Turbocharging System Design and Performance Analysis of a Marine Diesel Engine

The selection of turbocharging systems for 8-cylinder marine diesel engines is difficult due to the existence of scavenge interference between cylinders. The constant pressure and pulse converter turbocharging systems have been used to eliminate the scavenge interference by applying large volume exhaust manifolds or grouped exhaust branches according to the firing order. But, the performance of constant pressure turbocharging system under low speed conditions of propulsion characteristics and transient conditions is poor, because of less available exhaust gas energy. The structure and arrangement of pulse converter turbocharging system is complex, meanwhile, and the performance at high speed and loads is not as good. In this paper, three new turbocharging systems, such as, MIXPC (mixed pulse converter) system, dual-turbocharger system (DTS) and controllable exhaust system (CES) were designed to improve the performance of a marine diesel engine. In the upstream part of MIXPC system, the separated small diameter branch pipes were used to isolate the exhaust gas interference. In the downstream part of MIXPC system, the single main pipe was connected with one entry turbocharger to improve the operation efficiency of the turbocharger. In the DTS, two one-entry turbochargers were used, one of which connected with 4 cylinders by two branch pipes and a mixer. The two cylinders with firing intervals of 270 crank angles were connected with one branch pipe. In the CES, a control valve was used to control the exhaust gas flow. The valve was opened at high speed and load conditions and closed at low speed and load conditions. The steady and transient performance of the three turbocharging systems was analyzed by simulation. The experimental studies were also carried out to compare the performance of the three turbocharging systems. The experimental results showed that the CES had the best fuel efficiency under low speed and load conditions, and the DTS had the best fuel efficiency under high speed and load conditions. Compared with the MIXPC system, the overall brake specific fuel consumption under propeller operating conditions was reduced by 11.3g/kWh with DTS and 5.3g/kWh with CES. But the uniformity of exhaust gas temperatures of cylinder heads was the best for MIXPC system. In general, the DTS was superior considering the structure simplicity and performance of the engine.