Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(34) Koerting Two Stroke Cycle Engine.

One of the most prominent of the two stroke cycle scavenging engines built for heavy stationary service is the Koerting engine. Because of its peculiar scavenging arrangement, and as it is of the double acting type, it will serve to illustrate the cycle of that class of engine equipped with independent air pumps. Several of these engines are in use in Europe that have an output of over 4,000 horse-power, the general arrangement of which is the same as shown in the accompanying diagram Fig. F-11.

Fig. F-11. Koerting Two Stroke Cycle Engine with Scavenging and Charging Cylinders.

Since the engine is double acting, two similar combustion chambers are provided at each end of the piston as shown by C and C₁, and as each of the chambers gives one impulse per revolution because of the two stroke cycle, the single cylinder shown in the figure delivers two impulses per revolution to the crank-shaft. In order to have one exhaust port serve for both combustion chambers, the annular port E is placed in the center of the cylinder so that it is alternately opened to C and then C₁ as the piston travels to and fro, the port being covered by the piston at intermediate points in its travel. As the piston must cover the port for a considerable portion of the stroke, it is made very long, nearly as long as the stroke. The piston rod R that connects the piston with the crank passes through the cylinder head of chamber C₁, surrounded by a gas tight packing that prevents the leakage of the charge from C₁.

Unlike the ordinary type of two stroke cycle engine, the two combustion chambers are provided with mechanically operated inlet valves, V-V₁-V₂-V₃ that are opened at definite points in the stroke by the lay shaft X which is driven from the crank shaft. As the exhaust port E serves all of the functions of an exhaust valve, there are no valves provided at this point. Exhaust pipes connected to E carry the burnt gases to the atmosphere.

Two auxiliary air pumps of the double acting type are provided, shown at A and A₂, one pumping gas and the other air. They are driven from the crank-shaft through the connecting rod Y, and are proportioned so that together they force a mixture of the correct proportion for complete combustion into the working cylinder at a pressure of about ten pounds per square inch. Air and gas are compressed on one side of each pump piston in the spaces B and B₂, and the air and gas are drawn in on the other side as at H and H₂. The connections from the compressor cylinders to the working cylinder are arranged so that the two crank ends of the compressor cylinders discharge into the crank end of the working cylinder, and the front ends of the compressors discharge into the front end of the working cylinder, the exact moment of discharge being controlled by the inlet valves V-V₁-V₂-V₃. The pumps are arranged so that only pure air is admitted at first in order to force the products of combustion through the exhaust port so that they will not contaminate the following mixture of air and gas. The inlet valve opens immediately after the piston of the working cylinder uncovers the port E and reduces the pressure of the burnt gases to that of the atmosphere.

By the action of the admission control, the scavenging air first admitted, is prevented from mixing with the residual gas from the previous explosion, and in the same way the device prevents the loss of fuel through the exhaust ports, thus overcoming the principal objections of the simple two stroke types described earlier in this chapter. The compressor cylinders provide only enough air and mixture for one stroke and no reservoir is provided for a surplus of air or mixture.

As the piston moves forward, on the compression stroke and covers the exhaust port, the inlet valves also close, and the compressor pistons arrive at the end of their stroke so that no more air or mixture is delivered to the inlet valves. At the end of the compression stroke ignition occurs and the expansion or working stroke begins. The piston again moves to the right on the working stroke until the front edge uncovers the port E where the exhaust gases escape to the atmosphere.

The valve gear on the gas compressing cylinder is arranged so that no gas is delivered to the inlet valves of the working cylinder until the air cylinder has provided sufficient air to insure perfect scavenging of the products of combustion, this preventing the fuel from becoming contaminated with the burnt gas. Speed regulation for varying loads is effected by shifting the valve gear of the gas pump so that the gas is delivered at an earlier or later period in the stroke of the working piston, thus causing a variation in the quantity of gas delivered to the working cylinder. This is controlled by the governor directly on the valve gear of the pump or upon a by-pass in the pump cylinder or both. The by-pass, when open returns all of the gas in the passage leading to the inlet valve, that is beyond a certain pressure to the cylinder, so that the gas is delivered to the cylinder at a constant pressure, and therefore in proportion to the load and point of cut off.

This method of governing produces a mixture that varies in richness with the different loads that are carried by the engine, but as the air enters the cylinder first and is prevented from mixing to any extent with the gas by the shape of the cylinder heads, the igniting value of the mixture is not disturbed particularly as the rich gas remains in the cylinder heads and in contact with the igniters.

Like all large engines, the Koerting is started by compressed air taken from a reservoir. A special starting valve is provided for each end of the cylinder which is operated from the cam shaft by means of an eccentric. The air valves may be thrown in or out of gear by a clutch.