Understanding Oil Flow

The flow of oil through a Lycoming reciprocating aircraft engine is known to be a necessary function during the operation of the engine. Pilots are often not at all concerned about how this function occurs, as long as the oil pressure and oil temperature indicators show a proper reading. A&P mechanics, on the other hand, often need to know how the system works and what parts control the flow of oil during various phases of operation. Because of the large number of calls on this subject, which are received by Lycoming Service Specialists, we can be sure that there are many who do not have a good understanding of the oil system.

It is not surprising that many A & P mechanics do not have a firm grasp on the operation of the oil system. There is room for confusion since there are two basic systems and several variations on each of these.

Except for the screens, filter and oil cooler, the flow of oil through the engine is completely pre-determined by the designed engine-running clearances and by the passages which are drilled in the crankcase and accessory housing during engine manufacture. This flow of oil serves three purposes. First, it lubricates, but cooling the engine by carrying away the heat generated by combustion is a second purpose, which is often just as important. Many engines, particularly those which are turbocharged, have oil squirts in each cylinder which are designed to direct cooling oil on the back side of the piston. And finally, the oil cleans the engine by picking up dirt and depositing it in the screens or filter, or by keeping that dirt in suspension until the oil is changed.

The oil which has done its lubricating, cooling and cleaning flows by gravity back to the oil sump. From the sump, the oil pump pulls the oil through the suction screen. This screen will filter out large particles of carbon, dirt or metal. The pump then forces the oil through one of the two basic systems. In each of the two basic systems, there is a valve which forces the oil through the oil cooler when the valve is seated or allows the oil to bypass the cooler when the valve is open. Lycoming Engines were originally equipped with a bypass valve which was controlled by a spring. Referred to as a spring and plunger type, it functioned as a result of the amount of pressure in the oil system. The spring-controlled bypass system was superseded by a system controlled by a Thermostatic Oil Cooler Bypass which reacts to oil temperature changes.

The operation of the spring-controlled bypass system is the result of thick oil which causes an increase in differential pressure across the bypass valve and causes the bypass valve to be open, thus bypassing the oil cooler. As the oil warms up, oil viscosity and pressure in the system are reduced, allowing the bypass valve to close and forcing oil flow through the oil cooler. Although the bypass valve helps the engine to warm up more quickly by routing cold oil around the oil cooler, its primary function is for system safety; should the oil cooler become plugged for any reason, system pressure will rise and the differential pressure across the bypass valve will again cause the valve to open. This bypasses the oil cooler and prevents a possible rupture of the cooler and loss of the oil.

The Thermostatic Oil Cooler Bypass Valve was designed to provide better control of the engine oil temperature while also maintaining the safety of the oil system by bypassing oil around an oil cooler which is plugged for any reason. The thermostatic oil cooler bypass valve may be used on engines which use the pressure screen system and on engines which have a full-flow oil filter. For most engine models, an oil filter also requires an oil filter adapter. While the oil is cold, this system allows oil to flow through the oil filter without passing through the oil cooler. As oil temperature rises to approximately 180˚ F, the valve closes and forces the oil to pass through the oil cooler. The oil then returns to the accessory housing where it is routed through the oil filter adapter, the filter and then again through the filter adapter, accessory housing and finally into the crankcase.

The oil filter is another part of the system where blockage could cause serious problems. For this reason, an oil filter bypass is built into the oil filter adapter, or in the case of engines utilizing a dual magneto, into the accessory housing. These bypass valves are built-in safety features which activate as a result of excessive pressure in the oil filter. The oil filter bypass is not adjustable.

Oil enters the crankcase of most Lycoming Engines near the top of the right rear cylinder where it passes through the pressure-relief valve. There are three types of pressure-relief valve. With either the short or long dome valve, pressure is adjusted by removing the dome and adding or deleting washers which are located under the controlling spring. There is also a third style of pressure-relief valve which may be adjusted with the twist of a wrench or screwdriver.

An individual looking for the pressure screen housing may not find exactly what he or she is looking for since there are two possible variations. The housing for the pressure screen may have one hole facing the rear of the engine. This housing is used on engines incorporating a spring and plunger to control oil flow, and the single hole will be used for oil temperature probe. Another style of pressure screen housing has two holes facing the rear of the engine. The small hole is used for oil temperature bulb connection, and a Thermostatic Oil Cooler Bypass Valve is installed in the large hole.

Even more attention to detail may be required when an oil filter is installed. The pressure screen housing must be removed and oil-filter adapter installed in its place. With the oil filter adapter installed, either a spring-controlled bypass valve installed in the accessory housing just above the adapter, or a thermostatic bypass valve installed in the bottom of the adapter may be used to control oil flow to the oil cooler. Because of the better oil temperature control, use of the thermostatic oil cooler bypass valve is preferred by Lycoming. For engines shipped from the factory with an oil filter, and requiring an oil cooler in the aircraft installation, it is standard procedure for Lycoming to supply a thermostatic bypass valve. The hole in the accessory housing, which is needed for the spring-controlled bypass valve, is capped with a plug. A hole on the top of the adapter is provided for the oil temperature bulb.

A possible variation for the flow of oil which may be found with a Lycoming Engine. Some airframe manufacturers have utilized small engine models without an oil cooler. At the request of these airframe manufacturers, these engines are not machined to accommodate an oil cooler. Individuals who acquire these engines for use in their home-built aircraft may need an oil cooler to keep temperatures within operating limits. This can be accomplished by utilizing an adapter—Lycoming part number 62418. Utilization of this adapter will allow the engine to be used and the oil to be cooled, but there are limitations. An oil filter cannot be installed, and only the one-hole pressure screen housing can be used. This limits the system to use as a spring-controlled oil cooler bypass valve which is installed in the adapter.

There are several bits of information which may be helpful to those who have now acquired a better understanding of the Lycoming engine oil system. Lycoming Service Instruction 1008B gives instructions for installation of a Thermostatic Oil Cooler Bypass Valve on engines which have a pressure screen housing and no filter. Special Service Publication (SSP) 885-2 gives instruction for the installation of engine-mounted oil filter kits. And finally, a kit (Number 05K21437) for a remotely mounted oil filter has been developed.