Is it Diesel or diesel?
We often see ‘diesel’ written in modern texts.
Having said that, we should be aware that there are many words which derive from proper nouns, ie someone’s name, usually the inventor. When they do derive from proper nouns, we give a nod to this by using an initial upper-case letter.
The German engineer Rudolf Diesel (1858–1913) has this type of engine named after him – and so this should be shown, when writing the word, by using an initial upper-case letter.
The same is true of Macintosh (Charles 1766–1843) who invented a waterproof substance used in clothing.
Other examples include some animals’ names; here, we can see the difference between those animals named after someone and those which are not: a mute swan; a Bewick’s swan.