Cooling and Lubrication

Why the engine must be cooled

The reasons for cooling follow from what unchecked heat does. Roughly a third of the fuel's energy ends up as heat in the engine's metal, and if it stays there the cylinder head temperature climbs into the territory the fundamentals lesson mapped: detonation and pre-ignition become likely, the metal itself loses strength, pistons and valves distort and burn, and the oil breaks down and stops protecting. Cooling exists to hold every part inside its working temperature range. The range has a bottom as well as a top: cooling an engine too fast, as in a long idle-power descent, shocks the hot metal with uneven contraction, so the aim is a controlled temperature, not the lowest one.

Air cooling by design

Most aviation piston engines are air-cooled, and the design features that make air cooling work all serve one goal, forcing enough airflow over enough surface:

Cooling fins — the cylinder and head are cast with deep fins that multiply the surface area the air can carry heat away from.

Baffles — sheet-metal walls inside the cowling that block the easy paths and funnel the entering air through the fins of every cylinder, front and rear alike, so no cylinder sits in stagnant air.

The cowling itself — shaped so that ram air enters at the front at high pressure and exits at the rear at low pressure, driving flow through the engine compartment.

Cowl flaps — adjustable doors at the cowling exit that let the pilot set how much air is pulled through.

Cowl flaps

The cowl flaps are the pilot's cooling control. Open, they stick into the airstream, lower the pressure at the cowling exit, and pull more air through the fins, at the price of extra drag; closed, they streamline the cowling and let the engine run warmer and the aircraft faster. The operating pattern follows the engine's need: open for ground running, take-off and climb (high power, low airspeed: maximum heat, minimum airflow), closed or trailing in cruise, and closed in the descent to keep the cooling gentle. They are operated manually or electrically from the cockpit, against the cylinder head temperature gauge.

Liquid against air

The two cooling philosophies compare like this:

Air cooling — simple, light and robust: no radiator, pump, hoses or coolant to leak, freeze or boil, and nothing extra to maintain. Its costs are uneven temperatures (the rear cylinders and the head run hotter than the rest) and total dependence on airflow: on the ground, in a slow climb, the cooling is at its weakest exactly when the power is high.

Liquid cooling — a coolant loop with radiator and pump holds every cylinder at a nearly uniform, controlled temperature, permits tighter cowlings, and cools well regardless of airspeed. Its costs are weight, complexity, and vulnerability: a single hose or radiator leak can end the flight.

Watching the temperature

The cooling system's cockpit witness is the cylinder head temperature (CHT) gauge: a temperature probe fitted to the head of the hottest-running cylinder, chosen at certification, on the reasoning that if the worst-cooled cylinder is within limits, the rest are too. The CHT is the number the cowl flaps are flown against, and its unexplained rise is evidence in two directions at once: failing cooling, or the detonation story of the fundamentals lesson.

What the oil does

The lubrication system's functions go well beyond friction:

Lubricating — keeping a film of oil between moving surfaces so metal never touches metal.

Cooling — oil sprayed and splashed inside the engine carries heat away from parts the airflow can never reach, the piston undersides and bearings above all.

Sealing — the oil film helps the piston rings seal the ring-to-wall gap.

Cleaning — the circulating oil washes wear metal and combustion residue to the filters.

Protecting — the film coats steel against corrosion between flights.

Viscosity and the grade numbers

Viscosity is a fluid's resistance to flow, and its defining behaviour is that it falls as temperature rises: cold oil is thick and reluctant, hot oil thin and free-flowing. An engine needs both ends managed, oil thin enough to circulate at start-up yet thick enough to keep its film at running temperature. The aviation grade numbering expresses viscosity: the higher the grade number, the more viscous the oil, and the commercial aviation grades run at roughly double the equivalent SAE numbers (SAE is the automotive grading scale; an aviation grade 100 corresponds to about SAE 50). Multigrade oils span a range, flowing like a thin oil when cold and protecting like a thick one when hot.

Oil tank — carries the supply, with an air vent, an expansion space above the level for the oil's growth as it heats, a hot well (a small compartment that keeps the circulating core of oil together so it warms quickly after start) and a de-aerator that strips the air out of the frothy returning oil before it rejoins the supply.
Check valve (non-return valve) — stops tank oil seeping down into the engine while the aircraft is parked.
Pressure pump and pressure-relief valve — the pump drives oil into the engine; the relief valve spills excess back to the pump inlet, capping the pressure whatever the rpm or the oil's thickness.
Filters — at the tank outlet (suction), after the pump (pressure) and in the return (scavenge), each protecting the stage after it from debris.
Scavenge pump — drains the engine and returns the oil to the tank. It has a larger capacity than the pressure pump, because what it pumps is froth, part oil and part air, and it must keep the engine dry regardless.
Oil cooler with bypass valve — a small radiator in the return line sheds the heat the oil collected. Its bypass valve routes oil around the cooler when the oil is cold and thick (thermostatic) or when the thick oil's pressure across the cooler spikes (anti-surge), protecting the cooler and speeding the warm-up.
Pressure and temperature sensors, and the lines — the sensors feed the cockpit gauges; the lines tie the circuit together.

Wet sump, and the choice between them

In a wet-sump system there is no tank and no scavenge pump: the oil lives in the engine's own sump at the bottom of the crankcase, and a single pressure pump lifts it from there around the engine, gravity bringing it home. The trade between the two:

Wet sump — simpler, lighter and cheaper: fewer pumps, no tank, no external lines. Its limits: the capacity is what the sump holds, the oil cools poorly (it sits inside the hot engine), and unusual attitudes or sustained manoeuvres can slosh the oil away from the pump's pick-up and starve the pressure feed.
Dry sump — a bigger, cooler supply that keeps feeding in any attitude, suiting aerobatic and high-power engines. Its costs: more components, more weight, more plumbing to leak.

Oil consumption and the gauge triangle

Every piston engine burns a little oil, and the factors that set how much:

Oil grade — a thinner grade than specified slips past the rings and burns faster.
Cylinder and piston wear — worn clearances let oil through into the combustion space.
Piston ring condition — broken or stuck rings stop sealing the oil below the piston.

A slowly rising oil appetite is therefore one of the earliest signs of internal wear.

Pressure, temperature and quantity interact, and the exam wants the triangle read as one system: less oil aboard means the same heat is carried by fewer litres, so the temperature rises; hotter oil is thinner, so the pressure falls. Falling pressure with rising temperature is the classic signature of a falling quantity (a leak, or heavy burning), and it is trusted over any single gauge alone: one failed instrument moves one needle, a real oil problem moves both, in opposite directions.