Patent ID: 8151747

Claim:
An engine ( 10 ), comprising: a crankshaft ( 16 ) rotatable about a crankshaft axis ( 17 ); an expansion cylinder ( 14 ) including a centerline axis ( 62 ); an expansion piston ( 30 ) slidably received within the expansion cylinder ( 14 ) and operatively connected to the crankshaft ( 16 ) such that the expansion piston ( 30 ) is operable to reciprocate through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft ( 16 ), the expansion piston ( 30 ) including a top surface ( 50 ) and an outer perimeter ( 74 ); a cylinder head ( 33 ) disposed over the expansion cylinder ( 14 ) such that a bottom surface ( 52 ) of the cylinder head ( 33 ) faces the top surface ( 50 ) of the expansion piston ( 30 ), the cylinder head ( 33 ) including a crossover passage outlet ( 27 ) and an exhaust port inlet ( 53 ) disposed therein, the exhaust port inlet ( 53 ) and the crossover passage outlet ( 27 ) each being proximate the expansion cylinder ( 14 ); a crossover passage ( 22 ) connecting a source of high pressure gas ( 12 / 20 ) to the expansion cylinder ( 14 ) via the crossover passage outlet ( 27 ); a crossover expansion valve (XovrE valve) ( 26 ) disposed in the crossover passage outlet ( 27 ), the XovrE valve ( 26 ) operable to allow fluid communication between the crossover passage ( 22 ) and the expansion cylinder ( 14 ) during a portion of the expansion stroke; an exhaust valve ( 34 ) disposed in the exhaust port inlet ( 53 ), the exhaust valve ( 34 ) operable to allow fluid communication to or from the expansion cylinder ( 14 ) via the exhaust port inlet ( 31 ) during a portion of the exhaust stroke; a recess ( 60 ) disposed in the top surface ( 50 ) of the expansion piston ( 30 ), the recess ( 60 ) including a bottom surface ( 64 ); an expansion piston clearance ( 80 ) being a shortest distance, along a line parallel the centerline axis ( 62 ), between the top surface ( 50 ) of the expansion piston ( 30 ) and the bottom surface ( 52 ) of the cylinder head ( 33 ) when the expansion piston ( 30 ) is at its top dead center (TDC) position; a recess depth ( 82 ) being a shortest distance, along a line parallel the centerline axis ( 62 ), between the bottom surface ( 64 ) of the recess ( 60 ) and the top surface ( 50 ) of the expansion piston ( 30 ); an expansion ratio being the ratio of the enclosed volume in the expansion cylinder when the expansion piston is at its bottom dead center (BDC) position to the enclosed volume in the expansion cylinder when the expansion piston is at its TDC position; wherein a portion of the recess ( 60 ) overlaps a portion of the crossover passage outlet ( 27 ); wherein a portion the exhaust port inlet ( 31 ) does not overlap any portion of the recess ( 60 ); and wherein the recess depth ( 82 ) is between 1.0 and 3.0 times the expansion piston clearance ( 80 ).