diesel vs diesel engine

diesel and diesel engine both are nouns.

Word	Noun	Adjective	Verb	Adverb
diesel	Yes	No	No	No
diesel engine	Yes	No	No	No

As nouns, diesel and diesel engine are synonyms defined as:

diesel and diesel engine: an internal-combustion engine that burns heavy oil

Other synonyms of diesel include diesel motor.

diesel (noun)	diesel engine (noun)
an internal-combustion engine that burns heavy oil	an internal-combustion engine that burns heavy oil