In late February 2023, SAE International announced the release of a standards document to provide a common testing procedure to rate the maximum power of electrified powertrains. The new J2908 standard, titled “Vehicle Power and Rated System Power Test for Electrified Powertrains,” is a voluntary procedure to more easily measure and compare the maximum power of electrified powertrains used in hybrid-electric vehicles (HEVs), plug-in hybrid-electric vehicles (PHEVs) and battery electric vehicles (EVs) or fuel-cell electric vehicles (FCEVs).
The J2908 standard establishes the framework for testing, data post-processing and reporting of SAE System power for an electrified vehicle. The SAE system power is defined as the sum of the mechanical shaft powers of all powertrain components used for propulsion during peak wheel power. It is a voluntary procedure, said Michael Duoba, research engineer, Argonne National Laboratory, and sponsor of the document.
“The results are a familiar system power definition in the same manner as well-established engine power ratings, thus benefitting consumers who are considering making the switch to an electrified vehicle,” Duoba said, adding that he believes having a standardized testing procedure also may help automakers and other driveline developers validate previously derived physical-testing or simulation values for individual drivetrain components.
“The flexible methodology of J2908 includes ways to incorporate known bench test data or make direct system measurements during a vehicle test. The flexibility was designed to accommodate testing by manufacturers or independent testing laboratories,” Duoba said.
“It’s a process and a definition that we can all use,” Duoba said in an interview with SAE Media to discuss the new standard. “Every car company has been giving their (power rating) numbers in good faith, but maybe with a different approach. It was obvious that this is not sustainable the way things were headed.” He said the power-split designs of contemporary hybrids can be particularly complex, as can the distribution of blended drivetrain output to individual axles. “So we came about this with a very technology-agnostic approach,” Duoba explained.
"Another aspect that was important was that this was compatible other international standards, he added. “I would say that SAE J2908 may be a little bit more specific in some areas but it is definitely compatible with the others and you should arrive at similar results [peak power value].”
Duoba said the J2908 sets test specifics such as environmental conditions, battery state of charge and vehicle warmup and preconditioning. Vehicles with temporary, extra-power “boost” functions can use those setting to derive the peak-power rating, but he said the boost feature must be readily accessible to the driver.
Unlike with the widely adopted J1349 standard to derive “SAE Certified” power for an internal-combustion engine by testing on an engine dynamometer, Duoba stressed that because of the complexities of hybrid and all-electric drivelines, the new J2908 procedure relies on a wheel-power test (WPT) that measures power at the driven axle(s). Rather than using an engine dynamometer, “You can use a chassis dynamometer or a wheel dynamometer. OEMs have both,” Duoba said. “It's up to everybody to figure out the best way to get the most accurate numbers in their application.”
Duoba would like to see automakers begin using the J2908 procedure, although he concedes adoption “might take some time.” He noted that Volkswagen may be the first automaker to deploy the standard. On the consumer web page listing power for the 2023 ID.4 electric vehicle, Volkswagen’s power-rating disclaimer states, “Maximum hp calculated via peak performance of the electric motor(s) at peak battery power pursuant to SAE J2908.” Duoba stressed that feedback from industry will be key to continue refinement and improvements.
The J2908 procedure has been under Duoba’s study since early 2013. OEMs have the capacity to generate comparatively accurate predictions of peak system power, Duoba conceded, “But I’m like, ‘No – I just can’t do it. We have to base it on a physical test.'”
The J2908 standard document can be accessed here.Continue reading »