5L still wont put anything out unless its early stages. in our firefighting training we used 9L AFFF and 9L dry powder extinguishers and on their own even 9L has limited capability against a fast burning fuel/oil fire. when we did it for real on a couple of occasions against only simple paper bin fires even with 2 9L extinguishers a small bin fire we could only just contain and slow the spread for a minute or 2 until the back ups arrived with hoses.

Fire is an amazing animal and hard to control even when you know what your doing.

BCF was great stuff but gone now due to environmental issues. MoD have got rid of most BCF systems. we used to use it loads for machinery fires on ships. Ironically in 90's upgraded from CO2 to BCF/halon then mid 00's went back to CO2!!

BCF is good for fasgt knock down but in a car its weakness is being gaseous it dissipates quick and because its not taken any heat out of the fire it can easily reignite. Thats why in many car fires it seems to go out with the first extinguisher firing in the car then it flashes up again if the marshalls pop the bonnet too much. Also the reason they tell you to only pop bonnet on catch on road cars and squirt between gap.

IMO youd be better off with a plumbed in AFFF system backed up with a dry powder handheld. The first shot both takes out a lot heat (the water) and smothers the oxygen (the foam) then you back it up with a modern dry powder to form the solid powder "crust" over the engine bay to keep oxygen away while it cools below flash over temp.