Translation:The engine is really efficient and the man loves it.
I think it has to do with how invested the speaker is in whatever is being qualified with 'very' or 'really'? Like if you say somebody is a very good friend, as opposed to a really good friend, the second implies more of an emotional investment on your part, like said friend is truly a tremendously good friend and has done something that merits the title, even more so than the first one. Or if you say 'it's a very bad book' vs 'it's a really bad book', I hear in the first one that you didn't enjoy it, and in the second one that you hated every sentence and are considering using the book as kindling.
That's just how I understand it :D (although in this case I suspect it is simply that, unlike with 'effective/efficient', Danes see 'very' and 'really' as different and thus there is a different word for each, as is the case in English.)
The question is, what does the man love? The engine itself or the fact that it's effective?
The Danish sentence uses den at the end, which means that it refers back to the engine. So in the English translation it would either be "the man loves it" or "the man loves that one", but the latter version creates some more problems.
If the man were loving the effectiveness of his engine instead, which you would refer to if you write "the man loves that", you'd use the neuter-gender pronoun, since there's no specific item you're referencing: "og manden elsker det."