Wouldn't want to inconvenience anyone by making them fill up with gas AND water.

Similarly, you can eliminate much of the nitric oxide and other damaging chemicals by mixing diesel with a bit of propane. Propane injection has been used by tuners for years to make their diesels go really fast.

https://www.sae.org/publications/technical-papers/content/872095/

So why don't we do that? Well, because it is more convenient to replace diesel exhaust controls (the blue liquid) every 6 months than it is to add propane every fill-up, despite it being a better for the environment and better for the engine.