You are looking at a 3,500 horsepower, 24-cylinder radial Pratt & Whitney Wasp Major engine, developed at the end of WWII for use in Boeing's successor to the Superfortress. It is mesmerizing.
Reader ttyymmnn points out that while the engine was reliable in flight, if it was started improperly, all 56 spark plugs from its four banks of eight cylinders would have to be changed. It's not a big surprise that even this, the most advanced and complex reciprocating engine in USAF history eventually was phased out.
Have you ever seen a cooler, weirder looking engine?
(Hat tip to ttyymmnn!
Photo Credit: U.S. Air Force