Tuesday, May 13, 2008

Breaking in your Engine


Original publication by Norman Osborne
Adapted to current views by Pé Reivers

Standard fuel blends are:
Glow fuel: 72% methanol 10% nitro 18% oil (castor or synthetic)
Glow fuel for break-in: 80% methanol 20% oil (castor or synthetic)

The object of running in an engine, is to get the engine to the point where all the rubbing surfaces are perfectly mated to each other at all temperatures likely to be attained, while causing as little wear to the engine as possible in the process. The benefits of doing this properly are twofold; the engine would be a 'better' engine throughout it's life, and that life would also be extended. A good tip is to use an old glowplug when first running in an engine, as often minute pieces of metal from the running in process or swarf left from manufacturing can destroy a glowplug in seconds.

The method used to achieve this is simple. Run the engine very rich and lightly loaded at first and gradually increase the amount of work the engine is allowed to do, at the same time gradually increasing the temperature that the engine is allowed to attain, by judicious use of the main needle valve. This gradual process is spread over approximately the first half hour of the engine's life when it should be ready for its first full speed run. During all running of new engines, the setting should be on the rich side of peak power.

All running-in during the first tankfuls is done with the throttle fully open.

Ringed Engines

Start by using one of the smaller propellers recommended by the manufacturer and a minimum of 20% of oil in the straight or low nitro fuel. When using synthetic oil, addition of castor for the first run(s) is recommended by most manufacturers and engine authorities.

Run the first tankful absolutely soggy-rich, keeping the glow-plug lead attached if necessary to keep the engine running. Keep all runs of short duration of one-half to one minute, with a few minutes cooling down time between each. The number of heat cycles makes the metal "set" and speeds up the final fit of the components. The rich mixture and the short runs prevent the temperature from rising too much. Debris that forms will be washed away by the excess fuel and oil.

For the next few runs set the main fuel needle to give a very fast four-stroke with just the occasional hint of two-stroking. Allow engine to run for 30 seconds and then stop for a two minute cooling period, Start again for 30 seconds then allow to cool again. Gradually increase the length of the full throttle runs, unless the engine shows a tendency to bind, which can be heard by the labouring sound and unwillingness to maintain a steady rpm. At the end of two tankfuls lean the engine out to the point where the engine is on the verge of two-stroking and four-stroking and go fly, reducing throttle from time to time to allow the engine to cool a bit. Keep this up for about four litres of fuel. By now the engine should be steady running, without any tendency to sag, and can be leaned further to the point, that max. power is achieved. Always back off the needle a bit until a clear drop in rev's can be noticed. That is the standard flight setting for longevity and strong running in the air. During flight, the engine will lean out a bit, so you will be on the safe side of disaster with this setting.

If the engine is to be used under more harsh conditions (e.g. with a tuned pipe or high nitro fuel), then it will need some extra running-in under the short-run procedure, using the same fuel as for it's intended use, but with extra oil added. 25% oil is not too much for the first runs of a racing engine and will do it a lot of good.

All this might seem a long-winded process but it is necessary to get the best surface finish with the least wear inside your precious engine.

The reason for everything being done in small and gradual steps is that the rubbing surfaces have to be mated at gradually increasing pressures and to further complicate this the shape of the parts change as temperatures are increased. As an illustration, the cylinder and piston are round and parallel sided when made. As the engine warms up the top half of the cylinder gets hotter than the bottom half and so expands unevenly, worse than this the exhaust side of the liner runs hotter than the transfer side, then to add to the problem the front of the engine (in the airstream) runs cooler than the rear of the engine, so you can see the liner would be anything but perfectly round and parallel when thoroughly hot. That is the reason, that a well run-in engine has a bit less compression during starting than a new engine.

The piston and ring are subject to similar stresses. The ring alters its length depending on temperature and is also going up and down a bore which is no longer round or parallel sided and is guided through this operation by a piston which gets hotter at its head where it is in contact with the burning fuel mixture and therefore the diameter of the head is bigger than the walls. The exhaust side of the piston is hotter than the transfer side so the piston is no longer round or, as mentioned earlier, parallel sided either. All these distortions are larger or smaller depending on the temperature of the engine, so the running-in process has to allow the engine to make the mating working surfaces suitable for all these varying conditions.

During the run-in check all screws and bolts for security and if you have to tighten any cylinder head bolts, remember to tighten a little at a time and in diagonal rotation. You might also find the glow-plug has been affected by small metal particles fired at it during running-in. If you have any doubts change it and keep the old one only for running-in only, or throw it away. It did serve it's duty.

Running in ABC engines

An 'ABC' engine is one with special liner and piston metallurgy, e.g., the piston is aluminium (A), the liner is brass (B), and the brass is chrome plated (C). Generally these are performance orientated engines.

When an ABC engine is warmed up, the top of the liner, which made of brass, will expand more than the piston which is made of aluminium. Consequently, as the engine reaches working temperature the piston seal would not be very satisfactory. The manufacturers have taken steps to counteract this undesirable state of affairs by making the liner and piston the correct sizes for when the engine is hot. This means that when cold the piston is a very tight fit at the top of the liner, to the point where some make a light groaning noise when forced over Top Dead Centre (when the piston is at the very top of its travel, or TDC for short). Warning: a new ABC engine should never be turned over slowly, especially when still lubricated by the original preserving oil that the engine is shipped with. That will cause slight, but immediate damage to the top part of the piston fit, because the surfaces are still rough.

Little running-in is required with these engines, as the cylinder temperature must be raised to full working temperature as quickly as possible to avoid excessive piston wear due to the very tight fit at lower temperatures. As a matter of fact, it is best to fly these engines out of the box, using a rich setting with occasional four stroking. It is important to use an extra 5% of oil on top of the normal mixture.

When bench running, my usual method is to use about 5% extra castor oil in the fuel and set the engine for just below full speed running (throttle fully open, main needle just a little bit rich), and run the engine in short cycles of approximately 30 sec's. full speed and 5 sec's. at 1/3 speed, for the first 15 minutes. This keeps the cylinder temperature up and the slow running should give time for any hot spots to cool down somewhat. For the next 45 minutes use the engine normally but keep it just a touch rich (just 2 or 3 clicks). After that, normal fuel can be used as per manufacturers recommendations.

Setting the main needle

This is a most important setting as not only does it set maximum power but it also controls the running temperature of the engine and from there the length of the engine's life, the life of the glow-plug and the overall reliability of the engine's running in flight or elsewhere. Engines don't very often cut out in flight because they are set slightly too rich, but they most certainly do when set too lean.

It takes about five minutes to learn the drill for correct needle valve setting so it's worth taking the trouble for the long term benefits gained.

The drill is to start the engine on low throttle, for safety and usually easier starting due to higher gas speeds through the venturi and consequent better atomization of the fuel droplets. Once started, open the throttle fully and set engine to just four-stroking rich. Leave the engine to warm through thoroughly at top speed for a few moments, then adjust the needle until the engine is just off four stroking and running smooth. At this point, further leaning of the mixture results in RPM increase. From this established point richen up slowly again until a small but definite drop in RPM is noticed, with the engine still two-stroking. Lift the nose of the model up vertically and if the small loss of RPM is regained then you should have a good flight setting. If the model has a tendency to go rich or go lean in flight then an extra allowance will have to be made for this on the final setting.

The aim of this is to give maximum power from the engine when it is needed most, either when the model is climbing or when turning sharply and will also give sweet and cool running during level flight, the best of both worlds.

What happens when an engine is set too lean

The glow-plug engine keeps running because the heat of compression and the catalytic action of the glowing plug material with the methanol mixture ignite the charge in the cylinder at the correct moment. This catalytic action also keeps the plug lit. Sometimes an engine will start without adding glow to the plug after it has been stopped for several minutes. A little thought will let you see that the whole system is very temperature dependant. No spark to set it off, no injection of fuel to set it off. Just the combination of these two properties.

When an engine is set lean i.e., for maximum RPM at the start of a tankful of fuel, anything which slows the passage of the fuel through the needle valve will make the engine run overlean. Some engines will not continue to run in this condition and so cut dead but the majority will continue to run to some degree.

As the tankful of fuel gets used the fuel pressure, as seen by the needle valve, gradually reduces and as a result the engine gets a progressively leaner mixture as the flight continues. As the engine started set in a lean condition it must progress into the overlean condition and maybe go so far as to cut out, suffer from overheating and get damaged.

When the engine runs flat out, ignition timing and engine temperature should be as the designer intended but as the mixture gets into the overlean area temperatures will start to rise. This is due to lack of extra internal evaporation of excess fuel, and hotter burning of lean mixtures. Over-lean mixtures can change combustion characteristics in a way that imparts more heat to the surrounding engine structure leaving less for useful work. The fit of the moving parts becomes worse, and friction rises.

As a result of this the incoming charge is heated a little more than it should be and when compressed is at a higher temperature than it otherwise would be. The secondary effect of the engine structure being at a little higher temperature is that the glow-plug element is also a little hotter, which advances the onset of ignition.

As already explained, ignition timing is controlled by the temperature of the compressed fuel/air mixture combined with the temperature of the glow-plug element. The net result of both of these being raised is early ignition in the next cycle.. This early ignition gives just a little more time for even more heat to be transferred to the engine structure before the hot gasses are exhausted from the engine and so jacking up engine temperatures just a little more, resulting in the next cycle jacking up temperatures yet again.

This is the slow build situation that causes engine seizure in some cases and in other cases runaway early ignition that makes an engine stop quickly in the air and even throw the prop, as though it has seized, but in fact feels perfectly O.K. and runs O.K. when it has cooled down again.

Many of the modern engines do not reach these extremes but keep running at a steadier but much higher temperature than they were ever designed for. These engines run other risks which are much less obvious.

One risk common to ail engines which are run at elevated temperatures is that the oil gets very hot, thins out too far and so can no longer lubricate as effectively as it should, resulting in excess wear and shortened engine life. Synthetic oil will evaporate at very high temperatures without leaving a trace of lubrication.

The less obvious damage that can occur is not usually laid at the door of a lean fuel setting but I believe much of the time it is a relevant factor. When the engine has overheated and reached the point of too early ignition the burning mixture expands the gasses well before top dead centre and so the pressures inside the cylinder are much higher than they should be. These extra high pressures at the wrong time in the combustion stroke try to force the piston down with greater power than the designer intended and therefore overload the gudgeon pin in the piston and the little end bearing, the con-rod is more highly stressed, the big end bearings and the main bearings on the crankshaft are also subjected to higher loads. In four stroke engines it is not uncommon that the engine will knock and even kick back and throw the prop. Knocking can be heard, and is a typical sound like tin foil being rubbed.

These extra stresses may only result in wear being more rapid than necessary but if it is a regular occurrence it can result in very early bearing failure, and if it happens to be the con-rod that gives up first the resultant damage can be very expensive.

With all this in mind, when adjusting the main needle of your engine the catch phrase should be 'The future of your engine is in your hands'.

Setting the low speed

The need for a low speed adjustment on the carburettor is because as the throttle is closed it lets less air through into the engine and to keep the fuel air mixture within combustible limits the fuel flow. has to be altered, it's too fussy to leave to chance.

Most manufacturers have opted for a two needle configuration or something which works in a similar manner.

To adjust this low speed mixture start the engine, warm it up and make sure the main needle is properly set. Connect the glow-plug lead to energize the plug and slow engine down by gradually closing the throttle until the engine starts to run badly. At this point adjust the slow run needle to give smoothest running characteristics just a little on the rich side of fastest setting. Having made this adjustment slow the engine further until it runs badly again, now adjust slow needle until engine runs smoothly once more and continue this step by step procedure until the desired tickover has been reached, with the throttle barrel barely 1mm open. Now remove the plug lead and repeat the whole process. If the engine cuts dead in the middle of adjustments it's usually because it is too lean.

Having reached a slow tickover we have to check if the engine will pick up properly. Open the throttle to full speed with a smooth sweep, without jamming the controls. I know of many people who demand their engine to respond well to slamming the throttle open, but in two-stroke engines, this is asking for trouble and not to be recommended. If the engine picks up but splutters a little whilst doing so, the low speed needle is a little on the rich side. If the engine appears to miss and then picks up suddenly the low speed needle is slightly lean, and if the engine cuts dead when the throttle is opened richen the low speed needle 1/4 turn and try again, adjusting in small increments.

If the engine starts to pick up pretty well and then cuts at about 1/3 speed or so, try opening the main needle two or three clicks.

On many engines there has to be a compromise or two to get the low speed, midrange, top speed and pickup to a useable whole, and usually the compromise is that somewhere in the range the carburettor has to be set a little richer than would be considered ideal. Rich mixtures are an assurance for longevity and seldom cause the engine to stop, unless extreme.

Four-Stroke engines

The previous sections are written specifically for two-strokes, but exactly the same principles apply to a four-stroke, just some of the symptoms are slightly different.

When running-in from new use a fuel with 20% oil of which at least half is castor oil for the first runs. A four-stroke cannot be made to four-stroke when rich, it's already doing it, but what does happen is the engine misfires in a rather uneven manner, the richer the setting the more pronounced the misfire. The main needle, after running in, is set in exactly the same manner i.e., just on the slightly rich side of maximum RPM., when the engine is thoroughly hot. The use of a rev. meter is invaluable here, because adjustment by ear is more difficult than in two-strokes.

If the main needle is set too lean the engine may slow down with the. exhaust exhibiting a more leaden note than usual and may progress to the point where the engine stops with a bang due to 'detonation' and throws the propeller off in flight, or on the ground and perhaps at whoever is standing in the way.

'Detonation' is like early ignition but the difference is that instead of the mixture burning progressively from the glow-plug outwards, the temperature and pressure in the cylinder are such that the whole fuel charge ignites at the same time, before TDC, stops the piston dead, and blows it back the other way.

'Knocking' or 'Pinking' are like a slightly milder form of detonation that happens late enough in the compression stroke for the flywheel effect of the propeller to get the piston over TDC and therefore for the engine to keep running. Always close the throttle and open the main needle a bit more when this happens as it over stresses, and may damage the engine.

The main defence against these phenomena is to run the engine a little richer, and mix in some acetone (1% - 3%) in the fuel to stabilize combustion. Adding some 2% of water to the fuel may also suppress Knocking. (in hot, humid weather). This works in two distinct ways, first the richer mixture will keep the engine a little cooler, thus making the conditions for commencement of 'knocking' harder to attain and secondly and very importantly, a rich mixture is much less prone to detonation than a lean mixture. These two effects work together, either for you or against you depending whether you set the main needle rich or lean. Again, the engine's fate is in your hands.

The low speed needle is adjusted in the same manner as a two-stroke except generally speaking it needs to be noticeably richer than a two-stroke, to ensure a good pick-up when the throttle is opened and also on some engines to stop 'knocking', and the consequent risk of throwing the propeller, whilst the engine is picking up speed. In general, speed pick-up from idle is far more better in four-strokes than that in two-strokes

A Few General Do's and Don'ts

  • Always keep your engine clean.
  • Choose the propeller that allows the engine to run in the RPM range that the designer intended.
  • Always filter your fuel from fuel bottle to tank and from tank to engine, and keep the filters clean.
  • Don't wipe model with cloth that sheds fibres, some will undoubtedly find their way into the needle valve or somewhere just as inconvenient, and upset the mixture setting.
  • Don't use a damaged propeller, It has a tip speed of over half the speed of sound, and to shed a blade can obviously be lethal.
  • Always balance propellers and if possible spinners as there will be less vibration to affect the engine, model and radio.
  • Make sure your glow-plug battery is charged before you go out.
  • At the end of a days running 'dry' the engine out by pulling the fuel line off with engine running. Apply glow, and keep flicking the prop, until no sign of ignition is left. Then put a few drops of oil (not fuel) in the carburettor and ensure it is dispersed throughout the engine. Methanol fuel attracts water and is corrosive to steel, aluminium and copper bearing alloys. It is also possible, to make a mix with your standard mix oil and cleaning naphtha, drop a bit in the engine intake, ad glow and let the engine run on that mix with the fuel line disconnected. Now all traces of methanol, nitromethane and nitric acid will be expelled from the engine.
    Beware! The Naphtha mix can not be used on engines which have silicone parts inside the crankcase, like some YS four-strokes
  • Never store the model nose down in the corner of your garage, going home in the car, or even while cleaning it at the field, if it is fitted with a tuned pipe or an extra large silencer. The exhaust residue which collects in these is a dirty mix of condensed water, methanol, nitric acid and oil which is highly corrosive and would run straight back into the engine.
  • If the engine is badly 'flooded' i.e., liquid fuel in the crankcase turn the model over in such a manner that the excess fuel in the crankcase runs up the transfer passage, into the cylinder through the transfer port (make sure it's open-piston at the bottom of it's stroke), across the cylinder, out of the exhaust port and out of the silencer.

Good Engine Break-in Procedure

Adapted from Ed Moorman

Pre-lube the engine with some after run oil. Make sure the oil content in the fuel is 18% or more. 20% is better for break in. Castor oil or a castor oil/synthetic blend offers the most protection as synthetic oil burns off during a too lean run and offers no protection. I won't use a 100% synthetic oil fuel as all engines will always lean out in flight, and I want the added protection of castor oil.

Start the engine, let it run rich and at slow speed, not necessarily an idle, until the cylinder fins are warm, then go to full throttle, quickly lean it out to slightly richer than MAX RPM. After 5 seconds, pull the fuel line off or pinch it to kill the engine quickly, so that it doesn't linger at a too lean mixture. Let the engine sit until it is relatively cool to the touch. (The heat/cool cycle)

Now, do it again, but let it scream at full throttle for 10 seconds, kill it the same way and let cool. Another heat/cool cycle.

Increase the hot run by 5 seconds each run until you have a 30 second hot run.

Then do (5) 30 second hot runs. Now it's essentially broken in, so NOW adjust the idle mixture and you're ready to fly. I usually run a tank through at this point at various throttle settings, while I'm adjusting the idle mixture.

NEVER go to full throttle after any engine start until the head is warm to the touch.

O.S. Engine Break-in Procedure

Taken from O.S. .46 FX manual

All internal-combustion engines benefit from extra care when they are run for the first few times known as running-in or breaking-in.
This allows the working parts to mate together under load at operating temperature. Therefore, it is vitally important to complete the break-in before allowing the engine to run continuously at high speed and before finalizing carburetor adjustments.
However, because O.S. engines are produced with the aid of the finest modern precision machinery and from the best and most suitable materials, only a short and simple running-in procedure is called for and can be carried out with the engine installed in the model. The process is as follows.

1. Install the engine with the propeller intended for your model. Open the needle-valve to the advised starting setting and start the engine. If the engine stops when the glow plug battery disconnected, open the needle-valve to the point where the engine does not stop. Run the engine for one minute with the throttle fully open, but with the needle-valve adjusted for rich, slow "four-cycle" operation.

2. Now close the needle-valve until the engine speeds up to "two-cycle" operation and allow it to run for about 10 seconds, then reopen the needle-valve to bring the engine back to "four-cycle" operation and run it for another 10 seconds. Repeat this procedure until the fuel tank is empty.

3. Re-start and adjust the needle-valve so that the engine just breaks into "two-cycle" from "four-cycle" operation, then make three or four flights, avoiding successive "nose-up" flights.

4. During subsequent flights, the needle-valve can be gradually closed to give more power. However, if the engine shows signs of running too lean, the next flight should be set rich. After a total of ten to fifteen flights, the engine should run continuously, on its optimum needle-valve setting, without loss of power as it warms up.

5. After the completion of the running-in adjust the carburetor at optimum setting.

Optimum needle setting(1)

Slowly advance the throttle to its fully open position, then gradually close the needle-valve until the exhaust note begins to change. (4-cycle to 2-cycle) At this point, disconnect the battery from the glowplug, taking care that the battery leads or glowplug clip do not come into contact with the rotating propeller. If the engine stops when the battery is disconnected, close the needle-valve about 30° and restart.

Optimum needle setting(2)

As the needle-valve is closed slowly and gradually, the engine rpm will increase and a continuous high pitched exhaust note, only, will be heard. Close the needle-valve 10-15° and wait for the change of rpm After the engine rpm increases turn the needle-valve another 10-15° and wait for the next change of rpm. As the speed of the engine does not instantly change with needle-valve readjustment, small movements, with pauses between, are necessary to arrive at the optimum setting.

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