An engine is a combination of cards that work well together, but contains too few cards to make a full Deck. If an engine focuses on raising consistency, it is called a Draw Engine.
Rank 3 engine
These engines use the effect of a specific monster to quickly Xyz Summon Rank 3 monsters such as "Leviair the Sea Dragon", "Wind-Up Zenmaines", "Dante, Traveler of the Burning Abyss", "M-X-Saber Invoker", and "Totem Bird" (if using WIND monsters. There are three notable variants.
"Tour Guide From the Underworld" is used to Summon Level 3 Fiend-Type monster from the Deck. This engine was largely replaced by "Speedroid Terrortop" and "Speedroid Taketomborg", which could achieve the same purpose without wasting a Normal Summon. After "Terrortop's" Limiting, the "Predaplant Ophrys Scorpio" and "Predaplant Darlingtonia Cobra" engine became more prominent; while it uses up the Normal Summon, once per Duel it can also search out a "Fusion" or "Polymerization" Spell Card.
The Diva Engine is similar to the Tour Guide engine. "Deep Sea Diva" is used to Special Summon another Sea Serpent monster, setting up a Synchro or Xyz Summon. It is more commonly used in "Mermail" Decks, where is possible to search an "Atlantean" monster, specially "Neptabyss, the Atlantean Prince", and provide access to other combos.
The "Machina" engine consists of "Machina Fortress" and "Machina Gearframe". It gives more power to Machine Decks, Including "Gimmick Puppets", "Gadgets", "Geargia" and "Karakuri". "Gear Gigant X" and "Machina Peacekeeper" can be used to search "Gearframe", but the latter is rather not used due to its slow speed.
Dragon Ruler engine
When the infamous "Dragon Rulers" were Limited, they were commonly used as engines on Decks of their corresponding Attributes. For example, "Redox, Dragon Ruler of Boulders" could be used to revive "Scrap" or "Karakuri" monsters; "Tidal, Dragon Ruler of Waterfalls" could trigger the effect of "Atlantean" monsters while setting up the Graveyard, and "Tempest, Dragon Ruler of Storms" could fit in any Dragon-oriented Deck, mainly "Dragunity".
Fairy Tail engine
In Decks that rely on filling the Graveyard, specially "Lightsworn" builds, it has been common to use copies of "Fairy Tail - Snow", that can be used as free Xyz/Synchro materials, while banishing useless cards from the Graveyard or cards that trigger their effects upon being banished. Also, since it can be Special Summoned during the opponent's turn, it can also be used as a way to frustrate an opponent's move, acting like a "Book of Moon".