On iOS, there is an app called xSearch that integrates into Safari and sneakily hacks around the limited search engine options by watching your browsing history

On iOS, there is an app called xSearch that integrates into Safari and sneakily hacks around the limited search engine options by watching your browsing history for queries to the search engine you've selected in Safari, then immediately rewriting the URL and navigating to the search engine you actually want. Obviously this has security implications, but I don't ordinarily search for anything sketchy on my iPhone so I'm personally not too worried about it.