A compressor may include a compression cylinder, a compression piston, a crankshaft, an injection bore, a position sensor, and a valve assembly. The compression piston is disposed within the compression cylinder and is operable to compress a vapor disposed within the compression cylinder from a suction pressure to a discharge pressure. The crankshaft is operable to cycle the compression piston within the compression cylinder. The injection bore may be in fluid communication with the compression cylinder and may be operable to selectively communicate intermediate-pressure vapor at a pressure between the suction pressure and said discharge pressure to the compression cylinder. The position sensor may measure a rotational position of the crankshaft. The valve assembly may be associated with the injection bore. The valve assembly may be operable to control passage of fluid from the injection bore into the compression cylinder in response to data provided by the position sensor.