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sumed by the cooling system of the average gas engine in an ordinary pail, while five gallons of fuel will operate it for some time. The steam tractor must have two tenders. These comprise a tank wagon for water and another conveyance for wood or coal. Even if liquid fuel is burned under the boiler, the water tank will be necessary and twice as much liquid fuel will be needed than with a gas engine of the same power.
Fig. 9.The 1. H. C. Twenty Horse-power Tractor Utilizes
Single Cylinder Engine for Power.
Considerable time is needed to steam up and great care is necessary if the steam tractor is to be operated in cold weather. It is not profitable or desirable to use the anti-freezing solutions in the boiler that can be so conveniently carried in the cooling system of the gas tractor. The larger and heavier steam tractors need two men to operate them just as a steam locomotive does. The engineer is occupied in driving and controlling the machine and a fireman is needed to keep fuel under the boiler. The heaviest gas tractor can be controlled by one man.
A steam tractor must be relatively heavier than a gas-operated machine of the same power. While this may be considered an advantage from some points of view, it is a decided disadvantage in others. It is reasonable to assume that it takes more power to move a heavy machine than a light one. The more massive construction will pack the ground more than that having less weight because there is a certain limit to the size of the driving wheels beyond which it is not desirable to go. When a heavy machine becomes mired it is more difficult to pull it out of the hole by other forms of power, or by its own energy.
Gas tractors range in size from 12 to 110-brake horsepower, while steam ploughing tractors are not economical in sizes much less than 25 horse-power. Ordinary steam-traction engines, when ready for work, range in weight from 10 to 25 tons. The gas tractor is also made in many special designs that are not practical with the steam-propelled types, such as orchard tractors and combined self-contained ploughing and traction engines. When one considers the wide range of different gas tractors available, their generally lower cost and lighter weight than the steamers, the fact that a specially licensed engineer is not necessary and their wide adaptability, one can easily understand why they have displaced the steam-propelled machine in practically all lines of work.
REVIEW OF CONDITIONS ON WHICH TRACTOR DESIGN
Elements of Tractor Design Outlined-Selection of Power
Plant-Power Needed for Hauling-Energy Absorbed by
Elements of Tractor Design Outlined.—The main part of the gas tractor is the power plant. Next in importance comes the traction mechanism. For the most part the gas engines used in tractor work contain the same essential components as stationary gas engines do. Most of the earlier tractors used the common form of horizontal cylinder stationary engine as a power plant in the one and two-cylinder forms. Many of the present day successful machines utilize engines that are sold by the same manufacturers for stationary work. Some of the latest tractors depart from the old method of construction and the power plants are based more on the lines of automobile motors than of the stationary engines. The supporting frame, driving wheels, differential and power transmission gearing are not radically different from the same parts of a steam tractor.
The engines of single-cylinder tractors usually run at three hundred to four hundred revolutions per minute. At higher speeds than these, the vibration becomes excessive and all parts of the mechanism are severely stressed. Two and three-cylinder engines, which have a better running balance, are usually run at somewhat higher speeds, and the four-cylinder type, which is practically in perfect balance, both in that of the mechanical parts and torque, due to rapid sequence of explosions, are run at higher speeds. Some of these attain seven and eight hundred revolutions per minute. Gas tractors may be roughly divided into low, medium, and highspeed types, depending on the type of power plant employed.
The group usually considered as the power plant consists of other elements besides the engine. Some form of vaporizer must be provided to change the liquid fuel to a gas that can be exploded in the cylinder. It is imperative that some means of igniting the compressed charge of gas be included. The power plant must be automatically lubricated, this calling for various devices for supplying a regular quantity of lubricant. To keep the engine from overheating some method of cooling the cylinder is always included in the power-plant outfit.
It will be seen that the power-plant group consists of, first, a gasoline or kerosene engine; second, a method of supplying and vaporizing liquid fuel, these members usually forming part of the carburetion system; third, some means of igniting the gas, usually composed of a number of electrical devices to form a complete ignition system; fourth, a water tank or radiator, or an oil-cooling tank, a circulating pump for keeping the liquid in motion around the cylinders and forcing it to the cooler where the heat absorbed is dissipated to the air. These