Running in your Engine

By Norman Osborne

 

The object of running an engine in is to get the engine to the point where all the rubbing surfaces are nicely mated to each other at all temperatures likely to be attained (by the engine) 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 should 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 hour of the engine's life and at the end of this hour it should be ready for its first full speed run.

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

Ringed Motors

Start by using one of the smaller propellers recommended by the manufacturer and a minimum of 20% of castor oil in the straight or low nitro fuel.

Make 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 with a few minutes cooling down time between each.

For the next few runs set the main fuel needle to give a very fast fourstroke with just the occasional hint of two-stroking. Allow engine to run for 30 seconds and then close throttle or richen up needle to slow engine for 20 or 30 seconds cooling period, open the throttle again for 30 seconds then allow to cool again. Gradually increase the length of the full throttle run over two tankfuls of fuel. At the end of these two tankfuls lean engine out to the point where the engine is two-stroking and four-stroking (ratio about 60/40) and repeat previous procedure of short runs, gradually being increased in length for the next two tankfuls. The next stage is to lean the engine out until a full two-stroke is attained, but make sure it is only one notch from the four-stroke coughing, i.e., still set richer than maximum rev's. Repeat above procedure of short runs gradually increasing in length for two tankfuls.

The engine is now ready for full speed running, but still needs the first couple of tankfuls to be short runs gradually increasing in length.

The engine is now run-in for normal use, but if the engine is to be used under more harsh conditions (e.g. with a tuned pipe), then it will need further running-in under those conditions using the above short-run procedure.

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 facing 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.

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 working surfaces suitable for all these varying conditions.

Now that the engine is 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.

 

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 liner, 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 note of this and 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).

Very little running-in can be accomplished 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.

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 secs. full speed and 5 secs. 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).

 

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 atomisation of the fuel droplets. Once started, open the throttle fully and set engine to maximum speed. Leave the engine to warm through thoroughly at top speed for a minute or so then adjust the needle to see whether an improvement in RPM can be achieved. From this established point richen up slowly 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 is a semi diesel which means that it is partly the heat generated by compressing the fuel/air mixture and partly the temperature of the hot glow-plug element which ignites the charge in the cylinder at the correct moment. 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 temperatures.

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.

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 over-lean mixtures changing combustion characteristics in a way that imparts more heat to the surrounding engine structure and less to useful work. 

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.

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, 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.

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.

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 catchphrase 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 energise 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. 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 as fast as a servo would move it. 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.

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 'gel' into 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.

 

Four strokes

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% castor oil for the first hour. 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.

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 when this happens as it overstresses the engine.

The main defence against these phenomena is to run the engine a little richer. 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.

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.

 

A Few General Do-'s and Don'ts