A glow plug mounted in the cylinder head is used to ignite the fuel mixture in the combustion chamber of the engine, much like a spark plug does in an auto engine. However, unlike a spark plug, which only sparks at predetermined intervals, a glow plug contains a coiled element of platinum wire that "glows" red hot all the time the engine is running. For starting the engine, a 1-1/2 volt battery must be hooked up to the plug to make it glow initially. Once the engine starts running, the battery can be removed. The heat from the burning fuel in the combustion chamber will keep the element glowing red hot while the engine is running.

There are 3 basic varieties of glow plugs on the market:

STANDARD PLUGS - For 2-stroke engines that do not have a throttle. Normally that means engines that are designed to be used in free-flight or control-line models.

R/C PLUGS - For 2-stroke engines that do have a throttle. Normally these engines are designated as "R/C" engines. R/C glow plugs have a small "idle bar" across the end of the plug that is designed to protect the glow element from direct blasts of excess raw incoming fuel when the engine is in a low speed throttle setting. This is a common cause of R/C engines quitting during idle.

4-CYCLE PLUGS - Special heat-retaining glow plugs designed to enhance the performance of 4-stroke engines.

Two types of Sig glow plugs: a four-cycle plug (left) and an R/C plug (right). Notice the idle bar on the R/C plug.

In addition, each variety of glow plug is normally available in "short" or "long" size. This refers to the length of the threaded portion of the glow plug. As a general rule, .09- .15cu. in. engines uses a short glow plug, while . 15 and larger engines will use a long plug. Some new engines come with the correct glow plug already installed. Engines occasionally "blow a plug", which simply means the element has broken and the plug will need to be replaced. When you need a replacement, consult the engine manufacturer's instruction sheet for their recommendation on the correct plug to use in that particular engine.

A PRIMER ON GLOW FUEL

Glow fuel consists of three basic ingredients - methyl-alcohol, oil, and nitromethane. You should never try to run your glow engine on anything but top quality glow fuel, professionally blended from these key ingredients. Auto gas will not work in a glow engine!

METHYL-ALCOHOL: Also commonly referred to as "methanol". This is the major ingredient in glow fuel, usually comprising about 60% to 75% of the total fuel formula. As the primary power producing ingredient in the fuel, methanol is ignited by the glow plug and completely burned during the combustion cycle.

OIL: Special oils are blended directly in the glow fuel to provide lubrication for the engine. Two-stroke engines need 20% to 22% total oil content in the fuel blend, while four-strokes run better with a little less, about 15% to 18% oil content. For many years, castor oil was considered the best model engine lubricant, providing exceptional protection even when the engine is overheated by a carelessly lean needle valve setting. Now modern synthetic oils have been developed that have the advantage of lower varnish and carbon buildup inside the engine than castor oil. However, the synthetic oils do not provide as much high-temperature protection. Here at the Sig factory, we believe in using a blend of castor oil (with its high-heat protection) and synthetic oil (with its clean burning properties) in all of our glow fuels to provide the best all-around protection for your model engines. The oil in the glow fuel seeps throughout the engine as it is running, lubricating all the moving parts. The oil does not burn, most of it is expelled out the exhaust port.

Sig fuel has been the choice of champion flyers for over 20 years.

HOW MUCH "NITRO" TO USE: Nitromethane is a high-energy power booster. While many glow engines will run OK on a straight methanol/oil mixture, the addition of a little nitro in place of some of the methanol can increase the engine's total power output and improve the idle characteristics. Most standard R/C engines are designed to run best with glow fuel containing 5% to 15% nitro (10% nitro fuel is by far the most popular type we sell). Very small engines (.020 to .10) run better on 15% to 25% fuel. Some very specialized high-compression racing engines are designed to burn 40% to 60% nitro fuel. In truth, the nitro content should be matched to the engine. Too much nitro in an engine that is not specifically designed to handle it can cause problems. Fuels with a lot of nitromethane are also very expensive, because nitro costs much more than the alcohol it is replacing. Unless you are running a special high-performance racing engine, there is no need for extra nitro. Consult the Sig Catalog for further recommendations on fuel selection.

Some very small engines, like this Cox T.D. .051, use a glow "head" rather than a glow plug. The element is actually built into the cylinder head, which can be easily replaced with a new one if it burns out. Glow heads are threaded into the top of the cylinder.