You're building your Deck and you feel like something is missing. The Deck is below 40 cards as it stands and you need more. You have the core cards of your strategy and all the staples you need. What else could you even add? Upstart Goblin is at 1, so you don't get many free spots. That is where an engine can come into play.

An engine in Yugioh is a series of cards that work well together, but not enough to make a full Deck. As a result, you put the "engine" into another Deck to hopefully add something the Deck previously lacked. Another purpose of an engine could be to make a Deck more consistent. 

Today, I would like to go over the classic engines in Yugioh and discuss their power level in multiple Decks. A good engine is one that doesn't feel like you're making your Deck worse off by having it. The best engines have done this within multiple Decks either in the same format or throughout the history of the game. 

Tour Guide Engine

One of the earlier engines in the game's history is running Tour Guide from the Underworld with Sangan. Tour Guide was able to summon any Level 3 Fiend from the Deck, making it a great Rank 3 tool. The best early target for this was Sangan. Sangan, before an errata, was able to search any monster with 1500 ATK or less from the Deck upon leaving the field. There was no downside to this, meaning many Decks could use this to great advantage.

Sangan's Role

At the beginning of the Xyz Era, it was believed that Sangan could search after being detached as Xyz Material. This was later clarified to not be the case, but Sangan still remained the best summon target off Tour Guide due to its search potential. You could get early hand traps like Effect Veiler, Maxx "C", or D.D. Crow for any Deck. You also had the chance to search for the best cards in your Deck.

Dino Rabbit could search for Rescue Rabbit. Wind-Ups had options like Wind-Up Rat, Shark, or Magician. Inzektors could get to their Inzektor Dragonfly or Hornet. Most meta Decks, past or present, are likely to run low ATK monsters, which made Sangan the best Tour Guide target pre-errata.

With all that said, Sangan's search wasn't the most important. What mattered was that Tour Guide got you to a Level 3 monster for a Rank 3 play. Whether you wanted Number 17, Wind-Up Zenmaines, Leviair the Sea Dragon, or an archetype's own Rank 3, Tour Guide likely got you to it.

The engine did become outdated, as summoning a Rank 3 alone wasn't good enough, and Sangan was eventually banned. However, this does not take away from Tour Guide's power in the early ZEXAL era. She even found a new home later on with Burning Abyss, as now she had a whole archetype to summon. Tour Guide will always be one of the best cards in the game, even if it isn't as an engine anymore.

Speedroid Engine

After Tour Guide's time as the top Rank 3 engine, we were able to find a new alternative. The Speedroid engine consisted of Speedroid Terrortop and Speedroid Taketomborg. All you had to do was summon Terrortop and use its effect to search Taketomborg. Taketomborg could then Special Summon itself since you control a WIND monster (Terrortop).

Now, what made this engine so special compared to Tour Guide? Well it was the simple fact that this engine didn't require your Normal Summon. Speedroid Terrortop has the ability to Special Summon itself if you control no monsters. This is a generic Rank 3 without the need for a Normal Summon, which is outstanding.

PK Fire

One of the first Decks to use this engine to its fullest capability was PK Fire. PK Fire was a mix of the Phantom Knights and Burning Abyss strategies. The goal of this Deck was to summon Rank 3s like Dante, Traveler of the Burning Abyss, or The Phantom Knights of Break Sword. It didn't matter what monsters to used to get to these Rank 3s, and that's where Terrortop can come into play.

Terrortop and Taketomborg helped get you to a Dante as quickly as possible. Without your Normal Summon used, you could take advantage of PKs or BAs for more Rank 3s. This meant 2 Rank 3s, or a Number F0: Utopic Future, while you could get the PK engine going to search Phantom Knights' Fog Blade for negation.

PK Fire was eventually phased out as a top Deck, but the Speedroid engine added much power during its time at the top. The engine proved as a reliable Rank 3 engine, which was important for its next Deck.

Zoodiac

The next Deck that Terrortop found a great home in was the Zoodiac Deck. Zoodiacs upon release until the September 2018 F/L List was the undisputed best Deck. The archetype really wanted to get to Zoodiac Ratpier in order to climb into Zoodiac Xyzs like Drident or Broadbull. This is where Terrortop was useful. All you needed to do was get the Speedroid engine going to get into M-X-Saber Invoker, who could summon Ratpier from the Deck. This meant you got your Zoodiac plays going without a Normal Summon, making it easy for another Rank 4 like Daigusto Emeral to be useful in the Deck.

It was the Tier 0 powerhouse that got Terrortop hit on the F/L List finally, as the card went to 1 in June 2018. The engine became unreliable after the limit, as you didn't want to play 1 Terrortop and 1 Taketomborg, so the engine stopped seeing play in Zoodiac. If Terrortop did come back to 3, it would be the best Rank 3 engine once again, since it can now be a Link-2 as well. All the power Terrortop offered without a Normal Summon is why it's unlikely to return to 1 anytime soon.

Phantom Knight Engine

This was mentioned with PK Fire, but the Phantom Knights were useful outside Burning Abyss. The engine usually consisted of 3 Silent Boots, 2 Ancient Cloak, and 1 Ragged Gloves. This helped summon Rank 3s with ease, which was useful with Break Sword to utilize its Scrap Dragon-like effect. If you popped Break Sword, then you could access DARK Rank 4s like Dark Rebellion Xyz Dragon or Evilswarm Nightmare.

The engine was highly consistent, as there was some good search ability in it. Ancient Cloak was able to search for Silent Boots. Silent Boots was Special Summoned from the hand if you have a Phantom Knight on your field. Not only that, but it could banish itself from the grave to search for Phantom Knights' Fog Blade. You even have Ragged Gloves to dump a Phantom Knight for its graveyard effect if need be.

Early in the engine's lifetime, its only real home was PK Fire as they worked well with Burning Abyss. It wasn't that the engine was bad, it just didn't have a good home to offer synergy with. Eventually this would change, and the archetype would have a new tool to use as well.

Rusty Bardiche and Isolde

The introduction of Isolde, Two Tales of the Noble Knights was a good reason to revisit the PK engine. All the PKs were Warriors, which was what Isolde needed to be summoned. This meant easy access to getting Warrior combos going. PKs were best in DARK Warrior Decks, especially making better of use of The Phantom Knights of Shade Brigandine Trap for another potential Warrior.

Not only that, but The Phantom Knights of Rusty Bardiche really added new life to the Phantom Knight engine. Bardiche really accelerated the Phantom Knights engine and only needing DARK monsters meant any DARK Deck could use it. One of the best Decks was Orcust since all of its cards were DARK. You also had Galatea, the Orcust Automaton as a Link-2 to help climb into Rusty Bardiche.

This added more interruption to Orcust, as Bardiche easily helped get to 2 Phantom Knights' Fog Blade alongside Silent Boots. This was alongside Orcust being able to find ways to summon Dingirsu, the Orcust of the Evening Star on the opponent's turn, letting Rusty Bardiche pop a card, while setting up a potential Orcust Crescendo.

All that interruption and power got Rusty Bardiche banned, but Phantom Knights are still good enough in several Decks that can use Isolde. A searchable Fiendish Chain and an easy Scrap Dragon is always going to be good, which will keep Phantom Knights live for future options in other Decks.

Orcust Engine

Speaking of Orcust earlier, they were also an engine of their own. You usually saw additions like Orcust Harp Horror, Orcust Cymbal Skeleton, Orcustrated Babel, and Orcust Crescendo in the Main Deck. Sometimes you could throw in an Orcustrated Return as well. In the Extra Deck, all you needed was Dingirsu, the Orcust of the Evening Star and Galatea, the Orcust Automaton. Now, why was this engine so powerful? Well, mostly because of the Knightmares.

2 Monsters = Orcust Combo

Thanks to Knightmare Mermaid, any 2 monsters you can use to summon either Phoenix or Cerberus could get a full Orcust combo. You also had a Normal Summonable option to get the combo going with Knightmare Corruptor Iblee, but most chose to go into Phoenix/Cerberus for their Mermaid.

Mermaid was able to easily summon Orcust Knightmare from the Deck at the cost of a discard. This gave you all the materials you needed for Galatea, and Orcust Knightmare could dump Harp Horror for more Orcust plays. It easily got you to Rusty Bardiche as well when that was legal, making the Orcust combo more lethal, even setting up Cymbal Skeleton and Dingirsu in the grave. Orcustrated Babel let you summon Dingirsu on the opponent's turn for disruption with it and Rusty Bardiche.

Eventually, the Orcust combo was too good in every Deck, so Knightmare Mermaid was banned. This killed off any Deck using the Orcust combo, and took Decks that relied on that combo out of the meta. You had Cyber Dragon Orcust and Lunalight Orcust just to name a few. The Orcust combo made the engine one of the best in history, since any Deck could access it.

Sky Striker Engine

Next is a simple engine consisting of Sky Striker Mobilize - Engage! with Sky Striker Mecha - Hornet Drones in the Main Deck. That was also added with  Sky Striker Ace - Kagari in the Extra Deck. You had some other tech options with this engine like Widow Anchor and Shizuku, but it mostly had the three main cards. All for a simple reason, quick Link Climbing.

1 Card Link-2

Engage was an immediate Link-2 when it was legal. Well, it was really Hornet Drones, but your Engage searched for Hornet Drones. All you needed was to activate Hornet Drones and use the token to summon Kagari. Afterwards, Kagari would fetch the Hornet Drones back for a 2nd token. Off of 1 card, you had an instant Link-2. You could go higher, but you at least had a Link-2 at your hands with Hornet Drones. 

It was a simple engine that many Decks used. You even had some not using the Kagari due to Hornet Drones being a Warrior, which was Isolde access. There was a lot of versatility in this engine, which got Hornet Drones limited. It was still an option for some Decks, but it was less consistent with 4 Hornet Drones instead of 6. The engine was dead once Engage was banned, but some could still use Hornet Drones if they wished.

Invoked Engine

The next engine gave any Deck access to the Fusion Summoning mechanic. All you needed was 3 Aleister the Invoker with any number of Magical Meltdown and Invocation in the Main Deck. The latter depended on preference, though 3 Meltdown would be preferred for 6 Aleister. Aleister is great with Invocation since Aleister can be fused with any Attribute of monster. You later also had another option for Aleister the Invoker that could search the Invoked Spells/Traps with Aleister the Invoker of Madness. Add with with Invocation letting you use either grave and you could access many different Fusions.

Fusion Toolbox

With any LIGHT monster, Aleister turned into Invoked Mechaba. Mechaba is great considering it's a free negation if you have the same card type in the hand as the card you want to negate. WIND gave you Book of Moon on legs with Invoked Raidjin. FIRE monsters became an OTK machine with Invoked Purgatrio. EARTH was a simple big body with Invoked Magellanica. WATER gave an untargetable defender that could attack in Defense Position with Invoked Cocytus. Finally you had DARK giving you a massive floodgate with Invoked Caliga.

The most popular were LIGHT, WIND, and FIRE. Any chance to get a negate is great, so LIGHT Decks would love a Mechaba. This part mixed well with the Artifact engine. FIRE being able to OTK easily was great when every Deck typically runs Ash Blossom & Joyous Spring. You also had good synergy with the Windwitch engine, which made Raidjin one of the better options. In more recent times, Invoked mixed well with Shaddolls, making Caliga much better. Any Deck that appreciates its Attribute-specific Invoked Fusion would love to have Aleister in their Deck.

Artifact Engine

An engine that is format dependent is the Artifact engine. This engine mostly consists of 3 Artifact Sanctum with a copy of Artifact Scythe and/or Artifact Moralltach. Early in the life of the Artifact engine, Moralltach was great with its pop effect in the slower pre-Duelist Alliance format. Thanks to the TCG, though, we got a better Artifact long-term with Scythe. All you need is Sanctum on the opponent's turn to summon Scythe from the Deck. Scythe then triggers, preventing the opponent from summoning from the Extra Deck that turn. It's a very powerful engine that could get Scythe banned, especially with Artifact Dagda plus Tornado Dragon. It's a simple, yet effective engine.

Windwitch Engine

Probably the simplest engine of them all. The Windwitch engine consisted of 3 Ice Bell with 2 Glass Bell and 1 Snow Bell. All of this let you immediately use Crystal Wing Synchro Dragon on the first turn that's indestructible by card effects. Ice Bell + Glass Bell meant Clear Wing  or Winter Bell to climb to Crystal Wing. This mixed best with the Invoked engine to end on Crystal Wing and Raidjin, but any other WIND Deck like Speedroids could use it as well. While it largely fell out of favor during the VRAINS era, it could definitely make a comeback with Master Rule 2020 opening back up Extra Deck summons.