They most likely use their own in-house engine with Bitdefender, I doubt they use any other additional engines (of course anyone reading, feel free to correct me if I'm wrong).
If you have the product installed you can check the settings and see if you can find any settings regarding enabled engines; alternatively you could become an explorer and dig deep into the files used by the AV product from the installation directory (e.g. check the properties of the files and see if any are digitally signed by any other external vendors, etc).