Upvote:4
The most comprehensive (free) search engine I know is https://matrix.itasoftware.com/search
You can specify which airlines to search (or to exclude) in the routing codes. For example if you want to only search United, Delta and American, you would add a routing code like UA,DL,AA+
. These don't reset on date changes and you can copied and pasted as text for a new search.
There are routing codes for all types of things (layover cities, aircraft types, alliances, layover duration, number of stops, etc.). It's a powerful engine but has a bit of a learning curve to use it to its full potential.