Experimental Efficiency Characterization of an Electrically Assisted Turbocharger 2013-24-0122
Electrically assisted turbochargers consist of standard turbochargers modified to accommodate an electric motor/generator within the bearing housing. Those devices improve engine transient response and low end torque by increasing the power delivered to the compressor. This allows a larger degree of engine down-sizing and down-speeding as well as a more efficient turbocharger to engine match, which translates in lower fuel consumption. In addition, the electric machine can be operated in generating mode during steady state engine running conditions to extract a larger fraction of the exhaust energy. Electric turbocharger assistance is therefore a key technology for the reduction of fuel consumption and CO2 emissions.
In this paper an electrically assisted turbocharger, designed to be applied to non-road medium duty diesel engines, is tested to obtain the turbine and electrical machine efficiency characteristics. The radial turbine is of the variable geometry type where the mass flow characteristics are changed by varying the vanes angle. The motor/generator is of the switched reluctance type and has a maximum speed of over 135,000 rev/min.
The turbine and the motor/generator have been characterized with two different experiments. The turbine has been tested in cold flow on a specifically built dynamometer available at Imperial College. This allowed extending the turbine maps available from the manufacturer and to separate the bearing and heat transfer losses from the aerodynamic performance. To test the electrical machine, a prototype of the assisted turbocharger has been modified by removing the compressor and by placing the bearing housing on gimbal bearings. By measuring the speed, the reaction torque on the bearing housing and the electrical power, the efficiency of the electrical machine has been calculated.
Turbine testing results show a peak turbine efficiency of 69% at a velocity ratio of 0.65 with 60% vane opening. Results in the electrical machine testing have shown peak efficiency exceeding 90% at 120.000 rev/min with a maximum shaft power of 3.5 kW in motoring mode and 5.4 kW in generating mode.