Oil Engines to Run on "Anything"
Page 49
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Government Want an Engine That Will Operate on a Wide Variety of Fuels• By P. A. C. Brockington, A.M.I.Mech.E.
EXPERT opinion has vacillated in recent months between forecasts that the oil engine will be eventually superseded by a high-efficiency petrol engine and that the lower specific fuel consumption of the former power unit, will more than offset its disadvantage with regard to fuel availability.
'these views have no direct relation to the fuel emergency, but are based on considerations regarding progress with the cracking process in the treatment of crude oil, the increasing consumption of heavier fuels by industrial concerns, the relatively high demand for aircraft jet fuel and many other extraneous factors.
The production of high-octane lietrols; which advances in the cracking process have facilitated, is in part the basis for the view that the petrol engine will gain an ascendency over the oil engine because of the higher compression ratios, and increased efficiency that will be made possible.
Government Interest
Of great significance to the future of the compression-ignition engine is the interest of the Ministry of Supply and the Fighting Vehicles Research and Development Establishment in the development of an "omnivorous" engine that will operate on a large variety of fuels, including oil fuel, petrol and most of the lighter-fraction fuels, or mixtures of different ones.
Extensive tests have been carried out by the Ministry, the F.V.R.D.E. and a number of engine manufacturers with compression-ignition engines running on petrol and other lighter-fraction fuels. These have shown that the performance of some power units, with minor modifications 'to the injection equipment, is not affected by the change to the lighter fuels, notably to commercial-grade petrol. High-octane fuels are less' suitable.
Emphasis on the suitability of a commercial-grade petrol may be important, It is less costly to produce and the distillation equipment required is of rela tively simple design. In wartime a cracking plant would be particularly vulnerable to enemy action.
Fostering the development of the omnivorous power unit is linked, in the Ministry's view, to the possibility that the characteristics of petrol may enable a compression-ignition unit to be developed which Will operate satisfactorily at a substantially higher speed than its oil-burning counterpart.
Such a unit would be less costly to produce in terms of pounds sterling per
b.h.p. than existing types of oil engine and would have a wider application. If this were combined with an omnivorous appetite (presumably the oil-burning engine would have a lower rating) the unit would represent a revolutionary advance in power-unit development bo. th with: regard to user requirements and the national economy of this and other countries.
Existing evidence that this does not represent a designer's pipe dream is derived from experiments with a quality-type petrol engine that was converted to operate on the compressionignition principle. Its performance when running on petrol was similar to that obtained with the power unit in its carburetted form.
Fears that the use of petrol in compression-ignition engines will reduce reliability and increase ,maintenance, costs, cannot as yet he disproved by quoting factual -test data, but evidence has yet to be produced that the usc of petrol, even when employed without a lubricant additive, hastenswear or deterioration of the cylinder bores, pistons, valves or parts of the injection system, apart from the cams of the injection pump.
No Higher Temperatures Contrary to the experience of some operators Who have experimented with mixtures of petrol and oil fuel, an increase in exhaust-gas temperatures would not,appear to be the inevitable or even normal outcome of using petrol. This is indicated by the Meadows experiments (described on the opposite page), and it is reliably reported that the exhaust temperature of another make. of engine was reduced bY 80° C. when the fuel was changed from oil fuel to petrol. Dilution of the lubricating oil by petrol 'would not appear to be a likely cause of trouble in most cases unless the combustion efficiency were low and the engine were over cooled.
Mixing lubricating oil with the petrol is also favourable with regard to the cetane value of the fuel, a small percen7 tage providilig a relatively large increase in the cetane value of some fuels. The gains offered by improving the cetane value vary with the characteristics of the fuel, but are of particular importance in the case of a number of lighter fuels available, notably jet fuels.
Of potential importance, therefore, to the oil-engine industry is the development of a cetane-improver, which will, it is rumoured, be commercially available in the near future.
With reference to the practical details of the more advanced experimental work being performed, it is generally found that without modification to the injection settings the reduction in output is proportional to the difference in specific gravity of the two fuels.
Another important consideration to which the experiments_ give validity, is that raising the smoke limit, or possibly eliminating the limit, will give greater latitude with regard to an original design of a petrol-burning compression-ignition engine.
According to one authority, erratic running on petrol is most likely to he experienced at full speed with a 'light load, arid the overall combustion time is no greater under load than the normal combustion period of an oilburning engine, despite the greater lag before combustion starts after injection of the petrol.
Vaporization in Fuel System
Research engineers who have successfully run compression-ignition engines on petrol, attribute the rough running and poor performance, reported by some of those who have experimented With mixtures of the two fuels, to vaporization in the fuel-injection system. This would, they believe, also account for increases in exhaust temperatures.
It is of related interest that technicians of the Birmingham Midland Motor Omnibus 'Co., Ltd., do not consider that a mixture containing, more than 10 per cent, of petrol is a "practical proposition."
The complaint has been made by engine manufacturers that the Govern. ment and the oil companies have not indicated the proportional use of the different fuel fractions which would best match the economic interests of the country, and of those countries importing British engines. It is not so long ago that it was commonly assumed that boiler fuel would be in the most plentiful supply and that a substantial' proportion of oil engines would have to be adapted to burn the fuel. In a comparatively few years the relative abundancy of petrol has necessitated a complete re-orientation of outlook on the part of engine makers.
Despite the encouragement given by the Ministry of Transport to operators to experiment with petrol-dery mixtures, the number that has done so appears to be comparatively small, and there is no evidence of any haulier running a goods vehicle on a part-petrol mixture. Reports suggest that the fear of "wrecking the engine" is the general deterrent.
It is known, however, that at least one operator of oil-engined tipping vehicles on site work is running satisfactorily with a mixture comprising 20 per cent. paraffin and 80 per cent. derv. No trouble is experienced with starting and the performance and fuel consumption are virtually normal.