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THE PARAFFIN-EN( ED FARM TRACTOR
nNE of the aspects of farm1.-"machine operation, which is somewhat foreign to the experience of the road-transport engineer, is that of the fuel commonly employed. So far as the great majority of tractors is concerned, this is not petrol or Diesel oil, but paraffin.
To clear the ground for a note upon the paraffin-burning machines, it may be stated at once that petrol as fuel is restricted, on account of its relatively high costs, to the singlecylindered air-cooled engines of Ji-6 h.p. employed in the two-wheeled motor ploughs and cultivators used in horticulture, as well as to a few lour-wheeled. tractors, which are generally of much less than 20 h.p. There is also a small crawler with a twin or four-cylindered engine burning petrol.
• Good Results Obtainable • Inasmuch as the farmer enjoys a rebate of 7d. per gallon on paraffin used as fuel, it is obvious that in the larger-size tractors the heavier fuel must be accepted as the standard, and there is no question that the kerosene tractor, properly designed and handled, gives good results.
So far the number of oil-engined tractors in this country is very limited. Some of the more powerful crawler types have such power units and there are some wheeled tractors, mostly single-cylindered machines, operating on heavy oil. Of these, some employ high and some low compression, the latter being blow-lamp starters.
In handling and servicing tractors operating on kerosene it is important to have a full appreciation of the character of the fuel and of the motor designed to burn it. In this country the standard grade is vaporizing oil, which is more volatile than lamp oil. Although a tractor will run on lamp oil the result is, commonly, very unsatisfactory and a heavy loss of power is experienced.
Tractor vaporizing oil requires the application of heat, in order to secure complete combustion, and in this respect the structure of the motor differs from that of the road vehicle designed for operation on petrol. Hot-sprit manifolds are, of course, incorporated in petrol engines, but the idea is extended for paraffin burning to a full-blown vaporizer, heated by the exhaust gases and, usually, with a control by which the degree of heating can be varied.
The tractor is equipped with a small petrol tank from which the carburetter can be fed for starting purposes, and after a short run on this fuel, with the vaporizer properly adjusted, sufficient heat is generated for a change-over to paraffin.
The speed at which heating-up may be achieved is assisted in some
tractors by the installation of thermostatic control of the water circula.tion. A valve is kept closed and prevents free float., of the water until the desired temperature has been reached. Furthermore, most makers now supply radiator blinds or shutters to be closed while warming up. If such be not fitted they should be improvised.
Whilst it is undesirable to use an excessive amount of petrol for warming up, it is not wise to changeover before the engine is hot enough to burn kerosene without misfiring. The warming-up period . is best utilized by driving out to the field or, if already there, by setting out on the first stages of the -work.
• Maintaining Engine Temperature Once the tractor is operating on heavier, fuel, attention must be directed towards keeping up engine temperature to a satisfactory level. This, in practice, means as near as possible to boiling the radiator water without actually doing so. A temperature indicator is an excellent fitment to guide the driver, and tractors not equipped in this respect by the makers should be furnished with a proprietary type.
A common fault is to let the engine temperature drop by leaving the machine idling for a spell, during which, possibly, some adjustment is being made to the plough. With a cooler engine, misfiring commences and a defective plug commonly
ensues. It is better to keep the motor running at half-speed in such cases, or if a long stop be evident, to shut down and to start up again on petrol.
The ill-effects of incomplete combustion, with paraffin motors, are the same as experienced with petrol as fuel, but magnified in degree. A slight adverse engine condition will result in unburnt portions of the fuel getting into the sump and diluting the crankcase oil.
When properly operated, a kerosene tractor motor will not run more
than 50 to. 60 hours on ,one -Charge of lubricant. §Citrietimes the oil
• becomes unfit at art earlier period.' Such contaminated oil needs to be thoroughly, drained out and replaced. The old oil can be utilized for many other purposes on the farm.
Attention to the use of the right type of sparking plug and its maintenance in sound condition are essential for good results and, as may be readily imagined; the conditions in such engines are extremely severe for the plugs. The correct gap, for any particular engine, should be maintained by checking at regular intervals.
Most engineers who have been accustomed only to road vehicles are impressed by the care of tractor manufacturers to • provide ample • means for filtering, not only lubricating oil but also the intake air. In connection with the tractor motor, one other point may be unfamiliar. This is the provision of a special coupling to the magneto, which results in a constant-value spark. This aid to hand starting operates only at low speeds and effects this result by a device which holds back the armature shaft against a spring until a predetermined point, when it is released and the spring drives the shaft forward at high speed. One can, therefore, pull the crankshaft up slowly against compression and yet secure a spark with a real kick in it when firing point is reached.
Few tractors are fitted with selfstarters, and farmers, generally, do not favour their adoption.