11.1 Reciprocating Engines, Mixture, Detonation, and Preignition

ACS PA.I.G.K1c (powerplant and propeller) and the combustion half of PA.I.G.K2 live in one picture: a four-stroke spark-ignition engine turning a propeller, cooled by air and oil, and kept honest by the mixture. The Private Pilot Airplane knowledge test does not ask you to time a magneto with a light. It asks which stroke expands, whether you lean for a sea-level takeoff, what detonation is versus preignition, and which lever sets RPM on a constant-speed propeller.

The four-stroke Otto cycle

Most PAR trainers use a horizontally opposed, air-cooled, spark-ignition reciprocating engine. Each cylinder runs the Otto cycle — four strokes, two crankshaft revolutions, one power event per cylinder.

PHAK is explicit: the gaseous mixture is expanding on the power stroke. Intake is not “the explosion.” Compression is not ignition. A two-stroke packs those events into two strokes and is not the light-trainer default.

Cylinders fire in a designed order so the crankshaft sees almost continuous torque. The propeller is a rotating airfoil on that crankshaft (or on a geared shaft). Thrust depends on blade shape, blade angle of attack, and RPM. Blade twist (a steeper pitch near the hub, flatter toward the tip) keeps angle of attack more uniform along a blade whose tip is moving much faster than its root.

Mixture: rich to cool, lean to match density

The mixture control sets how much fuel joins a given mass of air. Fuel/air ratio is a weight ratio. As you climb, air density falls while an unadjusted carburetor or injector still meters roughly the same fuel, so the mixture becomes richer. Spark-plug fouling, roughness, and wasted fuel follow. The basic purpose of leaning at altitude is to cut fuel flow to match the thinner air.

PAR default, unless the AFM writes a high-density-altitude exception:

• Takeoff and climb — full rich (or the AFM takeoff-rich setting). Extra fuel absorbs heat as it vaporizes, holds cylinder-head temperatures down, and gives detonation margin at high power. Leaning a sea-level takeoff for “economy” is a classic miss.
• Cruise — lean per the AFM/POH. Many handbooks use peak RPM on a fixed-pitch airplane, or an exhaust gas temperature (EGT) procedure on an injected or well-instrumented engine.
• Descent and landing — enrich on the way down so the engine is rich before you need go-around power.

EGT measures how hot the exhaust is leaving the cylinder. Peak EGT sits near the chemically correct (stoichiometric) mixture. Rich of peak, unused fuel cools the charge and EGT falls. Lean of peak, there is less fuel to burn and EGT falls again. Best power is typically a slightly rich mixture. Best economy is leaner — at or lean of peak only if that AFM allows it. Do not invent a universal “100 °F rich of peak” number that is not in that airplane’s book.

Cylinder head temperature (CHT) is the more honest heat story. Oil temperature is slower and indirect. High CHT plus a lean mixture at high power is the detonation setup.

Detonation versus preignition

Normal combustion is a progressive burn from the spark plugs across the chamber. It is not an explosion.

Detonation is an uncontrolled, explosive ignition of the remaining fuel-air charge after the spark has already started a normal flame. Pressure spikes, CHT climbs, the engine may run rough and lose power, and pistons can be damaged. PHAK’s usual causes:

• Fuel grade lower than specified
• High manifold pressure with low RPM
• High power with an overly lean mixture
• Extended ground operations or prolonged steep climbs (poor cooling)

If you suspect detonation on a fixed-pitch climb-out, PHAK’s first cooling move is often lower the nose to increase airspeed and cooling flow — then enrich, reduce power, and open cowl flaps if you have them.

Preignition is ignition before the spark fires — a hot spot (carbon, a damaged plug insulator, an overheated electrode or exhaust valve). The charge starts burning too early and fights the still-rising piston. Power falls and temperatures soar. Detonation and preignition can feed each other. The vocabulary trap is swapping the two words: preignition is before the spark; detonation is the explosive leftover burn after a normal spark.

Overheating, oil, and shock cooling

Most leftover heat leaves through the exhaust. The rest is dumped by cooling fins and by oil, which lubricates, seals, carries away contaminants, and moves heat. Typical trainers are wet-sump: the oil lives in an integral sump. Oil pressure is the quick health check; oil temperature lags. Low oil can show up as high oil temperature. Cowl flaps, when fitted, open to dump heat and close to keep heat. On many PAR airplanes the only “hydraulic” circuit you will ever service is the brake system; some constant-speed governors also use engine oil pressure to change blade angle.

Overheating (high CHT or oil temp) is treated by more cooling air and less heat made: increase airspeed, enrich the mixture, reduce power, open cowl flaps. Excessively high temperatures cause loss of power, high oil consumption, and possible permanent damage.

Shock cooling is the opposite conceptual problem: a sudden large power cut in a high-speed descent can drop CHT fast enough to worry cylinders. The knowledge-test idea is gradual power reductions and some residual power in a long descent — not a published “never more than X degrees per minute” private-pilot statute.

Fixed-pitch versus constant-speed

A fixed-pitch propeller is one blade angle, most efficient at one airspeed/RPM pair. A climb prop is flatter (better takeoff, worse cruise); a cruise prop is steeper. The tachometer is the power instrument. RPM rises if you dive and falls if you climb, even at a fixed throttle. At altitude you may need more throttle to hold the same RPM because the air is thinner.

A constant-speed propeller uses a governor to change blade angle so RPM stays where you set it. Two cockpit controls:

• Throttle sets power, read on the manifold pressure (MP) gauge — absolute pressure in the intake manifold. Engine off, MP reads approximately ambient (about 29.92 inHg at sea level on a standard day).
• Prop control sets RPM.

High load or high airspeed: blades coarsen to hold RPM. Low load: blades flatten. When the blades are on the pitch stops (common in the landing flare), RPM again changes with power like a fixed-pitch prop.

Order that protects the engine: increase RPM before increasing MP; reduce MP before reducing RPM. High MP at low RPM is a detonation invitation. Confirm the AFM’s power-setting table; do not memorize an unpublished “24 squared” as law for every trainer.

Scenario: Maya’s high-DA takeoff

Maya is departing a 6,000-foot field on a hot afternoon. The AFM has a high-density-altitude leaning procedure for takeoff. That is the exception, not a license to lean every sea-level takeoff. She leans only as the book says, keeps the mixture rich enough for cooling at high power, and watches CHT. If the engine starts to feel rough and CHT climbs in the climb, she treats it as possible detonation: lower the nose, enrich, and reduce power rather than “lean a little more to smooth it out.”