As stated earlier the dry powder extinguishers do need to be shaken and "thumped" free every so often (monthly or more often in some cases) as the vibration in a vehicle does settle and compact the powder enough for it not to discharge effectively or sometimes not at all. I serviced and inspected extinguishers for a few years so have some experience with them. This discharge rate of a (quality)1kg dcp is around 8-10 seconds so not long but when used properly pretty effective. They are the best type to have in a vehicle as the cover a range of fires but do have limitations. They work by interrupting the flame chain reaction of a fire (displace oxygen in a way) rather than cooling or removing fuel so if a fire has an extremely high amount of heat then they don't generally put the fire out and prevent it reigniting, more just slow the process a little. Often though the slowing down is enough to prevent more heat developing and the fire will be extinguished. A note worth mentioning too is that once a dry powder extinguisher of any size has been discharged by even the tiniest amount they need servicing and recharging. The fine powder gets in between the sealing oring and head which allows the charging gas (nitrogen) to escape over a few minutes which renders the unit useless until it's serviced. Well worth having in the car and can prevent a major fire is used early enough. Cheers.
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my Dad and I tried to put a fire out with one of the smaller extinguishers, definitely need a bigger one but at the time its all we had. This was the aftermath. Only had 62,000 kms and he had just finished polishing it.
