Fig. 2 Hunting Phenomenon in the Scavenging and Exhaust Circuit of diesel Generating Plant with SCR
4] This results in excessive decline of the SCR system temperature, which in turn causes the turbocharger speed to go down excessively after a delay in time.
5] As a result of (4) above, the turbocharger charges far less air into the engine cylinders then is warranted for the engine output causing the engine exhaust gases to rise in temperature. It therefore is the above events taking place in repeated cycles that constitutes the hunting.
As can be seen from the above, the hunting in the scavenging and exhaust circuits of diesel generating plant of the configuration referred to is attributable to large heat capacity of SCR system causing a delay in time of the engine exhaust-gas energy being transmitted to the turbocharger.
Fig. 3 Mechanism of Hunting Occurring in The Scavenging and Exhaust Circuit
2-4. How to Prevent Hunting
As described in the preceding section, the hunting in the scavenging and exhaust circuits of diesel generating plant is attributable to the delay in time of exhaust-gas energy flowing into the turbocharger at a rate corresponding to the reduced engine output. Maintaining the plant heat cycle properly by controlling the flow of exhaust-gas energy into the turbocharger following the change in engine load, therefore, was considered important to the prevention of hunting.
Accordingly, as a practical method of controlling the flow of exhaust-gas energy into the turbocharger, the authors conceived the idea of installing a gas-extraction valve in the exhaust-gas duct near the turbocharger gas inlet to release part of treated exhaust gases from the SCR system into the atmosphere through the smokestack.
Explained using the diagram of Fig. 3, when the exhaust-gas energy at the turbocharger gas inlet is greater then it should be in relation to the engine output, the excess energy will be released by means of the gas-extraction valve to control the exhaust-gas energy to the level commensurate with the engine output so that the excessive decline in exhaust-gas temperature which occurs in reducing the engine output and triggers the hunting can be prevented.
The use of the gas-extraction valve was found effective in restraining the hunting through a study by numerical simulation, as reviewed in the following chapter.
