A Stirling engine can take advantage of adiabatic compression (which heats working gas leaving the cold cylinder) and adiabatic expansion (which cools working gas leaving the hot cylinder) to increase efficiency. In some implementations, partially-heated gas leaving the cold cylinder and partially-cooled gas leaving the hot cylinder can be routed directly to a regenerator using bypass paths that are opened using one-way valves. The resultant relatively reduced temperature difference across the regenerator, e.g., as compared to a typical Stirling engine, can reduce thermal loss and improve efficiency. In some implementations, the compression ratios of the Stirling engine can be adjusted such that the temperature of the adiabatic heated gas is the same or higher than the temperature of the adiabatic cooled temperatures, thus eliminating the need for a regenerator.