The four-wheel drive is not common in commercial service, as it has only been found necessary under a limited number of very severe conditions. A good deal has been done, however, in this direction, particularly in France. The resulting vehicle need not be purely a tractor. In fact, we often find heavy lorries employed not only to carry a substantial load, but to haul an additional lighter load in a trailer. As a rule, these trailers have iron-tyred wheels, but for service in which economy of engine power is more important than economy of money, rubber tyres are usually fitted, since they have the effect of reducing the power absorbed in hauling the trailer by about 25 per cent.
Another development which is due mainly to the difficulties of adopting the internal combustion engine for the haulage of heavy loads without shock, is the petrol-electric system. In this system the power of the car engine is used to drive an electric dynamo. This dynamo generates current which is either supplied direct to electric motors or else stored in a battery of accumulators, the former method being the better and more likely to survive. Sometimes one electric motor is used, taking the place of an ordinary gear box, and driving the back wheels through a universally jointed shaft and a differential gear. In other cases, two balanced electric motors are employed in or near the driving wheels. In others again, two motors are used, each driving through shaft and differential gear to one axle of the vehicle, and so providing an electric four-wheel drive. Another arrangement is the provision of four electric motors, one for each wheel. The vehicle is controlled through the medium of a “controller”; that is to say, an apparatus which, by the movement of a handle, varies the electrical connections and so makes the installation suitable for providing either a big torque at low speeds, or a comparatively light torque at high speeds. Electrical machinery is also in a sense self-regulating, and consequently a well-designed petrol-electric transmission is tantamount to the provision of an infinitely variable change speed gear. One of the strongest arguments against the petrol-electric method is that, when the machine is running fairly light and fast, the electrical machinery involves certain unnecessary power losses. Consequently, systems have been devised in which mechanical and electrical drive are combined, the latter only operating the vehicle under conditions equivalent to an increase of load on the engine.
Efforts have been made for many years past to evolve a satisfactory internal combustion engine using paraffin or some other heavy and comparatively cheap oil in place of petrol. While these attempts have by no means failed, the practical results are up to the present more or less limited to the use of paraffin fuel in tropical or semi-tropical countries, where the higher temperature facilitates its employment. Among the disadvantages of paraffin are difficulties in starting up, a tendency to soot up the sparking plugs, the need of more frequent cleaning of cylinders, and a certain amount of disagreeable smell, partly due to the creeping of the liquid through every available crevice.
So far as the ordinary petrol van or lorry is concerned, various types have been developed to meet a variety of commercial needs. A certain number of light vans are run on pneumatic tyres, but the solid tyre is preferred wherever economy is more important than speed. It of course goes without saying that, if a chassis is to run on solid tyres, it must be of substantial construction, and so designed that its mechanism will not be injured by the fact that the solid tyre is not so capable as the pneumatic of absorbing small vibrations.
A very popular type of motor van is designed to carry about 25 or 30 cwt. These machines are capable of speeds up to about 25 or even 30 miles per hour in emergency, and can average comfortably 14 to 16 miles. Under reasonable conditions, they can cover daily journeys of 100 to 120 miles. Among larger types the 3-tonner predominates. This class of machine can be generally used for daily journeys of 70 to 90 miles, averaging perhaps 11 or 12 miles per hour. It usually consumes petrol at the rate of about 1 gallon to 8 miles, though better results are obtainable under good conditions. There are also a large number of 5-ton petrol lorries in commercial service. These can be advantageously used to cover 60 or 70 miles a day, consuming about 1 gallon of petrol to every 6 miles run. The motor cab runs about 20 to 25 miles on a gallon of petrol, and the motor omnibus about 7 to 10 miles. This question of fuel consumption is, of course, distinctly important in military service, when adequate supplies are only maintained at the right points with considerable difficulty.
In later chapters some account is given of the attempts made by various governments to influence the development of motor traction into the directions dictated by their military needs, but this brief sketch of the general trend of events will be sufficient to indicate the present position, and to provide the necessary knowledge for the appreciation of the facts and considerations to which we shall now turn.
CHAPTER II
The Importance of the Military Motor
The Opinions of German and British Military Experts—The Old and New Methods of Transport and Supply—How Troops in the Field are Fed.