Editoriali

 

2/2007

 

2 Get Primed! Glow engine essential

Gerry Yarrish, Model Airplane News, 2/2007

 

Radio control model airplane engines are marvelous pieces of engineering. Two-stroke and 4-stroke glow engines are by far the most popular ways to power model airplanes, but if you are making the transition from electric to glow power, you should learn a few basics. Each type of engine has its benefits and drawbacks, and the choice of which to use is up to you. Let’s take a closer look at these impressive power systems.
TWO-STROKE ENGINES
Two-stroke engines are the most used engine types, and they have a good power-to-weight ratio (you get a lot of power for each ounce of engine weight). They have few moving parts and are relatively inexpensive. Maintenance is very easy, and with proper break-in, a 2-stroke engine will last many years.
These engines range from .010 cubic inch (ci) to over 3ci, but .40 to .60ci are the most used sizes. Also very popular are engines with displacements of: .049 (1/2A), .10, .15, .25, .32, .45, .46, .50 and .90ci. All model airplanes have a recommended engine-size range, but most perform best when powered by an engine that’s toward the top of that range. Nothing is worse than having an under-powered model, especially when you are learning how to fly.


ENGINE TERMINOLOGY
You should know a few basic engine terms (see Figure 1).

  • ABC—refers to the materials used to make the engine: aluminum piston (A), fitted inside a brass sleeve (B) that’s chrome-plated (C) [non-ringed engine].

  • CrankCase—the main body of the engine.

  • Conrod—the connecting rod; the part that attaches the piston to the crankshaft. It has a bushing at either end and is connected to the piston with a wristpin and to the crankcase with the crankpin.

  • Head—the part on top of the engine; it’s usually bolted on with four or six bolts. In its center is a threaded hole for the glow plug.

  • Ports—channels (openings) inside the engine case that transfer the fuel/air mixture from the crankcase to the combustion chamber. The ports are opened and closed by the piston’s upward and downward motions.

  • Sleeve—the cylinder lining; it houses and guides the piston and is separate from the engine case; it has openings (ports) cut in its side.

ENGINE BREAKDOWN
The engine case usually has three parts:

  • Front housing—the case that surrounds the crankshaft.

  • Crankcase—the main case on which the cylinder head sits.

  • Backplate—the part that seals the back of the engine.

Some engines have a two-piece case, but the “internals” of all 2-stroke engines are the same. The crankshaft is supported within the front housing by ball bearings or bushings, and it has a threaded front end. Bushings, like bearings, are machined parts in which other parts turn, and they are often used in less expensive engines. A prop nut and a prop washer secure the prop against the thrust washer at the front of the engine. At its rear, the crankshaft has a counterweight web and a crankpin that engages the bottom end of the conrod.
The connecting rod is attached to the piston with a wristpin. The piston fits inside the engine’s sleeve, which fits into and is supported by the engine case. The head sits atop the cylinder and the inner sleeve, and the space between the top of the piston and the bottom of the head is called the combustion chamber.

 

Fig. 1

This Enya .50SS has a 2-needle carburetor. The large needle adjusts the high-end fuel mixture, and the small screw (see arrow) adjusts the idle mixture.

This older Enya .29 has an air-bleed hole (arrowed) at the front of the carb body and an adjustment screw to control the idle mixture.

 

Four-stroke engines such as this Saito FA-56 are very popular. Instead of intake and exhaust ports, cam-driven valves and rocker arms control the fuel flow into and out of the combustion chamber.

 

BASIC 2-STROKE OPERATION
A 2-stroke engine makes one revolution for every power cycle (see Figure 2). As the piston moves upward in the cylinder, it compresses a fresh fuel charge. The fuel/air mixture heats up and is ignited by the glow plug. The piston’s upward motion creates negative pressure inside the crankcase below the piston, and this draws air and fuel in from the carb when the intake valve opens. The combustion of the fuel/air mixture forces the piston downward, and that compresses the fresh fuel charge. As the piston travels down and the hollow crankshaft rotates, the intake valve closes and the intake ports are opened. The compressed-fuel charge passes upward through the ports and into the combustion chamber.
This happens just as the last of the spent fuel charge exits the combustion chamber through the exhaust port. As the piston moves upward again, it closes the exhaust port and opens the intake valve, and the entire process is repeated.

 

FOUR-STROKE ENGINES
Many modelers enjoy using 4-stroke engines because they have a wide powerband (they provide more torque at lower rpm) and sound so nice while they’re running. They are, however, somewhat more expensive and more complicated than 2-strokes and require a bit more maintenance to operate properly.
Instead of intake and exhaust ports, a 4-stroke engine has intake and exhaust valves (see Figure 3). The crankshaft drives a cam assembly and lifter rods, and tappets and valve springs open and close the valves at the right times. Four-strokes produce a fair amount of power, but they’re at their peak at a lower rpm range than 2-stroke engines of the same size. In comparison, the power of a typical .90 4-stroke is roughly equivalent to that of a .60 2-stroke engine.


GLOW PLUGS
A glow-powered (nitro) engine uses a glow plug to ignite the fuel. In the center is a coil of platinum wire that glows bright red when it is attached to a 1.5 to 2V glow driver. To start the engine, the glow plug is energized with the glow driver; after the engine has started, the driver is removed because the piston’s compression of the fuel charge in the combustion chamber fires the glow plug to ignite the fuel mixture. If an engine fails to start or begins to lose power, check the glow plug’s condition and replace it if you need to (see the “Reading the glow plug” sidebar).
 

"READING" THE GLOW PLUG
The pro’s monitor how well their engines are running by “reading” their glow plugs; after they’ve flown their models, they examine the glow-plug’s coil.
If an engine has been tuned properly 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 fuel 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.

 


PLUG 1. TOO LEAN
This plug came from an engine that was tuned to run too lean. The housing is slightly oil-stained, but the coil is clearly compressed into the housing, so we know it got too hot and was nearly melted by combustion heat. If the engine had continued to run in this condition, the element would have melted and come out of the plug. At that point, you’d be lucky if it just fell out through the exhaust port; it could easily cause some type of internal engine damage.
PLUG 2. NORMAL
Here’s what a reasonably fresh plug from a well-tuned engine should look like. The housing is relatively clean, so it hasn’t seen too much hard duty, and the coil is shiny and hasn’t been deformed by excessive combustion temperatures or hydraulic locking caused by being run with too much fuel.
PLUG 3. TOO RICH
This was taken from an engine that had been tuned with an excessively rich fuel mixture. The deposits on the plug housing and the coil were formed because the plug couldn’t burn off all the excess fuel; this would eventually make the engine difficult to start and to tune.
PLUG 4. OLD AND WORN
This plug is closer to ideal, but it’s old and may fail soon. The lubricants in the fuel and the temperatures generated during combustion lead to glow-plug-housing discoloration. Notice that on the first few coils, the element is relatively clean and a white-ish color. This indicates an optimum to slightly lean fuel mixture. An engine with this plug makes good power, but it might be too lean if there’s a slight change in the weather (ambient temperature).

 

Figure 2: Two-stroke engine operation

1. As the piston reaches top dead center, a new charge of fuel/air mixture is drawn into the crankcase because of the low pressure created by the piston’s upward motion.
2. The current fuel/air mixture is then compressed by the piston in the combustion chamber, and it heats up and is ignited by the glow plug. This forces the piston downward.
3. As the piston comes down, it opens the exhaust port, and the spent fuel charge begins to exit the combustion chamber. At the same time, the piston compresses the new fuel/air mixture charge in the crankcase.
4. At bottom dead center, the piston opens the bypass port, and the new fuel-mixture charge flows from the crankcase into the combustion chamber as the last of the spent charge leaves.
5. The piston comes back up and seals the exhaust and bypass ports, and the entire sequence begins again.


Figure 3: Four-stroke engine operation

1. Intake: the piston comes down, the intake valve opens, and the fuel charge is drawn into the combustion chamber.
2. Compression: the piston comes up as the intake valve closes and compresses the fuel charge.
3. Ignition: when the piston is at top dead center, the glow plug ignites the compressed fuel charge.
4. The fuel mixture expands rapidly and forces the piston downward.
5. Exhaust: the piston comes back up again while the exhaust valve opens and the spent fuel charge is expelled.
6. The piston goes back down, and the entire sequence of events begins again.

 

STARTUP AND BREAK-IN
A just-out-of-the-box, brand-new engine needs special handling before it can be run at peak output. You shouldn’t just bolt a new engine to your model and go to the flying field. Some modelers often break in engines while flying their models, but with your first engine, you should play it safe and break it in at home where you have all your tools and supplies. Short, well-lubricated runs break an engine in gently because this allows the parts to fit together and seat gradually.
When you break in an engine, it wears all the parts slowly so that they will match one another precisely. To operate properly, all engines must be broken in, and some engines take longer than others. If you run your engine without breaking it in, it will get very hot because of excessive friction, and the localized heating will damage the internal parts—primarily the piston/sleeve fit.

Model airplane engines use glow plugs to ignite the fuel charge. Several kinds of plugs are available, so check your engine’s instruction manual, and use the one recommended by the manufacturer.

When you first get the engine, avoid the temptation to turn the crankshaft by hand. Carefully open the backplate, and look inside the crankcase first to make sure that there isn’t any debris inside that could damage the engine.

First, install a new glow plug. Gently snug it down; don’t overtighten it, or you could strip out the aluminum threads in the engine head. Fill the fuel tank with a 2-stroke fuel that contains 5 to 10 percent nitromethane and 18 to 20 percent oil. Attach the fuel line to the needle-valve assembly; make sure that the line doesn’t kink or touch the engine case, which will get very hot. Attach a propeller of the recommended size, and snug the prop nut down firmly with a 6-inch wrench. Don’t use pliers; they will damage the prop nut.
Open the engine’s needle valve at least four full turns counterclockwise, and open the throttle sleeve fully. Put your thumb over the air intake, and flip the prop counterclockwise several times until you see fuel start to flow through the fuel line and into the carb. When the fuel reaches the carb, close the sleeve to about 1/4 throttle and attach the glow-plug battery. Flip the prop over with a chicken stick or an electric starter until the engine starts to run. When the engine has warmed up a little, open the throttle all the way and let the engine run at a very rich, low power setting. After about 10 minutes, stop the engine and let it cool completely. Repeat this process several times, and gradually lean the fuel mixture each time by turning the needle valve in (clockwise) a couple of clicks. Don’t run the engine at high rpm and at a lean mixture setting until you have run at least six or seven tanks of fuel through it.
You’ll know that the engine has been properly broken in when it runs consistently without overheating and transitions smoothly from idle to full throttle. Remember: it’s always better to operate your engine a few clicks too rich than a few clicks too lean!
LOW-END SETTING
For your model to land safely, its engine must have a very reliable idle. Some engines have a single-needle valve and a small air-bleed hole in the carb body that is used for the low-end, or idle, fuel/air mixture setting. Other engines have two needles: a large main needle for the high-end setting and a smaller one for idle. Start with the engine’s factory setting; it’s usually pretty close to the correct setting. If you bring an engine to idle and it dies, then the low-end, fuel/ air-mixture setting is set too lean; you must increase the amount of fuel being drawn into the carb at idle. If the engine settles into an idle but then burbles or dies when the throttle is opened, the low-end mixture is too rich, so you must decrease the amount of fuel that enters the carb at idle. It’s a balancing act.
With a twin-needle-valve carb, the idle-needle valve is used to adjust how much fuel enters the carb. In an air-bleed design, the air-bleed screw is used to adjust how much air enters the carb during idle. Both types work well, but the more powerful engines usually rely on the twin-needle-valve carb.
CARE AND MAINTENANCE
If you take care of your engine properly, it will provide maximum power and last for a very long time. This care should begin on the day that you bring it home from the hobby shop. Most engines come with Allen wrenches; keep them in a safe place, and if you lose one, replace it with one of the same size.
An engine-maintenance check begins with the removal of the backplate and a look inside the crankcase for metal shavings and other debris. Also remove the head and check the combustion chamber. Squirt a little 3-In-One oil into the engine, and turn the engine over to lubricate the bearings and the conrod bushings. Make sure that the ports in the sleeve are aligned with the ports in the case.
Reassemble the engine, and tighten the screws in a “crisscross” pattern. Don’t use thread-locking compound on the engine-case screws or head screws. It isn’t required and will make it more difficult to remove the screws for future maintenance. It can also cause the threads in the screw holes to strip. Just snug the screws down using Allen wrenches and two fingers. Never force any part that won’t go on or move easily. An engine is made mostly of aluminum, and it is very easy to strip the screw-hole threads.
After the day’s last flight, drain the fuel out of the tank and run the engine dry of fuel. Clean any debris from the engine, especially from the cooling fins around the cylinder; if the fins are blocked, the engine won’t cool properly when it is running. Squirt after-run oil into the carb and glow-plug hole to coat the inside surfaces; this helps to prevent corrosion. Alcohol-based fuels attract moisture, and unprotected engine surfaces will corrode—especially the ball bearings. Oil is inexpensive insurance for long engine life.
That’s about it! To keep your engine happy, don’t run it too lean. With proper break in and maintenance, it will provide you with many hours of carefree flight time, and that’s what the hobby is all about!