A direct drive unit compressor system, comprising: a power generation unit comprising a truck engine, the power generation unit including a first power transfer device; a direct drive unit comprising a driven element, the driven element being a compressor, the direct drive unit including a second power transfer device; and a speed-governed transmission unit comprising a belt coupled to the first and second power transfer devices, wherein the speed-governed transmission unit is configured to transmit a substantially constant level of power generated by the power generation unit to the direct drive unit in order to drive operations of the compressor at or near the current requirements thereof independently of the current engine output; wherein the first power transfer device comprises a first crank-shaft, which is rotatable about a first rotational axis, and the second power transfer device comprises a second crank-shaft, which is rotatable about a second rotational axis; and wherein the first crank shaft is rotatable about the first rotational axis at a rotational speed based on the current engine output, and the second crank shaft is rotatable about the second rotational axis at a rotational speed based on the current requirements of the compressor; the system characterized in that the first power transfer device is operable at varying speed modes (411,412,413,414) based on current engine output; and the second power transfer device is operable at varying speed modes (511,512,513,514) based on current requirements of the compressor, and in that the speed-governed transmission unit further comprises: first and second sets of hub gears (62,63) of varying size disposed on the first and second crank-shafts (410,510), respectively; and first and second mech elements (64,65) disposed to guide the belt between selected ones of the hub gears of the first and second sets of hub gears, respectively. a power generation unit comprising a truck engine, the power generation unit including a first power transfer device; a direct drive unit comprising a driven element, the driven element being a compressor, the direct drive unit including a second power transfer device; and a speed-governed transmission unit comprising a belt coupled to the first and second power transfer devices, wherein the speed-governed transmission unit is configured to transmit a substantially constant level of power generated by the power generation unit to the direct drive unit in order to drive operations of the compressor at or near the current requirements thereof independently of the current engine output; wherein the first power transfer device comprises a first crank-shaft, which is rotatable about a first rotational axis, and the second power transfer device comprises a second crank-shaft, which is rotatable about a second rotational axis; and wherein the first crank shaft is rotatable about the first rotational axis at a rotational speed based on the current engine output, and the second crank shaft is rotatable about the second rotational axis at a rotational speed based on the current requirements of the compressor; the system characterized in that the first power transfer device is operable at varying speed modes (411,412,413,414) based on current engine output; and the second power transfer device is operable at varying speed modes (511,512,513,514) based on current requirements of the compressor, and in that the speed-governed transmission unit further comprises: first and second sets of hub gears (62,63) of varying size disposed on the first and second crank-shafts (410,510), respectively; and first and second mech elements (64,65) disposed to guide the belt between selected ones of the hub gears of the first and second sets of hub gears, respectively. first and second sets of hub gears (62,63) of varying size disposed on the first and second crank-shafts (410,510), respectively; and first and second mech elements (64,65) disposed to guide the belt between selected ones of the hub gears of the first and second sets of hub gears, respectively.