This application concerns systems and methods for compressing natural gas with an internal combustion engine. In a representative embodiment, a method is featured which includes placing a first cylinder of an internal combustion engine in a compressor mode, and compressing a gas within the first cylinder, using the cylinder as a reciprocating compressor. In some embodiments a compression check valve system is used to regulate pressure and flow within cylinders of the engine during a compression process.
ACKNOWLEDGMENT OF GOVERNMENT SUPPORT
 This invention was made with government support under DE-AR0000259 awarded by the U.S. Department of Energy. The government has certain rights in the invention.