Computational Study of Diesel Engine Downsizing Using Two-StageTurbocharging 2004-01-0929
To reach high levels of boost pressure for downsizing on Diesel engine, new turbocharging systems for in line 4 cylinder engines have been investigated. The purpose of this work is to evaluate the two stage turbochargers potential for passenger car applications by simulations. A 2.0l diesel engine has been taken as reference (Torque and Power for all engine speeds), and the aim of this work is to develop a 1.5l diesel engine able to reach this target.
The boost system architecture investigated consists two turbochargers in series with one intercooler after each compressor.
Simulations have been performed with GT-Power. Turbine and compressor wheels have been modified on both stages in order to define the best combination.
Results have shown that the steady state performance can be reached quiet easily. These results made us focus on transient aspect of this concept. A vehicle inertia model has been set up. To compare the system response for transient situation, a full load acceleration from 1000 to 4000 rpm in 3rd gear has been simulated and a full load acceleration in 5th gear from 80 to 120 km/h.
Transient simulations show “take off” problems due to low engine displacement at low RPM whatever the boost pressure architecture tested and the turbochargers considered.
For the acceleration in the 5th gear we achieved turbochargers matching as successful as the reference.
This computational study tends to show the interest and the necessity of the transient approach for such systems: steady state performance can be reached by several turbocharger matchings whereas transient performance needs a real optimisation. The transient aspect of this kind of boost pressure system must be taken into account to optimise the definition of turbine and compressor wheels for both low pressure and high pressure stages. Steady state performance with a specified turbocharger matching does not guarantee acceptable transient performance.