Two-stroke glow engines are by far the most popular engines used to power RC model airplanes. They are relatively inexpensive for the power they produce, they are easy to start, run and maintain, and once you learn how to properly take care of them they will last for many years. Let’s take a closer look.
Starting at the top of the engine is the cylinder head. It is bolted into place and has a threaded hole in the center for the glow plug.
The area between the bottom surface of the cylinder head and the top of the piston is the combustion chamber. The bottom of the glow plug and the platinum element can also be seen here.
All model kits and ARFs have a recommended engine size range. Typically this will be something like .25 to .32, or .40 to .60, etc. You should choose an engine that is within this range, and for better climb performance you should pick an engine closer to the higher side of the range. When it comes to choosing a propeller for your engine, you should also follow the recommendations found in the engine’s operation guide.
If you have never owned or run a 2-stroke engine then some of the terminology needs to be explained. Here’s a glossary of 2-stroke engine terms:
ABC– refers to the materials that make up the engine’s piston and sleeve; an aluminum piston (A), fitted into a brass sleeve (B), that has been chrome plated (C). An AAC engine is one with an aluminum engine fitted into an aluminum sleeve that has been chrome plated.
Case -the engine’s main body. Most are cast in one or two parts from aluminum, though some specialty engines are made from fully-machined aluminum stock.
Connecting rod -also referred to as a “conrod,” this is the part of the engine that connects the piston to the crankshaft. The conrod has bushings at each end and is connected to the piston with the wrist pin (top end), and is connected to the crankshaft with the crank pin (bottom end).
Cylinder head- the top part of the engine usually bolted into place with either four or six bolts or screws. A threaded hole in its center is where the glow plug is installed. The underside of the cylinder head is machined to form the top of the combustion chamber.
Ports– openings and channels machined into the sleeve and engine case that allow the transfer of the air/fuel mixture from the engine case into the combustion chamber and, after combustion, out through the exhaust.
Sleeve– the cylinder’s internal lining or ìliner.î A tubular, brass structure that houses and guides the piston, the sleeve has a flat rim flange that fits between the engine case and cylinder head to hold it in place. Port openings are machined in the side of the sleeves that align with the transfer channels in the engine case.
The part of the engine that the piston fits into and is guided by is the sleeve. Made of brass, it has openings (ports) machined into it to allow the flow of fuel mixture.
The crankshaft converts the up and down motion of the piston into rotation force to spin the propeller. The opening in the middle of the crankshaft is the intake port. It aligns with the carburetor’s venturi and allows the air and fuel mixture to enter the engine case through the holes in the back of the crank web.
TYPICAL TWO-NEEDLE CARBURETOR EXPLODED VIEW
With most engines, the case usually has three parts: the front housing that houses the crankshaft and main bearings, the crankcase that is the main case that the cylinder is attached to, and the back plate that seals the back of the engine. It is usually held in place with four bolts or screws and can be sealed with either a thin gasket or an internal O-ring.
The crankcase is supported in the front housing with a large rear bearing and a smaller front bearing. In less expensive engines, solid bronze bushings can be used in place of the ball bearings. A prop nut and a thrust washer hold the prop securely to the front end of the crankshaft and at the rear is a large counter-weighted web and crankpin used to connect the crankshaft to the conrod. The conrod is attached to the piston with the wrist pin and the piston fits within the sleeve, which fits into and is supported by the cylinder part of the engine case. The head fits on top of the sleeve and the space between the top of the piston and the bottom of the head forms the combustion chamber.
Depending on the design of the engine, the piston can be sealed with either a piston ring that fits between the piston and the sleeve, or the piston can be sealed with a slight taper (smaller at the top) in the sleeve. This is how an ABC engine is set up.
The piston is connected to the connecting rod with the wrist pin. A spring wire clip holds the wrist pin within the piston body.
BARREL-TYPE THROTTLE CARBURETOR
The engine’s power is controlled by its carburetor. The carburetor is made up of the main body, the throttle barrel, the high-end and low-end needle valve assembly, the spray bar and the venturi. Air enters the carburetor through the venturi opening and the amount of air is controlled by the rotating throttle barrel. A throttle arm is attached to the barrel so it can be rotated open and closed by the throttle linkage and servo.
The fuel enters the venturi through the high-end (main) needle valve and it sprays into the venture through a hole in the side of the spray bar. By turning the main needle valve in (clockwise) you lean out the fuel air mixture by lessening the amount of fuel relative to the air. By turning it counterclockwise you richen the mixture by letting more fuel flow in. The low-end (idle) needle valve is usually located at the center of the carburetor’s throttle arm and it adjusts the mixture while the engine is operating at idle to about º throttle.
It may require a thin screwdriver to adjust.
The glow plug lights up when you energize it with a glow driver battery. It ignites the fuel mixture in the combustion chamber.
The glow plug
is used to ignite the fuel mixture within the combustion chamber. The glow plug has a 1/4-28 thread and it screws into place in the hole in the center of the engine head. In the middle of the glow plug is a coiled element made of platinum wire. The glow plug is first energized with a 1.2V glow driver battery, and then the compression of the fuel mixture and the heat generated by that compression causes the glow plug to ignite the fuel charge, much like how a diesel truck engine operates. Once the engine is started and warms up for a little while, remove the glow driver battery and the engine will continue to run. Catalytic action between the methanol fuel and the platinum in the glow plug element as well as the engine heat keep the element glowing once it has been lit with the starting battery.
BASIC 2-STROKE ENGINE OPERATION
A 2-stroke engine makes one revolution for every power cycle. As the piston moves upward in the sleeve, it compresses a fresh charge of fuel/air mixture. The compression heats up the fuel mixture and is ignited by the glow plug. As the piston travels upward it also creates a negative pressure zone in the crankcase below the piston. This draws air and fuel into the crankcase from the carburetor and into the intake port. The intake port is machined in the side of the crankshaft and it lines up with the carburetor’s venturi and it opens up to the hollow center of the crankshaft. The air and fuel travel through the hollow crankshaft to enter the crankcase.
As the piston travels downward after the fuel mixture is combusted, the con rod turns the crankshaft and this closes the intake port. The piston continues downward and starts to compress the new charge of fuel mixture. When the piston passes a bypass port this opens the port to allow the compressed mixture to flow up the transfer channel between the engine case and the sleeve. This happens just as the spent fuel mixture charge exits the exhaust port. The piston goes back up and closes the exhaust port and starts to compress the new fuel mixture charge thus opening the intake port so another new fuel mixture charge can enter the engine and start the cycle all over again. A complete power cycle requires 2-strokes of the piston in the sleeve.
CATALYTIC ACTION BETWEEN THE METHANOL FUEL AND THE PLATINUM IN THE GLOW PLUG ELEMENT AS WELL AS THE ENGINE HEAT KEEP THE ELEMENT GLOWING
ENGINE STARTING AND BREAK-IN
By removing the engine’s back plate you can see the piston’s conrod attached to the small crankpin at the rear of the crankshaft.
Don’t just bolt a brand new engine into the airplane and go flying. To produce maximum power, a fresh-out-of-the-box engine needs some special handling. The piston and the sleeve need to be gradually fitted together for a precise fit. This procedure is known as “breaking in.” If you don’t take the time to break-in your engine, excessive heat built up from friction can cause internal damage and the piston and sleeve will never seal properly.
First, install a new glow plug (the proper length and type is indicated in your engine’s instruction manual), tighten it with your fingers then tighten it down about ? turn with a glow plug wrench. Connect the fuel lines to the carburetor and fill the tank with fresh glow fuel. Use fuel with the same nitro content (typically 10 to 15 percent) that you plan to run the engine with and be sure your fuel contains at least 18 percent lubricating oil. Install the recommended prop and prop washer then tighten the prop nut with a 6-inch adjustable wrench. Close the main needle valve completely by turning it clockwise, and then open it about four full turns counterclockwise. Open the throttle fully then prime the engine by placing your thumb over the carburetor opening and flipping the prop several times. Continue until you see fuel flow through the fuel line and into the carburetor.
Close the throttle to about 1/4, attach a glow driver battery to the glow plug and use an electric starter to turn the engine over. Don’t use your fingers; if you don’t have an electric starter then use a “chicken stick” available from the hobby shop. Once the engine starts, let it warm up a little then open the throttle fully and let it run with a very rich needle setting for about 5 to 7 minutes, and then shut the engine down and let it cool off for 10 to 15 minutes. Repeat this process several times while gradually leaning out the needle-valve mixture a few clicks each time. Don’t run your engine at full throttle with a lean setting until you’ve run at least six to eight tanks of fuel through the engine.
A properly broken-in engine will run consistently without overheating and will transition smoothly from idle to full power. Try to avoid the temptation of leaning your engine to get every last ounce of power. This leads to overheating. It is always better to adjust your model for peak rpm (using a good digital tachometer), and then backing down the main needle to richen the mixture until you lose about 200rpm.
BASIC ENGINE/PROP COMBINATIONS
Basic support equipment: Nitro fuel and fuel pump, electric starter, glow driver batteries, a digital tachometer, spark glow plug, glow-plug wrench, assorted ball drivers and screwdrivers. An extra prop or two also come in handy. A SlimLine Nitro Power Station is also shown.
|ENGINE SIZE||RECOMMENDED PROP|
|.049||5◊4, 6◊3, 7◊3|
|. 20-. 25||8◊5, 8◊6, 9◊5|
|.35||9◊6, 10◊5, 11◊4|
|.40||9◊8, 10◊6, 11◊5|
|.45||10◊6, 10◊7, 11◊5, 11◊6|
|.50||10◊8, 11◊6, 12◊8|
|.60||11◊6, 11◊8, 12◊6|
|.75||11◊8, 12◊8, 13◊6, 14◊4 12◊6|
|.80||12◊8, 13◊6, 14◊4, 14◊5|
|.90||13◊8, 14◊6, 15◊6, 16◊5|
|1.08||15◊8, 18◊5, 16◊6|
|1.20||16◊8, 16◊10, 18◊6|
|1.50||18◊6, 18◊8, 20◊6|
|1.80||18◊8, 18◊10, 20◊6, 20◊8|
|2.00||18◊10, 20◊6, 20◊10, 20◊8, 22◊6|
LOW-END NEEDLE VALVE
It is also very important
to adjust your engine’s low-end (idle) needle valve so your engine will operate smoothly and consistently while at idle. A properly set low-end needle valve allows the engine to transition smoothly from idle up to full power. If the engine hesitates and dies when you open the throttle, the low end is too lean. If the engine burbles, coughs and has a rough transition, the low-end needle is too rich. Make your adjustments by ? turns at a time until your engine operates smoothly and consistently.
CARE AND MAINTENANCE
Here’s a list of things to do to keep your engine happy:
- ï Use fresh, clean fuel.
- ï Install a fuel filter in your engine’s fuel system and in your fuel container.
- ï At the end of the day, empty your fuel tank and run the last bit of fuel out of the tank by starting and running the engine.
- ï Never leave fuel in the tank for extended periods.
- ï Use an after-run oil after the last flight of the day and before you store your engine for an extended period of time. Add a few drops down the carburetor and remove the glow plug so you can add a few drops into the piston and sleeve assembly.
- ï Always balance your propellers and lightly sand the leading and trailing edges to remove any sharp flashing.
- ï Never use a nicked or damaged propeller.
- ï If all of a sudden your engine won’t start up readily, replace the glow plug.
Flying model airplanes powered with 2-stroke glow engines is exciting and very satisfying. Once you learn how to operate and properly maintain these impressive little powerplants they’ll continue to earn their keep for many years to come. They are a very good investment and can be used to power several different planes.
For detailed information on 2-stroke glow engines, see Dave Gierke’s excellent books: “2-Stroke Glow Engines for R/C Aircraft, Vol. 1” and “2-stroke Glow Engines Vol. 2: Power Beyond the Basics,” available from rcstore.com.