2-stage supercharging applied to HSDI Diesel engines appears a promising solution for enhancing rated power, low end torque, transient response and hence the launch characteristics of a vehicle. However, many open points still remain, in particular about the impact on emissions control and fuel economy at partial load conditions, generally requiring both high airflow and high EGR rates.
The paper analyzes and compares two types of 2-stage supercharging systems: a) two turbochargers of different size; b) one turbocharger coupled to a positive displacement compressor. The goal of the paper is to assess pro and cons of the most feasible configurations for a typical automobile Diesel engine, complying with Euro V regulations and beyond.
The base engine is the 2.8L, 4 cylinder in-line unit produced by VM Motori (Cento, Italy), equipped by a standard variable geometry turbocharger. A 1D thermofluid-dynamic model of the Euro V version of the engine was built and calibrated against experiments at the dynamometer bench, at both full and partial load.
Using the computational model as a starting base, a large set of alternative supercharging systems has been analyzed. The three best configurations are compared to the base engine at full load, under both steady and transient operations. Also a set of steady points, representing the operations in the NEUDC for a vehicle complying with Euro V regulations is simulated, in order to assess the influence of the supercharging system on fuel consumption and emissions.
Finally, some simplified calculations are performed to evaluate the potential of the Miller cycle in terms of NOx reduction at partial load.
The study demonstrates that the use of a supercharging system made up by a positive displacement compressor and a VGT turbocharger is able to noticeably enhance engine performance at full load, with further slight advantages, in comparison to the other analyzed systems, in terms of fuel consumption and emissions control. Finally, this system can support the application of the Miller cycle at partial load, enabling a relevant reduction of NOx, without drawbacks on the other pollutants.