Stirling cycle machine

An external combustion engine comprising:

• a crankcase, the crankcase comprising a crankcase gas and lubricating oil, the crankcase gas having a crankcase pressure;
• at least two heater heads;
• at least two pistons, each piston reciprocating within one of the at least two heater heads;
• a working volume defined in part by the at least two heater heads and at least two pistons, the working volume comprising a working gas, the working gas having a working pressure;
• at least one pair of piston rods, each piston rod coupled to a piston and located partially in the working volume;
• a drive mechanism located in the crankcase, wherein the drive mechanism constrains one piston rod to move 180 degrees out of phase with the other piston rod in the at least one pair of piston rods;
• an airlock separating the crankcase and the working space, the airlock comprising an airlock gas, the airlock gas having an airlock pressure, wherein one or more pairs of piston rods passing through the airlock and each piston rod is sealed to the airlock by a rolling bellows seal;
• and
• a bidirectional regulator for maintaining a pressure differential between the crankcase gas and the airlock gas

The external combustion engine of claim 1 wherein the bidirectional regulator comprises:

• an oil filter fluidically connected to the crankcase;
• a compressor fluidically connected to the filter;
• and
• a pressure regulating spool valve fluidically connected to at least one of the filter, the compressor and the airlock.

A bidirectional pressure regulator for maintaining a pressure differential between a first volume and a second volume, the first volume filled with a gas and liquid, the gas in the first volume having a first pressure, the second volume filled with the gas, the gas in the second volume having a second pressure, the bidirectional pressure regulator comprising;

• a filter fluidically connected to the first volume;
• a compressor fluidically connected to the filter;
• and
• a pressure regulating spool valve having fluidic connections to at least one of the filter, the compressor and the second volume, wherein the fluidic connections are based on a position of the pressure regulating spool valve.

A method for controlling the pressure difference between an airlock having a gas at an airlock pressure and a crankcase having a gas and liquid at a crankcase pressure, the crankcase containing a drive mechanism, the method comprising:

• measuring the difference between the airlock pressure and the crankcase pressure;
• preventing the motion of the drive mechanism if the airlock pressure is less than a first predetermined amount above the crankcase pressure;
• operating a compressor to draw gas across a filter from the crankcase, when the airlock pressure is less than a second predetermined amount above the crankcase pressure;
• fluidically coupling the airlock to the compressor when the airlock pressure is less than a third predetermined amount above the crankcase pressure;
• fluidically coupling the airlock to the crankcase via the filter when the airlock pressure is more than a forth predetermined amount above the crankcase pressure.