Cause: IC Glow Engine is detuned.
Fix: Tune the IC Glow Engine.
Comment: Tuning an IC Glow engine is not intuitive. There are a number of controls which have to be harmonised for the engine to run optimally. While there is nothing new in the tuning protocol listed below, the writers endeavour to give maximum detail and understanding.
In addition the writers wish to thank Brian Winch for his contribution to the article.
IC Glow engine carburettor nomenclature differences:
Nomenclature for the parts of an IC glow engine can vary, so for clarity, the following differences are highlighted for the reader.
This article uses the phrase High Speed Needle Valve. Elsewhere this can be referred to as the Fuel needle.
This article uses the phrase Low Speed Needle Valve. Elsewhere this can be referred to as the Idle needle.
This article uses the term Carburettor Barrel to refer to the rotating part of the carburettor which controls the amount of air the engine receives. Elsewhere this can be referred to as as the Rotor.
IC Glow Engine Tuning Protocol.
| What | Why |
| No more than half fill the fuel tank | This simulates landing fuel head |
| The head of fuel varies as the flight progresses. This change in head affects the idling. By half filling the tank we simulate landing conditions and so optimise idling. | |
| Ensure centre line of the fuel tank is level with the needle valve assembly. | This avoids fuel siphoning. |
| If the centre line of the tank is higher than the centre line of the carburettor, fuel syphoning could take place, leading to bad idling and flooding during starting. | |
| Overview of the steps to be taken during setup. | Helps create an understanding of the process. |
| 1. Set up the carburettor barrel positions. | This is done with engine stopped. |
| 2. Set up the throttle trim positions. | This is done with engine stopped. |
| 3. Setup the high speed needle valve to get full throttle running which is 500 to 1000 RPM lower than peak RPM. | Engine running. This initial setting is needed so that low speed needle valve setting can be addressed. |
| 4. Find the approximate position of the optimum low speed, (idle), needle valve setting, quickly, by using the blow test and/or pinch test. | The setting of the low speed needle valve needs to be accurate to 1/8 turn, (45 degrees). Zeroing in quickly on this precise setting is facilitated by the blow test and pinch test. |
| 5. Make final small adjustments to the low speed needle valve setting to achieve optimum acceleration pickup when changing instantly from idle RPM to full throttle RPM at the Tx stick. | |
| 6. Reset high speed needle valve as needed. | |
| 7. Reset throttle trim position to get the target idle RPM. | |
| Detail of the steps to be taken during setup. | |
| Make throttle control throw adjustments such that: | |
| 1. At full throttle demand on the Tx, (throttle stick fully up), the carburettor barrel should be fully open, allowing maximum air into the engine. | Sets the air for full throttle. Fuel metering is set by high speed needle valve later. |
| 2. At idle throttle demand on the Tx, (throttle stick fully down), the carburettor barrel is almost closed, allowing minimum air into the engine. This critical size of the air gap is approximately the width of a dressmaker's pin. | Sets the air for idle throttle. Fuel metering is set by low speed needle valve later. |
| 3. On pressing the throttle stop button the throttle barrel should close completely, allowing no air into the engine. | Sets the barrel setting to no air and so stops the engine. |
| 4. With throttle trim fully forward the idle position of the barrel, (throttle stick fully down), should be almost closed to the clearance of a dressmaker's pin, as above. | These two trim settings, (points 4. and 5.),allow minute control of the idle setting, ranging from idling slightly faster than ideal, to just being able to stop the engine. More later. |
| 5. With throttle trim fully back the idle position of the barrel, (throttle stick fully down), should be fully closed, just. | |
| 6. Get the engine running at full throttle setting and adjust the main needle valve until the engine is running at 500 to 1000 RPM below peak RPM | This setting is initial and is needed to ensure the engine will run contiually at full throttle, i.e. it is not too lean which could damage the engine. |
| This is not the final setting of the high speed needle valve, as further smaller settings will be needed later. | |
| Safety. Stop the engine by pressing the throttle stop button. | Safety. The engine must be stopped before adjustments to the slow speed needle valve are attempted. |
| Safety. Make all adjustents from behind the propellor. | Safety. Model aero engines can seriously damage hands and fingers. |
| Note: between each of the following adjustments, the engine must be run at full throttle before cutting to idle, in order to clear the cranckase of any unburnt fuel, which could confuse the readings. | |
| 7. Run the blow test. With the engine stopped, and with the throttle control in the idle position, connect a length of clean fuel tubing to the carburettor fuel intake nipple, and blow while adjusting the slow speed needle valve | This allows you to set the approximate setting of the slow speed needle valve quickly. |
| With the slow speed needle valve fully closed, no air will pass through the jet when the blow test is run. With the slow speed needle valve fully open, air will pass freely through the jet when the blow test is run. With the slow speed needle valve set near to optimum, a little air will pass through the jet when the blow test is run. | |
| 8. Run the pinch test. With the engine idling, pinch the fuel line closed. | This allows you to set the approximate setting of the slow speed needle valve quickly. |
| If the engine stops immediately when the pinch test is applied, the low speed needle valve is set too lean. Turn the low speed needle anticlockwise, richening the mixture, until the engine will idle. If the engine continues to run when the pinch test is applied, and picks up speed slowly, the low speed needle valve is set too rich. Turn the low speed needle clockwise, leaning the mixture.The near optimum setting occurs when the pinch test causes the idling to pick up speed very briefly, (approx) half a second,and then stops the engine. | |
| 9. With the aproximate optimum setting of the slow speed needle valve now set, run the engine at idle for 60 seconds, then quickly open to full throttle. | This checks the acceleration response after first allowing the engine an extensive idle, as this is the worst case test for the optimum setting of the low speed needle valve.. |
| If set correctly the acceleration will be immediate. If lean, the engine will die. If rich it will sputter as the speed picks up. | |
| 10. If acceleration is not crisp and immediate, continue to make fine adjustments to the slow speed needle valve, richening or leaning as needed. Test after each adjustment by allowing a sixty second idle run, before instantly opening to full throttle. | Acceleration from idle to full throttle must be crisp and secure. Anything less can lead to deadstick, forced landings. |
| 11. Measure the idle RPM. | Need to check idle RPM is between 2500 and 3000 RPM. |
| If the engine is not fully run in, then idle should be approx. 3000 RPM. to be reliable, given the extra friction present in a new engine. If the engine is fully run in, then idle should be approx. 2500 RPM. to be reliable, given the lower friction levels present in a fully run in engine. | |
| 12. Adjust the throttle trim slightly while running at idle and measuring the idle RPM. | In order to set the carburettor barrel opening to give the desired idle speed, selected as above. This trim adjustment will, if needed, be very small. |
| 13. Check the running at full throttle and again adjust the main needle valve untill the engine is running 500 to 1000 RPM below peak, i.e. not too lean. | Lean runs can wreck engines. |
| 14. Record the needle valve settings. | So that they can be reinstated easily if needed. |
Comments
18th. April 2016.
i.) Added credit for Brian Winch's input.
ii.) Added carburettor nomenclature differences the reader needs to be aware of.