Aircraft Piston Engine Operation

The principles which govern the relationship between the pressure, volume, and temperature of gases are the basic principles of engine operation.

An internal-combustion engine is a device for converting heat energy into mechanical energy.  Fuel (Avgas) is vaporized and mixed with air, forced or drawn into a cylinder, compressed by a piston, and then ignited by an electric spark.  The conversion of the resultant heat energy into mechanical energy and then into work is accomplished in the cylinder.  There are various engine components necessary to accomplish this conversion and for efficiency of the engine.

The operating cycle of an internal combustion reciprocating engine includes the series of events required to induct, compress, ignite, burn, and expand the fuel/air charge in the cylinder, and to scavenge or exhaust the by-products of the combustion process.

When the compressed air fuel mixture is ignited, the resultant gases of combustion expand very rapidly and force the piston to move away from the cylinder head.  This downward motion of the piston, acting on the crankshaft through the connecting rod, is converted to a circular or rotary motion by the crankshaft.

A valve in the top or head of the cylinder opens to allow the burned gases to escape, and the momentum of the crankshaft and the propeller forces the piston back up in the cylinder where it is ready for the next event in the cycle.  Another valve in the cylinder head then opens to let in a fresh charge of the fuel/air mixture.

The valve allowing for the escape of the burning exhaust gases is called the exhaust valve, and the valve which lets in the fresh charge of the fuel/air mixture is called the intake valve.  These valves are opened and closed mechanically at the proper times by the valve-operating mechanism.

The bore of a cylinder is its inside diameter.  The stroke is the distance the piston moves from one end of the cylinder to the other, specifically, from TDC (Top Dead Centre) to BDC (Bottom Dead Centre), or vice versa.

Extinguishing System

These systems are provided for power plants, APUs and baggage compartments.  A system generally consists of:

·         A number of metal containers or bottles, containing an extinguishant eg. methylbromide, bromotrifluoromethane or bromochlorodifluoromethane (BCF) also known as Halon 1211.

·         Tubing to carry the extinguishing agent to areas that require protection.

·         Control valves.

·         Indicators.

·         Control circuitry.

Systems vary considerably on different aircraft but the basic elements are similar.  HRD or high rate discharge is the term applied to most systems in common use.

The extinguishant is pressurised with an inert gas and sealed in the container by means of a discharge or operating head.  When operated, either by selector switches  (fire handles) on the flight deck or crash switches, an electrically fired cartridge or squib ruptures a metal diaphragm within the discharge head and the extinguishant is released.  It then flows through spray pipes, spray rings or discharge nozzles into the appropriate firezone.  The electrical power is 28 volts dc and is supplied from an essential services busbar.

Most aircraft use a ‘two shot’ extinguishing system for the power plants.  This uses connections between the individual power plant systems.  In this system the fire extinguishers for each power plant are interconnected.  This allows two separate discharges of extinguishant into any one power plant.  On many aircraft two fire bottles are installed in each engine nacelle.  This allows two separate discharges of extinguishant into each power plant.  Indication that a fire extinguishing circuit has been operated is indicated by a warning light.

In some installations special switches are incorporated to automatically operate the extinguishers in the event of a crash.  These switches also connect cabin emergency lights to the aircraft battery power supply.  Two types of crash switch are in common use:

·         The inertia control type

·         The frangible type.

An inertia controlled switch generally consists of a heavy piston supported on its own spring and so arranged that at the required degree of deceleration (a typical value is 3g), it compresses the spring and causes a bow spring to snap over thereby bridging contacts connected in the extinguishing system circuit.  To allow resetting of the switch after operation or rough handling during transit, a reset plunger is incorporated.

Frangible switches consist of two electrical contacts mounted in a hermetically sealed glass envelope.  The contacts are prevented from closing by a spring, but in the event of the glass envelope being shattered, the contacts will close and complete the circuit to the extinguishing system.  The switches are located in positions such as the wing tips, the underside of engine nacelles, at various points on the underside of the fuselage, etc.  This is so that in the event of a crash, at least one of the switches will be shattered.