lever on each driving wheel which causes a series of sharp spikes to project beyond the periphery of the wheel to increase traction when desired.

A machine of this nature is shown at Fig. 18 which clearly outlines the general construction. As will be seen in general form it does not differ radically from the usual type of motor truck. As it is provided with a wagon bed it will haul grain, hay, stock, coal, lumber, milk, fruit, vegetables and other kind of merchandise. It has sufficient tractive power to pull three 14-inch ploughs and a harrow in ordinary stubble ploughing and will plough an acre an hour. It will pull two disks, two spike harrows, two seeders, two binders, corn planters, a road grader, a train of loaded wagons, or any other machinery. It has a draft equal to ten horses. This general purpose machine, therefore, is ideal for the small or medium sized farm as it will not only haul loads of all kinds on its own body but will do field work and serve as a portable beltpower plant when desired.

Many other special tractor forms have been evolved, ranging from crudely fashioned home-made contrivances to highly specialized types adapted to do only certain kinds of work. Obviously the general purpose tractor must suffer if compared to one designed to do a specific kind of work, but its wide adaptability gives it an advantage over the specialized forms that more than compensates for its lack of capacity in any specific work.

CHAPTER III.

DESIGN AND CONSTRUCTION OF GAS TRACTOR POWER

PLANTS.

Power Rating Basis-Indicated, Brake and Drawbar Horsepower-Types of Gas Engines-Comparing Two and Four Stroke Cycle Power Plants-Operating Principles of FourCycle Engine-How Two-Cycle Engine Works—Advantages of One Cylinder Motors-Features of Multiple Cylinder Motors-Heavy Duty and Medium Duty Engines-One Cylinder Tractor Engines-Two Cylinder Engines-Three Cylinder Power Plants-Tractor Motors with Four Cylinders.

Power Rating Basis. In referring to the power of the various internal combustion engines used as prime movers, a number of terms are employed in dealing with the subject that may prove confusing to the layman. Therefore, before going into the theory of engine operation to any extent it will be well to define the various methods of rating. Many believe that there is a difference between a gasoline or electrical horse-power, that produced by a steam engine or from the animal itself. The unit of work remains the same regardless of the source of energy and whether the power is produced by steam, gas, electricity, wind, water or horses. Various equivalents for a horse-power are used, for example an electrical horse-power is 746 watts. There are actually four common methods of horse-power rating in use, each of which carries an intelligent idea of its meaning.

Indicated, Brake and Drawbar Horse-power.— Indicated horse-power is the amount of energy developed within the cylinders of any heat engine. It is

determined by an instrument called the indicator, which gives a card or graphic chart showing the pressure in pounds per square inch exerted upon the piston of the engine. This is most commonly used in steam-engine practice and is a unit of interest mainly to the engineer or designer because it does not give the actual power developed by the motor. There are mechanical losses that must be considered before the actual power output of the power plant can be ascertained.

FIG. 20.-Simple Diagram to Show One Method of Making a Brake Test of Small Engine.

Brake horse-power represents the indicated horsepower, less the losses in friction of the working parts of the motor itself. It is the power actually delivered to the crankshaft or belt pulleys of the motor. The brake horse-power is easily determined by any form of absorption dynamometer of which the Prony brake is the simplest form. The application and use of this form of power indicator can be clearly understood by referring to Fig. 20 and from table given in Appendix. Any one can make the simple testing device illustrated, without any other instructions than given in the cuts. Good stout 4X4-inch lumber, free from knots; two

blocks curved to conform to the periphery of the belt pulley; two bolts as indicated and an ordinary platform scale are all that is needed. The length of the horizontal brake beam represented by the letter B is not arbitrary, it being necessary only to make sure that this should be short enough and have sufficient strength to prevent breaking when under test.

When making a test, the first thing to do is to have the engine running at its normal speed and under proper conditions to get steady power. The brake blocks do not bear against the belt pulley with any force except that produced by their weight. The nut on one of the bolts is then tightened to bring the brake blocks in forcible contact with the pulley. This tends to depress the end of the brake beam resting on the load transfer member, which is supported by the platform of the weighing apparatus. The brake beam should always point in the direction toward which the fly-wheel is turning.

The rule for obtaining the power delivered by the motor is a simple one. Multiply the speed at which the engine pulley is turning by the number of feet in the circumference of a circle having a diameter twice that represented by the radius B and then multiply this result by the net pounds lifted on the scale when the two nuts on the top end of the bolts have been screwed down sufficiently so that the brake blocks are bearing on the engine pulley as hard as possible without reducing the motor speed. Divide this result by 33,000 and the quotient will equal the actual power of the engine.

As an example, consider that the distance B between the center of the brake pulley and the portion of the beam resting on the scale is 4 feet. The circumference of a circle of this radius would be 2×4×3.1416 which equals 25.1328 feet. If the engine pulley is running 200

revolutions per minute and the lift on the scales is 100 pounds, we have the following expression:

At Fig. 21 the method of making a brake test of a tractor power plant in the field is clearly indicated.

Drawbar horse-power is the amount of power actually exerted in doing useful work, such as in hauling. We have seen that the difference between indicated horsepower and brake horse-power represented the amount of power lost by friction in the engine. If we deduct the power lost by friction in the power transmission mechanism, owing to transmitting it through bearings and gears, and the power necessary in propelling the tractor itself over the ground from the brake horse-power at the pulley, we obtain the drawbar horse-power.

This cannot be determined as accurately and as easily as brake horse-power because it depends to a considerable degree upon the condition of the ground and the