A compressor assembly (600, 700) comprising: a compression cylinder; a compression piston disposed within said compression cylinder and operable to compress a vapor disposed within said compression cylinder from a suction pressure to a discharge pressure, said compression piston movable within said compression cylinder between a top dead center position and a bottom dead center position; a crankshaft operable to cycle said compression piston within said compression cylinder; an injection port (454, 608, 708) in fluid communication with said compression cylinder and operable to selectively communicate intermediate-pressure vapor at a pressure between said suction pressure vapor and said discharge pressure vapor to said compression cylinder, said injection port (454, 608, 708) exposed by said compression piston when said compression piston is approaching said BDC position to permit communication of said intermediate pressure vapor into said compression cylinder, characterized by: an injector (604, 704) received in said injection port (454, 608, 708) and communicating said intermediate-pressure vapor to said compression cylinder, wherein said injector (604, 704) is controlled to inject said intermediate-pressure vapor into said compression cylinder at predetermined times based on a position of the compression piston within said compression cylinder; and a position sensor configured to measure a rotational position of said crankshaft, wherein said injector (604, 704) is operable to inject said intermediate pressure vapor into said compression cylinder in response to data provided by said position sensor. a compression cylinder; a compression piston disposed within said compression cylinder and operable to compress a vapor disposed within said compression cylinder from a suction pressure to a discharge pressure, said compression piston movable within said compression cylinder between a top dead center position and a bottom dead center position; a crankshaft operable to cycle said compression piston within said compression cylinder; an injection port (454, 608, 708) in fluid communication with said compression cylinder and operable to selectively communicate intermediate-pressure vapor at a pressure between said suction pressure vapor and said discharge pressure vapor to said compression cylinder, said injection port (454, 608, 708) exposed by said compression piston when said compression piston is approaching said BDC position to permit communication of said intermediate pressure vapor into said compression cylinder, characterized by: an injector (604, 704) received in said injection port (454, 608, 708) and communicating said intermediate-pressure vapor to said compression cylinder, wherein said injector (604, 704) is controlled to inject said intermediate-pressure vapor into said compression cylinder at predetermined times based on a position of the compression piston within said compression cylinder; and a position sensor configured to measure a rotational position of said crankshaft, wherein said injector (604, 704) is operable to inject said intermediate pressure vapor into said compression cylinder in response to data provided by said position sensor. an injector (604, 704) received in said injection port (454, 608, 708) and communicating said intermediate-pressure vapor to said compression cylinder, wherein said injector (604, 704) is controlled to inject said intermediate-pressure vapor into said compression cylinder at predetermined times based on a position of the compression piston within said compression cylinder; and a position sensor configured to measure a rotational position of said crankshaft, wherein said injector (604, 704) is operable to inject said intermediate pressure vapor into said compression cylinder in response to data provided by said position sensor.