Glow plugs are an essential tool in making a nitro vehicle work. But, like many aspects of nitro vehicles, they are a mystery to most people that use them - how they work, why they work, and how to pick the right glow plug. Let's look at the essence of a glow plug, separate fact from fiction, and explain the science behind glow plugs.
Glow Plug, Noun
A device for each cylinder of a diesel engine with a heating element to heat the incoming fuel and air so that combustion will take place more readily when the engine is cold.
So, in simple terms, a glow plug provides ignition for the fuel/air mixture in a diesel (or nitro) engine, somewhat like a spark plug does in a gasoline internal combustion engine.
How Does a Glow Plug Work?
Engines need three things to work: fuel, oxygen, and an ignition point. The glow plug provides that ignition by heating the element (which is the small coil of wire inside the plug). This is usually done with a 1.5v battery contained in a glow ignitor. Some of the more recent vehicles have on-board batteries that ignite the glow plugs as part of an electric starting system. Either way, once the element is heated and the engine is started, no more power is needed to keep the element hot and provide constant ignition to the engine.
How the element stays hot after the battery is removed all hinges on the fuel we use in glow engines and on the material the element is made of. The fuel contains methanol, which is a type of alcohol. The element is made of several different metals, which when alloyed together, make it strong enough to handle the heat and vibration. But one of the metals, platinum, is special. When the platinum in the element comes in contact with the methanol in the fuel, there is a catalytic reaction between the two. This heats the platinum while causing the methanol to ignite. This is the foundation upon which the entire nitro hobby is built.
What Determines the Ignition Point if the Element is Always Hot?
The catalytic reaction depends on two things to work: heat and pressure. The hotter the element is, the easier it will ignite. Similarly, the higher the pressure inside the combustion chamber, the easier things will ignite.
Glow Plug Temperature - Hot, Medium, or Cold?
Glow plug temperature is controlled by using different heat range plugs. There are many different heat ranges, but most fall in one of three categories: hot, medium, or cold. If you're not sure which kind to use, consult with the engine manufacturer to determine what they recommend for their engine. Using a hotter than normal glow plug will advance the ignition point, and using a colder than normal plug will retard the ignition point.
Combustion Chamber Pressure
There isn't much you can do to alter the pressure within the combustion chamber, as this is usually set by the manufacturer. You can add head shims to increase or decrease the size of the chamber, but this is something only experienced nitro users should attempt, as you can easily brick your engine if you make a mistake.
Glow Plugs and Nitro Fuel - What You Need to Know to Pick the Right Plug
There are very few glow plugs that are considered universal plugs (OS' #8 plug is an example). For the most part, the type of fuel you are running will impact the kind of glow plug you need to use. Another component in the nitro fuel is nitromethane. The Nitro Percentage of your fuel determines the ignition point as well; in other words, the more nitro you run, the more you advance the ignition point. Ideally, the ignition point will be when the engine is at top dead center (TDC). This will force the piston down and back up again for another stroke. But when you run higher nitro content and don't switch to a colder plug, you will advance the ignition point and result in less-than-optimum performance, since the piston is still on its compression stroke (the upward stroke) when the air/fuel mixture is ignited.
In general, the higher the the nitro percentage, the colder the plug should be. Conversely, the lower the nitro percentage, the hotter the plug should be.
I hope this helps to dispel some of the myths surrounding glow plugs and why they work. There's no mystery here - just science. And picking the right glow plug for your boat, car, or airplane or helicopter shouldn't be hard. You just need to consult your manufacturer's manual for a recommended plug and fuel. Once you know that, you can adjust your nitro content and glow plug temperature as necessary. Now that you have the knowledge, next time you come in and ask for a plug, be specific about what you want. And your engine will thank you.
Brian Schneider / Brian is the manager, webmaster, & social media guru for Roger's Hobby Center. He's been in the hobby industry over a decade, teaching people the essentials of the R/C world. He's written a number of helpful guides, including A Guide to LiPo Batteries.