Timing belt drives used in automotive engines are subjected to pulley angular vibrations and span tension fluctuations induced by various excitation sources such as fluctuating camshaft torque and crankshaft acyclism. Excessive vibrations and tension fluctuations can lead to undesirable phenomena such as noise, belt fatigue, desynchronization of engine components and overconsumption.

Faced to this issue, car manufacturers have recently introduced non-circular pulley profiles in timing belt drives. A non-circular pulley causes periodic elongations of its adjacent belt spans. Hence, the pulley behaves like a corrective exciter able to counteract the effect of the excitation sources acting on transmissions. It is now known that optimal pulley profile shape and phasing with respect to other pulleys can improve considerably the vibratory performances of the transmissions.

For determining the optimal values of these parameters and obtaining an efficient design of the belt transmissions, it is necessary to fully understand the key-phenomena related to the use of non-circular pulleys.

Previous experimental studies have provided a rigorous description of the proper effect of an oval pulley on the angular dynamics of an academic two pulley transmission with no excitation source (no acyclism on the driving pulley rotation and constant load torque on the driven pulley).

Following these works, the present paper aims at showing experimentally how an oval pulley can generate a corrective effect able to counteract an excitation source such as a fluctuating load torque. For that purpose, the oval pulley is mounted on the driving axis of a transmission whose architecture is similar to that of a 4-cylinder car engine timing belt drive. It comprises one driving pulley (crankshaft), one driven pulley subjected to a second order periodic load torque (camshaft) and two idler pulleys located on each span.

The experiments are carried out following a wide experimental plan: large speed-range and various pulley phasing. The test-bench used for the experiments delivers a complete vibrational diagnosis of the transmission thanks to a very efficient instrumentation providing the measure of:

- the pulleys' rotation (optical encoders) and torques (torque-meters),

- the transverse vibrations of the belt spans (laser distance sensor),

- the belt span tensions thanks to idler supports instrumented with strain gages.

The results obtained from the experiments are analyzed in the angular and angular frequency domains. The study points out the corrective effect of the oval pulley by showing how the different tested phasing values influence the level of angular vibrations and tension fluctuations in the transmission.

Keywords: timing belt, non-circular pulley, angular vibrations, vibration reduction, experiments.