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 primarily on drawing cards or adding cards to the hand, it is called a draw engine. These are often put into Decks that are inconsistent on their own in order to increase their consistency. There are many draw engines that exist, so when making a Deck, players typically include those that work best with the Deck they are trying to make, e.g. if their Deck's monsters are mainly Spellcaster-Type, they may include a Spellbook draw or searching engine.
Rank 3 engine
These engines use the effect of a specific monster to Special Summon Level 3 monsters to 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) with minimal effort and investment. There have been three notable variants throughout the history of Xyz Monsters:
- Normal Summon "Tour Guide From the Underworld" and use its effect to Special Summon a Level 3 Fiend-Type monster from the Deck. This was extremely popular while "Tour Guide" was Unlimited and "Sangan" was Limited, but became much less popular when "Tour Guide" was Semi-Limited, and fell out of favor almost completely when "Sangan" was Forbidden.
- Summon "Speedroid Terrortop" and use its effect to add "Speedroid Taketomborg" to your hand, then Special Summon "Taketomborg" from your hand with its condition. This engine largely replaced the "Tour Guide" engine when its cards were released because, as "Terrortop" can be Special Summoned from your hand if you control no monsters, it achieved largely the same effect without requiring the player to use up their Normal Summon.
- Summon "Predaplant Ophrys Scorpio" and use its effect to Special Summon "Predaplant Darlingtonia Cobra" from the Deck. After "Speedroid Terrortop" became Limited, this engine became more prominent, as while it once again uses up the Normal Summon and has the additional cost of discarding a card, once per Duel when "Darlingtonia Cobra" is Special Summoned it can add a "Fusion" or "Polymerization" Spell Card from your Deck to your hand, and it can be used even if "Ophrys Scorpio" is Special Summoned, so you can use a card like "Lonefire Blossom" to get this engine going.
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".
Crusadia Guardragon Engine
This engine involves Link Summoning "Crusadia Equimax", then using it to Link Summon "Crusadia Magius". When a monster is Special Summoned to the zone it points to, it can add "Crusadia Draco" to your hand, then use the two monsters to Link Summon "Crusadia Spatha", and Special Summoning "Draco" with its own effect. Finally, the player can use it to Link Summon "Guardragon Elpy", activate the effect of "Spatha" to move it to another zone, then use "Elpy" to Special Summon any Dragon monster from the Deck.
Mannequin Cat Engine
The "Mannequin Cat" Engine involves Link Summoning "Isolde, Two Tales of the Noble Knights", using said effect to Special Summon 3 "Hero Kid", using 2 of them to Xyz Summon "Number 29: Mannequin Cat", then use both "Isolde" and "Hero Kid" to Link Summon "Summon Sorceress". Using Sorceress's effect, Special summon 1 "Beast" monster to your opponent's field, then use "Mannequin Cat"'s effect to Special Summon 1 monster from the Deck. Alternatively, Link Summon 1 "Knightmare" monster using "Isolde" and "Hero Kid", use that "Knightmare" monster to Summon "Knightmare Mermaid" , using Mermaid's effect to Special Summon "Knightmare Corruptor Iblee", using "Iblee" to "Link Summon" 1 Link 1 Monster , then using "Iblee" 's effect to Summon herself to the opponent's side of the field,then use "Mannequin Cat" to Special Summon 1 DARK monster from your Deck to the Field. This engine is best used in 60 card "Warrior" Decks.
Sky Striker Engine
This engine uses Sky Striker Mecha - Hornet Drones to Summon a Token,then using it to Link summon Sky Striker Ace - Kagari. Using Kagari's effect to add Hornet Drones back,then use Hornet Drones again for an Instant Link 2. If you Normal Summon a Tuner, the Tuner can be used with Sky Striker Ace - Kagari to Summon Crystron Halqifibrax. Using its effect you can Special Summon Jet Synchron from your Deck, starting multiple combos and swarming the field with monsters.
This engine uses Supreme King Dragon Darkwurm along with a method to send it to the GY with Foolish Burial or Dragon Shrine. With "Darkwurm"'s effect, you can Special Summon it from the GY and add Supreme King Gate Zero from your Deck to your hand, allowing for an instant Scale 0 Pendulum monster that you can use with a higher Scale monster to Pendulum Summon and have potential to use "Darkwurm" as a Link material. You can also set "Darkwurm" in the Pendulum Scale and use its effect to add "Supreme King Gate Zero" to your hand, or Normal Summon "Darkwurm" for a similar effect.
This engine uses Aleister the Invoker, Invocation, and the Invoked monsters to power up your Fusion monsters and Fusion Summon powerful monsters. With "Aleister"'s effect, you can Normal Summon it to add an "Invocation" to your hand, using the materials in your hand, and banishing materials on your field or in either GY. The effect of "Invocation" allows for you to shuffle "Invocation" back into the deck and return a banished "Aleister" to your hand. The potential of using either GY allows for you to lock out future plays by banishing your opponent's monsters so they can't activate their GY effects later. Also, you can use "Aleister"'s effect to give one of your Fusion Monsters 1000 extra Attack and Defense, giving it potential to stay alive through a turn or destroy more monsters.