Prelube - Pressure Verses Flow

To explore this topic let us start by considering the crank bearings of a running engine. The tight clearance in the bearing area between the crank shaft and the crank arms is filled with pressurized oil from the service oil pump. As the engine turns the load rotates around the crank shaft. This rotating load is therefore constantly climbing onto a fresh pillow of oil film substantially preventing wear as there is never any metal to metal contact.

Now let us consider the engine after having sat for some long period of time. The oil has drained back to the tank leaving only the slightest of oil films. The weight of the crank arm has pressed the pillow of oil out of the way and left it sitting in near metal to metal contact. If the engine is started at this point without prelube, wear will occur at a far greater rate than during normal running until oil from the service oil pump reaches these surfaces to reestablish the oil volume needed for the pillow. As the engine wears from this kind of event the bearing clearances get bigger requiring a greater volume of oil to fill them. Thus an engine with more wear will wear more at startup than a new engine. This leads to progressive failure.

The greatest value of the prelube system is that it reduces to near instantaneous the time between when the engine starts to crank and when the pillow of oil is established to prevent metal to metal contact. If the prelube pump has moved enough oil during its cycle to fill the clearance voids around all running surfaces, then as soon at the engine starts to move it immediately rolls onto a thick film of oil and very little wear takes place. For most applications prelube oil flow should be sufficient to extend the starting wear failure mode well beyond other failure modes that would take the engine out of service.

Very little wear however is still wear and that is where pressure comes into play. In order to prevent that last little bit of wear, the engine bearings must be designed in such a way as to cause oil pressure to lift the components out of metal to metal contact. Obviously this is an innate property of the engine and is not under the control of the end user. Having lifted the components out of metal to metal contact during the prelube cycle the parts are temporarily set back onto the oil film if the prelube cycle ends just prior to engine crank. This however is insufficient time for the oil film to dissipate and the components are still protected.