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12-26-2012 | 09:12 PM

hugger-4641

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From: McKenzie, TN

Introduction to Glow Engines

I was perusing the "RC University" link while helping a newcomer to the hobby and I noticed there was not, as of yet, any basic information on Glow Engines. Here is an article I wrote to help some of my students, it may be of use to someone else as well so I thought I would post it. Feel free to add it to the "RC University" if it is deemed worthy.

Two Cycle Glow Engines by Gerald (Jerry) Hughes aka Hugger-4641<o

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>There is no substitute for one-on-one help from an experienced person when it comes to operating and tuning a glow engine. If you really want to learn how to use one of these engines safely and properly, I highly recommend getting some help from a club or experienced modeler.<o

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>This article is intended to help gain a basic understanding of how a two cycle glow engine works, how to set up the fuel tank and fuel supply system, and how to safely start and tune a basic glow engine. If you already have a basic understanding of how IC (internal combustion) gas and diesel engines work, then you will have no problem understanding the concepts discussed here. If you have no idea how a typical IC engine works, you should probably get a basic understanding of this first. A quick “Google” on “internal combustion engines” should lead you to an adequate tutorial on the basic concepts of compression, combustion, timing, and ignition for two cycle and four cycle internal combustion engines.<o

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> Just like any other IC engine, in addition to compression, there are four basic things required for a glow engine to run. These are: Fuel, Air, Ignition, and Timing. Let’s briefly address these and how they are used in a two cycle glow engine.<o

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>Fuel for a glow engine is a usually a mixture of Methanol , Nitro-methane, and some type of oil for lubrication of the engine. This has historically been Castor oil, but now days there are many fuels that use a synthetic oil component or a blend of both Castor and synthetic oils. These fuels are sold with different ratios of oil and Nitro (nitro-methane). Just like race cars can be modified and make use of different fuels, glow engines can give different results with different fuels. Engines that have been modified for higher compression can make use of less Nitro, while a lower compression can make use of a higher Nitro content. We are addressing the entry level RC enthusiast here, so generally, a fuel with 10% to 15% nitro content is preferred for most glow engines. Fuel with an oil content of 15% to 20% is also typical, and as said before, this can be castor, synthetic, or a blend of the two. <o

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>Air is mixed with the fuel at the carburetor. The two most common kinds of carburetors used on glow engines mix air with the fuel in a slightly different way. One is called a “two needle” carb, and the other is called an “air bleed” carb. The two needle carb is the most common and most reliable of these types. As the name implies, it uses two needle valves to regulate the fuel and air delivery for idle and high rpm operation. The High Speed needle regulates the amount of fuel being mixed with the air that is drawn though the intake of the carb. This mostly controls the higher rpm performance of the engine. The low speed needle controls the idle mixture and as you might guess, controls the idle and lower rpm performance of the engine. <o

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>The other type of carb is called an “Air Bleed” carb and is more common on cheaper, lower performance engines. These engines are typically found on RTF trainers. Most experienced modelers do not prefer engines with these carbs, but they are an economical entry level choice and are simple and reliable enough for most beginners to use effectively. The air bleed carb also has a high speed needle, but controls the air for the idle mixture by using a small hole in the exterior of the carb instead of using another needle to control fuel. Many “trainer” engine carbs such as the Evolution engine have locking screws or other mechanisms that prevent or limit the amount of adjustment that can be made to the idle and high speed settings. This is designed to keep an inexperienced person from making an extremely improper adjustment to the engine that would prevent it from running at all. <o

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>Ignition in a glow engine is a little strange compared to the typical IC engine. A glow engine is kind of a hybrid of a two cycle gasoline engine and a diesel engine. Glow engines have a “glow plug”, but this is different from a “spark plug” in a gas engine and more similar to the glow plug used for starting in some diesel engines. The glow plug in an RC engine has a filament made of Platinum, very similar to the Tungsten filament in a light bulb. A battery or some electrical source is used to energize the glow plug for starting the engine, just like in a diesel, but once combustion occurs, a chemical reaction between the the fuel and the Platinum in the filament keeps the filament “glowing”, and the electrical source is no longer required. The fuel/air mixture is compressed by the piston as it travels towards the head, when the correct compression and fuel/air mixture is present, if the glow plug is “glowing”, then combustion occurs. Unlike a diesel engine which continues to produce combustion from compression alone, if the glow plug in a glow engine stops working, the engine will not run. Likewise, if the glow plug is working but not working properly, the engine may start, but may run erratically, may not want to idle, or may not want to run at full speed. <o

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>Timing of the fuel delivery is achieved by a port in the crankshaft that is “connected” to ports in the cylinder walls, much like a typical two cycle chain saw or weed-eater engine. As the crankshaft turns, this “port” rotates past the opening in the carb at the same time that the piston has created a “vaccum” in the cylinder, causing the fuel to be pulled into the cylinder. This “timing” is not easily adjustable, so we won’t get into the details of how it works other than to say that it pulls fuel from the carb and delivers it into the cylinder. Timing of the ignition is not directly adjustable either. Ignition timing can be affected by the amount of compression, the amount of nitro content in the fuel, and the glow plug. Using the same fuel, a hotter or colder glow plug can slightly affect the timing of the Ignition, which may or may not be a good thing. The engine manufacturer will usually recommend a glow plug and fuel that is best for a particular engine. In the absence of manufacture's information, using a fuel with 10-15% nitro content and a “hot” glow plug such as an O.S. #A-3 is usually a good starting point that will run fine in most beginner level .25 to 1.20 sized two cycle glow engines. <o

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>Fuel Supply to the carb is one of the most problematic areas of a two cycle glow engine. Let’s discuss the typical components involved in a basic system and the problems they can present. This may sound strange to some, but the muffler is actually the first link in the “chain” of the fuel system, following the chain from the muffler, next is the tank, clunk and pick up line, bung seal, High Speed Needle, carb fuel inlet, barrel, and low speed assembly.<o

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>The fuel tank is the next link in the chain. The vacuum created by the engine is usually not enough to pull fuel from the tank reliably, so the tank is “pressurized” from a tube connected to the muffler on the engine. If this tube is not connected to the tank and to the muffler, or if the muffler has been modified improperly so that “back pressure” is lost, the fuel supply to carb will not function. If this happens the engine may not run, or may start and run briefly and then die. Another common mistake is to accidently switch the “pressure” line and the “pick up” line. The “pressure” line must be connected to the muffler. The “clunk” and “pick up” line must be connected to the High Speed needle valve. If these are switched, fuel will not flow properly. The High Speed Needle may built into the carb itself, or it may be a remote needle valve assembly attached to the firewall or elsewhere. Either way, the High Speed needle is next in the chain after the fuel leaves the tank through the clunk and pick up line.<o

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>Fuel is “pulled” or “pushed” (which ever works best for you to think about it) from the tank to the High Speed needle through the “pick up” line and “clunk”. The clunk is basically a weight with a fuel tube connected to it that allows fuel to be “drawn” from tank no matter what position the tank might be in as related to the ground. If the clunk is installed properly, the plane can be right side up, inverted, or anywhere in between and the clunk will fall to the same area of the tank as the fuel, thus fuel flow is not interrupted during maneuvering. I should clarify that this may not be true if the plane experiences a prolonged “nose down” attitude. Most clunks and fuel lines stay in the rear portion of the tank and are not flexible enough to fall completely to the front portion of the tank. In a prolonged “nose down” attitude of the plane, the fuel can fall to the front of the tank where the clunk cannot reach, thus fuel supply can be interrupted. For this to happen, you would have to make a very prolonged dive from several hundred feet up, but it can and does happen. <o

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>The bung is the opening in the fuel tank where the pressure line and pick up line enter and leave the tank itself. This is a common area for problems. If the bung is not sealing properly you may have a fuel leak. You may also have air getting pulled into the system at this point. If you have fuel leaking, the problem is usually obvious, but many times the problem won’t show up until the tank is half empty of fuel. You can observe the fuel line to the High Speed needle valve with the engine running and look for air bubbles. If you see any bubbles you have a leak in the bung or some where in the pick up line and clunk assembly. You must correct whatever is allowing the bubbles into the fuel line or you will be wasting your time trying to adjust the High Speed needle. <o

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>The High Speed Needle is the next link in the chain. As mentioned before, the High Speed Needle assembly can be attached directly to the carb or it can be a remote assembly. Remote assemblies are often attached to the back plate of the engine crank case or on the firewall of the engine compartment, and this presents another place for fuel to leak or air to enter the system. This is easily identified if you observe the fuel line with the engine running. If you have no air bubbles between the tank and the needle valve, but you have bubbles between the needle valve and the carb, then the o-ring in the needle valve is probably the culprit. A temporary test can be done by removing the needle and applying grease or petroleum jelly to the threads and o-ring area, then re-install and set the valve. If this stops the bubbles, then you know the needle valve needs a new o-ring. <o

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>Setting the High Speed needle. A general rule of thumb for getting started is to open the High Speed Needle 2 ½ turns. Usually, turning the valve clockwise closes the valve and turning the valve counter clockwise opens it. Close the valve completely, then open it 2 1/2” turns or 900 degrees of rotation. This is good starting point that should allow the engine to start, but may not be the optimal setting for performance. We will discuss starting the engine and fine tuning the needle setting later. <o

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>Carb fuel inlet. This is the point where fuel enters the carburetor. If the high speed needle valve is mounted remotely, then this is the point where fuel enters the barrel and ultimately goes into the engine. The barrel is the mechanism that rotates as the throttle is moved and regulates the amount of air allowed to enter the carb. If the needle valve is attached directly to the carb, then the fuel exits the needle valve and goes directly into the barrel.<o

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>If the carb has a Low Speed Needle, the adjustment screw is usually located on the opposite end of the barrel from the fuel inlet. Most often it is a very small slotted screw recessed inside the barrel assembly and throttle linkage. It requires a very small flat head screw driver to make any adjustments. We will discuss fine tuning the low speed needle later, but like the high speed needle, a good starting point is 2 ½ turns open. There are a few engines, such as some Supre Tigre, engines which are left hand thread. This means that to open the needle, you must turn clockwise instead of counter clockwise. Most O.S., Magnum, Asp, GMS, and other entry level brands have needle valves with right hand threads and must be turned counter clock wise to open them. If the carb is an “air bleed” type, it will usually have a small external screw head that may have a slotted, hex, or Phillips head, as well as a small locking nut.<o

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>Starting the engine. Let’s talk about safety at this point. Starting a glow engine can be dangerous even if the engine is not running yet. First of all, the aircraft needs to be strapped down securely to a starting stand designed to prevent any forward motion once the engine starts. Examples of these starting stands can be found in various threads on RC universe and other forums.<o

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>Starting a glow engine by hand is commonly done by experienced folks, but to be done safely requires some habits and precautions that are best taught in person. A propeller can “kick” back during starting and do a lot of damage to fingers if not done properly. The safest way to spin a glow engine is by using an electric starter. This is basically an electric motor held in your hand and applied to the nose cone of the engine. These can be cordless, or powered by connecting to a car battery or some other external power source. But this just gets the engine spinning, more is needed! As mentioned earlier, the glow plug requires some electrical source to begin the process. The most common method of doing this is by using a “glow starter”. This is basically a rechargeable battery with a long neck that is attached to the glow plug temporarily while the engine is being started. There are two potentially hazardous situations that occur at this time:<o

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>First you must apply the engine starter and remove it when the engine starts without putting any body parts in the path of the propeller. A very real danger is that the propeller or spinner could come loose or break apart during starting or right after the engine starts. If a spinner or propeller fails, it usually flies forward and/or off to one side. So, it is best to position your head and as much of your body as possible behind the propeller and reach around with one hand to apply the starter.<o

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>Second, once the engine starts, you must remove the glow starter. This places your hand in a very close proximity to the spinning propeller. You must remove the glow starter carefully keeping your eye on the propeller and keeping your hand from slipping into it. I paint the tips of my propellers red or yellow to help keep track of where the arc of the blades is.<o

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>Once the plane is secured and safety concerns are addressed, the starting procedure is simple:<o

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><![if !supportLists]>1. <![endif]>Fill the tank with fuel and turn on the rx and tx. Set the throttle so that the barrel of the carb is just slightly open. You should see a very small crescent shaped opening just big enough to put the head of a pen or pencil into. Make sure you have pre-set the High Speed Needle as mentioned previously, or no fuel will be able to reach the carb.<o

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><![if !supportLists]>2. <![endif]>Place a finger over the exhaust and rotate the propeller counter clockwise by hand several times until you see fuel in the fuel line to the carb inlet. Plugging the exhaust helps force the fuel into the lines quicker, but be careful not to let the prop slip or kick back on your hands. Using a “chicken stick” is a good idea for this task as well as for the next one. <o

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><![if !supportLists]>3. <![endif]>Rotate the propeller clockwise by hand until you feel some resistance, leave the prop in this position. This gets the engine back off of the compression stroke and lets the prop build some momentum before reaching compression. If you don’t do this, often the engine starter will slip on the nose cone or may just stall out when the compression stroke is reached.<o

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><![if !supportLists]>4. <![endif]>Attach the glow starter securely to the glow plug.<o

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><![if !supportLists]>5. <![endif]>Apply the engine starter and spin the engine. If the engine does not start within a couple seconds, re-check all the previously mentioned items.<o

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>Once the engine does start, carefully set the engine starter aside, then carefully remove the glow starter.<o

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>Now we are ready for tuning the High Speed Needle and possibly the Low Speed also. There are many threads devoted to this topic in the “Glow Engine” forum, but it will be discussed in another article titled “Tuning a Glow Engine”.<o

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