Prerequisite Knowledge:
Oil is not a requirement for the combustion process, but it is the lifeblood of the engine. Oil seals parts of the engine, lubricates the moving parts, cleans the engine of debris, and helps regulate temperature.
Oil Viscosity
The viscosity of a fluid is a measure of how much it resists motion, with more viscous fluids being more gel like and less viscous fluids being free flowing. The viscosity of oil varies greatly with temperature, with cold oil being a congealed gel, and warm oil flowing freely around the constricted parts of the engine.
When an engine is first started, the oil begins heating up, and its viscosity reduces, allowing it to start moving through the engine. In cold weather, the oil may be unable to heat up to its proper operating temperature. For this reason, the manufacturer of the plane may recommend a different type of oil with a lower viscosity rating for cold weather flying.
Oil is not a requirement for the combustion process, but it is the lifeblood of the engine. Oil seals parts of the engine, lubricates the moving parts, cleans the engine of debris, and helps regulate temperature.
Oil Viscosity
The viscosity of a fluid is a measure of how much it resists motion, with more viscous fluids being more gel like and less viscous fluids being free flowing. The viscosity of oil varies greatly with temperature, with cold oil being a congealed gel, and warm oil flowing freely around the constricted parts of the engine.
When an engine is first started, the oil begins heating up, and its viscosity reduces, allowing it to start moving through the engine. In cold weather, the oil may be unable to heat up to its proper operating temperature. For this reason, the manufacturer of the plane may recommend a different type of oil with a lower viscosity rating for cold weather flying.
Purposes of Oil
As stated above, oil has several roles inside a piston engine:
Types of Oil Systems
There are two main types of oil systems: wet sumps and dry sumps.
Wet Sumps
Wet sump oil systems use an oil pan at the bottom of the engine (also known as a sump) to collect and store the oil. When the engine is running, oil is pumped to the top of the engine, and is allowed to trickle through, eventually reaching the sump at the bottom. Wet sump designs are relatively simple, but have several disadvantages. The oil is stored in close proximity to the engine, which makes cooling of the oil a potential issue. Aircraft with wet sumpe systems also can't fly in negative-gs (e.g. inverted flight), as oil will flow up into the engine from the sump. This can over-oil the engine, and cause several other issues.
Dry Sumps
Dry sump designs are more reliable in negative-g flight, and allow for better cooling. This is because the oil is stored in a separate tank, rather than the sump itself. Oil is still collected from the bottom of the engine at sump, but a scavenge pump moves any oil from the sump to a separate oil tank, which is generally located away from the engine. This allows for better engine cooling, and it means there is less of a risk for over-oiling the engine in negative-g flight.
Both of these systems use oil pumps, oil filters, temperature and pressure gauges, and oil breathers. The seals within the cylinders of an engine are not perfect, and some of the pressure inside the cylinder combustion cylinders can escape through the piston ring seals into the crankcase below. An oil breather acts as a vent, preventing the pressure from reaching excessive levels.
The dry sump system is the only system to use a separate oil tank.
Importance of Oil
Without oil, an engine would not operate for long. The pilot can monitor the oil system's performance through the oil temperature and pressure gauges inside the cockpit. An oil leak is usually followed by iminent engine failure, and is typically shown as a rise in oil temperature (less oil to absorb the engine heat, so the remaining oil has to absorb more heat), and a drop in oil pressure (less oil moving through the system). When the oil pressure drops below the minimum value, the engine will no longer be able to lubricate or seal itself (cooling and cleaning are not major issues in the short term). What will kill the engine is the friction between the moving parts, as this will "rapidly dismantle" the engine.
As stated above, oil has several roles inside a piston engine:
- Lubrication: The main purpose of engine oil. Your standard piston engine has many moving parts, all of which need to be lubricated in order to increase efficiency and reduce wear. Oil coats all of the moving parts of the engine, including the pistons, crankshaft, and camshaft, in order to prevent wear on these parts.
- Sealing: The various seals inside the engine (e.g. piston ring seals) are also coated with oil in order to help them make a better seal.
- Cleaning: Engine oil also picks up any debris inside the engine (e.g. metallic shavings from engine wear) and carries them away from the moving parts. The debris is eventually filtered out through the oil filter.
- Temperature regulation: Oil also has a slight effect on the temperature of the engine, preventing it from changing too quickly. As metal changes temperature, it expands and contracts, which can cause fatigue cracks to form within the engine. For example, rapidly reducing power to idle for descent can cause shock cooling, where the engine cools rapidly, and the components contract. Oil helps the engine maintain a constant temperature, and acts as a buffer to any sudden temperature changes.
Types of Oil Systems
There are two main types of oil systems: wet sumps and dry sumps.
Wet Sumps
Wet sump oil systems use an oil pan at the bottom of the engine (also known as a sump) to collect and store the oil. When the engine is running, oil is pumped to the top of the engine, and is allowed to trickle through, eventually reaching the sump at the bottom. Wet sump designs are relatively simple, but have several disadvantages. The oil is stored in close proximity to the engine, which makes cooling of the oil a potential issue. Aircraft with wet sumpe systems also can't fly in negative-gs (e.g. inverted flight), as oil will flow up into the engine from the sump. This can over-oil the engine, and cause several other issues.
Dry Sumps
Dry sump designs are more reliable in negative-g flight, and allow for better cooling. This is because the oil is stored in a separate tank, rather than the sump itself. Oil is still collected from the bottom of the engine at sump, but a scavenge pump moves any oil from the sump to a separate oil tank, which is generally located away from the engine. This allows for better engine cooling, and it means there is less of a risk for over-oiling the engine in negative-g flight.
Both of these systems use oil pumps, oil filters, temperature and pressure gauges, and oil breathers. The seals within the cylinders of an engine are not perfect, and some of the pressure inside the cylinder combustion cylinders can escape through the piston ring seals into the crankcase below. An oil breather acts as a vent, preventing the pressure from reaching excessive levels.
The dry sump system is the only system to use a separate oil tank.
Importance of Oil
Without oil, an engine would not operate for long. The pilot can monitor the oil system's performance through the oil temperature and pressure gauges inside the cockpit. An oil leak is usually followed by iminent engine failure, and is typically shown as a rise in oil temperature (less oil to absorb the engine heat, so the remaining oil has to absorb more heat), and a drop in oil pressure (less oil moving through the system). When the oil pressure drops below the minimum value, the engine will no longer be able to lubricate or seal itself (cooling and cleaning are not major issues in the short term). What will kill the engine is the friction between the moving parts, as this will "rapidly dismantle" the engine.
That's it for aircraft oil systems! The next article is to be decided.