Are plug gaps with LPG/CNG narrower?

[CNG]

LPG does look cheap in Russia.

There seems to be quite a few knock/detonation-sensor kits. I was hoping they did more than just indicate knock. I was hoping they operated like a TAP in reverse. Thus when knock is found the output can be used to back-off the ignition. None do much that I've seen. They're very pricey too.

[LPGC]

You're not likely to get knock on LPG or especially CNG regardless of how advanced timing is but you can stll have too much advance that reduces engine power.

A knock sensor would be of more use for the petrol mode under high engie load conditions though.

[Gilbertd]

Agree with Simon. Propane/LPG has an Octane rating of 114 while Methane/CNG is between 120 and 130, so no matter how much compression and how much ignition advance you run, you're unlikely to get it to knock. If you advance it too far it will just try to run in reverse before it knocks!

[CNG]

Oh. That'd explain why I've never heard it knock. Ever. I did wonder. I had no idea!

That leads to asking, if the gases won't knock that's one level of engine-protection/emissions control that LPG/CNG can't offer. Is knock control done all the same but by different means?

[Gilbertd]

It's a level of engine protection that LPG/CNG doesn't offer as it isn't needed because you are never going to get it to knock. One of my cars requires 98 RON Octane or higher fuel so if I put 95 RON in it if I need fuel but can't get the full fat stuff, the knock sensors retard the ignition to prevent it knocking but it feels noticeably flat.

This is how the knock sensors operate on my P38 and it is done in much the same way on all other engines (they can't physically reduce the compression ratio so do it by retarding the ignition tlming):

The knock sensor produces an output voltage which is proportional to mechanical vibration caused by the engine. A sensor is located in each cylinder bank between 2/4 and 3/5 cylinders. The ECM calculates if the engine is knocking by taking camshaft and crankshaft sensor signals to determine the position of the engine in the combustion cycle.
The ECM can also work out exactly which cylinder is knocking and progressively retards the ignition on that particular cylinder until the knock disappears. It then advances the ignition to find the optimum ignition timing for that cylinder.
The ECM can simultaneously adjust the timing of each cylinder for knock. It is possible that all eight cylinders could have different advance angles at the same time.