With the engine, everything is clear, the fuel burns in the cylinder or chamber, pressure is created, etc.
That it is clear to you how an engine works, but it is
not clear to you how a
brain works, is not cause for questioning
that it does work.
The engine does not make decisions on its own. The driver does.
You are mixing the analogy.
Q: What organ in the body makes decisions?
A: The brain.
Q: What part of the brain makes the decisions?
A: No one part. All of it operates to produce a decision.
Q: What device in a car makes the 'go'?
A: The motor.
Q: What part of the motor makes the 'go'?
A: No one part. All of it operates to produce 'go'.
And how does the brain make decisions? Why, for example, does a person (the brain) decide that he needs to rush at a tank?
Tens of billions of neurons all have pre-existing levels of excitability, partly from genetic and partly from real-world experience. These come together with current input (say, from the colour of a stoplight ,the gumlnih in you belly ,the stress toons in your blood, etc), along with innumerable other factors that are encoded in the synapses rom reetn and log-term experiences - in a vastly complex way - but we might boil them down to subconsious thoughts ("Am I stressed?", "Is it late?", "Is my dog missing me?", "Am I hungry?" "Did I tape my show?", "Did my father survive that crash in a similar stoplight?") etc etc etc. They all compete, and present a range of options, one of which biols to the top and induces an action (or inaction).
This is the essence of a decision. And decisions are the essence of free will. I have multiple, competing options. I choose one, because I can.
It is not germane
why I might have chosen the action I did; th essence is simply that I did
choose it - willingly and with forethought. I coiuld tell you beforehand what my action was going to be, and I could tell you afterward what I was thinkngi that caused me to choose - whether or not there are other factors - I am aware, I am cogniant, I am sentient enough to preview and review may actions forward in time, backward in time and currently.
Bacteria can't do that. They do not rise to the level of what we consider free will. Neither do rocks.
The problem you're having is that you are trying to answer a question based on knowing very little about cognitioon and consciousness, and this is not the place for you to learn. The brain is essentially the single most complex object in existence. Do you expect to learn it all in a forum post?
More to the point, as I said above, just because you don't
understand it doesnt mean it doesn't work that way. That is called
argument by incedulity.