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Glow plugs explained

Glow plugs explained

Bess, Stephen

Glow-plug manufacturers produce a large variety of plugs-all with their own promise of great performance. They all have the same basic design, but with so many brands, a variety of “heat ranges” and ultra-hip-sounding “turbo” plugs on

the shelves, it’s difficult for average RC guys to know where to begin. Ever wonder what the difference between a spark plug and a glow plug is, or what makes a glow plug’s design so vital to an engine’s performance? Here are your answers.

GLOW PLUGS VERSUS SPARK PLUGS

Both glow plugs and spark plugs ignite the air/fuel mixture inside the engine. Glow plugs look similar to auto spark plugs-and both types cause the fuel to go “boom”-but that’s where the similarities between them end. Spark plugs (as their name implies) produce electrical sparks when connected to an electrical power source. Engine compression and combustion have no effect on producing sparks from spark plugs, and if the electrical source is disconnected, spark plugs quit sparking, and the engine quits running. Two-stroke model engines With glow plugs, however, operate under different conditions.

As its name implies, when a glow plug is connected to a 1.5V external power source (a glow igniter), its coiled element glows hot because of electrical resistance. When a 2-stroke engine’s cycle has begun, compression and combustion keep the plug’s coil glowing hot, even after the 1.5V power source has been removed. Unlike spark plugs, glow plugs will operate without an external electrical power source and will continue to glow throughout an engine’s combustion cycle.

UNDERSTANDING HEAT RANGE

It’s important to realize that there is no universal glow-plug heat– range standard; one manufacturer’s “cold” plug may be hotter than another manufacturer’s “medium” plug.

Plugs rated as “cold” could have cooler coils than “hot” plugs, and a coil’s temperature affects how an engine will run.

Several factors determine a glow plug’s temperature range; these include but are not limited to:

* The composition of the coil wire.

* The coil wire’s diameter

* The total length of the wire used to form the coil.

* The size of the plug opening in which the coil is installed.

* The plating and material used to make the glow-plug’s body.

Here’s how the factors contribute to glow– plug performance:

* Coil wire.

Thicker, shorter coil wire (fewer coils) makes a colder plug ; thinner, longer wire (more coils) makes a hotter plug.

* Opening size. The size of the opening that houses the element also affects a plug’s heat range. A larger opening exposes more of the coil to the air/fuel that enters the combustion chamber, and that promotes Combustion and contributes to the plug’s being “hotter.” A smaller opening around the coil makes the glow plug “colder.”

* Glow-plug housing. The type of plating used here affects a plug’s temperature range. Housings with chrome and shiny, gold-color plating reflect heat and contribute to a plug’s being hotter. Dull, black coatings are not as reflective and make for a colder plug.

“READING” A PLUG

The pro’s monitor how well their engines are running by “reading” their glow plugs; after they’ve run their vehicles, they examine the glow-plug’s coil.

If an engine has been tuned property and is running well, the coil will be shiny or a light gray, and its shape will be uniform even after a hard run. The leaner you tune your engine, the grayer and more deformed the coil will be, and these signs warn that you’re running your engine too lean! If that’s the case, replace the glow plug, richen the fuel mixture immediately, and retune your engine for good performance.

Before you unthread a glow plug, you must clean the area around it. When the engine has cooled, spray a generous shot of nitro cleaning spray around the glow plug to rid the opening of dirt and debris. If you don’t do this, you risk having this dirt fall into your engine as you remove the glow plug.

Glow-plug variants

Plugs with an “Idle bar” An idle bar runs across the glow-plug opening, and you’ll see these plugs on hobby– shop shelves. Will they work in your application? Properly adjusted engines don’t need a plug with an idle bar, and plug manufacturers say they aren’t made for engines smaller than .21 (3.5cc). The bar is intended to protect the coil from being extinguished by an overly rich fuel mixture. This type of plug was origi– nally designed to run in larger engines than we typically use. Such plugs may fit in your engine, but the idle bar may cause contact between the glow plug and piston or may increase compression and cause detonation.

“Turbo” plug. These do the same job as regular plugs-they ignite the fuel/air mixture inside the engine– but their design offers an advantage. A turbo plug is tapered and doesn’t need a washer; It seals the combustion chamber nearly perfectly. Regular, non-tapered plugs with copper washers can allow leakage out of the engine’s combustion chamber, and this causes a slight loss of performance.

The taper on a turbo plug and on the mating portion of the head also allows the combustion chamber to be smoother and less likely to disrupt the fuel/air mixture during combustion. ft doesn’t matter how long or short the plug is, and there isn’t any concern with the thickness or reusability of glow-plug gaskets.

Turbo plugs are sealed against the interior surface of the cylinder head; the tapered part mates with a taper in the cylinder head’s glow-plug opening (or “head button” on two-piece heads). This fit means that the glow plug is always installed at the proper height; the interior contour of the combustion chamber is never disrupted by a plug that is too long or too short, and there isn’t a gasket to worry about leaking. The result is a potential top-end performance increase of a few thousand rpm.

Are turbo plugs less durable than regular plugs, and are they suitable only for on-road engines? These are popular misconceptions. Having a tapered body does not make a plug less durable. Plug durability is determined more by Its coil construction and how the engine is tuned than by a plug’s housing. To seal properly, turbo plugs do, however, require perfectly clean plug and head-button threads.

Off-road engines encounter more dirt and grime than on-road engines, so turbo plugs are more suitable for on-road applications. If, however, you maintain a clean, debris-free head button and glow-plug threads, a turbo plug will work just fine.

NEW FOR NITRO O.S. 12CV-R & 15CV-R Turbo Heads

O’S’s newest pair of engines are turbo-plug versions of its popular .12 and .15 CV-R racing engines. They feature durable ABC construction, SG-style crankshafts, slide carburetors and perform ance-boosting turbo-plug cooling heads. You can expect these new turbo-plug CV-Rs to provide slight increases in overall horsepower and rpm.

O.S.; distributed by Great Planes Model Distributors Co. (800) 682-8948; osengines.com

CRC RACING STS .12 racing engines

Imported by CRC Racing, according to the company’s Frank Calandra, these new engines are very powerful. The ROAR-legal, blue-head 3-port STS MT-12 features ABC construction, a threaded crankshaft and a slide carb, and it uses standard glow plugs. The STS-RS12 is a ROAR-legal factory-modified 3-port with a gunmetal gray head (not shown). For the Outlaw crowd, a purple-head .12 with 5 ports and an ABC piston and sleeve is available. According to CRC, pull-start versions of these STS engines and big-block .21 engines are also in the works.

CRC Racing (315) 338-0867: teamcrc.com

head to create a strong seal without a gasket.

TECH Q & A

A I’m frustrated because the O.S.12TR in my Nitro TC3 is difficult to start, and when it does start, it takes forever on the

starter box to fire it up. My glow plug is fresh, and I know the glow igniter is charged and ready to go. I’ve even seen fuel spray coming out of the carburetor opening because it takes so long for the engine to start it almost floods.

A Luckily, I think there’s an easy answer and it lies with your starter box. Associated’s Nitro TC3 has a unique rear-facing engine orientation that most starter boxes are not designed for. You are probably trying to start the engine backwards! Unless you’ve set your starter box up to crank the engine in a counterclockwise direction (looked at from the rear of the NTC3), it is cranking the engine backwards. and your starting difficulties will continue until you reverse the starter-box wires to spin the engine in the right direction. An engine can start when spun backwards, but as you’ve found out, such starting is rare, and it takes quite a long time. If you see fuel spraying out of the carb, this is a sure sign that your engine is spinning in the wrong direction (engines are like air pumps; they always suck air and fuel in through the carburetor and pump it out through the exhaust-unless they’re spinning backwards). Reverse the starter-motor leads on your old starter box, or think about buying a new box. Check out Associated’s new Nitro TC3 starter box (item no. 1750) and OFNA’s new Multibox, which has built-in NTC3 alignment openings. As long as your equipment works properly and your engine is well tuned, you’ll be amazed at how easily the engine starts when the starter is spinning in the proper direction.

BY STEPHEN BESS

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Send your Piston Power questions and comments to Stephen Bess stephenb@airage.com

Copyright Air Age Publishing Jan 2003

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