Is diesel a long vowel word?

Answer 1

Diesel does have a long "E" sound, so yes.

Generally in long vowel words with two vowels together, the long vowel sound represents the first letter of the vowel pair, not the second, for example "died," where the long vowel sound is "I"

So "diesel" is unusual, since the long vowel sound represents the second vowel "E"

Answer 2

No, "diesel" is not a long vowel word. The vowel sound in "diesel" is short, as in "eh" and "ih".