Fig. 4.—Third cycle: Explosion and expansion.
The Fourth Period.—Exhaust: The piston returns a second time. The exhaust-valve is opened, and the products of combustion are discharged (Fig. 5).
Fig. 5.—Fourth cycle: Exhaust.
These various cycles succeed one another, passing through the same phases in the same order.
Valve Mechanism.—It is to be noted that in modern motors valves are used which are better adapted to the peculiarities of explosion-engines than were the old slide-valves used when the Otto engine was first introduced. The slide-valve may now be considered as an antiquated distributing device with which it is impossible to obtain a low consumption.
In old-time gas-engines rather low compressions were used. Consequently a very low explosive power of the gaseous mixture, and low temperatures were obtained. The slide-valves were held to their seats by the pressure of external springs, and were generously lubricated. Under these conditions they operated regularly. Nowadays, the necessity of using gas-engines which are really economical has led to the use of high compressions with the result that powerful explosions and high temperatures are obtained. Under these conditions slide-valves would work poorly. They would not be sufficiently tight. To lubricate them would be difficult and ineffective. Furthermore, large engines are widely used in actual practice, and with these motors the frictional resistance of large slide-valves, moving on extensive surfaces would be considerable and would appreciably reduce the amount of useful work performed.
Fig. 6.—Modern valve mechanism.