1. R.P. Pescara, Motor compressor apparatus. US Patent 1,657,641, 1928.
2. The Worthington–Junkers free-piston air compressor;Toutant;Journal of the American Society of Naval Engineers,1952
3. Present state and future outlook of the free-piston engine;Huber;Transactions of the ASME,1958
4. The GMR 4-4 “HYPREX” engine – A concept of the free-piston engine for automotive use;Underwood;SAE Transactions,1957
5. The future of the free-piston engine in commercial vehicles;Noren;SAE Transactions,1958