Increasing electric vehicle efficiency without adding significant development and vehicle unit cost remains a challenge for the global auto industry. But Matthew Hole, head of design engineering at Drive System Design (DSD), a transmission engineering consultancy, believes that a new transmission lubrication analytical technique can lead to significant energy and, ultimately, cost savings.
He revealed that joint programs undertaken between DSD, simulation software specialist Altair, and FluiDyna, a German computational fluid dynamics software and hardware specialist, could help achieve this, by extending what he terms as a new modelling approach to an area of transmission design that was previously unfeasible.
“Steady-state test results from a typical EV show that, up to approximately 80 km/h, the drivetrain is the largest energy draw on the system," Hole noted. "Greater EV drivetrain efficiency must be realized in order to secure the greatest possible real world operating range, without incurring additional cost.”
Although the effects of bearing and gear frictional losses are well understood, he said that the interaction between rotating components and the transmission lubricant has, historically, been difficult to explore.
“Finite Volume CFD modelling involves extensive run times, with each test point requiring several weeks’ computing just to generate a couple of seconds of real time data,” he explained. The new modelling approach reduces the time from weeks to only a few days by utilizing a Smoothed Particle Hydrodynamics (SPH) code: nanoFluidX.
The nanoFluidX factor
Developed by FluiDyna, the SPH method is a mesh-free alternative to the more common finite-volume CFD codes, said Hole.
“A model of a typical single-speed EV transmission would contain approximately 10 million particles, enabling the analysis of a couple of seconds of data in less than two days, with modest computational expense; an order of magnitude faster than previous techniques," Hole said. "This method typically solves on cost-effective graphics processors, such as those developed to solve artificial intelligence problems.”
DSD’s first application of nanoFluidX is on a planetary transmission design for an unnamed EV application. Hole explained that planetary geartrains present several lubrication challenges, such as obstruction of supply to the sun-planet mesh at the center of the system. Additionally, the planet gear bearings often run at high speeds and therefore require a robust supply of lubricant.
The company has applied nanoFluidX to an EV planetary transmission: “By accurately visualizing and analyzing the behavior of the lubricant and its interaction with the rotating assemblies, we were able to develop a highly optimized, passive lubrication system with low losses,” he reported.
Hole explained that to increase the oil volume for better thermal management without generating high churning and windage losses, a remote sump was created by segregating a chamber within the transmission casing. The sump is charged by a vaned impeller, designed as a low-cost molding that is clipped to the planet carrier. It scavenges the cavity containing the rotating components and supplies it to the sump under pressure “without incurring the cost and package penalty of an electric pump.”
Residual pressure in the sump forces oil from the exit via a series of baffles and guides to the center of the planet pins, where it is most needed.
Next steps in development
Hole said that unlike a test on physical components with limited visual access, the nanoFluidX model allowed key sections throughout the assembly to be examined, to confirm the satisfactory penetration of oil into those areas. It also enabled the confirmation of satisfactory, and rapid, pump priming and the correct functioning of the various guides.
The design process was validated by correlating the predicted results for a simplified system against physical tests for fluid movement and drag losses. Results from a single helical gear on a spin rig with a torque cell and high-speed video recording are stated to have provided enough confidence to proceed with the SPH method for analyzing the more complex planetary system.
The improved understanding of lubricant behavior provided by nanoFluidX modelling enabled DSD to introduce design improvements with confidence, noted Hole.
“The pump was actually more effective than necessary, so a revised impeller with fewer vanes and different geometry was introduced," he said. "Drag was successfully reduced without compromising lubrication. We then cut the oil capacity by 100 ml, and reduced drag further. In total, we cut drag losses by almost 30% in the final iteration while maintaining satisfactory lubrication of all the transmission elements.”