Magnetic Chip Detectors

The third component we will be discussing under AircraftLubrication systems is the Magnetic Chip Detector (MCD’s). As the name implies, it is a detector, which functions on the principles of magnetism.

Main function of the magnetic chip detector is to provide condition monitoring of the engine and its lubrication system components. To give an early warning of bearing failure, magnetic chip detectors are fitted in the system. 

They are normally of the bayonet type fitting with auto sealing adaptors thus,  can be removed, inspected and replaced very quickly, with no oil spillage. They are often found  in gearboxes and in the scavenge pump return lines where the metal debris are often found.

See below for details of a lubrication pump MCD.


 
 

In earlier versions of engines, the detection is done visually by removing and inspecting the chip detector at periodic intervals. However, On modern engines this is done electronically. If there are metallic debris that comes into contact, it completes an electrical pathway and provides a cockpit indications. Magnetic Chip Detectors (MCD’s) alerts / indication to the flight deck informs the pilot of contamination that requiring attention. 

Often the low resistance across the MCD, can be caused by small carbon deposits or tiny metallic deposits that are insignificant as far as engine wear is concerned. In such cases, there may be a ‘fuzz burner’ fitted that will pass an sufficient electrical charge through the MCD and clear off the fine debris and leave only major particles. If the warning light still remains ‘ON’ then the pilot knows that the debris is of a more serious nature.
 

Below illustration shows the location of MCDs in a Lubrication Pump scavange lines.

Aircraft Engine Lubrication Unit ( Lubrication Pump)

Lubrication unit Or simply said the Lubrication pump system is the main driving force behind the pressurized lubricants in a jet engine. Its main function is to provide oil under the required pressure for lubrication of the engine components and spool bearings, then perform the scavenging (returning) of the oil after lubrication process then finally to directs it to the oil tank via the oil/fuel heat exchanger.

In order to pressurize and to maintain positive displacement flow, it uses a series of pumps which are driven by a single shaft. Often the pump is a Gerortor pump (Click on to read on Gerotor pump construction) which is driven by a single shaft. Number of pumps in the shaft depends on the number of supply and scavenge systems available.  

Lubrication units has its main supply inlet from the Main Oil Tank , one or more Supply lines and one or more scavenge lines for returning oil. It also has various tapping for temperature and pressure measuring sensors. (See below)

The return lines are equipped with scavenge filter screens/mesh to remove any debris from entering the pump system. Each scavenge line is often provided with oil condition monitoring magnetic chip detectors. Magnetic chip detectors works by attracting metal substances to its tip and provide good indications on possible failures in the system ( based on metal analysis). It also aid in reducing  the amount of powdery metallic debris in oil system by attracting the metallic substances.(See below)

Ball Bearings

These bearings are used where it is not practical to use plain bearings, and where a high degree of reliability and precision is required.

The advantages are:

·         Low frictional losses

·         Wide range of loads may be accepted

·         Simple lubrication requirements

Ball Bearings

The most common type of bearing used on aircraft.  The ball bearing has many variations in design allowing it to be used in a wide variety of situations/

They carry radial loads and moderate axial loads in both directions and where a high axial load may be experienced, the balls run in a deep groove in the races.Two types of ball bearings are in general use.  The caged type and the crowded type.

Caged Ball Bearings

In general use, for engine applications and for equipment with rotational speeds in excess of 100 rpm.  When used within engine/gearbox casings they are lubricated with engine oil supplied by jets or by splash.  When used outside casings, they are lubricated by the application of grease which may be applied at specified intervals by grease gun, or may be of the pre-packed type where lubricating grease is packed and sealed into the bearing on assembly.

Crowded Ball Bearings

This bearing has filling slots in one or both races and has no cage or separators.  The balls therefore touch each other during operation, hence the term ‘crowded’.  They are suitable only where slow rotation or part rotation (oscillations) are found, and are usually of the sealed or pre-packed type.

Angular Contact Ball Bearings

Accept radial loads, and axial loads in one direction where a single bearing may be used.  For axial loads in both directions  an opposed pair of bearings is often used.

Thrust Reversers

There will be cases when the aircraft is required to land on runways that are shorter than those for which the aircraft can normally land.  It is also beneficial to use less runway than that available in order to have a safety margin in the event of a failure.  Although the brakes on the aircraft are designed to be adequate under all normal operating conditions, the engine can assist in shortening the landing distance by using the thrust reversal systems.In this system the engine thurst is vectored to the front to slow down the aircraft.

On a propeller engine this is very simply done by reversing the pitch of the propeller blades so that they now thrust air forward instead of backwards. Therefore the prop aircraft can change the direction of thrust by 180 degrees. But, This is impossible to achieve with a jet engine, The result of this is that the reversed thrust from a jet engine can never be as fully effective as that from a propeller.  The thrust level obtained from the engine can be as high in reverse as it is in forward thrust but, because of the angle involved, the effect can never be the same as the forward thrust.

types of reversers

As far as jet engines are concerned there are three basic types of thrust reverser: 

• Clamshell Door
• Retractable Ejector
• Cold Stream Cascade Reverser

 

Piston Engine Valves

Engine valves are used to  regulate the flow of gases into and out of a cylinder by opening and closing at correct  time in the combustion process.   Each cylinder has at least one INTAKE VALVE and one EXHAUST VALVE.  The intake valve controls the amount of fuel/air mixture that is drawn into the cylinder through the intake port while the exhaust valve allows the exhaust gases to exit the cylinder through the exhaust port.

These valves are faced into the combustion chamber of the cylinder and therefore those  are subject to high temperatures, corrosion, and extreme operating stresses.  Therefore, valves must be constructed of metals that are able to resist these stressful operating factors.  For example, intake valves operate at lower temperatures than exhaust valves and, therefore, are typically made of chrome, nickel, or tungsten steel.  

However, since exhaust valves must endure much higher temperatures they are usually made of more heat resistant metals such as inconel, silicon‑chromium or cobalt-chromium alloys.

As mentioned, each cylinder must have at least one intake and one exhaust valve;  however, on some high powered engines, two intake and two exhaust valves are provided for each cylinder for better operational performances. There are various types of valves used in piston engines and  the most common type of valve used in aircraft engines is the poppet valve which gets its name from the popping action of the valve.

Poppet valves are classified according to their head shape which may come in four basic designs:

• Flat head valve : As its name implies, this valve has a flat head and is typically used ONLY as an intake valve in aircraft engines
• Semi Tulip head valve : This valve has a slightly concave area on its head.
• Tulip head valve : This valve has a deep, wide indented area on its head.
• Mashroom head valve : This valve has a convex head and is NOT commonly found on aircraft engines

The valve face is that portion of the valve that creates a seal at the ports.  A valve face is typically ground to an angle of between 30° and 60° to form a seal against valve seat when then valve is closed.  In some engines, the intake valve face is ground to 30° while the exhaust valve face is ground to 45°.  The engine manufacturer specifies the exact angle which is based on airflow, efficiency, and sealing ability.  Valve faces are often made more durable by welding STELLITE, an alloy of cobalt and chromium, to the valve face.  Once applied, the face is ground to the correct angle.  Stellite resists high temperatures and corrosion and withstands the shock and wear associated with valve operation.

The Valve stem acts as a guide to keep the valve head properly aligned as it moves back and forth.  Most valve stems are surface hardened to resist wear and are joined to the valve head at the Valve neck.  The TIP of a valve stem is also hardened to withstand both wear and hammering.  A groove is machined around the valve stem near the tip for a SPLIT KEY. This key will keep the valve spring retaining washers in place and hold the valve in the cylinder head.

To help dissipate heat better, some exhaust valve stems are hollowed out and then partially filled with Metalic Sodium( Na).  When installed in an operating engine, the sodium melts when the valve stem reaches approximately 98°C.  The melted sodium circulates naturally due to the up and down motion of the valve and helps carry heat from the valve head into the stem where it is dissipated to the oil circulating at the cylinder head.

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.