Tuning Nitro RC Engine
This time I am dedicating a post to those of you who have decided to go down the Glow or Nitro Engine power route. Don’t worry if your preference is electric power, I will be posting plenty of material to keep you interested. In the meantime I want to look at how tuning nitro rc engine, glow engine or internal combustion (IC) engine, call it what you will, is achieved.
I have purposely made this drawing large so that you can see all the relevant part descriptions. Probably the most interesting feature of this two stroke glow engine is the rear remotely mounted High Speed Adjustment Needle Valve. Below is a picture of a carburettor where the High Speed Needle is part and parcel of the assembly.
Why would manufacturers want to separate this needle from the carburettor? Mainly for safety reasons so that ones fingers are as far as possible from the spinning propeller whilst making adjustments. When this needle is integral with the carburetor, it is very important to ensure knuckles and fingers do not come into contact with the propeller. Below are photos of glow engines using the two options.
You will notice that the rear needle valve is connected to the carburetor by a short length of silicone fuel tubing.
Why Do You Need To Tune Nitro RC Engines?
Our Two Stroke Glow or Nitro Engine runs on a special fuel made from a mixture of Methanol (the fuel content), Oil ( for lubrication) and, sometimes, Nitromethane (to help idling and transition from low to high speed). This is also the component that gives our engine its “Nitro” name. The manufacturer’s instructions will give you the correct relative percentages of these ingredients appropriate to their engine. Your local model shop is bound to have available supplies of suitable fuels.
Tuning your engine revolves around getting the correct mixture of air and fuel into the carburetor. Air is the largest component by volume. Mixing the two in the carburetor produces a wet fuel gas that becomes the right mix when the volumes of each are correct. When we talk about fuel/air mixtures we relate everything to the fuel content. You will hear reference made to “Rich” and “Lean” mixes. A “Rich” mix indicates to much fuel whilst a “Lean” mix indicates to little fuel.
Just to give you some idea of the mix ratio of Fuel to Air, for one kilo (2.2lbs) of fuel, we need approximately 4.5 kilos (9.9lbs) of air, a ratio of almost 5:1 air to fuel by weight. So we can start to appreciate that a small increase in fuel content will give a “rich” mixture whereas a small reduction in fuel content will give a “lean” mixture. The mixing of fuel and air for an engine aspiration system has the technical name “stoichiometric” which chemically means the relationship between the quantities of materials that are involved in a reaction. So stoichiometrically, more liquid fuel in the mix means a “Rich” mixture whereas less liquid in the mix means a “Lean” mixture.
Air enters the engine, not by suction, although this does account for a very small amount, but mainly as a result of atmospheric pressure. This can vary depending on the prevailing atmospheric conditions e.g. hot, cold, wet, etc. These conditions will affect the amounts of air entering the engine at any time. Such changes are very small but may affect your engine tuning at times.
To help you understand how your nitro engine works, let’s take a look at some diagrams.
For the purpose of this explanation the important areas are:-
b) intake port
c) combustion chamber
d) exhaust port
The main reservoir of air is in the “crankcase“. The air/fuel mixture reaches this area through the carburettor. The “piston” travels down the cylinder and forces the air/fuel mix (fuel gas) up the “transfer or intake port” to the “combustion chamber“. As a result of this transfer a vacuum is created in the “crankcase”. Atmospheric pressure takes over here and draws another shot of air/fuel mix through the carburettor. The two illustrations below should help to clarify this procedure.
How much of each component is drawn in depends on the settings of the “carburettor” so we need to look closely at the two possible adjustments that can be made at the carburettor.
Adjusting A Carburettor
Model aircraft engines usually employ one of two types of carburettors.
1) Fuel Metering
2) Air Bleed
Fuel Metering carburettors have mixture needles at each end of the barrel assembly. These enable adjustment for both High and Low rpm. In the photograph below the main high rpm adjustment needle is on the left and the low speed fuel adjustment needle is in the centre of the control arm nut on the right. When we come to discuss low speed settings we’ll see a view of this needle in close-up.
An Air Bleed carburettor has a main mixture needle and a small hole located somewhere on the main section of the body and a small bolt with a spring or locknut that can be adjusted to restrict the amount of air entering through this hole.
At this stage it is important to understand the distinct difference between the two types.
Air Bleed Carburettors adjust fuel flow for high rpm but adjust air flow for low rpm.
Fuel Metering Carburettors adjust fuel flow for both high rpm and low rpm.
Please note very carefully; DO NOT touch the slow running setting of a brand new engine! The manufacturer will have set this for you before despatching the engine and in 98 out of 100 cases this will be spot on. If you interfere with this setting you may have great trouble getting it right again so leave well alone.
Having said that, I need to give you the information you need should it be necessary to make an adjustment to this slow speed setting, so here goes. This should only be undertaken once your engine has been run in (see below). Carefully close the idle needle right down as far in as it will go (be careful not to over-tighten it) and open the main needle. Take a dressmakers pin and insert it into the “venturi” (air intake) of the main carburettor body and close the throttle barrel to hold the pin in its position. Fit a length of fuel tubing to the fuel intake nipple and start blowing. You should find that it is completely blocked at this point. Now very slowly unscrew the idle needle until the smallest amount of air from your blowing passes it. That’s it!
Run the engine, set the high speed needle, then come back to idle. Any further adjustment (if needed) will be very small – probably less than 1/8th of a turn. This illustration will show you the effects of idle needle adjustment.
To check whether your idle mixture is correct, with engine running at idle, push fully forward on the throttle transmitter control lever. If your engine stops dead your setting is too lean so open the needle a very small amount (about 1/8th of a turn) and try again. If it now hesitates when you throttle up, it has gone rich and you need to take the needle back almost all of the 1/8th turn you opened it.
Note that this method is for a fuel metering carburettor. An air bleed carburettor is adjusted for slow running the opposite way. turning the screw out ( anti-clockwise) increases the amount of air intake and as a result leans the mixture whereas screwing the bolt in (clockwise) reduces the amount of air bleed and richens the mixture.
Now, I will say it again, If at all possible avoid changing the setting of the Idle Needle especially on a new engine.
Running In a Nitro Engine
First of all, why is it necessary to “run-in” a new engine? Manufacturers produce the component parts of an engine to very fine tolerances and often minute bits of metal and other microscopic particles can get left behind. For this reason it is necessary to not only create a perfect mating fit between the various moving parts but to flush out any of these minute foreign bodies. To do this plenty of lubricant and flushing oil is desirable to keep rubbing parts cool and to flush the system.
The method of running in a nitro engine will depend on the type of engine you have. Basically there are two types of two stroke engines, “non-ringed” or “ringed“.
The ideal way to know when your engine is running at its optimum is to use a”tachometer” to observe the actual rpm setting.
Tachometers (Tachos) are readily available from most model shops and internet suppliers and if you intend to use nitro or glow engines long-term are a very worthwhile acquisition. The settings you make using this tool will be far more accurate than those made by relying on your hearing alone. If you would like to buy one from a UK supplier click the following link:- UK Tacho or if you prefer to buy from a US based supplier click the following link:- US Tacho.
Engines without a piston ring (non-ringed) require a fairly brief and simple running in process and can be done almost at full throttle rpm throughout. Just ensure that the high speed needle is opened a little more than optimum so that the engine is running slightly rich.
The way to do this is to open the high speed needle several turns to ensure a plentiful supply of fuel. Start the engine and gradually turn the high speed needle in a few clicks at a time, waiting between each adjustment, allowing the engine speed to settle. Keep adjusting until the rpm is hardly changing with each click of the needle. Once there is no further increase in rpm for the next click, you have reached the maximum rpm. Immediately turn the needle back at least two clicks and wait for the rpm to settle. This will richen the mixture and provide extra lubrication for the moving parts. The ideal running in rpm will be between 500 to 1000 below maximum. You need to do 6 to 10 runs at this setting by which time your engine should be ready to open up for full rpm.
again, 6 to 10 runs are necessary at a rich setting close to top rpm which you will establish exactly the same way as for a non-ringed engine. The technique I use is to start the engine holding the cylinder head. Allow it to run until you can no longer stand the amount of heat building up in the cylinder head. At this point, stop the engine and allow it to cool down completely to absolute cold (no residual heat).
Start the engine again and repeat the finger test. When you have completed the chosen number of runs using this hot/cold technique, start to turn the high speed needle in a click at a time whilst watching the tachometer to see that the engine speed continues to increase. Wait at least 30 seconds between adjustments, especially with remotely mounted needle valve carburettors to ensure everything has stabilised.
You should detect small increases in rpm with each click of the needle. Once you reach a point where the next click fails to cause an increase, you know your engine is running at full rpm. If you continue to turn the needle in a few clicks the rpm will start to drop off or fall back. If you reach a point where the fall of of rpm continues you have leaned your engine mixture too much and it is beginning to overheat. Immediately open the needle one full turn to richen the mixture. This is most important if you don’t want to do permanent damage to your engine. the additional fuel will help to cool the engine and increase the essential lubrication.
Re-adjust the needle to attain maximum rpm again using your tachometer. When you reach the point where the next click does not increase the rpm, you have reached the maximum. Turn the needle back two clicks and watch the rpm to see it doesn’t continue to reduce but settles just below maximum. If it continues to fall it needs a couple more running in sessions.
When you get to the point where your engine will sustain full rpm for at least 5 minutes, you can consider it fully “run-in”. At this point restart your engine and turn the needle out two clicks or until it is about 1000 rpm below maximum. This is the ideal run setting and you should now be able to leave this needle alone indefinitely.
Now that you have learned the technique of tuning Nitro RC Engine and before we leave this post I would like you to consider this point. How often does a car driver use maximum rpm? Ask any full size aircraft pilot how often he opens the throttle to maximum? You will be surprised at the answers. So why should you expect to need maximum rpm from your plane engine continuously? If you need maximum power from your engine all the time, your engine is too small for the plane or the plane is too heavy for the engine. An upgrade is needed!
Food for thought. Happy flying.