An excellent summary of all the in engine fuel saving means.

[Logic]

An excellent summary of all the fuel saving means.
(Well.. not all.. )

Improving Thermal Efficiency of Internal Combustion Engines: Recent Progress and Remaining Challenges
https://www.mdpi.com/1996-1073/15/17/6222

Low hanging fruit is the advanced corona ignition system (ACIS) etc coupled with leaner burn.
I NB that a couple of high voltage caps between the plug top and earth can get things going to the point where you can saw of the electrode that impedes flame travel.

Adding a constant ionizing voltage is also quite easy, but what is the ionization voltage during compression?
It's ~1000 volts (no amps) per mm at STP, but I think voltages decrease as gasses are compressed. (the above paper hints at that too)

There are also a good number of aftermarket systems that I cannot speak on.
Anyone using one?