LO 10-5 Identify and describe the types, functions, construction features, and applications of diesel engine lubrication system components

1.12. Oil Filter Bypass Valves

Oil Filter Bypass Valves

The bypass valve features pressure control valves and is incorporated into the filter, filter housing, or lubrication system module. The bypass valve allows oil to pass from the dirty side to the clean side of the filter if resistance or restriction is excessive across the filter media Figure 50. This prevents oil starvation of the engine in the event of a plugged filter. The simplest bypass valve design uses a spring-loaded filter element that causes the spring located beneath the element to compress if the oil pressure differential across the filter element becomes too high Figure 51. Compressing the spring moves the filter away from the base, allowing a direct connection between the filter inlet and the outlet. This condition permits oil to move directly from the dirty filter inlet through to the filtered center of the element without passing through the filter media.

FIGURE 10-50: Types of filter bypass mechanisms. Most HD diesels incorporate an additional bypass valve in the filter header.

FIGURE 10-51: A bypass valve located in the header of the oil filter. The bypass valve opens when filter restriction is too high, and dirty oil is then circulated through the engine.

To supplement the filter bypass valve, an oil filter bypass relief valve is often found in the oil filter header of many engines. Its purpose is identical to the filter bypass valve in a cartridge filter: it allows pressurized oil to bypass a plugged filter. The filter bypass valve operates on principles of pressure differential. Usually, when a new oil filter is installed, very little pressure drop occurs across the filter media. With increasing contamination, however, the filter becomes restrictive and a progressively larger pressure drop occurs across the filter. Typically, when the pressure drop exceeds approximately 3–30 psi (21–207 kPa), the bypass valve opens and allows unfiltered oil to flow into the main oil gallery. Without the bypass valve, the pressure differential across the filter media would burst or internally tear the filter.