Glow plugs

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      The glow plug is an integral part of a glow engine, and as every part of it is carefully designed and manufactured in order to faultlessly work along. Depending on the specifications of each engine, different types of glow plugs are produced depending on the size, the working temperature and the type of the resistance wire. So if any different type of glow plug is used the result is an unbalanced ignition and deterioration of engine’s performance. Also due to its flimsiness the glow plug is easily destroyed. In order to understand the engine’s behavior depending on the plug, firstly we will go through some basics such as: Internal combustion engines, glow plugs characteristics and usage, common causes of burn out, symptoms and how can be detected and which is the ‘’antidote’’ in each case.    

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      Internal combustion engines also known as thermal engines and according to the fuel ignition method are divided into three basic categories. The most used engines in cars, motorbikes, marine in and outboard engines and machines for every use, need a spark plug to ignite the fuel mixture. On the other hand also used in many applications the diesel engines do not require a spark plug but they depend solemnly to the mixture’s compression. One more category is the glow engines which are a hybrid between ignition and combustion.  In our Rc models all three of the above are being used. The glow engines are the best used due to performance, weight and cost.

      Nowadays 2 stroke petrol engines are also used extensively, equipped with spark plugs and electronic ignitions.

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       Finally diesel engines are being randomly used but disliked by many modelers because of their fumes and the hypothetically low performance.

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      Of course if a 0,45 ci Diesel spins a 13×6 prop with 10.000 rpm, idle less than 2.000 rpm and flight time 30 minutes consuming 270 ml of fuel, then second thoughts have to be made!                                            

      GLOW PLUG SIZE

      The glow plugs are divided into two categories according to their size, or better said their thread length.  The short ones are 4 mm long and the long reach 6 mm in length.

      The most proper way to choose the right plug for your engine is to read the engine’s manual something that many modelers overlook except the chapter that nominates the number of turns of the fuel needle in order to start the engine! As a rule of thumb small engines from 0.18 to 0.19 c.i capacity use short plugs and from 0.21 μέχρι 3,5 use long ones.              

                                                 

     Long  

                                                                                          Short

       The real reason of this separation depends on the thickness of the engine’s head which get thinner as the engine’s capacity reduces. That said is fairly simple to screw tight the plug with a washer and then unscrew the engine’s head in order to check how low the plug has been screwed in. The plug’s lowest point should not exceed at all in the combustion chamber. In such a case, the plug should be replaced with a short one. If the plug is buried deep in the engine’s head then a long plug should be used.  

       Of course there are always exceptions that verify the rule. There are engines that perform better when short plug instead of long is used. Never the less no accurate information exists about the longevity of these engines that are been submitted to extreme loads in order to provide the most of their power. Let’s go back to the majority of the engines that follow common criteria. If we are obliged to use non proper plug, it’s not tragic, unless certain measures are taken.  We better get used to have always at the flying field 2-3 plugs that suit the engine. If you have to use a short plug to an engine designed for a long one then as a result the compression is reduced and several hundreds of rpms are lost.

       Do not remove the copper washer to lower the plug because a malfunction may occur. In that case further lose of compression is certain to such an extent that the engine will not run properly or not run at all! In your flight box you do have some spare copper washers. It’s time to use them! On the contrary, do not use long plug to an engine that is designed for short one because a major malfunction of catastrophic proportions will occur! The result is the compression’s increase in this tiny engine, deregulation of the engine’s advance, overheat and non reversible engine’s failure. The proper solution is to add one or two washers to the existent one and unscrew the engine’s head top to check if the plug’s threads are still protruding. If not, then don’t hesitate to run it, otherwise simply let it. Does a single flight worth the money spend to purchase an engine?   

      RATED OPERATING VOLTAGE

      Every plug is engraved with its operating voltage. If not, then it is written on its packaging either 1,2V or 1,5V. Your choice depends on the power source that you will use. If Ni-Cad battery is used then a 1,2 V plug is to be chosen. In case of Lead-acid battery, a 2,0 V plug is your choice.    

      PLUG TYPES

      Depending on the manufacturer three basic plug types exist. The basic, the ones with idle bar and with combustion anteroom. The basic version is the cheapest one, suitable for any type of use except where there are specific instructions/recommendations by the engine’s manufacturer.

       The idle bar is nothing more than a bar welded in front of the coil wire in order to provide more stable idle and it’s been used in high combustion and performance engines.

       The combustion anteroom is a specially design cavity in the ‘’entrance’’ of the plug which maximize the local heat absorption. As a result, higher performance, more stable and low revs idle, smoother revs increase and occasionally less fuel consumption. In the last two types anti vibration, enchased coil plugs can be found.

       OPERATION TEMPERATURE

       Another parameter that defines glow plugs is the coil wire operating temperature. According to the graduation three major categories exist: Cold, Medium and Hot. Here is an example diagram:

        Of course several more subcategories exist which are been encoded by each manufacturer. The ‘’key’’ in this case is that the actual difference is not as visible as in size. The manufacturer’s manual has to be followed in order to achieve a high engine’s performance.

        GLOW PLUG TYPE AND IGNITION

         In an internal combustion engine the compression of a flammable fuel mixture of a certain proportion creates heat. If the compression rate is high enough, the fuel is self ignited as in diesel engines to which the compression rate is from 20:1 to 22:1. In glow engines the compression is from 7:1 to 9:1 which is not enough for the fuel mixture to self ignite. For that, a glow plug is used which heats and finally ignites the gasified fuel. This means that only during start up the plug is fed with power. Few minutes after ignition occurs, the power source is removed and the engine keeps running as a combination of fuel compression, fuel ignition heat and the methanol effect on the platinum coil that keeps it calsinated.

        From the above it’s obvious that the engine’s compression rate in conjunction with the plug’s thermal ability, defines the ignition point for each type of fuel. So if compression is too high, or the plug it’s too hot, then the ignition will occur before the piston reaches the top.  

         As a result the phenomenon of pre-ignition (2 stroke engines) and detonation (4 stroke engines) is noted. The result is hyper heating, loss of power and possible engine break down. The relative environment humidity greatly affects compression and ignition. If high percentage of moisture is present, then the gas pressure into the chamber increases and pre-ignition occurs. According to the law of Physics, the total pressure (P) of a gas mixture that fills a space equals the sum of the partial pressures of each gas. Having in mind that air consists of Oxygen, Nitrogen and Water and at the same time our fuel consists of Methanol, Nitro-methane and Oil, then the total gas pressure in the chamber will be:

        P (Total)= P (Oxygen)+P (Nitrogen)+P (Vapor)+P (Methanol)+P (Nitro-methane)+P (Oil)   

        It’s obvious that as the percentage of humidity increases, then as a result the total pressure increases along with the partial vapor pressure. Naturally the same happens if fuel remains exposed in the air. In this occasion regardless of the relative percentage of humidity, Methanol due to its hydrophilic attribute will absorb moisture from the atmospheric air. As a result unnecessary vapor will be present again in the combustion chamber.

        THE RIGHT GLOW PLUG

                                                                   

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        How you can be sure about which plug is the right one for your engine? First read the engine’s manufacturer’s manual. If not available, look for it in the internet. If none of the above happens, then follow the trials method. Choose a plug according to what is mentioned in the previous chapters. Start the engine leaving the plug connected.

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        After a few seconds disconnect the power source and watch if the engine’s revs drop. If this happens then you need a hotter plug or more Nitro-methane in your fuel mixture. Of course if the Nitro percentage is already high enough, then a hotter plug is required. On the other hand if after the engine’s start up the power source is disconnected and the engine by the time the throttle is advanced to its fully open position sounds like pre-ignition (2 stroke engines) or detonation (4 stroke engines), then either a colder plug, or less percentage of Nitro-methane in the fuel mixture is required.  

       A general rule dictates that:

       High operational temperature engine: Cold plug

       Low operational temperature engine: Hot plug

       So if you use a relatively old ‘’tired’’ engine, or you choose to use fuel without Nitro-methane then use a hot plug. If you use a high performance, high revs 2 stroke engine which always runs to its limits then choose a cold plug. Prefer plugs of 4 stroke engines in inverted positioned engines which are hard to start and maintain stable idle. Keep in mind that in special kind of high performance engines everything changes. Compression increase, tuned pipes, high Nitro-methane percentage and vibrations, practice excessive load to the plugs.

      WHY GLOW PLUGS BURN?   

      Several are the causes :

  1. Presence of humidity and the percentage of Nitro-methane.
  2. Poor mixture that means mixture poor in fuel and rich in air. It drives the engine’s temperature to high levels, to the point where plugs are burned and engine subjects to damage. Exception of the above are the engines equipped with fuel pump and/or valid tank pressure. So when the engine runs with its full rpms, turn the fuel needle 2-3 clicks to achieve a slightly richer fuel-air mixture to the point that light blue smoke is emitted from the exhaust.
  3. Excessive load. Means larger than suggested propeller. Always remember that load increase makes engines more sensitive to poor mixture.
  4. High compression.      

      POWER PANELS

      Power panels are a most useful and basic tool in the hands of every modeler. Nevertheless, lack of handling knowledge is frequently responsible for burnt plugs. Some manufacturers deliver panels that provide more voltage than required especially if connected to a fully charged battery. A more common cause is the misuse of the resistor which by many is used in its top position i.e maximum!

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      One simple trick is to connect the plug to the panel and check the coil’s color as slowly increase the electric current or to check the voltage using a voltmeter.               

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       When the coil glows red to orange or the voltmeter shows 1,5 or 2,0 V depending on the glow plug, then use a pen to make a mark by the resistor as an indicator of the upper end of it.

       VIBRATIONS

        If the engine suffers from excessive vibrations due to unbalanced propeller, or small accident, then most likely the plug’s coil will come off or brake. The engine keeps running but when it turns to idle it stops and do not starts up again. The malfunction is not visible but the plug remains inert. The only proper solution is to balance the propeller, the spinner and the engine itself.

        ENGINE’S WARE

        It’s very easy for a plug to be destroyed by a chunk of metal from a new or a misused engine. The usual cause is fuel with low oil percentage. A frequent check of the oil that comes out of the exhaust is advised.  

       EPILOGUE

        It’s worth mentioned that a significant number of modelers do not use the recommended by the engine’s manufacturer glow plug but one of their own choice because it may be cheaper, or because they are not aware of the difference or because they simply know better! As a natural consequence the plug is burned or the engine is damaged. Nevertheless the final finding is usually against the representatives or the manufacturers, but never against their own choice!

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