Plunger pump, low-viscosity fluid pressure building unit, pressure building system and engine

The invention provides a plunger pump, a low-viscosity fluid pressure buildup unit, a pressure buildup system and an engine. Wherein the plunger pump comprises a plunger, an oil inlet valve and an oil outlet valve; when the plunger moves in the first direction, the oil inlet valve is closed, the oil outlet valve is opened, and pressurized low-viscosity clean fuel is output; when the plunger moves in the second direction, the second direction is opposite to the first direction, the oil outlet valve is closed, the oil inlet valve is opened, and low-pressure low-viscosity clean fuel enters the plunger pump; wherein the plunger is provided with multiple seals and at least comprises a first sealing part and a second sealing part, the first sealing part provides main sealing for the plunger pump, and the exterior of the first sealing part is communicated with an oil return channel, so that a working medium leaked from the first sealing part enters a low-pressure oil inlet channel; and the second sealing part provid