THE INTERNAL -COMBUSTION TRACTOR 89 



bearings. The variation in number and arrangement of 

 cylinders requires material differences in the crankshaft. The 

 arrangement of the crankpins with respect to one another 

 has an important influence upon the sequence of the strokes 

 of the engine, as shown by the accompanying diagrams. It 

 will be noted that the power strokes of the four-cylinder 

 engine are the most evenly balanced, and those of the three- 

 cylinder next. In the two-cylinder engines the explosion 

 balance may be obtained by the opposed type, or else the twin- 

 cylinder type in which the pistons move together, both crank- 

 pins being on one side of the crankshaft. The opposed engine 

 with pistons working opposite each other gives also a rotative 

 balance, which in the twin-cylinder engine can be had only by 

 adding counter weights to the crankshaft or flywheel. In 

 the twin-cylinder engines which have the cranks on opposite 

 sides of the centre of the shaft, the pistons travel opposite 

 each other, thus obtaining rotative balance at the expense of 

 explosion balance. In the single-cylinder engine there can be 

 no explosion balance, and rotative balance is secured by counter 

 weights, which should be as close to the centre of the crank 

 shaft as possible in order to balance properly at all speeds. 

 The four-cylinder engine, which gives the most continuous 

 succession of power strokes, is, of course, much more compli- 

 cated than one with fewer cylinders. In the opposed engine 

 the necessity of operating the valves at such a distance from 

 each other is one disadvantage, and the distance of the intake 

 valves from the carbureter is another, which makes for higher 

 fuel consumption. The opposed engine is used on only a 

 few tractors. 



FLYWHEEL 



The flywheel of the gas engine is for the purpose of storing 

 up part of the power developed by one stroke of the piston in 

 order to insure the other three events of the cycle. In many 

 tractors a single flywheel is used, the belt pulley being mounted 



