Multifuel closed-loop thermal cycle piston engine, system and method

A multifuel closed-loop thermal cycle piston engine comprising:

• an externally-fired continuous combustion chamber configured to conduct continuous combustion of a fuel in a fuel burner chamber, the externally-fired continuous combustion chamber comprising a water injection stage succeeding the fuel burner chamber, the water injection stage configured to inject water into the externally-fired continuous combustion chamber post-combustion;
• a drive stage coupled to an engine exhaust manifold and comprising at least one drive cylinder;
• a compression stage distinct from the drive stage, the compression stage comprising at least one compression cylinder, and a pressure-operated exhaust valve of the at least one compression cylinder wherein the pressure-operated exhaust valve pumps air to the fuel burner chamber and comprises a variable pressure spring configured to provide minimum pressure to open the pressure-operated exhaust valve and maximum pressure to return a valve plug of the pressure-operated exhaust valve to a valve seat during higher RPM operation;
• and
• wherein a quantity of water injected into the externally-fired continuous combustion chamber post-combustion is configured to maintain an engine exhaust exiting to the engine exhaust manifold at or below a temperature set point.