Hashimoto, M., Aoyagi, Y., Kobayashi, M., Murayama, T. et al., "BSFC Improvement and NOx Reduction by Sequential Turbo System in a Heavy Duty Diesel Engine," SAE Technical Paper 2012-01-0712, 2012, doi:10.4271/2012-01-0712.
Reduction of exhaust emissions and BSFC has been studied using a high boost, a wide range and high-rate EGR in a Super Clean Diesel, six-cylinder heavy duty engine. In the previous single-turbocharging system, the turbocharger was selected to yield maximum torque and power. The selected turbocharger was designed for high boosting, with maximum pressure of about twice that of the current one, using a titanium compressor. However, an important issue arose in this system: avoidance of high boosting at low engine speed. A sequential and series turbo system was proposed to improve the torque at low engine speeds. This turbo system has two turbochargers of different sizes with variable geometry turbines. At low engine speed, the small turbocharger performs most of the work. At medium engine speed, the small turbocharger and large turbocharger mainly work in series. At high engine speed, the small turbocharger does no work at all, but the large turbocharger works mainly using a small turbocharger bypass. The basic engine, with six cylinders in-line and displacement of 10.5 L, is equipped with a high-pressure fuel injection system and a high- and low-pressure loop EGR system for using the high boosting and high EGR rate to reduce BSNOx and PM. Experimentally obtained results show that the sequential and series turbocharging system has 50% higher torque than the conventional, with improved fuel consumption achieved in the low-speed region.