Oil Injection Type. Injection pump P driven by eccentric E through rods G-H draws oil from tank K through M-N and sprays it into combustion chamber R through O-Q. Amount of oil sprayed is controlled by fly-wheel governor W-W shifting E on shaft S, thus varying stroke of P. Engine is started by heating R with torch U and injecting first oil with hand lever I. A second pump supplies constant level of oil to K, level being observed in glass L. C-C is the cylinder, and F is the fly-wheel. ] The accompanying diagram shows a diagram of a typical oil engine of the injection type, a pump P supplying the oil from auxiliary tank to the hot, extended combustion chamber R, this chamber being an extension of the cylinder C-C. Oil is kept at a constant level in K by an overflow pipe, the oil entering from the supply pump through pipe J, and entering the pump through M at N. By gauge glass L, the operator can tell whether he has a sufficient supply of oil. The injection pump P is driven from the eccentric E (mounted on the main shaft S) through the eccentric rod G and the rod H. The governor weights W-W alter the amount of fuel supplied by changing the stroke of the pump, thus keeping the speed constant under varying loads. The governor acts by shifting E in relation to the shaft S, a spring T controlling the throw of the governor. The entire governor mechanism is contained in the fly-wheel F. To start, the combustion chamber R is heated by the torch U, and after thoroughly heated, the starting fuel is injected by means of the hand lever I. This engine is of the two cycle type with scavenging air furnished by crank-case compression. (143) Construction of Gas Tractors. A gas tractor may be considered as being simply a special application of the gasoline or oil engine in which the engine drives the road wheels through a train of gearing instead of driving its load by a belt from the pulley. Four intermediate mechanisms must be provided between the engine and the road wheels in order that the tractor may properly perform its work. These devices are known as the “clutch,” the driving gears, reverse gear and the “differential” gear. It should be understood that these mechanisms do not change the construction or operation of the engine in the slightest, and that the principles that apply to the engines described in the previous chapters apply also to the engine of the tractor. The following will briefly describe the functions of the intermediate trains in their proper order, starting at the engine. The Clutch.