Wearing it Well

By Aliihsan Karamavruc, Senior Computer-Aided Engineering Analyst, BorgWarner Turbo Systems, Asheville, USA

Instead of predicting wear on nonuniformly loaded sliding surfaces using long and expensive physical tests, BorgWarner tested a new capability in ANSYS Mechanical to predict wear analytically. The company is on its way to reducing the time to predict wear on a turbocharger wastegate shaft from several days to one day. This capability will expedite design iterations between designers and analysts, and help to create more accurate wear-test rigs.

Save PDF
BorgWarner turbocharger

In turbocharged engines, exhaust gases drive a turbine that spins an air compressor to move additional air into the cylinders, so that they can burn more fuel with each combustion cycle. Turbochargers were originally developed to improve internal combustion engine performance, but today they are primarily used to reduce fuel consumption and emissions. The turbocharger uses wasted exhaust energy to contribute to engine efficiency and thus increase fuel economy. A turbocharged engine of equal power is also smaller, which decreases frictional and thermal losses to further improve the fuel efficiency of the vehicle. An engine with a turbocharger is lighter than a conventional engine with the same power, providing even more fuel savings.

ANSYS mesh

The majority of turbocharged gasoline applications require a wastegate (WG), a boost-controlling device that allows a portion of the exhaust flow to bypass the turbine wheel as necessary. This in turn reduces the power driving the turbine wheel to match the power required for a given boost level. A WG prevents the boost pressure from climbing indefinitely and consequently blowing the engine. An internal WG is built into the turbine housing and an external WG is constructed outside the housing.

BorgWarner is a global product leader in powertrain solutions, with a focus on developing leading powertrain technologies to improve fuel economy, emissions and performance. When designing WG actuators, BorgWarner must ensure the life of the actuators over millions of cycles of operation despite nonuniform loading that contributes to the difficulty of predicting wear patterns. A new ANSYS Mechanical feature that analyzes wear between sliding parts provides accurate predictions in one day — a big-time savings compared to the several days that were required in the past to perform physical testing.

"When designing turbocharger wastegate actuators, BorgWarner must ensure the life of the actuators over several millions of cycles of operation."

LIMITATIONS OF PHYSICAL TESTING

In a popular style of BorgWarner turbocharger, the WG actuator shaft is mounted vertically inside the turbocharger. One end of the shaft is connected to a flapper in the exhaust gas flow stream that seals the WG. Exhaust gas exerts a continual force on the flapper, while a spring at the opposite end of the shaft resists the force to keep the WG closed. The internal spring of an actuator is calibrated to a predetermined boost level. When this boost level is reached, the flapper opens and allows exhaust gas to bypass the turbine. From a wear standpoint, the primary concern is wear on the shaft and bushing, which is difficult to predict because of the nonuniform contact due to force exerted by the flow stream and rotational motion of the shaft.

Wastegate actuator diagram

Diagram shows how a wastegate actuator works.

Simulation vs. experiment
Simulation vs. experiment
Contact pressure as predicted by simulation closely matched actual wear in physical testing.

In the test rig, a motor drives an eccentric crank shaft connected to a crank arm that moves the shaft back and forth over the range that would be driven by the flapper. A 12.8 kg mass hangs on the end of the shaft to represent the force exerted by the flow stream on the flapper. Each revolution of the motor represents one open-and-close cycle of the device. The shaft is also maintained at a temperature of around 450 C to replicate the real-life operating temperature. The test rig accurately predicts the wear experienced by the WG actuator during turbocharger operation, but it requires building an expensive prototype. In addition, running the test rig through enough cycles to predict the wear on the actuator takes about a week.

Without analytically predicting the wear, BorgWarner engineers often found that their first design did not meet wear-life specifications, so the entire design, build and test process needed to be repeated, often several times. The ability to determine wear on the shaft and bushing prior to building a prototype would save time and avoid multiple prototypes. Until recently, the only method available to analytically determine wear had been using the Archard wear equation, which describes sliding wear based on the load, sliding distance, hardness of the contacting surfaces and a dimensionless constant K. While this equation is useful in predicting wear on evenly loaded surfaces, it does not address nonuniform loading, so it cannot be used in this case.

BorgWarner actuator test rig

CAD model of test rig

defection vs. crankshaft rotation angle
Deflections of key components vs. crank shaft rotation angle
Simulation of wear
Wear as predicted by simulation. These results showed a good correlation with test results.

ANSYS MECHANICAL NOW CALCULATES WEAR

Recent releases of ANSYS Mechanical have given engineers the ability, for the first time, to calculate wear based on nonuniform loading. In this case, BorgWarner engineers began with a computer-aided design (CAD) model of the test rig, including the crankshaft, crank arm, bushing, shaft and pendulum assembly (which holds the weight representing the flow stream pressure). The boundary conditions for the model included a fixed support holding the bushing in place, a mass connected to the end of the shaft, and rotational joints in the bushing and crank arm. Material properties were defined as a function of temperature. Material hardness was defined as a function of the yield stress of the underlying elements, but temperature was not included in this simulation. The generalized Archard wear model was used to predict wear based on the loads calculated at each point in the contact zone by the ANSYS Mechanical simulation. The value of K was determined based on an engineering handbook. A frictional contact was used between the shaft and the bushing.

Engineers ran the simulation over 720 degrees of motor rotation, which amounts to two open-and-close cycles of the actuator. The contact nodes were moved as per the wear increment at each time step. Additional equilibrium iterations for the corrected deformation were then performed. The software performed rezoning whenever the mesh became distorted due to wear.

Wear vs test

Wear results showed a good correlation with test results. Test duration was 4 days with 691,200 cycles at a temperature of 450 C and a rate of 2 Hz. ANSYS simulation represents only two cycles.

ACCURATE PREDICTION OF WEAR

The simulation results included deflection of components as a function of crank shaft rotation angle, which plays an important role in the resulting wear pattern. The simulation also calculated contact pressure as a function of crank rotation angle, an important predictor of wear. The contact pressure over the surface of the shaft as predicted by ANSYS software matched the wear patterns on a shaft that had undergone physical testing. The software also predicted the wear generated on each node of the shaft during two cycles of rotation. Engineers plotted accumulated wear over two cycles and extrapolated this information for the full one-week test period.

The new simulation capability in ANSYS Mechanical can predict wear with a high level of accuracy. This process reduces the time needed to investigate a design from several days to just one day. This capability will save BorgWarner time and money by making it possible to evaluate different design alternatives based on their wear performance prior to the prototype stage, so that just one prototype can be built with a high degree of confidence.

click below to start a conversation with ANSYS

Contact Us
Contact Us
Contact