ALCOHOL ENGINES.
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The Adaptation of Petrol Engines to -Use Alcohol.
AGREAT DEAL of very valuable information is contained in the Interim Report of the Special Committee on Alcohol Fuel and Engines appointed by the Australian Advisory Council of Science and Industry, especially when, in conjunction with the Committee's own investigations, we take account of the summaries and extracts quoted from the results of tests conducted under the direction of the United States Government. -Dealing first with -.generalities, it is stated that the main advantages accompanying the use of alcohol as compared with petrol and paraffin are :—
(a) Smoother running.
(b) Absence of carbonization.
(c) Absence of unpleasant exhaust.
(d) Greater safety from fire owing to the fact that alcohol mixes with water. This results in cheaper insurance costs and safer handling in bulk.
The heat obtainable from a pound of alcohol is, approximateljr, 11,000 B.T.U., whereas a pound of petrol contains 18,500 -B.T.U., and the same quantify of paraffin about 21,000 B.T.H. Against this advantage of the hydro-carbon. fuels, we must set the fact that an alcohol engine can be designed to give a considerably higher thermal efficiency than either a petrol or a paraffin engine. The report puts the thermal efficiency of a paraffin.engine at 16.2 per cent., of a petrol engine at 20.7 per cent., and of an alcohol en
gine at, approximately, a0 per. cent, .
The Higher Thermal Efficiency of Alcohol.
Probably the main reason why. higher thermal efficiencies are obtainable with alcohol is the presence of the oxygen which is at the same time responsible for the lower thermal value of a given weight of the fuel. Every fuel, of course, requires a certain amount of oxygen to render combustion possible, and. the usual method of introducing the oxygen is to supply the volume of air containing the necessary quantity. This also involves the introduction of about four times as much nitrogen. This nitrogen is compressed and heated, and is subsequently exhausted at a high temperature without doing useful work. 'Consequently, a fuel which contains a proportion of the oxygen necessary for combustion is thereby saved from the necessity for dealing with so much nitrogen, and a higher efficiency becomes obtainable.
A further consequence is that an alcohol engine of given cylinder dimensions and speed will, generate about 30 per cent, more power than a similar engine designed for petrol. Even a petrol engine' i modified as regards the carburetter and the pm-heating of the mixture to use alcohol, but having a low compression better suited to petrol, will, when using alcohol, have an available horse-power about 10 per cent, higher than When using petrol.
The best results will be obtained with alcohol if the compression is in the neighbourhood of f1.80 lb. per square inch, as against 70 lb. for.petroI, and-the possibility of using such high compressions without fear of pre-ignition is, of course, ,one of the reasons why the thermal efficiency of alcohol can be made so high and the power developed in an engine of given size so considerable.
A petrol engine merely modified to use alcohol will, of course, consume more of that fuel than of petrol for a given horse-power, the excess being about 50 per cent. However, a properly-designed alcohol engine with high compression will only .use about as much alcohol per horse-power as a petrol 'engine of the same size.
As regards the actual running of the engine, alcohol
has certain advantages and disadvantages due to the fact that its rate of flame propagation is very much slower. This means that it is difficult to secure high efficiency coupled with high piston speed. On the other hand, the alcohol engine is much less liable to knock than the petrol engine. This is partly attributable to the fact that alcohol has a greater range of explosive mixture with air.
It has sometimes been suggested that alcohol engines would give trouble as regards lubrication, but tests do not bear out this theory. Ills, in fact, found that the cylinders are particularly free from carbon deposits ; also that, even when lubrication is excessive, the exhaust shows very little sign of the bluish smoke which indicates a waste of lubricant. Thus the conclusion is that the alcohol engine should show a considerable saving as regards lubricating oil.
It also has the advantage in respect of general cleanliness, such as absence of smoke and disagreeable odours. Its one outstanding disadvantage is the difficulty in starting from cold, due tp the low vapourpressure of 'thea fuel. This difficulty is,n,ot so serious in stationaryas in vehicle engines, and various means are suggested for overcoming it in various cases. Of these, the first, is to pre-beat the .carburetter with a spirit lamp or flame. As alcohol does not light so freely as petrol, no considerable danger is involved ; but the plan would obviouslY be inconvenient to apply to vehicle engines. The next alternative is to use a little petrol for starting, subsequently maintaining the temperature of the fuel by utilizing the exhaust gases. This plan is satisfactory enough in it working, but involves complication in the mechanical ar, rangements, since it is difficult to design one carburetter to be adjusted satisfactorily and easily, while running, to meet the changed requirements when changing over from petrol to alcohol. Moreover, any plan which involves the partial use of some other fuel can only be regarded as a partial solution of the problem. „ Perhaps the best method is to heat the jet locally by mechanical or electrical means, and preferably the latter. This involves no difficulty at all if the car is equipped with an electric lighting system. A simple form of resistance coil need be supplied with current only until the engine his begun to warm up, after which the exhaust gases would be used. There, are also ways in -which the jet could be heated chemically, and it is, suggested that the carburetter might be provided with a "'foot warmer" of sodium acetate.
Mixtures with Alcohol.
The last plan of all is one which is likely to he extensively used during the.early stages before the true alcohol engine has been fully developed. This is to employ, the alcohol as part of a mixture with sonic other fuel such as benzole or ether. On this point, the conclusions reached in Australia that the benzole or ether could be put to better uses will not necess sadly, be endorsed here, because it is possible for us to create considerable home supplies of bonnie which could, in effect, be much increased' by mixing with alcohol. If this fuel bedarne general, there would be a natural tendency gradually to modify engine design n so as to enable larger and larger proportions of alcohol to be employed until finally the benzole percentage would be very .small, and would be retained mainly as an aid to denaturing the fuel and facilitat
ing starting. , It will be interesting now to detail the alterations necessary to existing types of engine to fit them to give good results with alcohol. The first of these is an increase in the compression. It is advised that
this can conveniently be effected in many cases by putting liners between the big-end of the connecting rod and the crankpin brass. The alteration should, however, be made with care, as in some types of engines it might lead to trouble owing to the increased stresses put upon the various parts. If, when the compression has been increased, the engine can still be turned over by hand, it will start more easily than before ; but when the compression has been made very high, a half-compression device will be necessary for starting. The next. requirement is to increase the size of the carburetter jet by about 50 per cent., the exact increase being determined by experiment. In some instances, where the fuel pipes are small, it may be necessary to enlarge them and also the fuel filters, so as not to impede the delivery of the larger quantity of alcohol that will be needed.
Various Engine Modifications. .
We now come to the question of pre-heating the fuel mixture. It is recommended that the incoming air shall be thoroughly heated by being made to pass down a sleeve 2 ft. or 3 ft. long surrounding the exhaust pipe ; also that it is desirable that the mixture of air and alcohol vapour after the addition et the extra air should be further heated by the exhaust gases. An adjustable air supply should be provided between the carburetter and the engine, and the fullest possible quantity of air should always be used to reduce the possibility of corrosion and pitting of valves that -might result if anyacids were present in the exhaust. All the tests made at present indicate that no corrosion is at all probable ; nevertheless, the precaution is desirable and; for the same reason, provision should be made to drain off any moisture that may tend to collect in the exhaust pipe or silencer. Finally, it will generally be desirable to reduce by some means the efficiency of the 'circulating system, either decreasing the rate of circulation or the surface of the radiator. An alcohol engine does not tend to heat the circulating water to the same extent as a petrol engine, and it is desirable, in the interests of efficiency, that the engine should run with the cylinders as hot as possible.
These hints are the results of a series of very careful experiments carried out by the Australian Committee and corroborated by official experiments in the -United States. They should, therefore be very useful to anyone who has occasion to modify an engine to give better results with alcohol or with any fuel of which alcohol forms a considerable part. While, in Australia, the first development of the use of alcohol is likely to be mainly in stationary engines employed upon farms, in this country the more probable development is the use of a mixture of alcohol and bensole in the engines of industrial vehicles,, tractors and motor boats.
It is to be hoped that the British Government will show itself ready to adoptwthe programme now recommended in Australia. This involves granting permission to use better and cheaper denaturants. The present denaturants add materially to the cost of the fuel, but if suitable distillates of coal tar oil were Permitted to be used, the result would be, it anything, to make the fuel cheaper than alcohol itself. It is also proposed that an allowance should he made to manufacturers of power alcohol to balance the increase in coat involved by conforming to Government restrictions as regards mgnufacture, and that an additional bones of 3d. a gallon should be allowed in order to encourage the development of an important new industry