Like I keep saying, I believe this technology will get refined someday, but....
This is a nice design but I truly doubt it will run even a very small IC engine. I think you will get approximately 500-700 mL of HHO per minute with that maximum and that's not enough to run a motor. You might, and I stress might, be able to idle a very small generator or lawnmower motor on that but it wouldn't handle any useful load. There are dozens, even hundreds, of plans around the web similar to that. I have copies of many of them.
Shoot me an email (it's in my profile) and I will send you some more detail on some of the plans I have. A few are not free for all plans and I respect the company that I got them from so I will not post them for the general public but if you contact me direct I'll share some info. I don't mean to discourage you, on the contrary. I think you should definitely build that one as it will help you learn about this stuff and it will work enough to do some testing and experimentation.
The plans you have is a parallel plate design. The series plate design I keep talking about will work much better but is also quite a bit more difficult to build. I'll try to explain the difference in the two designs.
A parallel plate, like the plans you showed, have either + or - applied directly to each plate. With the stacked plates yours has the only benefit is more surface area, which is certainly a good thing. The trouble is that your entire cell only drops about 1.5-2 volts so you are wasting 10-11 volts in the form of extra heat. Since each plate has the + or - applied to it, even if you had 100 plates you'd still be wasting energy.
In a series design you have a plate on either end, one + the other -. In between these two end plates you have multiple isolated plates that are only connected to the two end plates through the electrolyte between them. The current will pass from the end plates through the electrolyte through each plate until it reaches the other end electrode. Each plate will drop the 1.5-2 volts it requires at whatever amperage you set it up at based on your power supply and the electrolyte concentration. The number of center "neutral" plates is determined by the voltage you are applying. For example, in a car when running you'll have about 14 volts. For this application you would have the two end plates and 6 neutral plates in between them. Since each "cell" drops about 2 volts you will use 14 volts much more efficiently. Another benefit is that each of the neutral plates will emit hydrogen and oxygen gases. The important thing with the series design is to make sure each "cell" is isolated so the current cannot simply go around the neutral plate through the electrolyte to the other end plate. The current must be forced to pass through each of the cells in your system so work properly.
If this didn't make sense I can make a diagram of it and post it. The other big issue I mentioned earlier is controlling the amperage. With that design you've got the amperage will definitely run away on you as it heats up without some way of keeping it under control. The PWM will work but requires constant monitoring and adjustment. You can also wire the electrolyzer in series with a known load like your headlights. If your headlights draw about 12-15 amps that is the amount of current that will pass through the elelctrolyzer no mater how hot it gets, even if the cell were to get a direct short. The drawback to this method is that you have to leave your headlights on all the time and your headlights will be a little dimmer since the electrolyzer will drop the voltage by about 2 volts. There are circuits you can built that monitor the current and adjust automatically, basically an automatic PWM. You'll need to be pretty decent with electronics to build one. You can even open up your alternator and tap directly into the winding and extract over 100 volts from your alternator. This voltage will change with RPM which can actually be good for a hydrogen on demand system since you want more at higher RPM. You need the proper power supply circuit to use this power tho. You'd also use more neutral plates in your cell if you up the voltage. Basically 1 for each 2 volts you apply. The two end plates count as 1 by the way.
There I go again, rambling on....