You shouldn't need to understand the chemical reactions to project the car, though. You just need to project the fluids in question.
The fact is that we just don't know. Nasu has never addressed the question, or any similar one. I suspect that you probably couldn't project a car with Rin-level prana, but I am not willing to say for certain that you could not.
True, but you still need to understand how that powers the engine, and you need to get the chemical structure of those fluids exactly right otherwise you could unintentionally end up with a totally different fluid.
For that matter, this whole scenario is basically the problem with Lantz's OC's in a nutshell - they have no weaknesses and seem to be better at everything they do than their parents. I mean, for fucks sake Satoshi basically can never be killed, has absurd magical ability, is a fucking dragon, is really 2000 years old, has an 'oh my god so special and cool' sword, a second 'oh my god so special and cool' sword, and somehow for that matter killed Gilgamesh and defeated Arc (After which considering what lantz has written about him, he boned her) and then the icing on the cake - a reality marble. Oh, and he pisses off counter guardians for fun.
Seriously, we need to pull an intervention here.
The consequences are mine to show. Canon says it's possible. The effect on Sakura is my business. There is a point to why I'm doing this. And again I said built, she built the van.
Even if she did build the van, here's the kicker - SHE DIDN'T BUILD THE GAS, THE BATTERY, THE OIL, OR THE FUCKING WIPER FLUID! Lantz, I don't think you GET just how many individual moving parts are in an engine, that have to be millimeter precise or they simply won't work. And I can TELL this explanation is being pulled out of your ass here. In this case, when everyone else is calling bullshit (Except Mike, but mike isn't supporting you on this one and taking a neutral stance), you need to take a step back, and wait for a GM ruling. For that matter, even if she DID for the sake of argument project every individual piece, that would likely cost MORE mana and boil her brain from the sheer information overload required.