Breaking in your glow plug engine
The process of breaking in your glow plug engine is very important if you want the engine to have a long and healthy life. It doesn't take long to do and a properly broken in engine will run smoother and give you better performance.
Most gas rc trainers take a 40 or 46 size glow plug engine that runs on a special blend of fuel, the ingredients of which are oil (either Castor or synthetic), methanol and nitromethane.
These three ingredients of nitro fuel each have their own roles to play; the oil lubricates the moving parts inside the engine, the methanol provides the main source of ignition and the nitromethane continues the burn on the low presence of oxygen inside the combustion chamber, and also acts to keep engine parts clean.
Using the correct grade of nitro fuel for your engine is important, the 3 main grades for general flying are 10%, 15% and 20%; the percentage refers to the level of nitromethane content in the fuel. Your engine manufacturer's recommendations should be adhered to when buying fuel, particularly for the breaking in period.
Breaking in the engine - where to do it?
When breaking in your glow plug engine, you have two choices; you can either break it in on the ground, or in the air.
If you've bought an RTF trainer with engine and fuel tank already installed, then the obvious thing to do is to leave it all in the plane. But if you're building a kit and haven't yet installed the engine in your model, then you have the option of breaking in the engine on a bench stand or installing it in the plane and then breaking it in.
The bench stand option is the better one, the engine is easier to work with because it's not surrounded by model. Also, you'll be running the engine on a rich mixture which produces more gunge from the muffler - better to make this mess away from your shiny new airplane!
But, breaking in the engine in the plane isn't the end of the world, by any means. There's no need to attach the wing of course, and you can probably rig up some kind of temporary shield to steer the oily smoke away from the fuselage if needs be.
Breaking in a glow plug engine in the air is the other choice, and I've done this too. Of course, you should only do this if you've got some flying experience and can confidently fly solo. The advantage with doing the break in during flight is that the engine has a much better airflow over it, so cooling is more effective. The disadvantage is that you can't use the throttle too much, so you won't have much available power to hand, and you'll need to fly accordingly.
The process of breaking in
The reason you have to break in your new glow plug engine is to 'mate' the moving parts together and eliminate any roughness between surfaces. This beds everything in nicely; smoother surfaces mean less friction, and less friction means better performance.
For this reason, during the break in period the engine should be run rich, meaning more fuel than normal is let into the engine. This ensures a higher level of oil into the engine, to give better lubrication.
The mixture setting is controlled by the 'high end' needle valve on the right side (when viewed from the front) of the engine - screwing it in (clockwise) leans out the mixture (less fuel) and unscrewing it (counter-clockwise) richens the mixture.
Above, the location of the all important needle valve. Modern engines may have the valve at the rear (right), but most engines have the needle on the carburetor (left)
As a very general rule of thumb, the mixture is slightly rich if the needle valve is unscrewed 3 1/2 to 4 turns from being fully screwed in (ie closed). Again, this is only a vague indication and the engine manufacturer's instructions should be followed as you first and foremost guide.
Breaking in your glow plug engine should be done over a few tanks of fuel - but not in one go! It's important to give the engine lots of rest, particularly during the first tank. As you progress through the subsequent runs, you can let the engine run longer.
So, with the engine running, keep it at about 1/4 or 1/3 throttle with occasional 'blips' to over half throttle, and also fluctuate the revs between idle and half throttle. Never go full throttle during the break in period, it doesn't do the engine any good at all.
After a few minutes running, close down the engine by carefully putting your thumb over the carb venturi, and let it cool down. Each time you start a new run, let it run for a few minutes longer than the previous time.
As the engine breaks in, you will notice the revs increase and become smoother at the higher throttle settings. This is because the internal parts are getting bedded in nicely, and so the engine is starting to run smoother. But you need to keep the mixture rich to keep up the lubrication, so unscrew the needle valve a small amount more, say one quarter turn or so, and try to keep the engine sounding a bit lumpier than it should be.
Repeat this kind of run over a few tanks of fuel, and your glow plug engine will be nicely broken in and ready for action!
The pinch test
Once broken in, it's important to get the needle valve set right and, again, the manufacturer guidelines should be your primary reference. But there's a very simple test you can perform, the 'pinch test', to get the needle valve setting approximately correct.
With the engine running at full throttle, very quickly pinch the main fuel line, between your thumb and index finger. Only pinch the line for a second, then release it. If the engine dies, or stutters badly, then the mixture is too lean and you need to unscrew the needle valve slightly to richen the mixture.
If you pinch the fuel line and the engine RPM increases slightly, then it's running too rich and so you need to close the needle valve a bit. Every time you change the needle valve, repeat the pinch test.
When the mixture is set correctly, then engine RPM will increase very slightly when you pinch the fuel line, and then drop off again when you release it. This is what you're looking to achieve!
Why the pinch test works: if your glow plug engine is running too rich, pinching the fuel line starves the engine of fuel for a second, and so in the moment that you're pinching the line, the lack of fuel equates to a leaner mixture going in to the engine; a rich-running engine will suddenly perk up as the fuel leans out.
Conversely, if the mixture is already too lean, when you pinch the fuel line you're practically starving the engine completely, and so it will die.
It's important to remember that the needle valve setting will always vary, from flying session to flying session and even from flight to flight. This is because the mixture leans itself out as the fuel level in the tank gets lower. As it drops, so the pressure on the fuel being pushed through the line drops and this naturally weakens the supply into the carb.
Also, changes in the ambient air pressure can make a big difference to the needle valve setting, so the weather will also effect your engine's performance and how you need to tune it for the day!
Break in tips
A few pointers to bear in mind...
- Don't rush this very important process, impatience will cost you in the long run!
- Always adhere to the manufacturer instructions for your engine, including choice of fuel and glow plug as well as break in procedure.
- Go easy on the throttle - keep it between idle and halfway, only occasionally blip it fully open.
- New engines rarely run well - expect lots of cutting out to begin with.
- Be aware of noise - glow plug engines have a high 'nuisance' factor, so if you're breaking in your engine in the garden, try and do it when your neighbors are out!
- If you're breaking in on a bench stand, set up a temporary throttle linkage so the throttle can be safely controlled from behind the engine.
- Be very aware that the engine will be extremely hot, and will burn skin!
- Finally, respect the breaking in of your glow plug engine as a serious and important process.
Related pages
Model airplane engines - an overview of glow plug engines.
Glow plugs - understanding glow plugs for rc aircraft.
Gas rc airplanes - about them in general, with some popular examples.
RC nitro fuel - an overview of nitro (glow) fuel for rc engines.
Propeller size - a guide to prop selection.
