THE INTERNAL -COMBUSTION TRACTOR 95 



the cylinder. By this method the water cools the hottest 

 projecting points first, the exact reverse being true when 

 cooling is done from the outside. This means is particularly 

 advantageous with engines using the heavier fuels, as it allows 

 the use of higher compression, and thus materially increases 

 the power of the oil engine. It also produces a smoother 

 and quieter running engine, by retarding the spread of the 

 flame cap throughout the mixture. Instead of a sharp initial 

 blow, followed by a rapid decline in pressure as the gases ex- 

 pand and the cooling medium takes effect, the explosion pressure 

 is not only less violent but the piston receives a sustained 

 push after the manner of a steam engine. The water is usually 

 injected only at half load or above. 



FRAME 



No part of the traction mechanism or foundation can be 

 neglected without impairing the efficiency of the tractor. The 

 frame must be strong enough and large enough to support 

 the weight of the engine, radiator, and operator's platform, 

 and rigid enough to furnish a stable bed for the engine. The 

 work is necessarily rough, and if in addition there is constant 

 vibration of the tractor frame, a smooth-running, durable 

 motor cannot be expected. For this reason the frames are 

 usually of heavy, continuous, steel I-beams or channels, ex- 

 tending from front to rear. These are usually tied together 

 crosswise with other members of ample dimension. Since 

 the frame, if heavy enough, will never need replacing, many 

 builders are riveting it throughout into one solid block. 

 The frame should support the brackets for the impor- 

 tant shafts, including the rear axle. In order to provide 

 for all this without excessive weight in castings a sub- 

 frame, also of steel, is added often to the main frame. In 

 some cases the two are combined in one trussed frame of 

 bridge construction. 



