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Tune Your Engine
Secrets for finding your engine's "Sweet Spot"
 Finding the perfect ignition point Managing head temperature Tuning for cold we ather
Tuning for high altitude
by Dave Gierke
with illustrations by David Baker
 ave you ever swung a baseball bat or golf club hard …and hit the ball perfectly? You would know it right away—the ball travels long and straight with an effortless feeling at the instant of contact. These are examples of finding the sweet spot. Glow ignition engines also have a sweet spot. When found, a smooth, powerful, long-lived performance is the reward.
IGNITION AND COMBUSTION
Combustion within a piston engine is not an instantaneous, explosive process. Burning takes time. Maximum engine performance is realized when the air-fuel mixture is ignited before the piston reaches top dead center (TDC), while the peak cylinder pressure occurs slightly after TDC—the operating cycle's sweet spot location. The exact location of the sweet spot depends on many factors including the engine's design. By adjusting the ignition point timing, the operator can experimentally search for the engine's sweet spot … defined as the highest attainable rpm for a given propeller size.
This represents a normal pressure curve; notice how the ignition point occurs before TDC,
while the cylinder pressure peaks after TDC.
Using a tachometer, set the primary needle valve 100-200 rpm below peak. Note the position of the operator behind the engine; also note the eye and hearing protection. |
After disconnecting the start-up battery, ignition of the air-fuel mixture within the glow engine depends on catalytic action, compressive heating, and the ability to retain a portion of the plug's heat from cycle to cycle. These factors will be explained later. The resulting temperature of the Platinum alloy wire within the glow plug ignites the mixture, producing a flame-front that burns its way through the remainder of the charge. Burning is rapid enough to allow the engine to operate at the highest practical shaft speeds.
IGNITION POINT AND TIMING
Ignition occurs toward the end of the engine's compression operation; when ignition begins, compression is said to end. There are some variables that cause the ignition point to occur earlier on the compression operation—producing an advanced ignition point; others cause the ignition point to occur later on the compression operation—producing a retarded ignition point (e.g., a lean air-fuel mixture ignites earlier than a rich mixture, therefore it advances the ignition point timing). By advancing or retarding the glow engine's ignition point timing, its performance sweet spot may be determined.
If the ignition point moves away from TDC, the timing is said to advance.
If the ignition point moves toward TDC, the timing is said to retard.
ADJUSTING THE IGNITION POINT
Unlike spark ignition engines, determining a glow engine's exact ignition point is difficult without using sophisticated and expensive instruments. Fortunately, there is a roundabout method available. By using a tachometer and a glorified thermometer, the engine tuner has the necessary tools to find the illusive sweet spot. These variables affect the ignition point timing of a given engine and are listed in the approximate order of importance:
- Needle valve setting
- Cooling
- Compression ratio
- Nitromethane content (fuel)
- Propeller load
- Glow plug heat range
- Oil type and percentage (fuel)
Setting the needle valve. The most important adjustment an operator makes is setting the engine's high-speed needle valve. When backed-off (rich) 100 to 200 rpm from its peak wide-open throttle (WOT) setting, the maximum cylinder pressure will locate at the sweet spot … if all the other variables are adjusted correctly. However, set the needle valve a bit too lean and the combustion temperature will rise, causing the ignition point timing to advance. Overheating, possible combustion defects, and a missed sweet spot are the consequences.
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TO BE AVOIDED—
DETONATION
High compression ratios are directly related to detonation—a nasty engine damaging combustion defect that must be avoided. Detonation occurs when the advancing flame front pressurizes and heats the unburned mixture ahead of it, causing spontaneous combustion. The mixture's abnormal combustion produces very high flame speed resulting in a local temperature and pressure spike. This causes a "knocking", "pinging", or "frying egg" sound—a sure indicator that mechanical damage is occurring to the piston crown and/or cylinder head.
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Cooling. Air-cooled glow engines are also partially liquid cooled. Vaporizing fuel in the crankcase and combustion chamber helps keep engine temperature within an acceptable range. A very rich needle valve setting ensures that the engine will run cool at the expense of efficient burning of fuel. However, when the primary needle valve is optimized for performance, airflow through the cooling fins is required. If the temperature climbs beyond the established limits, ignition and burning begins earlier (advances). If the cylinder head temperature is too low, as often happens during winter conditions, ignition and burning begins later (retards), causing erratic combustion and premature wear in certain engine types.
Compression ratio. An engine's compression ratio (CR) is the comparison of cylinder volumes prior to compression and after the piston reaches TDC. As the compression ratio is increased, the air-fuel mixture is squeezed into a smaller volume prior to ignition, resulting in a greater heat release rate after ignition. Within limits, high compression ratios produce enhanced cylinder pressure, torque and power.
Raising an engine's compression ratio advances the ignition point timing; lowering it retards the ignition point timing. Raising or lowering the compression ratio by removing or adding head shims (gaskets) is the most common method of achieving this change.
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Removing a head shim (gasket)
increases the compression ratio; adding a shim reduces the engine's compression.
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Considered a radical adjustment, compression change is usually reserved for significant changes in altitude, atmospheric conditions, or nitromethane content in the fuel
Nitromethane content. Nitromethane is a powerful fuel ingredient. Under ideal conditions, increasing the fuel's nitro content (% by volume) will increase engine power. Because nitro is very slow burning, the ignition point timing must be advanced to maintain the correct pressure peak sweet spot after TDC. Raising the fuel's nitro content often requires adjusting one or more of the other variables to obtain satisfactory performance, as we will discuss.
Propeller load. Larger propeller diameter and/or pitch increases the engine's load, compelling it to run slower at WOT. Running slower allows more effective cylinder compression and increased heat release from the air-fuel mixture. Decreasing propeller load causes the engine to operate faster, effectively reducing its compression ratio and heat release.
Increasing the propeller size (load) advances the ignition point timing; decreasing propeller size retards the ignition point timing. When an engine is close to its sweet spot, seasoned operators experiment with minute changes in propeller diameter.
Glow plug heat range. Glow plugs have two functions—get the engine started and keep it running. By passing a low voltage electric current through the plug's wire element, a high temperature orange-white glow is produced; this allows the engine to be started when the proper mixture of fuel and air is present within the combustion chamber. After starting, the source of the electric current is removed from the plug … but it continues to glow, keeping the engine running.
Removing a head shim (gasket)
increases the compression ratio; adding a shim reduces the engine's compression.
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 A glow plug works by a combination of catalytic action, compressive heating, and the ability to retain a portion of its heat from cycle to cycle. First, the platinum alloy coil of wire within the plug's cavity heats-up as it comes in contact with the fuel's methyl alcohol vapor; this is a heat releasing catalytic action. Next, the temperature of the plug's wire element is further increased by the engine's compression of the air-fuel mixture.
Manufacturers produce glow plugs in various heat ranges: hot, medium, and cold. Many factors determine the heat range, but it's important to note that hot plugs advance the ignition point timing, while cold plugs retard it.
Fuel-oil type and percentage. We know that the fuel's nitromethane content plays an important role in locating the engine's sweet spot. However, there's another fuel-related factor to consider—the percentage and type of lubricating oil.
For most sport flying applications, the percentage of lubricating oil used in modern R/C aircraft engines ranges from 15-22% by volume. Since oil must be heated along with the fuel's other ingredients, the ignition point timing is affected—fuel with higher oil percentages require more time to heat, which retards the timing. Of course, lower oil percentages produce the opposite effect, advancing the ignition point timing.
The type of oil also affects the ignition point timing. In general, synthetic lubes are easier to heat than bean oils, such as castor. For fuels with equal oil percentages, synthetics advance the ignition timing more than blends containing castor oil.
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