With the pistons together at the inner dead center, the space between them is filled with highly compressed air from the previous combustion stroke. At this point the fuel is injected into the highly heated air, and the expansion of the charge begins, the combustion proceeding under constant pressure during the first part of the stroke, or during that part of the stroke in which the fuel is admitted to the cylinder. When the supply of fuel is cut off the working stroke continues by the increase of volume, or expansion of the gas, the gases being reduced to nearly atmospheric pressure at the end of the stroke with the pistons at the position shown by diagram (b). At this point the piston P is just opening the edge of the exhaust port M, allowing the products of combustion to escape to the atmosphere through the annular exhaust passage that surrounds the port M. As the pistons continue to move outwards the gases continue to issue from the exhaust port at practically atmospheric pressure until the position shown by diagram (c) is reached by piston P2. At this point P2 is just opening the inlet port N allowing fresh air to enter the cylinder for the purpose of scavenging the engine. The passage of the air through the intake port N and out through the exhaust port M continues until the pistons pass the outer dead center, shown by diagram (d), and begin to come back on the return stroke. In diagram (e) the pistons have traveled far enough to close both ports, and as the space between them is filled with pure air from that furnished by the port N, the pistons will continue to move toward one another on the compression stroke. When they have reached the end of their travel as shown by diagram A, the fuel is injected into the cylinder and combustion occurs due to the temperature of the high compression temperature. This is the complete cycle of events made in two strokes, and it will be noted that the cycle has been accomplished without the use of valves. The compressed air for scavenging the cylinder is provided by air pumps that are driven from the connecting rods by a link motion. One low pressure pump for the scavenging and one high pressure pump for spraying the fuel into the cylinder against compression are provided. As the inside of the piston is always exposed to the atmosphere through the open ends of the cylinder and is never exposed to the heat of combustion, perfect cooling is secured, and as a matter of course, perfect lubrication. In the two cylinder engine in which four pistons are used, the cylinders are arranged in tandem with the two adjacent pistons, and the two outer pistons connected respectively. In fact the second cylinder pistons are duplicates of those just shown and are connected to the linkage in such a manner as to have the corresponding pistons in one cylinder act with the corresponding pistons in the second. (34) Koerting Two Stroke Cycle Engine.