16.8 and 16.9 Piston engine lubricants, fuels and lubrication systems

Scope

A level 1, B1/B3 level 2.

• 16.8: properties and specifications of standard, alternate and drop-in fuels; lubricant properties and specifications; fuel additives; safety precautions.
• 16.9: lubrication system operation, layout and components.

Aviation gasoline (AVGAS)

Grade	Colour (dye)	Notes
AVGAS 80 (80/87)	Red	Low-lead; largely discontinued
AVGAS 100 (100/130)	Green	High lead
AVGAS 100LL	Blue	"Low lead" — but contains about four times the lead of grade 80 (FAA); the most common grade
UL91 / UL94	Unleaded (often undyed or specific colour)	For engines approved to use it
MOGAS (automotive)	Varies	Only where the engine/airframe is approved (STC/manufacturer); ethanol content prohibited unless approved

Key properties

• Anti-knock value (octane / performance number): resistance to detonation. Octane number = % iso-octane in an iso-octane/heptane reference mixture with the same knock resistance (80-octane = 80 % iso-octane, 20 % heptane). Above 100, performance numbers are used; the two numbers (e.g. 100/130) refer to lean / rich mixture ratings.
• Volatility — must vaporise readily for starting and distribution, but too volatile → vapour lock (fuel vaporising in lines/pumps and blocking flow) and evaporation losses. Measured by Reid vapour pressure and distillation curve. Causes of vapour lock: low pressure (altitude), high fuel temperature, excessive turbulence in lines. Prevention: boost pumps, routing lines away from heat, keeping fuel cool.
• Calorific value, freezing point, stability, cleanliness.

Additives

• Tetraethyl lead (TEL) — raises anti-knock value; with ethylene dibromide as a scavenger to remove lead deposits.
• Dyes for grade identification.
• Antioxidants, corrosion inhibitors, and (optionally) fuel-system icing inhibitor.

Alternate and drop-in fuels

• Alternate fuels: approved fuels with possible operating limitations (e.g. MOGAS under specific approval, Jet A-1 in diesel aero engines).
• Drop-in unleaded avgas replacements are certified to be used without engine/airframe modification once approved for the type.

Diesel aero engines

Burn Jet A/Jet A-1 (or diesel fuel where approved); kerosene's lower volatility reduces fire risk and vapour lock.

Fuel safety precautions

Correct grade check (placards at filler caps, nozzle sizes, decals), bonding before refuelling, no smoking or ignition sources, fuel sample checks for water and contamination (sumps after refuelling and before first flight), proper disposal of samples, skin protection (lead and benzene in avgas), ventilation.

Piston engine oils

Oil type	Use
Straight mineral oil	Break-in (running-in) of new or overhauled engines — no dispersant additives, so rings seat properly
Ashless dispersant (AD) oil	Normal use after break-in: dispersant additives keep contaminants suspended (removed at oil change) without forming metallic ash deposits (which could cause pre-ignition)
Multigrade / semi-synthetic AD	Better cold-start flow and high-temperature protection

FAA cautions: switching an old, dirty engine from mineral to AD oil can loosen sludge and block oil passages/screens — follow manufacturer guidance.

Viscosity grades: aviation grades such as 65, 80, 100, 120 (≈ twice the SAE numbers 30, 40, 50, 60); multigrades like SAE 15W-50. "W" prefix in aviation oil names (e.g. W100) denotes ashless dispersant oil.

Required properties: correct viscosity and high viscosity index (stable viscosity with temperature), high flash point and fire point, low pour point, chemical stability, resistance to oxidation and carbon formation, low cloud point.

Functions of engine oil: lubricate, cool (piston crowns, bearings), seal (piston rings), clean (carry away contaminants), protect against corrosion, cushion shock loads.