LV-T30 "Reliant" Liquid Fuel Engine
From Kerbal Space Program Wiki
The LV-T30 Liquid Fuel Engine is used to provide thrust to a rocket and is fueled by FL-T500 Fuel Tanks.
Usage
The LV-T30 is currently the only Liquid Fuel engine and is also the only engine with controllable throttle. This makes it very useful for precise, controlled maneuvers and conserving fuel. They are not placeable on radial decouplers and are typically reserved as a main engine after liftoff. The LV-T30 must be directly below a non-empty FL-T500 Fuel Tank to produce any thrust.