System and method for comprehensively utilizing residual heat of compressed natural gas engine (CNGE)

The invention discloses a system and a method for comprehensively utilizing residual heat of a compressed natural gas engine (CNGE), which relates to a novel energy-saving technology of an engine. In the method, high-pressure fuel gas output by a high-pressure gas cylinder is heated with tail gas and/or the residual heat of water in a cylinder sleeve by using a residual heat exchanger, and the high-pressure fuel gas is depressurized and treated by using two stages of expansion machine devices (a variable-expansion-ratio expansion machine serving as one stage and a fixed-expansion-ratio expansion machine serving as the other stage), so that the fuel gas pressure needed by engine combustion is reached; meanwhile, pressure energy and residual heat are utilized comprehensively by using the expansion machines, so that the output shaft power of the engine is increased, the system efficiency of the engine is increased, the driving mileage is increased, and natural gas consumption is reduced. The device disclosed by the invention is compact in structure, and is reliable in performance; 80 percent of energy consumed in a natural gas compressing process is recovered, so that the advantage of natural gas serving as engine fuel gas is improved remarkably.