Pressure washer pumps are normally always a positive displacement type. This means that all the water that enters the pump will discharge the pump. There is no “slippage” or losses like that of centrifugal pumps. This is the reason a positive displacement pump can produce high pressures and why it must have a bypass or unloader valve in the discharge if a trigger gun is used.

Most of these pumps employ one, two or three plungers reciprocating in chambers with seals and valves to move the water. Differences exist with the size (diameter) of the plungers, the stroke (back/forth distance) of the plungers and the type of mechanism used to move these plungers.
Very small (inexpensive) direct drive units will usually have short stroke, small diameter plungers driven by a wobble plate or eccentric. These pumps rotate at very high speeds (3600 RPM) and have a fairly short life span if used extensively.

Better direct drive pumps use larger plungers and/or longer strokes and are driven by an  eccentric crankshaft or true crankshaft similar to an automobile engine. These pumps rotate at significantly lower (1800 RPM) rotational speeds which increases longevity and reduces noise.

Heavy duty/commercial service pumps use even larger plungers and/or longer strokes and have a true crankshaft drive. These pumps will be belt driven at rotational speeds normally under 1500 RPM. They usually rotate at speeds from 500 RPM to 1000 RPM for the longest life possible.