Glow Plugs By James Goss      I have always found it fascinating that we can hand crank our engines with just a few flips of the prop when everything goes just right. Likewise I am also fascinated that at times you can flip the prop all day long and the engine will not hit a lick.  Considering the many parameters that must be right for an engine to start, I think temperature is probably one of the most important. On a 50 degree day it may take more than just a few flips of the prop to crank an engine because first of all the glow plug is not as hot as it would be on a 90 degree day. The ambient temperature being 40 degrees lower also means the element's temperature in the glow plug will be 40 degrees lower, so ignition of the fuel is not as readily achieved.  Also the engine has more resistance to movement of its parts and becomes sluggish due to the thicker oil  in our fuel. Even your glow plug battery will not be performing as well on a cold day because its chemical activity will be reduced thereby limiting the current through the glow plug. Here is a simple tip: always keep your ni-starter battery in your pocket between flights to keep it warm on a cold day and it will perform better. I have been thinking about building an insulating cover to go over the head of my engine so it will hold the heat between flights on a cold day, like the pizza guys have for their delivery. I think everybody will agree that a warm engine will crank better than a cold one.      Another factor is the element in a glow plug has a positive temperature coefficient. This describes how the resistance of the glow element responds to a change in temperature. Being positive indicates that its resistance will go down as the temperature goes down so here again on a cold day the glow plug will place more load on the battery which is already sluggish. I consider the glow plug to be the heart of the engine and without it you have no engine, so take care of it. What can you do to take care of a glow plug? First of all start out by using a good quality plug. If it cost $ 6.00 so what, your model cost several hundred and a dead stick might just do it in. It has taken me many years to comprehend this minor detail but it finely soaked in. Even though I am cheap I will now pay more for my glow plugs. I may not be getting a better glow plug, but by it costing more I at least think I am.  The glow plug is one of the most likely components in the whole system to go bad. I can't count the number of models that I have had to dead stick because of a bad glow plug and have lost several beautiful models. By the way, be sure to periodically check the tightness of your glow plug. It uses fine threads, ¼ - 32 but by heating and cooling so much they will become loose. A few years back I had a dead stick and brought the plane in for a nice landing. I was getting ready to crank the engine again so I refueled and started to connect the nicad battery to the glow plug. I realized the glow plug was missing, boy was that a funny feeling. Where do you connect the battery? The engine must have run fine down to the last thread before the plug was blown out. I say this because the engine was at max throttle when it quit and it was an instantaneous shut down. You would think that as the plug became loose the engine would start running rough. I am so cheap that I am still looking for that glow plug in the soybean field.      How many times have you had a good flying session one weekend with the engine running great and the next weekend you start to crank your engine and it will not hit a lick? So you finally determine that the glow plug is open and you replace it. Now you start thinking that if the last time it flew so well when did the plug go bad? The probable answer is that it opened when you first connected the battery to it. Having a positive temperature coefficient means that the element will have a large surge current when the battery is first connected to it, especially if you make and break the connection several times while you are trying to get the battery connected. This surge current will only last a few thousands of a second and most of the time everything will be ok. At other times the surge will blow the element apart. An example of this can be found in your home. When does your light bulbs blow, while they are on or when you first switch them on? They also have a positive temperature coefficient and when cold will have a low resistance, just like the glow plug, and a high surge current. Most metals will have a positive temperature coefficient, but some devices such as thermisters can have a negative coefficient. If you placed a thermister with a negative coefficient in series with an element that has a positive coefficient, the two would cancel each coefficient and there would be no surge current. Manufactures could build light bulbs for our homes that would last for 20 years using this concept, but they would put themselves out of business if they did.                                                                                          The glow plug is really a well-built device for the price we pay. Just try taking one apart and you will see. It is basically a ½ inch long bolt with ¼ - 32 threads and a 5/16 hex head. The bolt is drilled in the center where the element stud is placed in a tight fit but not pressed. The top of the bolt is rolled around this stud to hold it in place. The tip that you connect the battery to will not pull out, you must grind the rolled top off to remove the stud. The element stud is insulated from the main bolt body with mica washers. Mica is a very good insulator and can withstand the high temperatures encountered. Just how hot does the glow element get? With a glow plug removed from the head of an engine I checked its temperature with three amps flowing through the filament. I tested several different types of plugs and they all checked at about 650 to 680 degrees F. This is hot enough to ignite paper or wood if brought in contact. For the test I was using a thermocouple probe in direct contact with the element. Now I realize that the element being out of the head will not have the heat sinking action that the head offers, but while in the engine's head the element itself will not lower much in temperature.  So when the head of your engine reaches 200 degrees, not all of the heat is coming from friction and burning gasses inside the engine, some is coming from the little 650 degree furnace we call the glow plug as well. Here is a good tip to remember: never remove your engine's glow plug while the engine is hot. Let the engine cool to the point that you can touch it with your hand. If you try to remove the plug while hot, the threads are much tighter and may seize up and damage the aluminum head threads. You will then have to re-tap the threads with a ¼ - 32 tap if you are lucky, if not you will need a new head or install an insert that can be drilled and tapped.   I had this to happen at least two times on my engines and have seen other modelers with this problem.      The element itself is made from an alloy of platinum and may come in sizes 32 to 36 awg. If the element was straight it would measure between 1/2 of an inch to 7/8 of an inch in length, but instead it is formed in a coil with aboutfive or six turns. Being in a coil has the advantage of having heat from one turn to help heat its neighboring turn and also takes less space and is more rigid in structure. It requires about 3 amps of current, ac or dc to heat it to an orange/yellow glow and will measure about .1 ohm when cold and .16 ohm when hot. When at .1 ohm it will be dissipating about 2.5 watts of heat. At .16 ohms it will produce 1.56 watts of heat. Most elements that I tested would burn out at between 3.5 to 4 amps of current. Now you may wonder how your nicad battery which is rated at 1.2 volts and 1400 mah, 1.4 amps, can deliver three amps needed to heat the glow plug. It works like this: as long as you do not draw over 1400 ma the battery will maintain a terminal voltage of 1.2 volts. When you exceed 1400 ma the terminal voltage of 1.2 volts will start to drop. As current continues to increase, voltage continues to drop. When you connect your nicad battery to the glow element its terminal voltage drops down from 1.2 volts to only .5 volts. This is due to the internal voltage drop of the battery. The state of charge will determine the internal resistance a battery will have at any given time, which controls the amount of internal voltage drop. Remember that any battery, even a dead one, will show its normal voltage rating if there is no load connected across it when you measure its terminal voltage. Current must be flowing in order to have voltage drop. You may recall the old saying that you can have voltage without current but you can't have current without voltage.  At .5 volts and .16 ohms you will get 3.1 amps of current flow. So what I am saying is that you operate your glow plug with only .5 volts when using a nicad battery. If you had a power supply that could supply a large current and set its output for 1.2 volts you would burn out the element because at 1.2 volts the current will be 7.5 amps and produce 9 watts of heat. By only having .5 volts to work with you can't afford any voltage drop on the conductors while getting the current to the element. Always keep the glow plug tip clean of any foreign matter such as varnish and corrosion. Also clean your ni-starter tip with alcohol every month. I have noticed a lot of modelers do not use the plastic ni-starter tip cover. Keeping the tip covered will reduce corrosion. Remember that for corrosion to occur it needs moisture and oxygen so keep it covered when not in use.      If you are using a power panel to heat your glow plug then you can solve the problem of high surge current real easy. Always have the power panel set for minimum output when you connect it to the glow plug. After a few seconds bring up the current to its normal range. I call this soft starting and it will eliminate any possibility of plug damage due to surges. Your power panel operates a little different than does a battery. When you connect your glow plug to a power panel you are not putting dc across the plug, instead it is a form of ac.  A series of rectangular shaped pulses, square waves, are used to heat the element. The frequency of these pulses are in the neighborhood of 166 hz, and is said to be better than dc because it vibrates the element and helps keep fuel droplets slung off, thereby helping to prevent flooding. The peak amplitude of these pulses is around 14 volts, and at minimum setting the pulse width is .05 ms. At max setting the pulse width is 1ms in duration. So at max setting of your power panel you have 14 volts for 1 ms and zero volts for about 5 ms. The panel is simply switching the 14 volts on and off and averages out to give you the 3 amps needed for filament heating. This power panel was tested with a 330 ohm load resister across its terminals. With a power panel of this type a load is needed before it will turn on. Have you ever wondered what turns on your power panel? It has no switch to turn it on or off, the glow plug's element completes the circuit to turn it on. After the engine cranks and you remove the battery connection there is no current flowing through the glow plug element, it is an open circuit. It is now heated by the platinum's reaction with the hot gases in the engine. So if the element opens during flight it is not because of current flow through the element, but more than likely due to vibrations and too much heat.      To prove that there is no electrical activity in the glow plug after you remove the battery, try this simple test. Crank your engine as always and remove the battery, now place a jumper across the plug by connecting the jumper between the glow plug tip and the engine's head, what happens? Did the engine continue to run normal? This is a direct short across the element that removes any possibility of voltage being on the element. If it does reduce speed it is because the jumper is sinking some of the heat away from the element. This method could be used as a test to find out which plug is best in holding its heat in between engine cycles. When you connect an alligator clip to the glow plug's tip you are only ¼ inch away from the element so it is almost like putting a heat sink on the element itself. If the engine does not loose any rpm then this plug is good at holding heat and at low speed will have a better idle.           I will always try to start my engine by hand even if it takes a while because I feel it is better for the engine to get the parts lubricated before you hit a cold engine with an electric starter at three or four thousand rpm. If you are using an electric starter for safety reasons, which I highly recommend, leave off the battery connection and rotate the prop 15 or 20 times to get some lubrication in the engine and then apply the starter.  I don't think it is wise to leave your nicad connected for very long after your engine starts because you may over heat the glow plug. Not only will it have heat from the battery current, but also heat from the engine's hot gases will now be added. Exposing the mica insulation to this excess heat could shorten the plug's life in that it may have an insulation breakdown, if not now some time in the future. Now I know they make on board  battery systems that constantly heats the glow plug to achieve better engine idle, but it could shorten the life of the plug's insulation if they apply full glow plug voltage at max speed. With an on board ignition you do not need full voltage, like when you are starting your engine, but rather just enough voltage to increase the plug's current during an idle. The plug is already hot due to the element's action in the engine so we only need just a little more heat to go with it to keep a good idle. If there was just some way to monitor the heat of the glow plug, the on board battery system would know how much voltage to apply to the plug for best idle.      While running some experiments on glow plugs the other day I came across something very interesting that I didn't already know. I actually feel that very few modelers would know about this detail. While running a 40-size engine on a test stand I decided to place my digital voltmeter across the glow plug. I was really just reinforcing the statement that I made earlier about there not being any electrical activity in the glow plug after you take the battery off. Unexpectedly when I connected the voltmeter it measured a small DC voltage. The voltage would vary directly with the engine speed and ranged from .2 mv to 1 mv.  Each time I connected I got the same results, a stable voltage with amplitude that depends on speed. Boy howdy, this voltage can be used for something I know! How about an onboard tachometer that uses this voltage as the dc reference feedback voltage. Or better yet, how about using it on the onboard ignition system. Or even better how about---------! The list of possibilities is endless. The voltage is so tiny it needs to be amplified to be of real use. Knowing this I hurriedly constructed a simple op-amp that would allow the .0002 volts to be represented by 1 or 2 volts. Everything seems to work fine, but I think I will try an instrumentation amplifier. This is the most sensitive amplifier for amplifying tiny signal voltages and does not create any circuit loading at all. I will update you on this topic at some later date.      When you stop and think about it the glow plug is acting just like a thermocouple. A thermocouple works because of the Seebeck effect. If you take two dissembler metals and bond them at both ends and heat one end while the other remains cold, the electron transfer between the two metals will produce a dc voltage. If you have gas-heating appliances around your home that uses a constant pilot light, then you have a thermocouple to protect against your pilot going out and flooding the burner chamber with gas. You can make a thermocouple by twisting a paper clip and a piece of copper wire together. Heat the junction with a match and you will measure a dc voltage on the other end, about 7 or 8 mv. Likewise you can take a glow plug and do the same experiment. Place your milivolt meter across the plug and use a match to heat the element, a dc voltage will be produced. The glow plug forms a thermocouple by having the element connect to the stud in the center. This creates the two dissembler metals needed for the Seebeck principle to work.      Heat is the number one enemy to all electrical insulation. It doesn't matter if the insulating material is insulating 10,000 volts or .5 volts; excess heat will damage insulation. Another experiment I performed on a variety of glow plugs is called a high-pot test. For this test I used an insulation tester known as a megger.  Insulation resistance can't be checked with your standard ohmmeter that you would use to check the resistance of the element. It requires much more voltage for this test than your multimeter will furnish. Most multimeters will only use between 1.5 and 9 volts for measuring resistance. This low voltage does not have a strong enough electrostatic field associated with it to show up faults in insulation materials. The megger uses between 100 and 1000 volts dc. The model I have is a hand-cranked version and really works well. You can't use a megger on a good glow plug because you would read the resistance of the element. So I took several new plugs and removed their elements for this test. Now we are testing the resistance of the mica insulation between the element electrode, the tip, and the case. Even though the plug only receives 1.5 volts or less, and it doesn't require much to insulate at this low voltage, the higher the insulation resistance the better the quality of the plug. I started with 100 volts and all plugs passed by showing infinity. Next I applied 500 volts and most had very high megohm readings and they all passed. Even at 1000 volts most good brands passed, but some of the inexpensive brands like tower started to fault. Failing this test means that the insulation has some microscopic cracks in it and may have a short life, especially where a lot of heat is involved like 650 degrees.      I hope you enjoyed the above information and I would like to leave you with this thought: What is the difference between glow plugs designed for two-stroke engines and the ones designed for four-stroke engines? Also if anyone finds my glow plug in the soybean field at Munford, there is a nice reward for its return. Thanks!

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