The two sentence ?

The second one is a statement rather than a question. It means this is how an engine works, but you need to drop the question mark. There is nothing wrong with the first, but it's something you would use asking for clarification, you would ask your mechanic something like this.

The first one is a question, the second one is more of an introduction to answer the first one if you continue with 'is' for example, the second one is re-itterating the question, and therefore the 'does' is removed. The first sentence should be used in formal English writing in the sense that i'm assuming you will be using it in. Depending on the way in which they're used they're both grammatically correct, although as i have said it depends on WHO is the speaker, the second one needs the question mark removed though as it is not a real question.

EG,

Person One: How does an engine work?

Person Two: How an engine works is a question that is almost impossible to answer...

Both convey the same idea semantically, but the respective grammars employed are quite different.

1. "How does an engine work?" is a proper sentence with a subject and verb made into a question with the interrogative adverb "how", whereas
2. "How an engine works?" is only a noun clause beginning with a nominalising conjunction "how".

As a noun clause, 2 cannot serve as a sentence independently, but must be part of a longer clause in which it becomes a part, say the subject or the object.

examples:

"How the engine works is no mystery if you understand the Carnot cycle." (subject of verb "is")

"You need to understand how an engine works if you want to be a mechanic." (object of verb "understand")

Hence in formal English, one would say and write 1. but not 2. as a complete, self-contained grammatical sentence. Now this is true whether in the UK, the US, or any other Anglophone country (if we define Anglophone to exclude English pidgins and creoles.)

Nevertheless, pattern 2 does not hinder understanding as the speaker/writer's intention is clear. In casual speech, the creative arts, and everyday communication acts, pattern 2 is not shunned as incapable of communicating ideas.

No.1 is British English
No.2 is American English


Both has same meaning.