Product names which are derived after an inventor's name will often remain capitalized, though not always (e.g. the petroleum distillate used to power trucks and locomotives is called "diesel" rather than "Diesel" even though it's named after the inventor of the four-stroke compression-ignition engine for which that fuel was formulated). I suppose a more realistic example is the development of powered pumps, locomotives etc.

Understanding the Context

Newcomen's atmospheric engine did a vital job, but was bettered by Watt's improvements. Trevithick's locomotive of 1804 hauled a load, but modern diesel engines work far more efficiently and reliably. // I'm not sure this is really an English language question. Even with uncountable nouns, for specific instances/types, we have nouns preceded by indefinite articles as in the following examples.

Key Insights

It is cold outside! I could do with a hot tea! The old diesel What’s the difference between particulate and particle? Should it be diesel particulates or diesel particles, and why? Could you provide three or more examples where it should use particulate rat...

Final Thoughts

Gas is flammable, diesel vapour combustible. In England I was always taught that the difference between flammable and inflammable was that inflammable required a flame to permit burning.