by Bob Lang
As the title implies, it’s not rocket science, only common sense. With a little practice, simple tuning becomes second nature. Eventually you will settle on a procedure that works for you, the following is what works for me. It isn’t something that I have invented but is a compilation of things that I have read about, or heard of and tried. But before we get into the tuning of our engines here is some basic information that will help you understand why tuning is important.
Most engine manufacturers provide operating instructions with their engines. Don’t wait until all else fails before you read them. They usually contain recommended fuel nitro content and perhaps type of oil and amount. They also offer a propeller recommendation. Although, only test flying with different propellers will determine the one that gives the best performance and reliability from your engine. Also you will find recommended break in procedures and fuel mixture needle settings that will at least let you get your engine started.
A glow engine, either 2 cycle or 4 cycle needs four things to run properly. They are air, fuel, heat and a lubricant. All of these things need to be introduced into the engine in quantities that promote efficient combustion. Because all engines are different through design, manufacturing tolerances or ware from usage, every one will require different quantities of these four ingredients for proper operation. Let's take a look at these four ingredients and find out what part each play in the combustion process.
Air: Air is about three quarters nitrogen and one quarter oxygen by weight, along with various other gasses in substantially smaller quantities. The oxygen in the air we breath is what supports the rapid oxidation or burning of the fuel in our engine.
Fuel: Methyl alcohol or methanol is the base ingredient in glow fuel. Nitromethane is usually added in various quantities to improve ignition and increase power output. Maximum combustion efficiency is achieved with an optimum air to fuel ratio which is measured by weight. This ratio with alcohol is from 4.5 to 6.5:1. Increasing the percentage of nitromethane in the fuel, lowers the air/fuel ratio further.
Heat: Heat is what causes the fuel to ignite in the combustion chamber. In our glow engines, we use a battery to heat the platinum wire in the glow plug to initiate ignition of the fuel. The catalytic reaction between the platinum wire in the glow plug and the methanol during combustion causes the wire to glow after the battery is disconnected. Once the engine is running, combustion heat transferred into the engine also helps ignition.
Lubricant: The lubricant is mixed with the fuel. It could be castor oil, synthetic oil, or a mixture of both. Rust preventatives, anti-foaming agents and lubricity enhancers are also added. The lubricant has two basic jobs, preventing metal to metal contact of the engines moving parts and heat transfer.
Many things effect how a particular engine runs. Design, wear, load, weather and altitude are the main things that effect performance. Varying weather conditions and altitude cause changes to the air/fuel ratio resulting in a loss of performance. Choice of glow plug, fuel and mixture settings are the three things that we can use to tune our engines. At least they are the easiest changes to accomplish.
After you have broken in your new engine following the manufacturers suggestions, there are a couple things that you should check before you begin to tune it.
Check that the throttle will open fully without stalling the servo at high trim.
Be sure that you can shut the engine off with the throttle trim.
The fuel tank is the correct size and in the correct location and the clunk is free to move without touching the rear wall of the tank.
All fuel lines are of appropriate size and as short as possible without kinks. I also recommend that your fuel filter be installed in the filler line from your fuel pump and not in your airplane.
Use the recommended glow plug. Stay away from the $1.50 bargains because they will cause you a lot of grief.
Now that you have satisfied yourself that the above has been checked or corrected, you are ready to do your initial tuning.
Start your engine and let it warm up for 15 or 20 seconds at a throttle setting that will just keep it running. Always watch out for that spinning propeller as it can inflict serious injury to any part of you that comes in contact with it. During this time you can remove the glow driver.
If the engine quits immediately when you take the glow driver off of the glow plug, the idle mixture is excessively rich, the glow plug heat range is too cold or the plugs filament is contaminated. If you are satisfied that the glow plug is not the problem close the idle mixture screw or open the air bleed screw one quarter turn to lean out the fuel idle mixture and try again. Repeat this process until the engine will keep running as the glow driver is removed.
Slowly open the throttle fully. If the engine sags or quits as soon as you move the throttle, open the high speed needle a few clicks and try starting it again. Repeat this process until full throttle is achieved.
The high speed fuel mixture must be adjusted before you can adjust the idle mixture. To do this, turn the high speed needle in while the engine is at full throttle until maximum rpms are obtained. Then back the needle out three or four clicks. If you are using a tachometer to adjust your engine, only use the tach from the back side of the propeller. You will want to richen the high speed needle until the engine loses 200 or 300 RPM. This will help prevent the engine from leaning out as it unloads (picks up rpms) in flight at full throttle.
A 4-cycle engine only fires every other revolution as compared to a 2-cycle firing once every revolution. Because of this and the fact that a 4-cycle usually turns at a lower rpm than a comparably sized 2-cycle, it takes longer for mixture screw adjustments to change the rpms. I generally move the high speed needle on a 4-cycle one click at a time every couple seconds to avoid leaning the engine too much too fast. All 4-cycles will detonate if overly lean. Some have a tendency to throw the propeller which is why you should only tach the engine from the back side of the prop. 2-cycle engines generally lose rpms when leaned too much and eventually stop because the piston has expanded, due to excess heat and started to seize in the cylinder. The alcohol in the fuel also cools the engine as it is vaporized in the cylinder prior to combustion. Lean mixtures don’t supply enough fuel for proper cooling resulting in engine damage.
Now that we have the high speed mixture adjusted we can work on the idle. A reliable idle is essential to the longevity of our models. If your engine slows down gradually at idle and then stops, it is too rich. If it stops all of a sudden, it is too lean. If you pinch the fuel line closed at idle, the engine should speed up slightly before it dies. If it didn’t, it was too lean to start with. If it gains a lot of speed before it dies, it is overly rich. Adjust the idle mixture settings as required.
Engine idle is affected by the flywheel action of the propeller, heat range of the glow plug, amount of nitro in the fuel, idle mixture setting, mechanical condition of the engine and mounting position. Here are some general observations about the engine idle.
Generally a heavy or larger propeller lets the engine idle slower but loads the engine more at high rpms increasing engine heat.
Four cycle engines and low idle speeds are helped by a hotter glow plug because of it’s ability to retain heat longer.
Fuel with 10% to 15% nitro does wonders for engine idle--broadens the range of the high speed needle valve and helps the engine accelerate to full power.
Good compression is a must for a good idle and maximum power. Lower than designed compression can retard ignition of the fuel and create excessive heat at full throttle.
Inverted engines are usually less tolerant of overly rich mixtures due to the fuel charge cooling the glow plug because of its inverted location.
Watch the exhaust at idle. No smoke means that the engine is probably too lean. Increasing smoke means that the engine idle mixture is too rich. A slight but steady amount of smoke is what you are looking for.
An engine that is too lean at idle won’t accelerate smoothly when the throttle is opened. For proper acceleration, the idle mixture should be richened as necessary.
Idle mixture screws need less adjusting than the high speed mixture needle. With a little thought and observation, a reliable idle will become a reality.
Those of you that fly where the humidity can be high during the warm months of the flying season and just can’t get your engine to run well consistently, might try a thicker head gasket and less nitro in your fuel. The moisture in the air is causing over compression and inducing pre-ignition and detonation. If not corrected it will result in severe engine damage.
On a hot day, if you hear a sizzling sound out of your exhaust at high throttle settings, richen the high speed fuel mixture. A lower nitro content fuel or colder glow plug will also help. The heat is causing pre-ignition. Engine damage could be the result.
Now that you are satisfied with the mixture adjustments, make one final check before you taxi out for takeoff. Have a helper hold the plane nose up, being careful to keep the plane of the propeller above your heads. Go to full throttle for at least 15 seconds. If the engine is going to go lean and sag or stop, it will do it now. If you find yourself with an engine that is trying to seize because of a lean mixture setting, reduce the throttle setting and you will reduce the heat. The engine just might keep running at this reduced power setting long enough to get your plane safely back on the ground where you can rethink your tune up procedure.
Be aware of the weather conditions and listen to your engine while your flying. Understanding what your engine is telling you and learning how to correct it will add up to more time flying and less time in the shop.