Glow Plugs Explained

Glow Plugs Explained

With the ever popular use of electric power systems, nitro-burning glow engines have started to before less understood by newcomers to the hobby. Once the mainstay of the hobby, 2-stroke glow engines still have a lot to offer and produce excellent power for their weight. Once you’ve decided to give a glow powered airplane a try, ask for help and always break in your engine  before your first flight. Here are three of the most asked question about glow power.

WHY DOES THE GLOW PLUG CONTINUE TO WORK AFTER PULLING OFF THE STARTER BATTERY?
When hooked up to a battery, the glow plug’s coiled-wire element glows bright orange, which creates a temperature in excess of 1,500 degrees F.

Once the engine is flipped over, either by hand or a starter, the compressed fuel/air mixture will ignite. If the mixture is right, the engine will become self-sustaining so that when the battery is disconnected, the engine continues to run.

Simply put, what happens inside the combustion chamber is that the coiled-wires are heated up from the compression stroke and continue to glow for the next compression stroke, igniting the fuel/air mixture, which in turn, heats up the wire for the next cycle.

HOW DOES AN IDLE BAR HELP THE IDLE?
The idle bar is there to keep the glow plug from getting extinguished when the engine is throttled up. When the engine is idling, it has a tendency to pool up some fuel in the crankcase so when the engine is throttled up, that puddle is forced through the cylinder transfer ports. These ports direct the flow right at the glow plug. With an unshielded glow plug, that fuel hits the wire element and instantly smothers it. The idle bar in front of that wire element helps prevent the flow of fuel from hitting the wire and thereby keeps the glow plug lit. If you have a problem with the engine choking out when you go to wide open throttle, you may want to try a glow plug with an idle bar.

WHY ARE THERE DIFFERENT GLOW PLUG TEMPERATURE RATINGS?
We have different plug temperature ratings so they can be used to change the performance of the engine depending on the flying conditions. Because our engines have a fixed compression stroke and operating setup, the perfect ignition point will change with different running conditions. Some of these can include compression ratio, nitro or oil contents in the fuel, weather conditions and propeller load. By using plugs with different temperature ratings, we can adjust the ignition point so it’s not too early or too late. Once you find that sweet spot, your engine will produce the best performance.

Updated: July 23, 2020 — 5:22 PM

6 Comments

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  1. Your explanation of the reason for the glow plug element to remain hot without the battery, once the engine is running, is ridiculous. The real reason is that the catalytic reaction between the combustion gasses and the platinum in the wire used for the plug’s element keeps it hot. Ever wondered why all cars require a catalytic converter and why there always is a heat shield around it?

    1. Catalytic converters are for pollution control, the engine does NOT require one. The converter (why it is called a “converter”) takes unburned hydrocarbons and other exhaust components and “converts” them into a non-polluting exhaust, REDUCING the pollution the engine puts out. They are hot because of the chemical reaction and the hot exhaust gases. The hot combustion gases of a glow engine would keep any type of thin wire hot enough to keep the engine running, whether or not there was any platinum content. I believe the platinum content is for longevity of the coil.

      1. Calm down, catalytic comes from the root word catalyst. Catalyst is a chemical term for a substance that provokes or speeds up a chemical reaction. The coil on a glow plug is plated with Platinum. Platinum is a catalyst to cause combustion when there is the correct mixture of oxygen and ethanol. I have experienced spontaneous combustion when my HB20 started without the glow plug connected to power. I had flown a couple of flights so the engine mixture was properly set. I spun the engine with the starter motor to draw fuel up to the carb and the engine burst to life. Quite the surprise.

  2. Two things keep the plug glowing after the batteries are removed.

    The heat from the previous ignition stroke heats the element and the catalytic reaction between the alcohol in the fuel with the platinum element in the plug maintains the heat until ignition

  3. The ongoing saga of the glow plug: 3 explanations so far, all different. In close to 40 years I’ve never witnessed an explanation of glow plugs and their operation where people can be in agreement. It continues.

    1. J D Vesper explanation is the correct version and is corroborated by the plug manufacturers, should anyone wish to go to the source for their information, rather than relying on the misinformation of people that have a voice but no knowledge – a common issue these days where the internet operates to make lazy minds even lazier.

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