Fig. 18.—Spherical Combustion Chamber.
Fig. 19.—Enlarged Combustion Chamber.
The spherical form cannot continue during the stroke for mechanical reasons; therefore some proportion of piston stroke of cylinder volume must be found to correspond with a spherical form of the combustion chamber to produce the least loss of heat through the walls during the combustion and expansion part of the stroke. This idea is illustrated in [Figs. 18] and [19], showing how the relative volumes of cylinder stroke and combustion chamber may be varied to suit the requirements due to the quality of the elements of combustion.
Although the concave piston-head shows economy in regard to the relation of the clearance volume to the wall area at the moment of explosive combustion, it may be clearly seen that its concavity increases its surface area and its capacity for absorbing heat, for which there is no provision for cooling the piston, save its contact with the walls of the cylinder and the slight air cooling of its back by its reciprocal motion. For this reason the concave piston-head has not been generally adopted and the concave cylinder-head, as shown in [Fig. 19], with a flat piston-head is the latest and best practice in airplane engine construction.
Fig. 20.—Mercedes Aviation Engine Cylinder Section Showing Approximately Spherical Combustion Chamber and Concave Piston Top.
The practical application of the principle just outlined to one of the most efficient airplane motors ever designed, the Mercedes, is clearly outlined at [Fig. 20].