FLYER Exhibitions
As a addition to the piece, here is Justin's description of the anatomy of a spark plug.
Anatomy of a spark plug
Certified aviation sparking plugs consist of a high tensile steel body which screws into the cylinder head of the engine, and a centre electrode encased in a ceramic insulating material secured inside the body. The centre electrode protrudes out of the nose of the plug into the combustion chamber, and is surrounded by one or multiple earth electrodes forming the spark gap. Early spark plugs were designed so that they could be dismantled for servicing, but now all spark plugs are a throwaway item designed to be discarded when they have failed or worn out. Most plugs include a resistor between the high tension lead contact and the central electrode to help eliminate current spikes helping reduce electrode erosion and electrical interference typically affecting avionics. Typical automotive plugs have a ceramic top and a steel lower body which has the hexagon for installation/removal and the threaded portion that screws into the head. The ceramic top has a ribbed appearance. The ribs act to increase the distance along the surface of the ceramic from the HT connection at the top of the ceramic to the earthed plug body at the bottom of the ceramic. This dissuades tracking of the spark down the outer body.
Plug Cap
The certified ignition or HT leads have two types of plug cap connection, 5/8in and 3/4in. The 3/4in or all-weather connector is excellent at sealing out rain and sealing in the ambient pressure that the cap was last connected at. (It is capable of maintaining sea level pressure up to 50,000ft.)
The all-weather cap is typically fitted to high-performance aircraft that can operate at high altitude, minimising the risk of high altitude misfires. The HT lead is made up of a central core that carries the high voltage encapsulated in an insulating material, which is then surrounded by a woven, screened cable that is earthed to the engine via the magneto body. The screened cable, sometimes covered by an outer, coloured coating, screens the high voltage pulses, helping eliminate electrical noise interference with other systems.
The outer plug body acts as a screen and is connected to the HT screening by the plug cap, continuing the screen to earth via the plug body to the cylinder head, keeping interference to a minimum. Automotive plugs use a far less elaborate system with the familiar push-on plug cap, which is not required to operate at the high altitude of the certified engines. There is generally no screening on the plug lead on light sport aircraft.
Electrode design
Plug electrodes come in many designs, the common REM40E plug, utilising ‘massive electrodes’, has two earth electrodes protruding opposite one another towards the centre electrode. Massive electrode plugs can have one earth, as with the automotive plug, or up to four radially-positioned around the nose of the plug. The electrodes are typically made from nickel alloys which resist spark erosion well. Fine wire plugs utilise electrodes made from the element iridium. Iridium plugs are the hardest wearing plugs presently available, which typically wear three times slower and can go to the TBO of the engine without failing. Iridium is very rare and trades at about £362/oz, making the cost of the iridium plugs significantly higher than massive electrode plugs.
Plug reach
This is the distance from the shell gasket to the end of the shell threaded portion. The reach of the plug is determined by the design of the cylinder head. If the plug is too long, the threads will protrude into the combustion chamber and will pick up deposits, making plug removal difficult or even impossible. In some engine designs the plug, if too long, may mechanically interfere with the piston having possible catastrophic consequences. If too short, the spark gap will be incorrectly positioned for efficient ignition of the fuel/air mixture. Long reach plugs are typically used on turbocharged engines and short reach are used on naturally aspirated engines. There are some exceptions, notably the TCM O-470 fitted typically to C180/182 can use short or long reach depending on the model.
The reach is not to be confused with protruding tip plugs, or projected core nose, as with many automotive plugs and ‘Y’ designated aviation plugs.
Heat rating
The heat rating of a plug is its ability to transfer heat away from the plug to the cylinder. For a plug to work efficiently, it must be kept within a desired operating temperature; typically for an avgas-burning engine, the nose core insulator should be between 900 and 1,300°F. Below 900°F the plug will suffer from carbon and lead fouling, both being conductive and able to short across the plug gap stopping or hindering the spark. Above 1,300°F, the plug will suffer from lead fouling.
At idle the plugs operate at about 500°F, hence if the mixture is too rich they suffer easily from carbon fouling. A hot plug utilises a longer core nose, the ceramic surround seen around the centre electrode, which effectively slows the heat transfer. A cold plug has a shorter nose core and transfers the heat accumulated in the plug to the cylinder faster, consequently the plug runs at a cooler temperature. If the plug runs too hot it can ignite the fuel-air mixture before the plugs sparks, hence the term pre-ignition. This will significantly raise the combustion chamber temperatures actually compounding the problem, possibly causing damage to the engine. (Some people will be familiar with this phenomenon in petrol cars ‘running on’ after being switched off.)
The environment and the way an aircraft is operated can have significant effect on how the engine runs and the temperatures generated in the cylinder. Consequently, many manufacturers will state several plugs as being certified for a particular engine to help guard against pre-ignition and/or plug fouling. The heat rating of the plug should be of little concern to the owner operator as the engine manufacturer will have determined the best plug for the engine, specifying it within the engine type certificate; assuming that the correct plugs have been fitted.
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