I helped my buddy install some twin turbos on his Dodge Diesel PU, and has a a master spreadsheet of all the numbers he needed to understand his project.
Mass flow ratios for his PU are somewhere about 20:1 to 30:1 air/fuel when he's pulling 200 hp: the data point is appx 26 lbs/min intake air, 1.17 lbs/min fuel flow.
Diesel is largly cetane, (C10H22) and so a lb of diesel yeilds a 120 to 22 ratio of C to H. So 22/142 lbs (0.155 lbs/min H)
1 gal of water electrolyzed to hydroxy, you would get 8.345 lbs of gas, with 1 to 16 ratio of H to O. So, you get .49 lbs of H and 7.85 lbs of O.
The diesel burns, water does not. You need to add energy of the diesel, going thru the alternator, to electrolyse the water.
If the engine is say 33% heat efficient, and the alternator is say 85% efficient, and the electrolysis of (being extremely generous) 90% efficient, than you have a 25% efficient energy conversion to get your magic gas.