All About Engines

Fig. 75.—Hartnell governor with dash-pot The Modern Reciprocating Engine 125 then has to come back again. In fact, a sensitive governor, when disturbed, tends to oscillate so that it always appears to be “ hunting ” for variations in the engine which do not really occur. To a great extent this is prevented by a dash-pot: a rod fixed to the governor opera- ting gear is furnished at its lower end with a piston work- ing in a small cylinder, as shown in Fig. 75, There is also an adjusting spring which enables the speed of the engine to be varied. The most delicate control is obtained by using two separate valves for each end of the cy- linder, one for admission and one for release of the steam at each end. They are of two kinds — Corliss and “drop” valves, and a diagrammatic section of a cylinder fitted with the former is given in Fig. 76. There is a steam chamber above the barrel and an exhaust chamber below. The valves, which are cylindrical, are opened and closed by turning them a little way round and then back again. They are fixed as near the ends of the cylinder as possible, so that the steam ports are very short, and there is an extremely small