By: Rick Klages
This article is a short summary on what glow plugs do and how they work. I want to give you a brief summary about their use in our hobby, so you have some understanding of them. They are very simple and vital units of our unique hobby, so we should know a little about them.
Not a lot of modern model fliers are familiar with how a glow plug works and why we would want to use anything besides the plug that came with our engine. Other than knowing they burn out, and they are required to start an engine, most of us could not explain how they work. They have become so reliable, and fuels today so consistent and high quality that most fliers just use the same old plug again and again because they work. There are several misconceptions about glow plugs, so lets try to explain how they work using some very good articles and information from the manufacturers to back us up.
First off a glow plug is not a spark plug. They are two completely different things. We won’t go into spark plugs here as modern glow engines do not use them. A glow plug is just as it’s name implies. It is used to maintain combustion of the alcohol/air inside our glow engines. It works in conjunction with both the compression and the alcohol that is the largest component of a gallon of glow fuel. The element (coil) in the center of the glow plug is derived of mostly platinum (thus the price), and other elements (iridium or rhodium) which prolong the life of the element, as pure platinum breaks down under the caustic conditions inside a model engine. The platinum and the vaporized alcohol/air react as compression increases (as the piston rises in the cylinder) and ignition of the fuel/ air mixture is achieved. This reaction/process occurs each time the cylinder makes a complete cycle (every 360 degree rotation of the crankshaft in a 2 cycle or 2 stroke engine) as long as fuel/air is present. Thus the name glow plug, because as long as there is compression, and a correct fuel mixture the element glows and this process continues to keep our engines running. The size of the platinum coil and the construction of the plug body contribute to how much heat is maintained between each cycle of the piston in the glow plug. . This is why different plugs react differently with ambient air temp, altitude, humidity, fuel type and nitro content, and engine displacement.
Second, the temperature rating (hot,med,cold) of the glow plug does not mean how hot or cold your engine operating temperature will be. So therefore if you think putting a hotter glow plug into your engine will make it run hotter—NOT REALLY. Most of you engine temperatures are controlled by the fuel/air mixture that is regulated by the main needle for all intensive purposes, not the glow plug. If it’s running to cool-lean your mixture. If it’s running to hot—richen your mixture. That is another article.
Now that we have a little knowledge on how and why a glow plug works, let’s take a look at the different temperature ranges of glow plugs and what they do.
Glow plugs are rated from cold to hot. Again this has nothing to do with engine temps.
Glow engines, whether two or four cycle have no way to precisely “time” the ignition cycle of the engine. They are designed by manufacturers to have the best overall performance possible under lab conditions. The engineers try to optimize the power and reliability by spending many hours testing cylinder timing, using several mixtures of fuel. This is because they must produce an engine that is capable of operating in many different areas of the world with many types of fuels and climate. This is a “fixed” timing, using both intake and exhaust port size and position. The engine manufacturers needed a way to fine tune the engines for maximum performance, so they realized that by changing the point the at which the glow plug reached the temperature of ignition (usually by using a different diameter glow wire), they could slightly change the “timing” of the engine, much like turning a distributor on an automobile. Therefore a “hot” glow plug obtains it’s ignition temp sooner-it actually advances the dynamic timing of the engine slightly. Consequently a “cold” glow plug does the opposite, and “retards” the timing slightly. Now we have a way to fine tune the performance of our glow engines.
Again this all incorporates air temp, engine displacement, fuel type and nitro content, altitude, etc into the equation, so what runs to peak today may not tomorrow if there is a sudden weather change. That is why so many people just don’t bother “tweaking” an engine this way, because it does take some knowledge and skill and most of the time the performance gain isn't worth the effort for the average pilot. Modern glow engines run so well right out of the box. This is a definite indication the designers and engineers know what they are doing. But the perfectionist among us love to tinker and make everything work it's best. It's fun as well as educational.
Don’t let this scare you away from trying a different plug. Just realize that changing a glow plug often times ends up becoming an issue of several things to really work for optimum performance. Head shims, needle valve settings, and engine load all play into the equation. It’s fun, challenging, and gives you a sense of accomplishment when you have the skills and knowledge, as well as take your time to set an engine up to work it’s best for your type of flying---especially when your buddy with the same setup can’t believe how you heli blows his away and runs so smooth and perfect.
Have fun and check out these links to websites with much more in depth information for those who want to know it all. They are great reads.
http://www.osengines.com/accys/glowplugs.html
http://findarticles.com/p/articles/mi_qa3819/is_200408/ai_n9433814/pg_1?tag=artBody;col1
Thank you,
Rick Klages (from Rick’s “now you know” series) 11-2008
rick@heliproz.com
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