Nothing like HHO.

HHO is a snake oil scam, because the amount of "gas" created is so minuscule that it has zero effect.

Setups such as propane and CNG are actually introducing higher amounts of combustable fuel into the intake. The issue with both systems here is that when a fuel is introduced into the combustion chamber of a compression ignition engine via the air intake, rather than being precisely injected at a pre-determined time, you lose all ability to control start of combustion (SOC).

An earlier SOC event can increase horsepower and increase fuel economy (by reducing the amount of diesel needed during the injection event). However, the side effect of that is a rapid and often very high spike in cylinder pressures. It's this exact problem that is the cause of bent and/or broken rods, with windows in the blocks.

That's the inherent danger of propane and CNG. That's why propane kits, although widely popular years ago, were also quickly abandoned by the masses. CNG is just the same thing, under a different name and a slightly different fuel. Still doesn't change the fact that it's a fuel being introduced into the air intake, creating the same effect as propane.