Piston Engine Connecting Rosds

The connecting rod is the link which transmits forces between the piston and the crankshaft.  Connecting rods must be strong enough to remain rigid under load and yet be light enough to reduce the inertia forces which are produced when the rod and piston stop, change direction, and start again at the end of each stroke.

There are three types of connecting-rod assemblies:

• The master-and-articulated-rod assembly
• The plain-type connecting rod
• The fork-and-blade connecting rod

  The master-and-articulated-rod assembly

The master-and-articulated rod assembly is commonly used in radial engines.  In a radial engine the piston in one cylinder in each row is connected to the crankshaft by a master rod.  All other pistons in the row are connected to the master rod by an articulated rod.  The articulated rods are constructed of forged steel alloy in either the I- or H-shape, denoting the cross-sectional shape.  Bronze bushings are pressed into the bores in each end of the articulated rod to provide knuckle-pin and piston-pin bearings.

The master rod serves as the connecting link between the piston pin and the crankpin.  The crankpin end, or the ‘big end’ contains the crankpin or master rod bearing.  Flanges around the big end provide for the attachment of the articulated rods.  The articulated rods are attached to the master rod by knuckle pins, which are pressed into holes in the master rod flanges during assembly.  A plain bearing, usually called a piston-pin bushing, is installed in the piston end of the master rod to receive the piston pin.

Plain-type connecting rods

Plain-type connecting rods are used in in-line and opposed engines.  The end of the rod attached to the crank pin is fitted with a cap and a two-piece bearing.  The bearing cap is held on the end of the rod by bolts or studs.  To maintain proper fit and balance, connecting rods should always be replaced in the same cylinder and in the same relative position.

The fork-and-blade rod 

The fork-and-blade rod assembly is used primarily in V-type engines.  The forked rod is split at the crankpin end to allow space for the blade rod to fit between the prongs.  A single two-piece bearing is used on the crankshaft end of the rod.

Construction of a Piston

The piston of a reciprocating engine is a cylindrical member which moves back and forth within a steel cylinder.  The piston acts as a moving wall within the combustion chamber.  As the piston moves down in the cylinder, it draws in the fuel/air mixture.  As it moves upward, it compresses the charge, ignition occurs, and the expanding gases force the piston downward.  This force is transmitted to the crankshaft through the connecting rod.  On the return upward stroke, the piston forces the exhaust gases from the cylinder.

The majority of aircraft engine pistons are machined from aluminium alloy forgings.  Grooves are machined in the outside surface of the piston to receive the piston rings, and cooling fins are provided on the inside of the piston for greater heat transfer to the engine oil.

Pistons may be either the trunk type or the slipper type.  Slipper type pistons are not used in modern, high-powered engines.  All the rings will be fitted above the gudgeon pin(Piston pin).  The top face of the piston, or head, may be either flat, convex, or concave.  Recesses may be machined in the piston head to prevent interference with the valves.

As many as six grooves may be machined around the piston to accommodate the compression rings and oil rings.  The compression rings are installed in the three uppermost grooves;  the oil control rings are installed immediately above the piston pin.  The piston is usually drilled at the oil control ring grooves to allow surplus oil scraped from the cylinder walls by the oil control rings to pass back into the crankcase.  An oil scraper ring is installed at the base of the piston wall or skirt to prevent excessive oil consumption.  The portions of the piston walls that lie between each pair of ring grooves are called the ring lands.

In addition to acting as a guide for the piston head, the piston skirt incorporates the piston-pin bosses.  The piston-pin bosses are of heavy construction to enable the heavy load on the piston head to be transferred to the piston pin.

Piston Engine Cylinders

The portion of the engine in which the power is developed is called the cylinder.  The cylinder provides a combustion chamber where the burning and expansion of gases takes place, and it houses the piston and the connecting rod.

There are four major factors that need to be considered in the design and construction of the cylinder assembly.  These are:

·         It must be strong enough to withstand the internal pressures developed during engine operation.

·         It must be constructed of a lightweight metal to keep down engine weight.

·         It must have good heat-conducting properties for efficient cooling.

·         It must be comparatively easy and inexpensive to manufacture, inspect, and maintain.

The head is either produced singly for each cylinder in air-cooled engines, or is cast ‘in-block’ (all cylinder heads in one block) for liquid-cooled engines.  The cylinder head of an air-cooled engine is generally made of aluminium alloy, because aluminium alloy is a good conductor of heat and its light weight reduces the overall engine weight.  Cylinder heads are forged or die-cast for greater strength.  the inner shape of a cylinder head may be flat, semispherical, or peaked, in the form of a house roof.  The semispherical type has proved most satisfactory because it is stronger and aids in a more rapid and thorough scavenging of the exhaust gases.

·         The Cylinder Head
·         The Cylinder Barrel

At assembly, the cylinder head is expanded by heating and then screwed down on the cylinder barrel which has been chilled, thus, when the head cools and contracts, and the barrel warms up and expands, a gastight joint results.  While the majority of the cylinders used are constructed in this manner, some are one-piece aluminium alloy sand castings.  The piston bore of a sand cast cylinder is fitted with a steel liner which extends the full length of the cylinder barrel section and projects below the cylinder flange of the casting.  This liner is easily removed, and a new one can be installed in the field.  

Cam Shaft

The valve mechanism of an opposed engine is operated by a camshaft.  The camshaft is driven by a gear that mates with another gear attached to the crankshaft.  The camshaft Usually rotates at one-half the crankshaft speed. in other words, when the cam shaft rotates two cycles the cam shaft only complete one cycle.There are number of lobes which are machined on the cam shaft and  As the camshaft revolves, the lobes cause the tappet assembly to rise in the tappet guide, transmitting the force through the push rod and rocker arm to open the valve according to the sequence .

The profile of the lobe controls in terms of crankshaft degrees the point of valve opening, the rate of valve opening, the period the valve remains open, the rate of valve closing and the point at which the valve closes.

For radial engine the cam shaft operation is provided by a ring which is known as the Cam Ring. this is a circular metal ring with the lobes on it in a circler. this helps the radial engine rocker arms to be opened and closed as per the firing order. More information on radial engine cam ring will posted in next post. 

Piston Engine Crankshaft

The purpose of this component is to change the reciprocating motion(up and down movement) of the piston into rotary motion.  Crankshafts are usually alloy steel forgings with their journals and crankpins hardened to resist wear.  The crankpins and journals are usually hollow, to reduce weight, these spaces being interconnected by drillings in the crank webs to provide passages for lubricating oil.

A shaft is classified according to the number of ‘throws’ or cranks, for instance a ’six throw’ shaft has six crankpins.  The crankwebs are sometimes extended, the extra metal providing a means of balancing the assembly or provide provisions for attachment of damping weights. Suitable drives at each end of the crankshaft transmit the torque to the reduction gear and the accessory drives and In direct drive engines the crankshaft is connected to the propeller with or without the propeller governor.

The simplest crankshaft is the single-throw or 360° type.  This type is used in a single-row radial engine. It can be constructed in one or two pieces.  Two main bearings (one on each end) are provided when this type of crankshaft is used.

The double-throw or 180° crankshaft is used on double-row or 180° crankshaft is used on double-row radial engines.  In the radial-type engine, one throw is provided for each row of cylinders.

Piston Engines Crankcase

This is the name given to that part of the engine that houses the crankshaft and connecting rods.  It provides mounting faces for the cylinders or cylinder blacks, reduction gear, wheel case and other units.  It may be a single casing or build-up of several sections depending on the type of engine.  It will contain the main bearings which are usually plain metal bearings for in-line engines and roller bearings for radial engines.  The engine mountings for in-line engines take the form of our feet and a steel ring is usually used for radial engines.  Provision is made at the lowest point of the crankcase for collection of engine oil for recirculation which is known as the engine oil sump

.

The crankcase is subjected to many variations of vibrational and other forces.  Since the cylinders are fastened to the crankcase, the tremendous expansion forces tend to pull the cylinder off the crankcase.  The unbalanced centrifugal and inertia forces of the crankshaft acting through the main bearing subject the crankcase to bending moments which change continuously in direction and magnitude.  The crankcase must have sufficient stiffness to withstand these bending moments without deflections or deformations.  If the engine is equipped with a propeller reduction gear, the front or drive end will be subjected to additional forces causing engine case to be more stressful.

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.

Types of Fire Extinguishants

Methyl Bromide

This extinguisher boils at 4.6°C and was used for the protection of power plants in older aircraft.  It is toxic and should not be used in confined spaces, flight crew compartments or passenger cabins.  This extinguisher is no longer listed.  Existing bottles may be maintained but refills will not be made with Methyl Bromide.

Bromochlorodifluoromethane (BCF)

This semi-toxic extinguisher is particular effective against electrical and flammable liquid fires.  It is used in power plant systems, and for the protection of auxiliary power units in some aircraft.  It is also used in certain types of portable extinguisher.  It becomes gaseous at normal temperatures and condenses to liquid at -4°C (25°F), and can be stored and discharged at moderate pressures.  It has little or no corrosive effect, although halogen acids will be formed if its products which have been decomposed by fire comes into contact with water, eg. condensation caused by fire.  In contact with fire, BCF volatilizes instantly, giving rapid flame extinction with little or no harmful effect on metallic, wooden, plastic or fabric materials.

Also known as Halon 1211.

Bromotrifluoromethane (BTM)

This semi-toxic extinguishant is used for the protection of power plant and APUs.  It is also widely used in cargo compartment fire suppression systems of some types of aircraft.

Also known as Halon 1301.  It has a boiling point of -58°C.

Aircraft Towing

On aircraft with a nose-wheel landing gear, a steering arm should be fitted to the nose wheel to guide the aircraft Light aircraft can be moved and guided, by hand or by a tug.  

Special attention should be paid to the following:

• Force should not be applied to the thin trailing or rear edges of wings or control surfaces such as ailerons or elevators.

• Generally speaking it is better to push an aircraft backwards rather than forwards, because the leading edges of the wings and tailplane are stronger than the trailing edges.

• The struts, which support the undercarriage on some aircraft, are suitable for pushing the aircraft as these are the strength parts of the aircraft.

• The flat of the hands should be used when pushing, so as to spread the load over the largest area.

• When pushing on struts, the force should be applied as near to the end fittings as possible.

• A propeller must never be used to push or pull the aircraft, as the engine should always be regarded as 'live' and a propeller may kick if it is turned.

On aircraft with a steering nose wheel connected to the rudder pedals, care must be taken not to exceed the turning limits. Normally the maximum limits are marked on nose wheel doors as “NO TOW”

On this type of aircraft it is also important that the rudder controls are not locked during the towing operations.  This is because the rudder controls and the nose-wheel steering mechanism are interconnected and the excessive movements could damage the mechanical linkages.

When towing aircraft by tow bar and tug special care should be given to below facts,

The correct tow-bar should be connected between the towing attachment at the base of the nose undercarriage leg and the tug.

 

• A person familiar with the aircraft brake system should be seated in the cockpit/cabin to operate the brakes in an emergency.  The brakes should not normally be applied unless the aircraft is stationary.

• Once the tow-bar is connected, the brakes, and if fitted, the rudder lock, may be released and the aircraft towed at a safe speed.  A safe speed is considered to be walking speed.

• A close watch should be kept on the wing tips and tail, particularly in confined spaces, to ensure that they do not come into contact with other stationary or moving object.

• In circumstances where the ground over which the aircraft has to be towed is either boggy or very uneven, the strain imposed on the nose undercarriage may be excessive and it may be necessary to tow the aircraft by means of bridles attached to each main undercarriage.  If towing attachments are not provided on the main undercarriage legs, ropes should be passed carefully around the legs as near to the top as possible and avoiding fouling on adjacent pipes or structure.  A separate tug should be connected to each main undercarriage assembly.  Steering should be carried out by means of a steering arm attached to the nose wheel rather than by differential movement of the tugs.

The most common means of towing a large aircraft nowadays is by means of a tow bar-less aircraft handling tractor.These tractors tend to be front wheel driven and therefore when towing an aircraft, are acting to ‘pull’ the tractor/aircraft combination.The towbarless tractor consists of a low level tractor with a rear mounted cradle, comprising of a ‘scoop and gate’ assembly.

Owing to the tractor’s low height it can easily move-in under the aircraft’s fuselage to couple-up with the aircraft’s nose wheel. During operation the nose wheel is raised by about 20cms by the tractor and after the towing is completed the nose wheel is lowered and the nose wheel is released from the cradle.

Continuous Loop Type Detectors

These detectors are designed to provide maximum coverage in the particular firezone.  They are used mainly for engine and APU installations.  They may also be installed in landing gear wheel bays and adjacent to hot air ducting.

These detectors operate on either of two principles:

•  The resistance type.

• The capacitance type.

The method of operation depends on the type of control unit fitted to the system.

Detector elements are manufactured in various lengths and are joined together to form a continuous detector loop.  This is routed round the installations as required.  An element consists of a stainless steel or inconel tube, with one or two center electrodes insulated from the tube by a temperature sensitive material.  When two conductors are provided within the tube, one of the connectors is earthed to the outer shell of the connector at the end of the tube.  Electrical connectors are provided at both ends of the tube.  Sometimes the elements are enclosed in a sheath which gives protection from damage.

 Resistance Type

The resistance of the insulating material decreases with an increase in temperature(NTC material).  At the warning temperature, sufficient current passes to operate a warning circuit.  The element is supplied with a current which is passed through a control box to operate the warning system.

In a typical system the detector is a nickel wire embedded in a temperature sensitive material called a di-electric.  This is contained within a small diameter stainless steel or inconel tube joined together with special couplings to form a loop.  The loop is routed and clamped around a firezone as required.

The inner wire and the tube form an inner and outer electrode and are connected to the aircraft power supply through a control unit.  Some systems use 115V ac and 28v dc, other systems use 28v dc only.

Under normal conditions only a very small standing current passes through the separating di-electric.  The current passed is insufficient to operate the control system.  As the temperature increases the resistance of the filling material decreases as it has a negative co-efficient of resistance.  This will allow more current to pass to the control unit.  When the temperature has risen above a pre-determined level, sufficient current will pass to operate the warning circuit.

When the temperature drops, the di-electric (the separating material) will return to its previous characteristic, the current will fall and the warning circuit will switch off.  Thus the system is now re-set and awaiting a further input.

The capacitance type will be posted in next post.