Among the various types of hydrostatic transmissions, those based on radial piston multi-stroke machines are well-known solutions for off-road mobile machines. The balance between compactness, efficiency, control, comfort and price is the main reason for this. For the same reason, several car companies have tried to introduce hydrostatic transmissions into on-road applications.
All such efforts have failed, mainly due to the whine noise produced by the hydrostatic machines.
In particular, well-known standard solutions that aim to address the noise of radial piston multi-stroke machines, such as optimized relief grooves known as ‘notches’, are oriented toward reduction of the noise level. Unfortunately, the remaining whine noise, even with well-optimized notches, is unacceptable for automotive NVH standards.
This article presents a new, inventive, scientifically based, successfully tested method aimed toward the suppression of the whine noise of radial piston multi-stroke hydrostatic machines. This method is based on the application of a Pseudo Random Binary Sequence (PRBS). This type of sequence is well-known in the domain of signal treatment as a basis for signal scrambling. In this case, a PRBS is used to scramble hydraulic noise. Accordingly, unpleasant whine noise can be altered, which can help merge the sound of a hydrostatic transmission with the sound of a thermal engine. Specifically, using the PRBS, the standard hydrostatic machine power spectrum is transformed toward the ‘white noise’ spectrum. In addition, by applying the PRBS, the frequency of the characteristic hydrostatic machine repetitive sound sequence is reduced, falling closer to the frequency of a characteristic thermal engine repetitive sound sequence. Due to the frequency reduction and transformed power spectrum, the sound of the hydrostatic machine will appear merged with the sound of the thermal engine. More precisely, overall noise will be increased, but it will be perceived more as that from the noisy thermal engine and not as the whine noise of a standard hydrostatic machine. With the hydrostatic machine sound merged with the sound of the thermal engine, a new powertrain concept based on the inclusion of a hydrostatic transmission can be successfully proposed for on-road applications.
This concept, already proven with test vehicles, represents a significant achievement, as it will allow for new possibilities in powertrain design.
It should be noted that this solution has a patent pending, so not all technical details can be reported currently. Therefore, the presentation of the methods and data herein may not be as clear as they would be once the patent is published. Nevertheless, enough technical details are given such that specialists in the field of hydrostatic transmission can grasp the novelty of the main idea behind the concept presented herein and start to evaluate potential impacts of the proposed new method against standard, known methods.