The supercharging system investigated in this study is made up of a traditional turbocharger, coupled with a Roots-type positive displacement compressor. An electrically actuated clutch allows the compressor to be disengaged from the engine at high speed and under partial load steady operations (such as the ones occurring in a driving cycle).
This concept of supercharging has been applied to the downsizing of a reference engine (a 2.5 litre, turbocharged, four cylinder, high speed DI Diesel engine), without penalization on the maximum brake power (110 kW) and transient response.
For such a purpose, a “paper” engine has been theoretically characterized. The gross engine parameters have been optimised by means of 1-D numerical simulations, using a computational model previously validated against experiments.
Performances of the reference and the downsized engine have been compared, considering both steady and transient operating conditions, full and partial load. The two-stage supercharging system allows the “paper” engine to provide higher values of torque at low engine speed and full load, and to perform slightly better in terms of transient response. Furthermore, when considering operating conditions occurring in the European Driving cycle (Roots compressor disengaged), the downsized engine shows lower fuel consumption (from 6 to 24%), and lower pollutant emissions.