Overcoming Crankcase Sludging
Page 53
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Results of investigations at Thornton Research Centre into Performance and Properties of Fuel for Oil Engines T is now known that oxidation plays a relatively minor role and that the oil insolubes, which collect in the lubricant, raising its viscosity and forming sludgy deposits, are mainly derived from the combustion chamber," This definition of crankcase sluciging was given by Mr. A. Wilson, M.Eng.. A.M.I.Mech.E., A.M.Inst.Pet.. in a paper entitled " Diesel Fuel Research at the Thornton Research Centre," which was read in conjunction with Mr, C. G. Williams, D.Sc., M.I.Mech.E.,F.R.Ae.S., F.Insr.Pet., before the Diesel Engine Users' Association, in London. last week.
The oil insolubes. Mr. Wilson cantinned, progressively increased, but were colloidially dispersed through the oil irk the form of primary particles. If water were present, emulsions formed. and became evident as sludge. These insolubes were maintained in suspension by 'using a detergent additive to the lubricant, thus enhancing the natural dispersing powers of the oil.
Deposits Drained with Oil By preventing deposition, these additives served to keep the engine in a clean state, and when oil was changed the deposits would be drained from the engine, together with the oil. This, it was realized, was only a cure, but research would be continued to tackle the problem at its source, which was genekally recognized as being the cornbpstion chamber. Comparative tests had shown that one engine might produce insolubes in the oil at a rate 20 times greater than another.
This was not a simple question of completeness of combustion, or the amount of blow-by. It had been found that a high swirl around the periphery of the combustion chamber had the effect of removing the oil film, and not only induced high oil consumption, but deposited on the walls incompletely burned fuel particles, which eventually found their way into the crankcase.
Oil Consumption
Simply by changing the combustionchamber arrangements. the high gas movement had the effect of increasing oil consumption by 75 per cent. and the rate of insolube formation was trebled. Although this had taken plate, the engine performance remained unchanged, the exhaust was perfectly clean and there was no change in the amount of blow-by.
Mr. Wilson considered that ring sticking was one of the most serious forms of piston fouling. This could be caused by use of an unsatisfactory fuel or lubricant, but basically was frequently an engine characteristic. The temperature in the ring-band zone appeared to be the deciding factor, and trout* could be expected when the temperature in this zone exceeded 200 degrees C.
In many cases, the difficulty could be alleviated by using an appropriate additive oil which prevented the deposition of the materials giving rise to ring sticking. Deposits in the ring grooves did not constitute a limiting operational factor, although it was known for severe carbon packing to be the cause of ring sticking. During experiments at the Thornton Research Centre, carbon packing on the top groove had been found to be the most serious, and the collection of these deposits was essentially an engine factor depending to some extent on the fuel.
Nozzle fouling, he thought, was a serious and prevalent source of trouble in the oil engine. Internal lacquering was caused by overheating of the body of the injector, and could lead to sluggish operation of the needle valve, which would give a poor cut-off and encourage dribble with consequent loss. of power.
Nozzle Orifices Choked It had been found, in the case of some engines, that the degree of deterioration in performance had not been great, and might possibly pass unnoticed in service. The complete choking of the nozzle orifices was frequently caused by the presence of foreign material in the fuel, but it was considered that carbon on the external face of the nozzle could be built up to close the hole.
Basically, the fuel was responsible for nozzle fouling, but provided that the injector was kept cool at the body and nozzle tip, most of the troubles would be eliminated. In the discussion which followed this lecture, an operator stated that the injectors of his engines were cleaned weekly, and that the only time • trouble was experienced was when a new batch of fuel was received. Pilot injection had been tried at the Thornton ReSearch Centre, said Mr. Wilson, and the familiar "Diesel knock" had entirely disappeared. In fact, the engine was so quiet that the initial impression was that the unit could not be functioning properly.
The system had been tried with two separate fuel pumps and two injectors per cylinder, but it was realized that this would not be practical in operation. This experiment, which at first was directed towards reducing the high rate of pressure rise and corresponding "Diesel knock," had produced evidence of more complete combustion.
Blowing Smooths Combustion
Supercharging, it had been found, had the effect of smoothing out combustion, and greatly reducing engine noise. Almost perfect combustion had been achieved under appropriate supercharge conditions. up to loads at which the limiting factor tended to be the rate at which heat could be dissipated from the piston, rather than the limitation imposed by exhaust smoke.
The lecturer thought that exhaust smoke could be caused either by incomplete combustion, or quenchipg the combustion process by chilling of the burning vapours by cold walls. Smokedensity measurement, he found, was full of pitfalls, and up to the present, the instruments used by the Thornton Research Centre had failed to give consistent results.
In the discussion, Mr. Hopkins, of CAN., Ltd:, mentioned that a single pump and injector unit had been produced for pilot injection of an oil engine, and was at present undergoing experimental tests. [The Atlas injection system which was described The Commercial Motor" on July 23, is one of the forerunners of the pilot-injection system, and there is some likelihood of an injector and pump of this kind being developed in this country in the near future.]