GAS ENGINES 



509 



of the shape of the combustion chamber. It has been 

 asserted that the ideal combustion chamber is spherical. 

 The "I" head approaches this most closely. The "L" head 

 has one side pocket, and the "T" head two side pockets. 

 The total combustion space is about the same in each 

 shape. The combustion chamber in any engine may be 

 altered by the following: thickness of the gasket between 

 the crank case and the cylinders, or between the cylinders 

 and the cylinder head; the quantity of carbon deposits on 

 the walls of the combustion chamber; and the thickness of 

 the babbit in the upper half of the connection-rod bearings. 

 The bore of the engine is the diameter of the cylinder. The 

 stroke is the distance between extremes of the piston travel. 

 The displacement is the area of the piston head, times the 

 stroke, times the number of cylinders. Clearance space or 



I 



Fig. 403. Showing the " /' 



T L 



or dome-head, the " T"-hcad, and the " L"-head 

 cylinder construction 



combustion chamber is the space above the piston, including 

 all chambers and pockets, when the piston is at its highest 

 point. It is usually 25 to 30 per cent of the piston displace- 

 ment. The valves and spark plugs are located in the 

 clearance space. 



CYCLE 



A cycle is a series of operations through which a moving 

 part passes in regular order before repeating. The most 

 common gas-engine cycle is made up of 720 or four strokes 

 of the piston. An engine using this cycle is known as a 



