Small engines and medium size vertical engines usually have the water jacket cast in one piece with the cylinder casting and others have a separate head that is bolted to the cylinder.
In the latter type the water flows from the cylinder to the head through ports or slots cut in the end of the cylinder water jacket that register with similar slots in the jacket of the head.
Thus in this construction we have not only to pack the joint to prevent leakage of gas from the cylinder, but also to prevent the leakage of cooling water from the jacket into the cylinder, or outside. Thus there is always a chance of water leaking into the cylinder bore and causing trouble unless the packing is very carefully installed and looked after.
In large horizontal engines the gas and water joints are never made at the same point, as it would be practically impossible to prevent leakage into the cylinders of such engines.
When the cylinder and cylinder water jackets are cast in one piece without a water joint at the junction of the cylinder and the head, the water connection between the head and the cylinder being made by pipes external to the castings.
Small, portable, stationary engines are sometimes “HOPPER COOLED,” or cooled by means of the evaporation of the water contained in an open water jacket that surrounds the cylinder.
The hopper is merely an extension of the water jacket such as used on all water cooled engines, the only difference being that the top of the hopper is open permitting the free escape of water vapor or steam to the atmosphere. The water level should be carried within two inches from the top of the hopper.
Water when converted into vapor or steam absorbs a great quantity of heat, and of course the steam carries the heat of evaporization with it when it escapes to the atmosphere.
As the hopper is open to the air, the temperature of the cylinder cannot exceed 212° F. (temperature of boiling water) as long as there is sufficient water left to cover the cylinder.
The hoppers contain sufficient water for runs of several hours’ duration, and as the water boils away or evaporates, it may be replenished by simply pouring more water in the top of the hopper. Hopper cooling is used principally for small portable engines where the weight of a water tank or other cooling device would be objectionable and also where there is danger of freezing the pipes and connections of other systems.