I have noticed that, at least in larger diesels (such as Ford, Dodge and Chevy pickups and SUV's, as opposed to the Mercedes and VW's) the gains reported seem to be somewhat less than with a gasser.
Subjectively speaking, it seems like diesels reporting a baseline of 15-22 mpg are only reporting gains of about 3-5 mpg, or about 20-30 percent, with few exceptions. Gassers seem to be able to regularly realize 40 to 100% or better.
I have a theory as to why diesels don't realize as much gain.
Please don't jump all over me, guys, this is a THEORY based on totally SUBJECTIVE observations based on totally ANECDOTAL reporting...no scientific method involved, here, OK? Just throwing it out there for your perusal and amusal. Please scrutinize and criticize freely.
Gas engines ignite a mixture of fuel and air by a spark. The hydrogen and gasoline ignite at the same time.
Diesel engines compress air to well over the ignition temperature of the fuel, then inject the fuel to ignite it.
Except that when you introduce HHO into the intake airflow of a diesel, you are NOW compressing a MIXTURE of hydrogen AND air to well over the ignition temperature of the hydrogen. Theoretically, this would cause preignition, would it not?
So the question THEN is, what is the autoignition temperature of hydrogen, and when, during the compression stroke, is it reached?
932 degrees F (773K) is the autoignition temperature of hydrogen.
(there is a Diesel Cycle calculator about halfway down the page)
773 degrees K is reached at a compression ratio of just under 11:1.
Now, granted, this is theoretical. This assumes ambient air at 1 atm and 70 degrees F, perfect adiabatic compression, and no fluid flow losses.
Ford Powerstroke and Dodge Cummins engines run at 18:1. Chevy 6.5's run at 21.5:1. Chevy Duramax at 16.8:1 to 17.5:1, depending on the year.
When do you reach 10.9:1? Depends on the maximum compression ratio.
In a 17:1 engine, 11:1 is reached at about 20 degrees BTDC.
(My calculations may be incorrect on this one...someone check me please)
In a 21.5:1 engine, 11:1 is reached at about 23 degrees BTDC.
This, of course, assumes perfect compression...that the entire length of the compression stroke is spent compressing the air (it is not, because of valve timing, which would cause the 11:1 number to be reached later in the stroke)
The valves do not close until about 43 degrees ABDC
Injection timing starts at about 28 degrees BTDC
Depending on the engine of course...
Injection timing would have a mixed result on the preignition question. As the website above mentions, spraying the diesel fuel into the hot compressed air has the effect of cooling the air (as the fuel vaporizes and absorbs heat until IT reaches its' autoignition temperature) which obviously *delays* the ignition of the fuel.
But timing brings the fuel combustion event closer to the hydrogen combustion event. If the hydrogen combustion has already started, then spraying diesel fuel into it would make the diesel burn *somewhat* faster. How much? Who knows? I do not know how to calculate that.
And when you throw in fluid flow losses, turbocharging, etc. then none of the assumptions are correct, and frankly, I do not know how to calculate that, either.
But still, I would have to ask: are we experiencing preignition of the (tiny amounts of) hydrogen that we are putting into the cylinder, and thus, not burning it at the exact same time as the fuel?
And if we ARE, then could THAT explain why we do not seem to experience gains as dramatic as the gassers do?