first side 232 of a differential pressure switch gauge 234 a second side 236 of switch gauge 234 and the downstream side of second needle valve 230 are connected to a second tee 238 second tee 238 is also connected back to crankcase 20 through an oil passage 240 after the engine has been converted to form compressor 10 and the apparatus installed in package 12 it is ready for operation such as the compression of natural gas from a wellhead a line from the wellhead is connected to inlet valve 42 on tank 40 and the appropriate connection is also made to inlet line 60 on fuel vessel 58 similarly threaded end 88 of discharge line 86 is connected to whatever is downstream such as a storage vessel or pipeline if the gas being handled is suitable as fuel for the engine portion of compressor 10 this fuel flows from fuel vessel 58 through fuel line 64 into carburetor 26 pressure regulator 66 insures that the fuel pressure at carburetor 26 is maintained at a constant predetermined level as required by the carburetor the engine portion of compressor 10 which is the left side as seen in fig3 operates in a normal manner to.