Help regarding diode selction?

[petrolhead]

Does anybody have a better understanding of diodes than I, I need to select one to prevent flow back up to ignition circuit whilst wiring in an engine shutdown system linked to a fire extinguisher.

12volt (usual 14V ish engines running)
known current draw about 1.0 amp for stop solenoid only.
The circuits that would operate undesirably without a "blocking" diode will not draw much either ..ignition circuits, gauges, etc...I could put a meter on to work it out but just use a big safe diode is best I think.
The feed back of power to the ignition circuits will only be brief, whilst ignition keys are in stop position to energise the stop solenoid.

I could ignore it and let my ignition circuits remain powered up as the engines stop....but that is not a job done right!