A cryogenic fuel storage and supply system for an engine comprising: an insulated tank adapted to contain a supply of liquid cryogen with a headspace above the liquid cryogen; a liquid fuel line adapted to connect the liquid cryogen in the tank to an engine; a pump inside the tank and adapted to pump the liquid cryogen through the liquid fuel line toward the engine when the pump is activated, wherein the pump has a capacity sufficiently large to provide simultaneous supply of gas to an engine while also pressure building the tank or saturating the liquid cryogen in the tank; a heat exchanger in the liquid fuel line and adapted to vaporize liquid cryogen for delivery to the engine, wherein the heat exchanger has capacity to simultaneously deliver vapor to the engine and vapor to the tank; an economizer line communicating with the headspace in the tank and adapted to direct vapor in the headspace toward the engine when a predetermined pressure in the tank is achieved; a pressure build line configured to selectively permit an excess gas stream from the heat exchanger to be delivered back to the headspace; a saturated gas line configured to selectively direct a vaporized stream from the heat exchanger to liquid cryogen in the tank for purposes of saturating the liquid cryogen; a withdrawal line in fluid communication with the pump and the liquid fuel line so that the pump is configured to drive liquid cryogen through the withdrawal line towards the engine; a withdrawal line control valve disposed along the withdrawal line; a gravity withdrawal line in fluid communication with the withdrawal line and configured to communicate with liquid cryogen in the tank so that the liquid cryogen is delivered to the withdrawal line; and a gravity withdrawal line control valve disposed along the gravity withdrawal line. an insulated tank adapted to contain a supply of liquid cryogen with a headspace above the liquid cryogen; a liquid fuel line adapted to connect the liquid cryogen in the tank to an engine; a pump inside the tank and adapted to pump the liquid cryogen through the liquid fuel line toward the engine when the pump is activated, wherein the pump has a capacity sufficiently large to provide simultaneous supply of gas to an engine while also pressure building the tank or saturating the liquid cryogen in the tank; a heat exchanger in the liquid fuel line and adapted to vaporize liquid cryogen for delivery to the engine, wherein the heat exchanger has capacity to simultaneously deliver vapor to the engine and vapor to the tank; an economizer line communicating with the headspace in the tank and adapted to direct vapor in the headspace toward the engine when a predetermined pressure in the tank is achieved; a pressure build line configured to selectively permit an excess gas stream from the heat exchanger to be delivered back to the headspace; a saturated gas line configured to selectively direct a vaporized stream from the heat exchanger to liquid cryogen in the tank for purposes of saturating the liquid cryogen; a withdrawal line in fluid communication with the pump and the liquid fuel line so that the pump is configured to drive liquid cryogen through the withdrawal line towards the engine; a withdrawal line control valve disposed along the withdrawal line; a gravity withdrawal line in fluid communication with the withdrawal line and configured to communicate with liquid cryogen in the tank so that the liquid cryogen is delivered to the withdrawal line; and a gravity withdrawal line control valve disposed along the gravity withdrawal line.