Preliminary DoE Analysis and Control of Mapping Procedure for a Turbocharger on an Engine Gas-Stand

Abstract

The use of turbochargers on both gasoline and diesel engines is started to become a common strategy to comply with stringent limits on CO2. The main action towards lowering fuel consumption of powertrains is achieved by reduction of engine size and number of cylinders, annexed to the lower friction. However, this is directly linked to the worsening of deliverable output power under the natural aspirated configuration. Therefore, turbocharging is often adopted to overcome this problem where useful energy contained in the exhaust gasses is used to increase the air density at the intake. The increase in power from a natural aspirated configuration is a direct consequence of higher fuel quantity to be injected. In order to pursue a systematic evaluation of the powertrain system, engine, turbochargers and auxiliary components are included into 1D models. Several conditions can be simulated without the need of an extensive test plan. In 1D software like Ricardo Wave, turbochargers performance are imposed as input. These are previously measured in appropriate turbocharger gas-stand where hot or cold air is blown through the turbine while load on compressor is controlled by adjusting a back pressure valve. Compressor and turbine maps are generated for constant speed lines which are corrected for total temperature. Pressure ratio, mass flow and isentropic efficiency are also monitored as parameters to characterize performance maps of turbomachinery. In gas-stands, steady flow conditions are imposed at compressor and turbine. However, in turbocharged engines, pulsating flows induced by the engine valvetrain disturb continuously turbocharger conditions during the engine cycle. In fact, the effects that the conditions of the engine air-path could have on the turbocharger operations are excluded from the system modelling. In this study, an appropriate engine gas-stand has been developed in order to improve the accuracy on estimating the turbine extraction power in 1D powertrain simulations. In addition, future analyses on turbocharger transient operations could be investigated. The compressor outlet has been disconnected from the 2.2L Diesel engine intake so that the load on turbocharger and engine can be independently controlled. In order to extend the engine capability in delivering mass flow and pressure at the turbine inlet, an external boost rig has been installed with the capability to control pressure, mass flow and temperature at the engine intake. In a first instance, a 1D model of the system including turbomachinery, Diesel engine and boost rig has been developed using the commercial platform Ricardo Wave. In this way, a preliminary DoE study of the entire system has been performed in order to evaluate the effects of parameters and actuators on the turbocharger operations. Additionally, the control of the rig has been tested by confirming the previous DoE study. Approaches to create turbochargers maps are shown. Last section of the paper focuses on turbine pulsations and the interpretation of efficiency calculated in experiments and simulations.

Attempt to correlate simulations and measurements of turbine performance under pulsating flows for automotive turbochargers

“…A boost rig unit connected to the engine intake is able to target the temperature, pressure and mass flow through a series of valves, defined as the hot, dump and main valves, respectively. 9…”

“…In the case of the turbine outlet temperature, eight thermocouples at different depths are placed in a straight insulated duct to determine the presence of swirling flows. 9…”

“…To investigate turbine performance under pulsating conditions, an engine gas-stand facility has been developed, see Avola et al 9 Exhaust gases from a ford four cylinder 2.2L diesel engine provide power to the 41 mm variable geometry turbine (VGT), leading to rotation of the turbocharger. As shown in Figure 1, an external compressor back-pressure (CBP) valve generates a load on the compressor, which is operating under a steady flow.…”

“…On-engine applications are investigated through experiments and the validation of a 2.2 L diesel engine gas-stand. 9 To evaluate isentropic turbine efficiency and to reduce heat transfer effects, 29 the inlet temperature at the turbine rotor has been experimentally measured and compared to measurements at the exhaust manifold. This has improved the correlation of unsteady turbine performance between engine experiments and 1D model simulations.…”

“…8 In the presence of unsteady flows in a turbocharger turbine, performance measurements through the use of external instrumentation depend significantly on the positioning. 9,10 Deviation from steady efficiency is significant due to the presence of oscillating flow parameters in relation to the operational pulsation amplitude and frequency. 11,12 Due to the impossibility of measuring turbocharger torque, turbine efficiency is correlated to inlet and outlet temperature measurements.…”