A semiempirical reciprocating compressor model was developed to characterize the time response of the pressure within a compressed-natural-gas storage tank. The model assumes isentropic compression in the single-stage compressor that feeds into an isothermal reservoir, i.e. compression heat was assumed to dissipate between the cylinder and the tank. This low-order model utilizes a few key variables such as the compression ratio, the cylinder volume, the tank volume, and the engine speed and yields a reasonable approximation for the pressure inside a tank over time. The natural-gas compressor used to validate the model is a unique automotive internal-combustion engine that was heavily modified to have two distinct modes. In one mode, all engine cylinders fire normally providing locomotion for a natural-gas vehicle. In the other mode, one cylinder of the engine is used to compress low-pressure residential natural gas, in multiple stages, to a standard US compressed-natural-gas vehicle storage tank pressure of 248 bar (3600 lb/in(2) gauge). It was found that the first-order response model closely matches the trend in the tank pressure over time seen in laboratory experiments, with a typical error of less than 5%.