Conventional testing still remains a critical part of the product development cycle, validating designs and ensuring that performance, durability and reliability targets are met. It also provides a valuable source of data for calibration and verification of computer models, used for development of future products.

Cummins Turbo Technologies therefore maintains an extensive test capability, with facilities in both its technical centres in China and the UK. Their test work supports not only product development and validation but also supplier selection and research into advanced materials.

Test Rigs
Turbocharger testing may be conducted either on an engine on a test bed, or on a gas stand rig that simulates the engine exhaust conditions. Cummins Turbo Technologies has 19 gas stands, each using a burner system fed with a mixture of diesel and compressed air. By controlling the fuel and air pressure, and flow rate we generate a variety of turbine inlet conditions, driving the turbocharger through its complete operating range. Separate circuits within the test cell supply oil and cooling water.

Gas stands have two clear advantages over engine based testing. First, the elimination of the engine means tests can be controlled more precisely and more reliably. Second, they allow the compressor and turbine stages to be decoupled from one another so that we can explore the complete performance envelope. However, Cummins Turbo Technologies also has seven engine test beds, used when it is vital to understand the interaction between engine and turbocharger, such as determining the effect of exhaust pressure pulsing on turbine nozzle dynamic loads.

Precise measurements of turbine and compressor aerodynamic performance are among the key product tests conducted on our dedicated performance gas stands, quantifying pressure, flow and stage efficiency across the full operating range. When measuring compressor performance, we drive the turbocharger open loop and throttle the compressor outlet in order to map all flows from choke to surge. Inlet air is controlled very carefully and both compressor inlet and outlet pressures and temperatures are measured to ensure that we obtain accurate data for operating efficiency.

The compressor stage is replaced with a hydraulic dynamometer when testing turbine performance. This allows us to obtain mass flow and efficiency characteristics over a wide range of turbine pressure ratios and shaft speeds, unrestrained by the compressor and with great accuracy. The test is run on a steady state gas stand incorporating a dynamometer that absorbs the gross power ie. including the bearing reaction, produced by the turbine.

Burst Testing
One of the safety related tests we conduct on both the turbine and compressor stages is a containment or burst test. This is a vital part of product validation, ensuring that in the event of catastrophic failure the rotating components of the turbocharger are completely contained and that housings and fasteners remain intact. Performed on a gas stand, the test involves driving the rotor system beyond its maximum speed until a point when either the turbine or the compressor wheel fails, or bursts.

Proving Durability
Burst testing is designed to ensure Cummins Turbo Technologies' products are safe. Durability is another key requirement, particularly in the heavy-duty engine market. Impeller and turbine wheels are subjected to intense stresses at high speed. As speed and load vary through the engine's duty cycle these stresses also change and this can cause fatigue damage, commonly referred to as low-cycle fatigue. In order to ensure our products are capable of meeting the most demanding duty cycles, we carry out low-cycle fatigue durability tests on all our designs. They are conducted on a gas stand, with the output from the burner rig split to feed two turbochargers. Control valves mounted upstream of each turbocharger are opened and closed repeatedly, so that each turbocharger is cycled from minimum to maximum speed.

Each cycle takes about five seconds; over 15,000 cycles are completed each day. The test continues for days, or even weeks, until the wheel fails. Several wheels of the same design are tested to establish both durability and reliability. This data is then used to calibrate computer fatigue models, allowing us to make accurate life predictions for real world duty cycles. High-cycle fatigue due to resonant vibration is also a potential threat to the durability of turbine or impeller blades. Gas loading of the blades can be highly dynamic, so Cummins Turbo Technologies uses sophisticated CAE tools to develop designs that minimise these loads. We validate the designs by applying strain gauges to the blade surface to measure the level of blade vibration under realistic loading conditions. The gauge positions are selected by finite element modelling of the blade geometry and with gauge lengths in the order of just one millimetre, even gauge application is a highly skilled process.

Specialised high temperature strain gauges fixed with ceramic cement are needed to cope with the extreme temperatures when measuring turbine blade vibration. This is one of the tests we run on an engine test bed in order to understand the interaction between engine and dynamic blade loading.

Less Noise
Customers have become more discerning about noise levels in recent years. Tthere are several potential sources of turbocharger noise that have to be separated in order to measure them.

Cummins Turbo Technologies has developed a unique noise test cell capable of doing this and measuring each simultaneously. The turbine stage is driven by a gas stand rig with its burner system housed in an adjacent room to isolate the noise it generates. Body radiated noise from the turbocharger housing is measured in a hemi-anechoic chamber using a hemispherical array of microphones.

Compressor intake noise is ported to another hemi-anechoic chamber for simultaneous measurement. Special high temperature probes are used to take in-duct turbine exhaust noise readings. Data from each measurement point is analysed after major components such as blade-pass noise have been extracted, with results plotted against flow and pressure characteristics to generate noise maps.

Cummins Turbo Technologies' test rigs are computer controlled for optimum productivity and repeatability. There are routines with automatic control that allow standard tests to run unmanned. Data from tests are uploaded to a results database held within the Cummins Turbo Technologies global product data management system so that results can be shared with company engineers all over the world. Requests for test work are also handled via this channel, making it a truly global test management system.

This is a summary of only a small part of our worldwide test capability. Whether it is bearing durability, accelerated thermal cycle or vibration testing, the Cummins Turbo Technologies test department is dedicated to providing accurate and timely input into the product.