THE COMMERCIAL MOTOR
Page 7
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Paraffin-Fuel Difficulties.
Is Kerosene Suitable for Motor Vehicles ?—By "Alpha."
The proposals of the Chancellor of the Exchequer, so far as they relate to the taxing of petrol and increased taxation of motor vehicles, bear very hardly on a growing industry, and tend to retard the increased use of commercial motors. Present users of commercial vehicles have, too, received a sudden and unlooked-for extra call. This applies more particularly to public service vehicle owners.
Since the proposed tax on petrol has been made public, a number of writers in daily papers and in technical journals have expressed the opinion that the use of paraffin for internal-combustion motors will remove the difficulty of an increased cost of fuel from the minds of users of petrol vehicles, but this idea is a fallacy, and quite misleading. Careful investigation of the difficulties involved in the use of paraffin will show this to be so, and nobody is more alive to the faults of paraffin as a fuel than the man who has experimented with and tried to use it on motor vehicles.
Dangers of Paraffin.
It is often stated that petrol or any light spirit is exceedingly dangerous to handle, and so it is if due precautions are not taken. Against this, paraffin is set up as a safe fuel, since it has a higher flash point. In that respect, it is safer, hut paraffin is a very awkward liquid to deal with on motor vehicles. It possesses a penetrating power which is exceptionally great, and it will leak at joints and unions under the slightest provocation; it is exceedingly difficult to make a paraffin-tight joint on any paraffin containing vessel or pipe. If the slightest leakage does take place, paraffin vaporises very slowly, unlike petrol which vaporises quickly. The vapour of paraffin is heavy and is ignited fairly easily, so that the danger of fire is really very great by reason of the fact that the fumes hang about on parts of a vehicle, or in the trays below the frame level. Wood, asbestos and other materials absorb paraffin almost like blotting paper absorbs ink, so that, if the vapour by any chance ignites, the whole of the substance saturated with the paraffin is destroy9d by fire. There is also the unpleasant odour which paraffin possesses.
Is Paraffin Cheaper than Petrol in Use ?
Although paraffin can be bought much more cheaply -than petrol to-day, it is quite an open question whether, after taking everything into consideration, it is really cheaper to use than petrol. An engine run on paraffin does not develop the same power as on an equal bulk of petrol; usually, with the same size of engine, the power developed when run on paraffin is about 15 per cent. less than when run on petrol. An engine using paraffin carbonises very quickly on top of the piston, and in the combustion chamber. To ensure absence of pre-ignition, it is frequently necessary to take off the cylinders, and to scrape the piston bead and the combustion space of the cylinder.
If the lubrication system is not perfect and the engine is run short of lubricating oil, there is a greater risk of the piston's seizing in the cylinder, and so the cost in repairs is greater. The paraffin engine over-heats more rapidly than a petrol engine; the valves are burnt away more quickly, and the heat conducted to the bearings causes greater wear and tear at these parts. Against the high compressions which can be used in a petrol engine, considerably-lower compression has to be used in a paraffin engine, and the fuel consequently does not do work under the best conditions. The heat efficiency of a paraffin engine is less than a petrol engine, and it is rather difficult to get a multi-cylinder engine to run without misfiring when paraffin is used. This misfiring is more particularly experienced at the first starting-up of the engine, for, if the slightest trace of liquid paraffin gets into the eombus
tion space, it very often finds its way between the points of the sparking plug, causing short circuiting.
Practical Difficulties Encountered.
There are a fair number of paraffin carburetters on the market which perform moderately well, but a general statement may be made that none of them is really up to the work that is required for the greatly-varying load conditions under which the commercial-vehicle engine has to run. Extraneous heat must be applied to a paraffin carburetter, so that the liquid shall not only be vaporised at a certain point but. that it must be kept in a. vaporized condition until it is actually inside the combustion space. It is no use to heat the vaporising body of the carburetter, and then to have a cold induction pipe on which the vapour can impinge during its passage to the cylinder. If an induction pipe is fitted and is not thoroughly heated, the paraffin will certainly condense on the inside and will he carried into the engine in a liquid condition. In that case, the fuel is incompletely burnt, and is ejected from the exhaust pipe as smoke which has an offensive smell.
A special design of engine almost seenis to be a necessity, when paraffin is used as a fuel, and, even when such design is used, a certain amount of petrol is necessary for starting-up purposes, so that the vaporising surfaces shall receive preliminary heating before the heavier fuel is turned on. In this respect, the Parsons marine engine is one of the best, as the fuel is vaporised to a certainty right at the inlet valve of the engine, so that it is impossible for any liquid to get into the combustion space.
In the case where a separate vaporiser is used, there is always delay in heating it tip. Such. separate vaporisers are of necessity cumbersome and heavy, and must have a large reserve beating surface, in order that, if the engine is stopped for a short period, it can be readily and quickly started again. If there is no reserve heating surface, the whole operation of heating up the vaporiser has to be gone through every time the engine is stopped. Whenever an attempt is made to run an engine on paraffin and the compression pressure is higher than about 62 pounds to the square inch, knocking and pre-ignition occur when the engine is run at its full load for any extended period. To get over this difficulty, some firms make use of what is known as a drip feed. This device is automatic, and allows a certain quantity of water to be taken into the cylinder at every suction stroke of the engine. The water is turned into steam on making contact with the hot combustion-chamber head, and thus keeps down the temperature to a workable point.
If fairly-high compressions are required, it would appear that some mechanical method of injecting paraffin into the engine at the end of the compression stroke is qiiite necessary. The principle on which the Diesel engine works is one which is well adapted for the use of heavy fuels, and the only reason why this principle has not yet been brought into commercial-vehicle practice is the heavy construction which has to be employed to withstand the high compressions and explosion pressures. There are also a number of complications in MI and air pumps and compressed-air tanks, so that the Matter of installing this engine on motor vehicles becomes very difficult, unless for the heavier types. Taking all the advantages and disadvantages into consideration, it would appear that we are yet a long way from the point of commercial success in the use of paraffin as a fuel for the internal-combustion engines which are employed on commercial vehicles. Much costly experiment is necessary before the difficulties will be overcome. In the meantime, petrol is, as it always has been, the cleanest, safest, most easily worked, and probably the cheapest fuel—taking, as I again emphasise, everything into consideration—that eau be obtained.