Abstract

The conventional powertrain has seen a continuous wave of energy optimization, focusing heavily on boosting and engine downsizing. This trend is pushing OEMs to consider turbocharging as a premium solution for exhaust energy recovery. Turbocharger is an established, economically viable solution which recovers waste energy from the exhaust gasses, and in the process providing higher pressure and mass of air to the engine. However, a turbocharger has to be carefully matched to the engine. The process of matching a turbocharger to an engine is implemented in the early stages of design, through air system simulations. In these simulations, a turbocharger component is represented largely by performance maps and it serves as a boundary condition to the engine.

The thermodynamic parameters of a turbocharger are calculated through the performance maps which are usually generated experimentally in gas test stands and used as look-up table in the engine models. Thus, the operational of the engine is dictated by the air flow thermodynamic parameters (pressure, temperature and mass flow) from the turbocharger compressor; this in turn will determine the thermodynamic parameters for the exhaust gas entering the turbocharger turbine. The importance and its sensitivity dictate that any heat transfer affecting the experiments to acquire the performance maps will cause errors in the characterization of a turbocharger. This will consequently lead to inaccurate predictions from the engine model if the heat transfer effects are not properly accounted for. The current paper provides a comprehensive review on how the industry and academics are addressing the heat transfer issue through advancing researches. The review begins by defining the main issues related with heat transfer in turbochargers and the state-of-the-art research looking into it. The paper also provides some inputs and recommendations on the research areas which should be further investigated in the years to come.

Turbocharger Testing

The turbocharger is an important component of a modern diesel engine and increasingly so of gasoline engines. For many years turbochargers have been of a fixed internal geometry installed within a system wherein overpressurization of the engine has been prevented by the use of an internal, or external, waste gate (valve). This device acts as an air pressure relief valve by diverting a variable amount of exhaust gas flow from the turbine and sending it to waste, thus reducing the compressor speed and lowering boost air pressure. Recent advances in materials and engine design mean that the modern turbocharger is now often fitted with a variable-geometry turbine (VGT) that is electronically controlled through the ECU so that its performance is optimized to deal with the wide range of operating conditions and to support the correct function of exhaust gas recirculation (EGR) and other emission control strategies. Variable-geometry turbine systems may be servo-electric or servo-hydraulic and do not require waste gates, but in most applications still require antisurge valves.

Antisurge valves operate as a pressure relief valve in the boost air circuit and are designed to deal with the sudden rise in air pressure that arises between the compressor and the engine when the engine throttle is suddenly closed from near, or at, wide-open throttle (WOT) operation. A complication of the operation of antisurge valves is that the air relieved has to be returned to the compressor inlet and after the mass-airflow sensor, which has already registered its passing, to prevent incorrect fueling. The moving blades and parts of the actuation mechanism of VGT units have to work in the highest temperature zone of the turbine, because of the demands made upon the materials used; until recently, VGT designs have been confined to diesel engines, which have lower exhaust gas temperatures than gasoline engines. A problem for vehicle designers is the cost of VGT designs, which is often well over double that of fixed-geometry designs; therefore, in truck installations, where space may not be as constrained as in light vehicles, the use of two-stage or twin-turbo designs may be more cost-effective. A two-stage boost system can be based on two separate turbochargers, one HP and one LP, permanently connected in series, or on two units fitted in parallel with some modulation of the gas flow to one or both units, to give optimized boost and engine power. Turbocharger testing takes two forms:

1.Testing the turbocharger and its associated actuators and sensors, in a customer built test stand, as a separate module without any other part of an engine. In this case a combustion-gas generator is required to drive the turbine.

2.Testing the turbocharger mounted on an engine as an integrated system. In this case the engine provides the gas generation to drive the turbine.

Turbocharger Test Stands

A range of test stands that are designed for testing the automotive range of turbochargers are commercially available from companies such as Kratzer Automation [3]; these units typically use natural gas in hot-gas generators whose energy output is variable over the full operating cycle of the intended engine installation. Although gas burners provide a virtually particulate-free gas flow, it is important for some tests to simulate operation in poor gas conditions; therefore, such rigs have to be capable of simulating less than optimal gas conditions.

Non-engine-based turbocharger test rigs are cost-effectively used by manufacturers for the full range of development, quality assurance, and endurance testing, including:

1.Testing over the extreme thermal range of the turbine unit and thermal shock testing.

2.Testing over the full range of the pressure–volume envelope to confirm compressor surge and choke lines and the unit’s efficiency “onion rings” or “efficiency islands”.

3.Checking for turbine blade resonance, fatigue testing.

4.Seal and bearing module performance testing and blow-by tests (oil leakage into either induction or turbine side has serious consequences).

5.Mapping of both turbine and compressor speed–pressure–temperature performance.

6.Lubrication system and oil suitability testing, including resistance to “oil-coking” following shutdown while at high turbine temperature.

7.Running at over-speed and to destruction in order to check failure modes and debris retention.

Testing of Turbochargers Using an Engine

This must be considered as distinct from testing a turbocharged engine and an example of an instrumented test unit is shown in Figure 7.11. In the latter case the operation of the unit is one more variable to be mapped during engine calibration and, where appropriate, will be the subject of its own “mini-map” covering the operation of the servo-controller of the VGT vanes within the engine fueling map. For example, in engine calibration work the interaction of EGR and turbocharger geometry has to be optimized, but when testing the turbine using an engine, for speed–pressure sweeps, etc., the EGR will interfere with system control and may be switched out of circuit.
Similarly, it will be found that running the engine and dynamometer in manual control mode (position/position) will give the most repeatable results in much turbocharger testing because it eliminates the majority of engine and cell control system influences. Gas flow, from the exhaust of individual engine cylinders into the turbine, is complex and subject to pressure pulsing; therefore, the pressure regime of the whole exhaust manifold and turbine has to be understood in order to optimize its design. Such test work, which is practically impossible to accurately simulate on a test bench, has to be done in an engine test cell. Such testing has led to the development of twin-scroll turbine designs, which divide the gas flows into the turbine housing from pairs of cylinders in order to reduce pressure-pulse crosstalk evident only on a fully instrumented engine.