbsup: Not what I wanted to hear though! Looks like I have lost my argument on the LPG forum though :lipseal

[Nick Mann]

Surely the valves can't be open far? The piston with the wasted spark will be just very slightly past the top of it's cycle at that point, and our engines are of the interference variety.

[Turbo_Steve]

I didn't know it was an interference engine. In that case, as Nick says, I'd expect the Exhaust valve to be tightly shut before TDC (to prevent it getting smooshed) and the inlet valve should be open(ing) as the piston descends.

I thought 6a13tt could lose it's cambelt and not self destruct? Bummer.
This means no overlap......:(

[Nick Mann]

I have heard of one 6A13TT that has shed a cambelt and not bent any valves. I have heard of three that have. I'm assuming that means that they are interference and one failure was *very* lucky.

[Davezj]

i have a cambelt snapped engine (not one of mine) in the back garden with valves marks on every piston, not much of a mark but the one of the heads did have a couple of bent valves and there are the tell tail crescent witness marks on the piston.

[Turbo_Steve]

Well well.....live and learn. Suddenly regular cambelt changes seem a lot more important :D

It'll just be the inlet valve, then, definitely.