PRESSURE PUMP
The pump is normally a gear type pump driven by the engine. Its purpose is to deliver the oil to the engine at the correct pressure and ensure adequate lubrication. It consists of two gears, one of which is driven by the engine (termed the driven gear), which meshes with and drives the other gear. The two rotating gears capture oil in the spaces between the gear teeth and the pump housing on the suction side, transferring it to the pressure side.
The pressure obtained depends on the downstream flow restrictions created by bearings, etc.
The pressure pump must be able to deliver sufficient oil to ensure adequate lubrication over the complete engine speed range. It is a positive displacement pump, where the same amount of oil flows through the pump at a consistent rpm. To ensure that the system pressure remains constant because of engine speed changes and, therefore, pump speed variations, there must be some method of relieving excess pressure.
Fig. 4.4
Fitting a pressure-relief valve achieves this. If the pressure is below that for which the relief valve is set, the valve remains closed. When the pressure exceeds the pre-determined value, the valve opens returning oil to the sump or inlet side of the pump. This ensures that the system pressure is constant. The relief valve can be a simple spring-loaded ball arrangement or, for high altitude, a more complicated arrangement that senses atmospheric pressure as well.
COOLERS
Oil absorbs some of the internal heat developed within the engine. In order for it to do this efficiently, it needs to be cooled. Therefore, an oil cooler is fitted in the oil system of both the wet sump and dry sump systems and is normally cooled by ram air. Some systems incorporate adjustable shutters to control the amount of cooling air entering the cooler and can be automatically positioned in response to signals from an oil thermostat.
Chapter 4 Piston Engine Lubrication and Cooling
Fig. 4.5
If the oil is overcooled, a condition called coring can occur. The oil in the matrix congeals and stops the oil flow. As the oil flow stops, the relief valve opens, returning hot oil back to the engine or tank. A rapid rise in oil temperature is an indication that coring is present. To correct this situation, the cooler needs to be heated to encourage oil flow once more. Closing the shutters or reducing speed and descending to a warmer level can achieve this.
A thermostatic control valve allows cold oil to bypass the core of the cooler. As the oil warms up, a valve diverts the oil through the core so that the air can pick up excess heat. In some cases, the thermostatic valves can also function as a pressure relief valve and are both temperature and pressure sensitive.
FILTERS
Piston Engine Lubrication and Cooling Chapter 4
Filters are fitted to remove small particles of contamination passing through the lubricating system that can result in damage to the pump and bearings. Most piston engines use the full flow oil system, where all the oil passes through a pressure filter located downstream of the pressure pump. In a dry sump system, scavenge filters are normally located in the scavenge lines from the bearings and pumps. If the filter becomes blocked, engine failure is likely, so the filter has a bypass to allow unfiltered oil to be supplied to the engine. A strainer, usually a coarse mesh type filter, is in the oil sump of a wet sump engine to prevent contamination from entering the system.
PRESSURE GAUGE
Fig. 4.7
This is the most important gauge for satisfactory engine operation. Should the oil pressure fail, bearing failure occurs very quickly. A green arc on the face of the gauge shows the normal pressure range, a yellow arc for the caution range, and a red line for maximum oil pressure. Oil pressure should register on the gauge within 30 seconds of the engine starting or slightly longer on a cold day. Should the oil pressure not register within this time, shut the engine down.
TEMPERATURE GAUGE
Fig. 4.8
This gauge shows the oil temperature taken after the oil cooler and has a green arc for the normal temperature range and a red line for maximum temperature. The gauges used on light aircraft consist of a sealed Bowden tube filled with a fluid. As the temperature of the lubricating oil rises, it heats the fluid in the tube, causing it to expand. This expansion results in the pointer moving over a temperature scale.
Chapter 4 Piston Engine Lubrication and Cooling
OIL TANK
Fig. 4.9
This is only fitted to dry sump systems and is an external tank, which contains the correct amount of oil in order to provide proper circulation and cooling. Engines with constant speed propellers feed the pressure pump through a stack pipe in the tank with the propeller feathering pump outlet at the bottom of the tank. Therefore, should fault occur resulting in a loss of oil, a reserve of oil is retained in the tank for propeller feathering.
COOLING
Piston Engine Lubrication and Cooling Chapter 4
Air-cooling is the method employed on modern light aircraft and is achieved by passing air over the engine. To improve the cooling effect, the cylinder barrels and cylinder heads are fitted with fins to increase the surface area for efficient heat transfer to the airflow. It is a pressure cooling system where air from the propeller and the aircraft’s forward speed is forced through the cowling air inlets to the upper part of the engine, creating a high-pressure region. Baffles then direct the air over the cylinders to the lower part of the engine, which is the low-pressure region. The air exits via a fixed outlet or variable cowl flaps. To direct jets of cooling air over various accessories such as magnetos, alternators, and fuel pumps, blast tubes can be built into the baffles.