I am not an engineer this is what I know please ask an expert and the net for true details but i will give it a try.

In a normal engine during the compression stroke, as the charge of air and fuel is being compressed by the pistons movement up to wards top dead center, 5 to 15 degrees BEFORE piston reaches TDC the spark plug ignites the mixture , WHY ?

This action of pree ignition in intended to allow for a complete flame deflagration or complete combustion of gases,
Hence complete pressure build up before Piston is at TDC, this timing effort is intended to make best use of the gasses pressure, see you want to have the maximum and complete gas pressure al ready built up meet the piston at TDC or Just after TDC, so that gas pressure is at maximum and burning of gasses is already complete , other wise if you ignite the charge AT TDC then you will have flame deflagration following the piston down hill trying to catch up to it and pressure is in the process of building up or being generated as the piston is going down , in other words pressure is trying to catch up to the downward speeding piston , instead of acting on the piston with pressure.
I hope that is clear .

it takes a bit of imagination to imagine that but it is true you can test it on old cars with classical ignition systems.

Timing of the ignition depends of many thing , fuel type , load, air , RPM , engine design etc

That is why computer engine are more efficient and powerful you can perfectly time the ignition.

In the Wankel one of there two processes can take place but not the other

The same charge compression does take place in a Wankel, but not the pree ignition

In the Wankel the pree ignition is imposable to implement for lots of reasons, starting from the fact that the area of the rotor is large enough that the pressure of the pree ignited gases would prevent the rotor from completing its turn.
Also the combustion chamber of the Wankel is very small ,and is momentarily created it doesn’t last that long .
this creates more problems one of which is the fact that the flam deflagration is so slow to spread across all of the gas fuel mixture (charge) so they use two spark plugs to ensure as wide spread flam deflagration as possible.

So you cannot have gases pressure build up as the rotor is moving to TDC . you can only hope for best ignition exactly at TDC and since the rotor passes to fast at TDC the need to create as many as possible of ignition points to deflagrate the flam as much as possible.
If I know correctly Wankel don’t do good n low RPMs they have to Rev up to get any power .

By the way Wankel is rubbish, it is not even a rotary engine it is an eccentric rotary engine.

See my blog or email me if you want to know the true rotary engines that have the true power of rotary engine and none of the Wankel disadvantages .

Pree ignition is possible in my engine design,complete torque, no wobbly mass, very high compression ratio , all ready supercharged, higher volumetric efficiency , high power to weight ration etc .

I hope this answers your question.

sorry for the long post