adhesion between the driving wheels of the tractor and the surface over which they are moving. Under favorable conditions where the slip is very slight, the drawbar horse-power of a well-designed gas tractor would be about 2% of the actual brake horse-power. It will not be as high as this unless the power-transmission mechanism is very efficient and the weight distribution, wheel and gear construction is such as to get the most perfect tractive effort.

Gauge

To Resistance

Diaphragm,
Chamber

To Power

FIG. 22.-The Kennerson Traction Dynamometer, an Efficient and Simple Instrument for Measuring Draft.

The drawbar pull of a tractor is determined by means of some form of traction dynamometer, such as shown at Fig. 22. This instrument is placed between the source of power or drawbar of the tractor and some immovable body, such as a large tree. The tractor is started and the maximum amount of pull is indicated in pounds upon the gage just before the tractor-wheels

slip or the engine stops. This dynamometer may also be used to indicate the tension in tow line or draft gear. As will be apparent, an instrument of this nature may be used very easily in making comparative tests between the tractive power or drawbar pull of various forms of engine and will also indicate the amount of draft needed to haul a wagon, pull a plough or do any other work.

The most easily understood rating, and one generally used in describing a tractor to the average farmer is the horse equivalent horse-power. This enables one who wishes to know how the work of a tractor will compare with the actual work of farm horses to make an intelligent approximation. It is difficult to make exact comparisons inasmuch as no two horses can do exactly the same amount of work, and furthermore the average animal is more flexible in its exertions than any mechnical tractor. We have seen that the horse is able to exert two or three times its average power for a short distance in the field, while a tractor has no such great overload capacity.

The horse equivalent power of a tractor is considerably less than its drawbar horse-power and represents the number of actual average farm horses that a tractor can replace in every-day work. It may be approximated by dividing the drawbar pull, exerted by a tractor travelling at two miles an hour by 200. If the tractor shows a drawbar pull of 2,000 pounds at that speed its drawbar horse-power or horse equivalent power will be equal to that produced by ten animals of average size.

Types of Gas Engines.-Two types of gas engines have been applied generally to furnish power for transportation purposes. These differ in construction and operating cycle to some extent, though in all forms power is obtained by the direct combustion of fuel in the cylin

ders of the engine. In all standard engines a member known as the piston travels back and forth in the cylinder with what is known as a reciprocating motion, and this in turn is changed into, a rotary motion by suitable mechanical means to be described fully in proper se

FIG. 23.-Sectional View of Ellis Three-Port, Two-Cycle Engine, Showing All Important Parts.

quence. Gas engines may operate on either the twocycle or four-cycle principle, the former being the simplest in action, though the latter is easiest to understand.

Comparing Two and Four-stroke Power Plants.The sectional view of a two-cycle engine depicted at Fig. 23 shows the three moving parts employed. The gas is introduced into the cylinder and expelled from it through ports cored into the cylinder walls, which are covered by the piston at a certain portion of its travel and uncovered at other portions of the stroke. The three moving parts are the piston, connecting rod and crankshaft. If this type of power plant is compared with the four-cycle engine shown at Fig. 24 it will be apparent that it is much simpler in construction.

In the four-cycle engine the gas is admitted into the cylinder, through a port at the head closed by a valve, while the exhaust gas is expelled through another port controlled in a similar manner. These valves must be operated by mechanism distinct from the piston. In addition to the three main moving parts used in the two-cycle engine there are a number of auxiliary moving members that are part of the valve-operating mechanism. The four-cycle engine is more widely used because it is the most efficient type. The two-cycle engine is simpler to operate and very smooth running but it is not as economical as the four-cycle because a portion of the fresh gas taken into the cylinder is expelled through the open exhaust port with the burnt gases before it has a chance to ignite. As a four-cycle engine is more generally used its method of operation will be described first.

Operating Principles of Four-cycle Engines.The action of the four-cycle type will be easily understood if one refers to the illustrations at Figs. 25 and 26.