How it works

How the engine makes power

Each cylinder runs the four-stroke cycle: intake, compression, power, exhaust. Two spark plugs per cylinder, fired by two independent magnetos, ignite the mixture. The magnetos are self-contained generators driven by the engine, so ignition keeps working even if the aircraft's electrical system fails completely.

Because there is one magneto per plug set, the pre-takeoff 'mag check' shuts off one magneto at a time. A small RPM drop (typically within the POH limit, and with limited difference between mags) is normal because only one plug per cylinder is firing. A large drop or a rough engine suggests fouled plugs, a bad harness, or timing issues.

Fuel metering: carburetor vs injection

Older 172s use a float-type carburetor. Air flowing through a venturi pulls fuel in. Fuel vaporisation cools the air so much that ice can form inside the carburettor even on warm humid days, which is why carburettor heat exists: it routes unfiltered, pre-heated air around the throttle plate. Carb heat gives a small RPM drop when applied, and a rise afterwards if ice was melting.

The fuel-injected 172R/S has no carburettor, so no carb heat. Fuel is metered by a servo and delivered to each cylinder's intake port. Hot starting needs a different technique because fuel in the lines can vapourise, and the electric boost pump is used to purge vapour.

Mixture and why you lean

The mixture control sets the fuel-to-air ratio. At sea level full rich is correct; with altitude the air gets thinner but the engine still delivers the same fuel unless you lean. Too rich wastes fuel and fouls plugs; leaning for cruise saves fuel. Always follow the POH leaning procedure (typically by EGT or by RPM peak) and never lean for takeoff except as the POH specifies for high density altitude.

Pulling the mixture to idle cut-off is the normal way to stop the engine: it starves it of fuel and prevents the engine from running on after the magnetos are off.

Propeller

The fixed-pitch propeller is essentially a rotating wing. Blade angle is a compromise between climb and cruise. Because pitch cannot change, throttle directly controls RPM, and RPM changes with airspeed and attitude: in a dive RPM rises, in a steep climb it falls. The tachometer therefore also works as a rough power indicator.

Technical detail — combustion, compression and power

The IO-360-L2A has a compression ratio of 8.5:1 and a displacement of 361 cubic inches (5.9 L). Each cylinder fires once every two crankshaft revolutions (four-stroke Otto cycle), so at 2,400 RPM each cylinder fires 20 times per second. Power is controlled by manifold pressure (throttle) and RPM; with a fixed-pitch propeller, both change together, so the tachometer is the primary power indication.

Peak cylinder pressure should occur at roughly 15–18° after top dead centre. Ignition is timed at 25° before top dead centre so the flame front has time to develop. Too lean or too much advance moves the pressure peak earlier and raises cylinder head temperature (CHT) — the path to detonation. Detonation is explosive burning of the end gas; it shows as rising CHT and rough running, and can hole a piston within minutes.

Technical detail — lubrication and cooling

Wet-sump system: 8 qt capacity, minimum 5 qt for flight (7 qt recommended for long flights). The engine-driven gear pump draws oil through a suction screen, sends it through the full-flow filter and the thermostatic oil cooler bypass valve, then to the main gallery, crankshaft, cam and hydraulic lifters. Oil pressure normal range 50–90 psi; minimum 20 psi at idle; maximum 115 psi on cold start.

Cooling is by ram air through the nose cowl inlets, forced downward over the cylinder fins by the baffles and out the cowl exit at the bottom. Missing or torn baffle seals let air bypass the fins and are a common cause of one hot cylinder. CHT limit 500 °F; keep below about 400 °F for long cylinder life. Oil temperature limit 245 °F.