"I'd sure love to find a brand new compression coil for the steamer."

"And I'd like to be king of all Londinum and wear a shiny hat. Just get us some passengers. Them as can pay, all right?"

"Compression coil busts, we're drifting..."

"Best not bust, then." ―Kaylee and Mal [src]

The compression coil was a device necessary to run the main engine of a ship, including the gravity thrust, power plant, and life support. A compression coil was an integral piece of a Trace Compression Block, such as the one that the Firefly-class transport ship uses.

Part of the Serenity's compression coil, the catalyzer, blew out and caused a dangerous explosion. Being as Kaylee said it was the port one, there was likely a starboard one also. The compression coil rotated as part of the main engine. The coil was repaired by Capt. Malcolm Reynolds, who was forced to steal one from the captain of the SS Walden, who tried to murder him.

Kaylee said that she wanted to pick up a new compression coil when they are getting off the ship at the Eavesdown Docks, foreshadowing the events in "Out of Gas".