Timed injection of coolant during the onset of the compression stroke in a reciprocating compressor can achieve quasi-isobaric conditions during the initial part of the compression stroke. When followed by a quasi-isothermal process, due to continued coolant injection during the remainder of the compression stroke, the compression process can require less energy than an isothermal compression process. Rapid and significant cooling during the onset of compression is required to achieve quasi-isobaric cooling in a real reciprocating compressor. The suction valve must be kept close to prevent additional suction air inflow. The results from an experimental compressor, equipped with timed coolant injection and suction valve control were compared against a simplified 0-D dynamic polytropic process model and a 0-D dynamic model based on the First Law of Thermodynamics. Both modeling methodologies rendered good agreement with the experimental results, with the 0-D dynamic First Law model being better suited for design purposes.