CAA NZ AME Piston Engines key terms

The ideal thermodynamic cycle used to model spark-ignition piston engines, comprising intake, isentropic compression, constant-volume heat addition at combustion, isentropic expansion and constant-volume heat rejection.

The ratio of cylinder volume above the piston at bottom dead centre to the volume remaining at top dead centre. It strongly influences thermal efficiency, detonation margin and starting behaviour.

A pressure value representing the useful work delivered per cycle divided by engine displacement, allowing engines of different sizes to be compared for power output independent of speed and capacity.

The average pressure acting on the piston during the power stroke derived from the indicator diagram, representing work developed inside the cylinder before friction and accessory losses are deducted.

The ratio of the air mass actually trapped in the cylinder per cycle to the theoretical mass that would fill the swept volume at ambient conditions. It governs attainable power at a given speed.

Abnormally rapid uncontrolled combustion of the end gas ahead of the flame front, producing a sharp pressure spike, audible knock and risk of piston or valve damage if sustained.

The crankshaft angle, measured before top dead centre, at which the spark occurs. Correct setting balances peak cylinder pressure against detonation margin and varies with engine speed and load.

A cylinder arrangement in which cylinders lie on opposite sides of a common crankcase with opposing pistons sharing crank throws, giving a compact, well-balanced and air-cooled-friendly layout common in light aircraft.

The main structural housing that supports the crankshaft bearings, carries the cylinders and accessory housing, and on opposed engines is usually split on the vertical centreline into two halves.

The shaft that converts reciprocating piston motion into rotary motion through crankpins and throws, incorporating counterweights, oil drillings and a nose or flange for propeller or gearbox attachment.

Mass attached to the crankshaft opposite each crank throw to balance reciprocating and rotating forces, reduce bearing loads and torsional vibration, and smooth the torque delivered to the propeller.

The link transferring gas load from the piston pin to the crankshaft crankpin, comprising a small end, shank and big end, usually with a split big-end cap and shell bearings.

A load-carrying bearing with no rolling elements, in which a steel shell lined with a soft overlay supports a rotating journal on a hydrodynamic oil film.

A long stud passing through the whole crankcase and cylinder assembly to clamp the cylinders and heads to the crankcase, maintaining preload on the joint under thermal cycling.

The honed cylinder bore, usually of steel or chrome-plated or nitrided material, often with cooling fins, in which the piston reciprocates and which guides the rings and transfers heat.

The reciprocating component that receives combustion pressure, transmits force through the piston pin to the connecting rod, and carries the rings while managing heat flow to the cylinder wall.

The upper piston ring, usually of cast iron or steel, whose main function is to seal combustion gas against the cylinder wall; it also conducts heat from the piston crown to the barrel.

The lowest piston ring, featuring an expander and rails, which scrapes surplus oil from the cylinder wall and returns it through piston drain holes to control oil consumption and prevent fouling.

The clearance between the two ends of a piston ring when fitted in its bore; too small causes scuffing when hot, too large allows blow-by and lost compression.

The designed gap between the rocker arm or tappet and the valve stem tip, allowing for thermal expansion so the valve still seats fully when the engine is hot.

A self-adjusting tappet using engine oil pressure and a plunger to maintain zero valve clearance, quietening operation but requiring clean oil and correct preload.

A device that meters fuel into the induction air stream using a venturi pressure drop and float-controlled fuel bowl, providing a combustible mixture across the engine's operating range.

A narrowed throat in the induction or carburettor passage where air velocity rises and static pressure falls, creating the pressure differential that draws fuel through the main metering jet.

The cockpit or linkage control that varies fuel flow relative to airflow, enriching for power and leaning for altitude or cruise, to keep the fuel-air ratio near the optimum for the phase of flight.

The fuel reservoir in a carburettor in which a float and needle valve hold fuel at a near-constant level, so metering jets see a stable head of fuel regardless of consumption.

A self-contained engine-driven generator of high-tension ignition current using a rotating magnet and coil, requiring no external electrical supply and providing independent duplicated ignition.

The screened high-tension leads and connectors carrying current from magneto distributor outlets to individual spark plugs, with shielding to suppress radio interference and prevent flashover.

A device that screws into the cylinder head and provides an insulated centre electrode and grounded shell electrode across which the ignition voltage arcs to ignite the compressed mixture.

A system in which a pressure pump feeds oil from a separate tank to the engine and a scavenge pump returns it, allowing inverted flight and reliable supply in aerobatic or air-cooled installations.

A heat exchanger, usually air-cooled, that removes heat from engine oil to keep viscosity and film strength within limits, often with a thermostatic bypass to prevent over-cooling.

The measured temperature of the cylinder head, a key indicator of cooling adequacy; excessive values warn of detonation, weak mixture or baffle and airflow problems.

Sheet-metal ducting around air-cooled cylinders that directs cooling air through the fins in a controlled path, preventing leakage around rather than between the fins.

An exhaust-driven compressor that raises intake manifold pressure above ambient, recovering power lost with altitude; it requires wastegate control and careful mixture and temperature management.

A valve that bypasses exhaust gas around the turbine to limit boost pressure, controlled by a spring and diaphragm or controller so manifold pressure stays within the engine's approved limits.

A propeller whose blade angle is automatically varied by a governor to hold a selected rpm, converting engine power into thrust efficiently across the flight envelope.

A flyweight and oil-pressure control unit that senses propeller rpm and adjusts blade angle to maintain the selected speed, increasing pitch as rpm tends to rise.

Twisting oscillation of the crankshaft and propeller shaft caused by cyclic torque pulses; it must be kept clear of critical frequencies, which is why operating ranges may be restricted.

An instrument displaying absolute pressure in the induction manifold, used with a constant-speed propeller as the primary indication of engine power output.

The temperature of exhaust gases measured near the cylinder outlet, used to set mixture for best power or best economy and to detect lean-running or ignition faults.

A maintenance test in which air is applied to each cylinder at top dead centre and leakage or differential pressure is measured, revealing ring, valve or head gasket defects.

The process of inhibiting corrosion in an engine laid up for storage by fogging oil into cylinders, sealing openings and renewing desiccant or dehumidifying protection.