diesel engine vs diesel

diesel engine and diesel both are nouns.

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

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

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

Other synonyms of diesel engine include diesel motor.

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