AVIATION SCIENCE LESSON 5: SPARK PLUGS Teacher: Subject: Grades: Subjects: Paul Ladegard, Alan Dick Aviation Science Secondary Technology, Science Learning Objectives: Students can locate the sparkplugs on an aircraft engine, read the code, inspect plugs for wear, clinkers, and other usage, demonstrate proper extraction, cleaning of a plug, and describe how a spark plug works. Student Activities: Students read Student Reading below. Teacher reviews Student Reading with the class. Students learn to properly extract spark plugs, and put them back using the proper sized wrench. Students are shown clean and contaminated plugs, and how to inspect them for airworthiness. Students learn to compare the attributes of aircraft, snowmachine, truck, spark plugs. Students practice cleaning plugs, using proper tools, safety glasses, etc. Visit local hangar and observe sand blasting and checking of spark plugs under pressure. Students practice extracting plugs using proper wrench, and installing plugs using proper torque wrench. 1
Materials: Spark Plugs of various sizes and conditions Pick for cleaning Blue Gloves Q Tips Torque wrenches Socket wrenches Safety Glasses MEK cleaner Student Reading: Aircraft sparkplugs Sparkplugs have been a very important component in aircraft engine improvement and development since the very beginning of flight. The aircraft engine depends on the ignition of the fuel/air mixture at exactly the right time to drive the piston downward in the power stroke. The engine cannot be more reliable than the sparkplugs. If the sparkplugs fail there will be no ignition of the fuel, no power stroke, and probably no engine power. In the early days of aviation, sparkplugs were a point of vulnerability. The ceramic that surrounds the wire/electrode and keeps it from grounding presented many problems. The ceramic often broke, fractured or even disintegrated. Improvements in ceramics have improved engine reliability tremendously. The ceramic must be a good electrical insulator and able to withstand high temperatures as well as thermal shock. Keeping the high voltage from running to a wrong place at a wrong time has been a constant challenge for aircraft engineers. Two sparkplugs per cylinder. Modern aircraft engines have two sparkplugs per cylinder and two magnetos. Each magneto separately fires one sparkplug in each cylinder, so if one magneto fails, each cylinder is still supplied with a spark from the other magneto. Or, if both mags are good but one sparkplug fails, that individual cylinder has another working sparkplug. Each cylinder has one plug on the bottom and one on the top. 2
While two plugs serve as a safety factor by providing redundancy, there is a greater reason that prompted the need for dual plugs. An aircraft engine is working against air, and is best served with a large diameter piston with a shorter stroke and fuel with a much quicker flash point. With the development of a wider diameter piston came the problem of the flame front across a 5 cylinder. The dual flame front provides much better combustion across the wide cylinder. As pilots do a run-up before takeoff, one magneto, and therefore one set of plugs, is shut off at a time while the engine is held around 1800 rpm. When one mag is shut off, there is a corresponding drop in rpm s (50-100) that clearly demonstrates the loss of power an engine would experience if there were only one set of plugs. During preflight run-up, both mags are individually turned off while the other mag is checked. An aircraft engine can safely run on one magneto or one sparkplug per cylinder. The purpose of the mag check before takeoff is to be certain all systems are operational. In the air, two plugs provide a much smoother power pulse, and more efficient fuel burn. This is one of the reasons for two magnetos and two plugs per cylinder. If one sparkplug seems to be running poorly, it is easy to find which cylinder has the faulty plug. The engine is started on both magnetos then quickly switched to the mag that is running ragged. It is run for a short time, then the engine turned off. The exhaust manifold of each working cylinder will be sizzling, and the exhaust manifold with the bad plug will be cool or cold. Two kinds of plugs. There are basically two kinds of sparkplugs used in manufactured aircraft today: Massive Finewire 3
Massive plugs can have two, three or four grounding electrodes on the perimeter and one electrode in the middle that the spark jumps from to the grounding electrodes. Massive plugs can be easily cleaned and re-gapped and are less expensive than finewire. ($25 each compared to $70-95 each) During the 100 hour or annual inspection, the ceramic on the massive plugs is cleaned by sandblasting. However, this does damage the ceramic, as the roughened surface will pick up deposits more quickly than the smooth factory surface. The average life of a massive plug is around 300-400 hours. The gap is 0.019 set with a precise tool (not a pair of pliers!) and that gap is consistent whether the engine is high or low compression. Low compression engines are 7.0:1 and high compression are 8.5:1. Finewire plugs resist buildup by clinkers and last longer than massives, but cannot be cleaned by sandblasting as the sandblasting process would weaken the finewire element to the point of breaking. Clinkers are cleaned with a pic that resembles a dentist s pic. Plug Attributes: The numbers and letters on a sparkplug tell their story. The number might be R H B 37 E The first letter, R indicates that there is a resistor in the plug to cut down on radio interference. The second letter, H indicates whether the barrel is shielded or not. The third letter, B tells how long the threads are, and what size wrench would remove the plug. Threads might be long or short depending on the cylinder head design. The forth number 37 indicates the heat range. A higher number indicates a hotter plug. The fifth letter E indicates the type of electrodes, whether massive or finewire, and how many ground electrodes surround the inner electrode. This aircraft sparkplug system is comparable to the number/letter system used on snowmachine, outboard and fourwheeler sparkplugs. Electronic shielding: Every sparkplug can potentially cause radio static, so each plug is shielded by a grounded metal jacket. The 4
surrounding shield collects electronic interference that could threaten safe radio communication and runs it to ground. Cleaning Sparkplugs: Cleaning sparkplugs involves: Wirebrushing the threads. The threads of both massive and finewire plugs should be cleaned with a wire brush to prevent them from seizing in the cylinder head. Each aluminum cylinder has an embedded helix coil to hold the steel plug. If the plug grabs, loosens and twists the helix coil, an expensive cylinder replacement is the next step. Cleaning clinkers with a pic. Sandblasting the plugs to remove carbon from electrodes and porcelain. Rinse in MEK (Methylethylketone) to remove any remaining carbon and contaminants. Blast with compressed air to further clean any sand or other particles. If a sparkplug is dropped on the ground or concrete, it must be discarded, as the internal insulating ceramic could be fractured, resulting in grounding of the spark. Plug Condition: A good sparkplug has no clinkers, the center electrode is round, and is the color of good coffee. A worn sparkplug has a center electrode that is shaped like a football. Most massive plugs have an expected life of 300 hours, with adjustments and cleanings done before discarding. Sparkplugs can be cleaned, gapped and reused. However, cleaning involves sandblasting. When a plug comes from the factory, the porcelain has a smooth finish to resist carbon buildup. Once a plug is sandblasted, the protective smooth finish of the porcelain has been removed. It then collects carbon more easily and must be cleaned more often. A fouled sparkplug has clinkers. Clinkers are rounded, almost like shotgun shot. They might result from fuel problems and 5
chemicals not scavenged from the cylinder. Lead additives have a greater density than gasoline. If the additives settle in a tank, they can concentrate and, when later burned, leave deposits on the plug. The clinkers must be cleaned with a pic. The tank of a plane that has set for a long time would benefit from being stirred. A carbon fouled sparkplug is black in color and is the result of a fuel/air mixture that is too rich. This can be prevented by proper leaning of the engine and monitoring of the EGT s during operation. A carbon fouled plug is undesirable as carbon is a conductor of electricity, and the high voltage spark could migrate from the center electrode and down the side of the porcelain rather than jumping the gap to ignite the fuel. An oil-fouled plug is black with a thick sticky film. This might be an indicator of oil blowing by the piston rings and a low-compression problem, but it also could be an indication of oil getting past the intake valve. Circulating oil serves to cool the valves and bring the excessive heat back to the crankcase. Some oil might leak past the valve into the cylinder. If oil leaks past the exhaust valve it will be coked, and leave a sticky deposit. Installing sparkplugs: Anti-seize. In addition to cleaning the threads, anti-seize lubricant must be applied to the threads, but sparingly. If excessive antiseize leaks into the cylinder, it can short the spark plug and ground the spark. Older planes had a copper helix coil embedded in the aluminum cylinder head, but most newer engines have a steel helix coil. If that helix coil is backed out by a seized plug, the repair will be well over $1000. The plug should be inserted into the cylinder and turned at least three turns by hand before putting a wrench to complete the tightening. The plug wire is replaced and tightened with a 3/4 wrench. Rotation: During the 100 hour or annual, plugs are rotated in a pattern that changes their position from top to bottom in each cylinder, changes cylinders and changes magnetos 6
for each plug. The pattern follows the order shown on the right. The bottom plugs tend to collect more clinkers than the top, and rotating helps avert uneven wear of the electrodes. At the same time, sparkplugs are supposed to fire from the center electrode to the outside grounding electrodes, but sometimes this gets reversed. Rotating helps avert this as well. Copper washer: The copper washer on a plug should be changed each time the plug is extracted, as heat from the engine hardens the copper. A fresh soft copper washer seals the plug from cylinder pressures. Some mechanics anneal used copper washers by heating and plunging them into cold water, softening the copper. Understanding and monitoring sparkplug condition are absolutely essential to maintaining airworthiness. Assessment: Students demonstrate cleaning and inspection of an aircraft plug and evaluate for airworthiness use. STANDARDS: Technology C 1: A student should use technology to observe, analyze, interpret, and draw conclusions. Science A: A student should understand and be able to apply the processes and applications of scientific inquiry. Science E: A student should understand the relationships among science, technology, and society. 7