TROUBLES WITH EXHAUST VALVES.
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Notes on Some Interesting Experiments by Messrs. Rolls-Royce, Ltd.
SOME VERY USEFUL information has been circulated by Messrs. Rolls-Royce, Ltd., who, as is well known, are making the Bolls-Royce aero engine, which has proved to be one of the most successful engines of the type produced during the war. This information is expressly stated by the directors of the company to be circulated as wide as possible, to persons engaged in the design and manufacture of aero engines not, only in this country but abroad, in the hope that it may be of service, and it is the result of a series of experiments which have been made at the company's works.
Although experiments were expressly made with the aero engine in view, yet the information is of extreme use to anybody concerned in the manufacture and also in the running and use of internalcombustion engines of all types, and we feel sure that the information will not come amiss to any engineer,. whether he be responsible for the running of a single van or whether under his charge is a vast fleet of coin mercial vehicles, and that being so, we deal with it in our columns.
The subject that was investigated was the cause of the trouble which has occurred in connection with exhaust valves.
The engines had been submitted periodically to rigorous tests of long duration on the testing tackle, and in all these tests no exhaust valves have been burnt. These tests were assumed to be more adverse than service conditions. Engines had been in for repair which had run in service up to 200 hours without the valves having been changed or their condition impaired, yet there have been reported on several occasions, by the Service, instances of the burning of exhaust valves after a few hours run. The directors of Messrs. Rolls-Royce, Ltd., decided upon a thorough investigation.
A particular instance of this trouble is related. On investigation it was ascertained that the seating of the valve in the cylinder was tree, the tappet clearance was correct, and that the analysis of the steel was satisfactory. Messrs. Rolls-Royce, Ltd., however, claim to have proved exclusively that the valves had burnt through pre-ignition arising from the ignition plug. Further investigations have elucidated many facts that were hitherto obscure, and thrown light, upon them, and the information thus obtained will no doubt serve to eliminate many mistaken impressions at present held as to the causes of exhaust valves burning. It was found that the temperature of an exhaust valve in the hottest portions is 700 to 750 degrees Cent. (1296 to 1381 Fehr.), and that the temperature of the exhaust gases in the exhaust port is approximately 700 degrees Cent. It was found that the valve is hottest when the cylinder is working with the most efficient mixture, that is to say, when the maximum power is-obtained vith the minimuni amount of petrol. Whether the aixture be weakened or strengthened from this point he temperature is immediately reduced. When the nixture is weakened the power immediately "drops ; nit the mixture can be strengthened 'considerably without any loss of power. The general impression that weak mixtures cause the valves to get very hot was not borne out by these experiments. It was apparent that the effect the mixture has on valves burning is controlled by the amount of free oxygen in the exhaust. With a correct mixture there is a certein percentage of free oxygen in the exhaust, ea, k whereas with a slightly stronger mixture this does not occur.
Chromium steel is stated to possess the advantage over tungsten steel that it does not oxidize at ordinary working temperatures. The disadvantage in the use of tungsten steel is that under ordinary conditions the valves will scale and gradually thin. Once the valve reaches the temperature at which burning commences, however, the difference between tungsten and chromium steels is not appreciable. se The question of tappet clearance was investigated, this, of course, bong practically affected by the elongation of the valve at normal working temperatures. .Tests carried out showed that this elongation is .014 in. to .016 in. A set of valves was ran in with no clearance at all in the tappets when cold, so that at normal temperatures the exhaust valves did not seat by .015 in. After a long run the valves were in perfect condition, and it was noticeable while the engine was running that the valves were the same heat all over, whereas under normal working conditions they are cooled on the seating where it conies in contact with the cylinder. It was found that it was possible to run with the valves not seating by as much as .008 in. and still obtain a cool ring round the outer ridge of the valves. It is assumed that in this case the valve is cooled by the thin volume of gas passing around the valve at a considerably reduced velocity, the gas itself being cooled by c(Sntact with the seating of the cylinder. Investigations showed that, with no tappet clearance in the exhaust valves when cold, the power was reduced by 1.5 per cent., and, further, that it was possible to run with only .005 in. clearance with no loss of power. In carrying the tests still further the tappets were adjusted so that the exhaust valves could never seat when cold by .01 in. As a result the power was reduced by as much as 15 per cent., and the exhaust valve became excessively hot. In these conditions it was distinctly noticeable that the valves were hottest an the outer ring and cooler towards the stem ; this, of course, being the converse of the 'results with Correct tappet adjustment.
Why a bite is taken out of one portion of a valve seating, leaving the,rest of the seating in fairly good condition, is explained by the fact that the bite always occurs on the sector towards the top centre of the cylinder where the valve was found to be much hotter. When a valve turns round while the engine is running it takes a very much longer time to start to burn, but immediately burning commences the valve ceases to turn.
In tests made in order to ascertain the effect of pre-ignition by means of a starting plug which preignites, it was ascertained that it was possible to burn valves of certain types in from six to eight minutes, a number of valves actually being burnt out.. in this manner during the test. Attempts were made to imitate the conditions of a pre-igniting sparking plug by electrie means, but it was found not possible to reproduce the actual conditions arising from an overheated plug, but it was proved .that a sparking plug which pre-ignites under ordinary 'conditions causes the exhaust valve to run at an incandescent beat, and it was found also thatâ– ipre-ignitions are greatly influenced by the amount, of burning oil in -the combustion chamber.
Messrs. Rolls-Royce, Ltd., claim that a case of preignition in an over-oiled cylinder has never been known, and actual experiments prove that if a plug commences to pro-ignite this can be remedied immediately by syringing a little oil into the air intake. It was also proved by watching the temperature of the exhaust valve that it is possible to inject a cer tain amount Of oil without affecting the h.p. of the engine, but that it serves to seduce the temperature of the valves.
It was found in the course of the tests that the water circulation has a very big effect on the overheating of exhaust valves. When the water circulation was faulty, the first part to be affected proved to be the exhaust valve. If the failure of the water circulation causes the sparking plug to pre-ignite, which is a natural consequence, the valve is burnt in a few minutes.
From the result of the investigations and other information obtained Messrs. Rolls-Royce consider that the burning of the exhaust valves is due to two causes : (1) failure of water circulation, and (2) preignition, of which the first is more frequently the cause of the trouble, in addition to which it may set up condition 2. When the water circulation tends to fail the water commences to boil, and consequently is lost through priming and evaporation, a trouble that it has not been possible so far to obviate. As
the engine continues to run the conditions get worse' and in the hottest part, namely, that portion of the water jacket round the exhaust valve, steam is generated which tends to force the water out in the front and consequently exerts a pressure against the water pump which still further reduced the flow. By this time pre-ignition starts, the power drops, and the valves commence to burn. The only way to prevent the trouble is to avoid running the engine with the oil too cold, which reduces the quantity that escapes from the bearings, and so in turn the quantity that reaches the pistons. As a consequence, conditions are set up that considerably increase the tendency to pre-ignite. There is an encouraging side to the trouble, and that is that if an exhaust valve burns the engine is not put out of action, and but for this "safety valve" the piston roighttfair first, in which case the result would be more disastrous whereas the exhaust valve if burnt is easily replaced. Excellent results have been obtained by increasing the flow of water per minute through the cylinder jackets. Radiators and radiatoripiping offer by far the greater resistance to the flow of water, and any resistance in these should be -carefully watched.