Tune Your Engine page 2 return to page 1 return to the sample articles list
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The non-contact infrared temperature gun features a liquid crystal display. The infrared temperature gun must be held close to the glow plug, to ensure accurate readings.
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REQUIRED GEAR
Traditionally, model airplane people tend to ignore engine temperature unless they have an obvious overheating problem that requires attention; then they usually provide more cooling air to the cylinder until the nuisance goes away. Conversely, R/C nitro car enthusiasts rely on cylinder head temperature to determine the health and tune of high performance engines buried deep within the car chassis. Listening to the engine's exhaust note, and touching its hot cylinder head won't help determine its shaft speed or operating temperature. Human senses just aren't precise enough; you need help from an accurate tachometer and a method for measuring head temperature. Tachometers can be obtained at the hobby shop.
Car tuners typically use two types of temperature instruments: the contact-type thermocouple and the infrared temperature gun. Each has advantages and disadvantages.
The thermocouple unit is accurate and convenient to use; its liquid crystal display (LCD) is easy to read when mounted within the model. Its downside: trying to install the sensor terminal under a cylinder head screw—which isn't always accessible.
Temperature guns provide instantaneous LCD readings when pointed directly at the target from close range.
The negative side of infrared units is their accuracy. If pointed at a black, non-reflective surface such as blacktop, they work well. However, when aimed at the glow plug of a model engine, which often has a reflective surface, readings may be low by as much as 100° F. When experimenting with ignition point variables at the flying field, I use infrared guns for comparing temperatures. For absolute readings I use thermocouples. Whichever type you use, I can't overstate the importance of temperature knowledge when adjusting for ignition point timing. |
PUTTING IT ALL TOGETHER
Now that you know the factors involved in tuning your engine for optimal sweet spot performance, you can put them to work.
FOLLOW THE INSTRUCTIONS:
Before doing anything with your new engine—read the instructions provided by the manufacturer. It will tell you everything about running and caring for that new mill-fuel, glow plug, propeller, and break-in procedures. Please don't overlook the break-in. Even the most perfectly produced modern engine needs to be heat-cycled before it can deliver peak performance.
Choose the fuel and flight propeller (as a starting point) recommended by the engine manufacturer. Select a fuel that lists the nitromethane content along with the oil content and type, (e.g., 10% nitromethane, 20% synthetic oil). Some fuel companies provide this information, some don't. Don't sacrifice any information that may help you tune—know what's in the fuel blend.
Use a tachometer, infrared gun or thermocouple.
Have a notebook handy for record keeping.
Start the engine at partial throttle; advance the throttle to wide-open.
Use the tachometer when adjusting the needle valve; back off slightly rich (about 100-200rpm) after finding the engine's peak rpm. Allow the engine to temperature stabilize at this setting for approximately 15 seconds, then read and record the rpm and cylinder head temperature.
Note: If further RPM is lost at this setting, the engine is probably not completely broken-in. Go back to the test stand for further break-in running.
Here is where most operators simply taxi-out, throttle-up, and fly. This is also where ignition point timing, sweet spot tuning, begins. You can fly now, with uncertain results … or tune for superior performance and extended engine life. Which will it be?
NEEDLE VALVE TUNING AND COLD WEATHER
Cylinder head temperatures can be altered drastically by richening or leaning the primary needle valve at WOT. You could obtain the desired head temperature by doing this, but it's counterproductive. The goal of ignition point tuning is to operate the engine at a slightly rich 2-cycle for maximum sweet spot performance, while providing a "normal" head temperature. In other words, the engine's air/fuel ratio shouldn't be used to tailor the head temperature at the cost of torque, horsepower, and longevity. In cold weather, adjusting the needle to manage cylinder temperature can produce catastrophic damage to an overcooled engine. In a futile attempt to raise the cylinder head temperature, continued leaning of the air-fuel mixture produces a lubrication-starved engine with detonation-prone combustion. |
How to Reduce High Cylinder Head Temperatures
The ideal model airplane engine (.29 - .75 in³) cylinder head temperature occurs in the range of 350-375° F. If your temperature measuring instrument is providing these numbers—great! You're close to the sweet spot, but you can't be certain unless you experiment with some of the variables.
Your thermocouple or infrared gun indicates a cylinder head temperature of 400° F. This is too hot. There's a risk of: the ignition point advancing; additional temperature rise; leaning of the mixture; detonation, and engine damage. How can you tune this unacceptable temperature into the safe zone?
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Inlets at front of the Great Planes Lancair let cooling air flow over the engine.
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Check the needle valve. First, check the WOT needle valve setting. Do this by momentarily pinching the fuel line. If the rpm jumps up a couple hundred (use your tachometer), you're in good shape. If you're set too lean, the rpm won't increase. Turn the needle valve counter-clockwise until a 4-cycling rich mixture is achieved. Allow the engine to cool for about 15 seconds; reset the needle valve; pinch the fuel line again to check the setting.
Check air cooling. Next, check that your engine has adequate cooling. If the engine is mounted in the airplane and is cowled, the air inlet and/or outlet area may be too small. Remove the cowl and recheck the head temperature. If it drops to an acceptable level, the temp may be OK during flight (with the cowl in place). However, check it immediately after landing; if still too high, you may need to enlarge the cowl inlet and/or outlet.
Try a colder glow plug. If the engine is still running hot, it probably has advanced ignition point timing. Always try the easiest, least invasive variable first—one at a time, running the engine after each change (e.g., install a colder glow plug; reduce the propeller load [diameter and/or pitch]).
Increase lube and/or add castor oil. If this doesn't completely solve the problem, try a tank of fuel with a bit higher percentage of lubricating oil, or add some castor oil to the mix.
Try reducing nitro fuel content. In the unlikely case that the engine is still running hot, try reducing the nitro content of the fuel.
Add a shim. If all else fails, remove the cylinder head and add a head shim (gasket).
Compression ratio change is considered to be the most radical tuning change, and shouldn't be tried first. In this example, head temperature may drop into the acceptable range with the first or second "minor" adjustment, or a combination of others. Because of its subjective nature, ignition point tuning is often considered more of an art than a science. I believe that a combination of art and science is closer to the truth.
During this period you should write down all pertinent information in your notebook, especially the engine rpm and head temperature at WOT. Once the cylinder head temperature is within 350-375° F, changes in plug heat range, propeller and fuel should enable you to obtain the highest rpm figures; finding the optimum rpm is the final step in locating the engine's sweet spot.
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SAFETY FIRST
Use all recommended safety procedures when running engines. A staked-down holding system for the model's empennage, or an approved test stand; the operator should be positioned behind the engine, after starting; use safety glasses and hearing protectors; tachometer and temperature readings should be performed from behind the propeller's plane of rotation. |
Tuning For Wintertime Engine Operation
You want to fly during the wintertime, but the temperature seldom breaks the freezing point. When starting the engine you notice that the needle valve is difficult to adjust for WOT. Perhaps the engine misfires and runs rough. Check the head temperature; it's not uncommon to be lower than 200° F—much too cold for efficient, high performance operation. The ignition point timing is retarded; if allowed to operate at this head temperature, the delicate piston fit on ABC-type engines will wear away after only a run or two.
Aluminum tape must be wrapped completely
around the cylinder; its adhesive backing sticks
well to itself.
First, block off about half of the engine's crankcase cooling fins—just below the head, with aluminum tape. Cut and wrap a length completely around the engine. Manufactured by 3Mtm (and others) for heating and air conditioning contractors, this product can be purchased at most hardware outlets.
Run the engine again. Set the primary needle valve as before: 100-200 rpm on the rich side of peak at WOT. Allow it to warm-up, and note the cylinder head temperature. Add or subtract tape until the temp stabilizes at the low side of the acceptable temperature range (about 350° F).
Since the ignition point timing may still be retarded, try some of the other "tricks"—a hotter plug; more propeller load; more nitro in the fuel, etc., until the highest rpm is found. |
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Tuning For High Altitude Engine Operation
You've decided to attend a fun-fly in another state. Your engine has been running great at home, but you're concerned about the altitude at the fun-fly site—it's 2000 feet higher.
All non-supercharged internal combustion engines lose torque and horsepower performance when operating at higher elevations. The air contains less oxygen per volume and is less dense. Since all reciprocating piston engines are essentially air pumps, their ability to transfer and trap air in the combustion zone is reduced when operating at higher altitudes.
Check your notes to see if rpm has dropped. On arrival at the fun-fly, run your engine at WOT and set the needle valve as usual. Compare the rpm and head temperature with your records from back home—same propeller, fuel, and plug. Depending on the weather conditions, you will probably notice that rpm has dropped. The cylinder head temperature may be within the acceptable range (350-375° F). This is normal, but you might be able to retrieve some of the lost performance.
WHY USE CASTOR OIL?
I resist reducing the fuel's oil content (%) or the eliminating of castor oil. Not only is castor is an excellent, lubricant, but it's unsurpassed in removing excess heat from the engine's operating cycle. Castor oil also offers great protection by forming a tough varnish on the piston and cylinder—saving them from destruction in case of an excessively lean needle valve setting. A good chemical cleaner will remove varnish after about twenty hours of "normal" operation. This will often save you the cost of replacing damaged parts. |
Run more nitro. Experienced tuners have learned that some of the oxygen can be replaced by increasing the nitromethane content of the fuel; the nitromethane molecule contains oxygen that is released during the combustion process. If you use 10% nitro fuel at home, bring along some 20-30%. Keep in mind, higher nitro content reduces the run time for a tank of fuel.
Run the engine on the higher nitro content fuel, being sure to re-set the WOT needle valve. Carefully watch the cylinder head temperature. If it soars beyond 375° F, don't panic—simply shut the engine down, allowing it to cool, while you contemplate the next change.
If you are now running hot: If you're using a 12 x 9 two-blade prop, reduce the load to 12 x 8 and run the engine again. If it still runs hot, reach into your bag of tricks and try a colder plug, or more oil (%) in the fuel blend. Keep in mind, you're trying to retard the ignition point timing while moving toward the engine's sweet spot. If still too hot, try something else.
Reduce compression ratio. By adding a head shim (gasket) you will automatically retard the ignition point timing and the head temperature may now fall into the normal range. Although reducing the compression ratio may seem counterproductive, the results often produce better than expected performance. Lowered compression allows higher percentages of nitromethane to be used without causing detonation, while the ignition point timing moves (retards) toward the sweet spot.
Other options. Once you have achieved acceptable head temperatures with the high nitro fuel,
additional experimentation with plugs, prop load and oil content can further optimize the engine's performance.
If you'd like to comment on this article, please email Dave Gierke at daveg@flyrcmag.com. |
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