As well Q15 must be a PNP transistor, so this shunt regulator can work. If it is a NPN device the diode and resistor would have to swap places. It basically amplifies the current capability of the diode by the current gain ( up to the transistor device dissipation limits depending on mounting and ambient derating) of the transistor, so you can use a cheap close tolerance low power zener diode to handle a higher current.
Not as good regulation wise as a IC regulator, but good enough for a power rail, and can handle large current pulses quite well, with the input side providing a roughly constant current drive and turning off on overvoltage to protect the regulator. Power rail will be around 4V for everything, and will keep it reasonably constant as the engine runs. As the rest is either CMOS or Siemens IIL logic that is perfect for them. CMOS does not care much about supply voltage and the IIL logic has an internal current source generator to run it's internal logic devices.