Rotordynamics of Automotive Turbochargers

The enacted average CO₂ emission for new passenger vehicles in Europe is limited to 120 g/km from 2012 (65% produced vehicles) to 2015 (100%). This emission limit is reduced to the ambitious long-term target of 95 g/km from 2020. Additionally, the average CO₂ emission limit for new light-duty commercial vehicles is 175 g/km from 2014 (70% manufactured vehicles) to 2017 (100%); and it is reduced to 147 g/km (ambitious long-term target) from 2020. To reduce carbon dioxide (CO₂) and nitrogen oxides (NO_x) exhausted by passenger and commercial vehicles and to improve the fuel consumption of the engines, we have already carried out many measures, e.g. high-pressure direct injection (HPDI), exhaust gas recirculation (EGR), variable valve train (VVT), variable compression (VC), and hybrid techniques [3]. Two other important aspects are downsizing of engines by reducing the number of cylinders or volumetric size of cylinders, and turbocharging. Engines with less number of cylinders or small cylinder volumes induce less friction power between the pistons and cylinders. Additionally, the total weight of the vehicle is also reduced due to small engines, leading to less driving friction. Evidently, small engines needs less fuel consumption; in turn, they produce less engine power. Small engines consume less fuel and therefore produce less carbon dioxide (CO₂) and as well as nitrogen oxides (NO_x). In the point of view of energy and air pollution, they have done a good job to sustain our energy resources and to keep the environment less polluted and clean.

Some essential thermodynamic characteristics of gases are needed to know in the turbocharging. They have been usually applied to the turbocharging of engines, where the charge air and exhaust gas are assumed as compressible ideal gases.

Exhaust gas turbochargers used in the automobiles, such as passenger, on-road vehicles, and off-road engines have some discrepancies to the heavy turbomachines applied to the power plants and chemical industries. The first ones are much smaller and work at high rotor speeds in various operating conditions, such as variable rotor speeds, pressures, temperatures, and as well as mass flow rates. Contrary to the automotive turbochargers, the industrial turbomachines are bigger, heavier and mostly operate at a stationary working condition. Due to their large sizes of compressor and turbine wheels, the turbomachines only operate at low rotor speeds between 3,000 to 20,000 rpm. The maximum circumferential velocities of the turbine and compressor wheels used in the automotive turbochargers are approximately 530 and 560 m/s, respectively. The maximum circumferential velocities of the compressor and turbine wheels are determined by the durability of materials at various driving cycles. Their thermo-mechanical characteristics and lifetime depend on the using material, producing method, and as well as driving cycles.

Two important issues of the automotive turbochargers in the rotordynamics are resonance and instability of the rotor. Both have a common harmful effect that causes damages of the turbochargers during the operation. However, there is a big difference between the resonance and instability if we take a close look on them.

We have thoroughly discussed the rotordynamic stability in Chapter 4. In this chapter, it deals with the resonance that is a harmful effect causing damages to the turbochargers. When the rotor angular frequency equals its eigenfrequency (undamped natural frequency), the resonance occurs at the critical speed at which the rotor arrives at the maximum deflection amplitude.

To support the rotor in the operation, the bearing system including the thrust and radial bearings is necessary for turbochargers. The impulse forces of fluids in the wheels and pressures acting on the compressor and turbine wheels cause the axial thrust on the rotor that depends on different working conditions. Hence, the axial thrust is balanced by the thrust bearing to keep the rotor stable in the axial direction. On the contrary, the radial bearings induce the bearing forces to balance the unbalance forces acting upon the rotor in the radial direction.

Some uncertain boundary conditions play a key role for the stability and functionality of the rotor, as shown in Fig. 7.1. Unfortunately, it is very difficult or impossible to take all of them into account in the rotordynamic computation where only the rotor containing the compressor, turbine wheels, rotor shaft, radial bearings, and seal rings is considered. Normally, the boundary conditions are generally assumed ideal in the computation, such as sufficient supply oil, good parallelism of the thrust bearing, all wedges of the thrust bearing having the same slope, good quality of the radial bearing in terms of the bearing non-coaxiality, non-roundness, and characteristics of surface roughness. Therefore, the rotordynamic computation cannot cure the instability and malfunctions of the rotor, or prevent it from damages at inappropriate boundary conditions, like oil insufficiency, oil contaminated with hard particles, foamy oil, or oil coking in the bearing in the operation. However, the computational results help us better to understand the rotor response. Therefore, further improvements could be done for the rotor stability and reduction of the bearing friction. Additionally, experimental measurements also provide us with the real rotor response at such unknown boundary conditions that one could not consider in the computation.

In the following section, we focus only on the rotor balancing in turbochargers and not in the industrial turbomachines because we can find them in [1], [4], [6], and [10].