[DifferentPointofView]

Taken from that link.

So, this is a great technology and we should all rush out and bolt hydrogen generators onto our cars? Well, no. The problem is that the rest of the engine needs to be optimised as well to get the benefits. To take advantage of the higher octane rating, the compression ratio must be raised by redesigning the cylinder head or pistons. More critically, the ability to burn a leaner mixture is only useful if the engine ECU is configured to inject a very lean air/fuel ratio, and manage the required variations in spark timing and fuelling (which current production vehicles, without exception, are not). Simply speeding up the burn, even if theoretically advantageous, is likely to make economy worse if the ignition timing is not adjusted to suit.
As a result of these considerations, I am highly sceptical about the use of hydrogen generators as an aftermarket fuel "saving" device, and will remain so until the makers produce some good results.

Many devices that produce hydrogen via on-board electrolysis feed not pure hydrogen, but a mixture of hydrogen and oxygen, to the engine. This mixture (two parts hydrogen to one part oxygen) is commonly known as Brown's Gas. Various semi-magical properties are claimed for this gas, but these are widely disputed and should be considered with scepticism.