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Burning Heavy Oils
Page 82
Page 83
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in Petrol Engines
By L. Mantell
The Bi-Fuel System as Developed by the Solex Concern in Conjunction with Prominent Users. Changing Over from Petrol to Heavy Oil as the Speed and Manifold Tem perature Rise An Account of the Properties of Certain Heavy Fuels and How They Are Best Employed. Guarding Against Avoid.. able Troubles when Burning Creosote in Vehicle Petrol Engines
IN this paper, on September 29, 1931, the writer was privileged to describe for the first time the earlier experiments made by the Belfast Omnibus Co., Ltd., with the use of creosote, employing the Solex hi-fuel equipment, which was designed by the writer and the patents of which are conjointly held by himself and the Solex company.
The possibilities of using successfully a suitable grade of creosote were originally suggested by Mr. James Courtney, chief engineer of the Belfast Omnibus Co., Ltd., who had for some time been experimenting with various mixtures of this fuel, a good grade of which was produced by the Belfast Corporation Gas Works. Mr. Courtney, however, had been employing mixtures of creosote and petrol, and on attempting anything like an attractively large proportion of the, former had encountered the starting, idling and low, loading troubles which have always attended the use of high-boiling-point fuels.
Mixing Heavy and Light Fuels Appeared Wrong in Principle.
It had for some time been the confirmed opinion of the writer, based upon the observation of numerous efforts in the past conducted upon similar lines, that whilst the local heat and velocity which accompany medium to heavy loading made this part of the curve usable, any attempt to handle the low loading and idling ends of the graph by simple fuel mixtures were foredoomed to failure owing to the accompanying drop in charge velocity and heat which would inevitably result in the deposition of any fuel that contained an appreciable proportion of high-boiling-point fractions.
Although, therefore, such fuels might be quite practicable for marine engines which operate, after warming up on petrol, at sustained and tolerably constant loads fairly well up on the throttle curve, their use in road traction, where repeated periods of low loading and idling are inevitable, would continue to be impracticable owing to the rapid fouling of the bead, rings and valves, and to crankcase dilution from the constant percolation of unburnt fuel 'past the pistons.
After due consideration it seemed apparent that as heat and velocity were the dominant factors in ensuring combustion and as these ,incentives were evidently needful in proportionately higher degrees as the volatility of the fuel became less, the inverse view would hold good and the automatic reversion to petrol as the heat and velocity dropped would meet the case.
The employment of two carburetters or their mechanical equivalent
was, therefore, proposed, feeding in Parallel into a suitably designed hot manifold cast integrally with the exhaust pipe; a large carburetter with its idling and low-throttle feed system suppressed, to supply the creosote or other suitable oil, and a smaller one supplied with petrol only, and linked up in such a manner that it came automatically into operation as the closing of the bigger carburetter throttle reached a point where the creosote output ceased. In the absence of slow-running arrangements an adjustable overlap of the two fuels should be arranged for at this position to avoid a flat spat in the throttle curve, and an additional means would be provided completely to cut off the petrol when the creosote was effectively in operation.
Hi-Fuel System Put to Practical Test.
The practicahility of this system being agreed upon, a suitable equipment was duly prepared by Solex, Ltd., and fitted to an experimental bus by Mr. Courtney, who quickly found that the broad conception was quite sound and set to work on the solution of the various detailed troubles.
After a few months nine buses were equipped and ran on public service with tolerable satisfaction, the only troubles experienced which up to the time of the last article on this subject had not yet
been solved, were crankcase dilution (which was still too great) and a tendency to the formation of a tarry sludge in the sump and timing-gear case. The latter was thought at the time to be due entirely to unsuitable lubricating oil, for it seemed to occur more with some grades than with others ; oils, for example, containing paraffin wax were found to sludge more readily than those having an asphaltic base.
Considerable uncertainty also existed as to the best type of creosote, more especially as regards its phenol and naphthalene content and its distillation range, also as to the cheapest and most effective means for preventing the deposition of naphthalene crystals during cold weather.
At this point in the development the progress made in Belfast attracted the attention of the Associated Equipment Co., Ltd., as A.E.C. engines were used throughout by the B.O.C., and an intensive investigation was decided upon at Southall under the direction of Mr. E. C. Ottaway.
It was found that the tarry sludge was caused by the incomplete combustion of certain slow-burning fractions of the creosote and was very pronounced when tar acids were present above a certain limited percentage without a balancing proportion of accelerators. There were also convincing indications that if naphthalene in any appreciable extent were allowed to remain in the distillate a proportion of either tar acids or solvent naphtha was necessary to hold it in permanent solution.
Castor Oil as Lubricant.
Mr. Ottaway also found that by the use of first-pressure castor oil as a lubricant tendencies to gumming of the rings or valves were greatly obviated, and that any tarry matter formed in the sump by the escape of incompletely burnt phenols, was apparently held in suspension. Castor oil evidently had a solvent action on gummy organic oxides and a capacity for keeping the carbon in the colloidal form, so that, although the oil became discoloured, its action as a lubricant was not impaired and dilution became of less importance.
The question of the entire removal of tar acids and naphthalene or the retention of each in balanced proportions is one which is apparently not yet settled.
Whilst the knock rating appears to be improved by the presence of the phenols the risk of tarry formations by slow combustion is courted if careful stops be not taken to speed up their burning and if this be done by naphthahaw solvent provisions must be mode against cold-weather deposition. Some distillers, therefore, adopt this practice and add benzoic or solvent naphtha, whilst, others argue that as the fuel with these bodies removed is still composed mainly of ring compounds and aromatics which have a high octane value, it is preferable entirely to remove the phenols, for which a 'ready market is found, and freeze out the naphthalene.
The writer's limited knowledge of coal-tar distillation will not permit the expression of any opinion as to which procedure is the better, but would mention that the only serious troubles so far recorded with the use of creosote have been either tarry. sludging, or mechanical stopprigel in the -supply sys tern owing to naphthalene crystals. It would, therefore, appear that until a correct balance of these questionable bodies is finally determined and standardized bY the distillers their entire elimination may be the safer course.
The London General Omnibus Co., Ltd., which is running one test bus and preparing several more in conjunction with the A.E.C. concern, requires fuel to conform with the accompanying tentative specification.
In the ordinary way these equipments are supplied by Solex, Ltd., with two standard carburetters operated by a special linkage which involves an additional hand or foot .control of the small petrol carburetter throttle to permit of warming up under light load, or for temporary use in cases where a prolonged stop has allowed the manifold temperature to drop below the critical speed necessary before -the engine will "pick up" the creosote.
The A.E.C. concern, however, has amplified this system for L.G.O.C. use, and produced a special hi-fuel carburetter on Solex lines, but with a thermo: static device and electric relay to change automatically the main supply from one fuel to the other, thus avoiding any additional controls to cope with these conditions. Patents have been duly applied for in the joint names of Mr. Ottaway and the A.E.C. concern.
As regards the present position of saleable equipments these are at the moment only available for A.E.C. 416, Regent and Regal types, and for nearly all Leyland types. Manifolds for other makes ef commercial vehicle are in project, but are not yet ready for offering to the public. Solex, Ltd., controls the whole output with the 'exception of the special thermostatically operated equipment made by A.E.C. and not yet final as regards its design, and wishes it to
ha understood that whilst it is in a position to supply complete outfits for the above makes of engine the uncertain state of the creosote market at the moment compels the refusal of any responsibility for troubles incurred through the use of unsuitable fuel.
The tests carried out up to now show that, subject to a good quality of creosote, a suitable lubricant and care in maintenance an attractive net saving can be effected.
Although creosote is more particularly mentioned because of its high antiknock value, crude oils can be used if light loading only is required.
The knock rating of the paraffins is more or less inversely as their boiling points and unless treated, therefore, with some anti-knock factor their use, especially in a hat manifold or othee vaporizing device, is, owing to their detonative tendencies, only possible for modest power outputs. With creosote it is different ; this fuel, having an exceedingly high octane number, can he most advantageously used with considerably higher compressions than are normally possible with petrol.
The power curve, although generally a little lower at the bottom end, becomes progressively higher at the top as the ratio is raised to its optimum, which, with o.h.v. engines, appears to be about 6.5 to 1. The consumption figure also is better than that of petrol by 10 per cent. to 15 per cent.
The outstanding details of maintenance pttentioa necessary are broadly as follow : 1. Until a better lubricant is fotind first-pressure castor oil is best.
2. It should be changed every 4,000 miles, using average good quality creosote.
3. Attention must he given to a tendency to accumulation of carbon under the inlet valves, which must be cleared periodically. The lower the mean loading the quicker this accumulation.
4. Although the head and rings do not foul at a greater speed than with petrol there is a tendency with some grades of creosote to deposit carbon in the inlet pipe, which must he periodically cleaned out. Small orifices such as screen-wiper and vacuum-tank suction nipples tend to choke if not periodically attended
5. Before finally switching off at night it is desirable to allow the engine to idle on petrol for about five minutes to ,prevent it being " gummy " for the next cold start.