Hi Milkoni,
A pretty neat design.
Just a general observation: when testing injectors, ignition systems etc, it is best to be able to vary the on (dwell) period and the frequency. I think this can be done with a single 555 chip.
Even though a switching element (BJT, IGT, MOSFET) may be switching at a relatively low frequency, it is still important to minimize the switching element turn on and turn off time to minimize stress of the switching element.
In addition to Cruts' observations, here are a few from me.
(1) The BC548/BC558 transistors are not really up to the job. BC337/BC327 high conductance, higher current transistors would be more suitable. The effective gate capacitance of a single MOSFET is huge so four in parallel amount to quite a capacitance.
(2) It would be better to have an independent gate shaping resistor for each NMOSFET. I would recommend 22 ohms.
(3) R4 is a tough high. I would recommend 47 Ohms.
(4) The standard 555 timer needs at least 220nF ceramic capacitors directly across its supply pins.
(5) The NMOSFETs also need decoupling. I would recommend a 1000uF low frequency decoupling capacitor across the supply line and 220uF ceramic capacitors physically close to each NMOSFET
By the way, I have not looked at the deign of the 555 part of the circuit, but my impression is that it could be simplified.
Please do not think that I am criticizing your design as I said it is pretty neat, well drawn and well presented.
spec