A new SAE document aims to establish an industry standard for determining the horsepower ratings of EVs, hybrids, plug-in hybrids, and hydrogen fuel cell vehicles. The latest revision of SAE J2908, released in January 2023, details methods for measuring and calculating an electrified vehicle’s “rated system power,” which is comparable to a gas or diesel engine’s net horsepower.
Up until now, the lack of published guidelines has left automakers to chart their own courses. Compared to internal combustion engines, electrified powertrains present two key challenges when it comes to measuring power. The first stems from the fact that hybrids and many EVs combine power from two or more propulsion units. It’s easy to measure the power output of an electric motor or gas engine in isolation, but how these devices interact when installed in a vehicle affects the total system output.
This is particularly relevant for hybrids, since the engine and motor(s) rarely produce peak power at the same time. Because of this, the total horsepower of a hybrid is almost always less than the sum of the power made by the engine and each motor independently. Exceptions like the 2024 Chevrolet Corvette E-Ray, which makes a claimed 655 hp from its 160-hp motor and 495-hp V-8, always draw our scrutiny. With this new document, we now have a simple tool for questioning manufacturers: “Is that power output determined using the SAE J2908 methodology?” We’ve reached out to Chevrolet asking that very question, but the company hasn’t yet responded to us as this was published.
The second complication in measuring the power of electrified vehicles arises from the fact that the battery affects how much power an electric motor makes. The maximum electrical power available from a battery drops in extreme cold and as the battery charge falls. SAE J2908 defines acceptable ambient conditions for testing and processes for selecting a battery state of charge, warming up the vehicle, and conditioning the battery.
How Power Is Determined With SAE J2908
The J2908 procedure starts with a wheel power test on a chassis or wheel hub dynamometer to find the road speed where the vehicle produces maximum power. The power output at the wheels, though, is affected by the friction in the drivetrain, which means it can’t be directly compared to the SAE net power ratings of gas and diesel engines.
To arrive at an apples-to-apples number, the J2908 guidelines outline the process for determining the power of each propulsion device at the output shaft under the same conditions as when the vehicle makes maximum wheel power. (This is a critical detail.) There are actually four different techniques manufacturers can use for this part, although they only need to choose one. One method directly measures the output shaft power of each propulsion device during the chassis dyno test. Another calls for bench-testing individual components while simulating the conditions of the wheel power test. A third technique determines the efficiency of the drivetrain components through bench testing and works backward to calculate output shaft power.
The fourth method has the potential to be less scientifically sound. It allows automakers to use development data, models, or real-time CAN (controller area network) data from the vehicle to calculate the contributions of the individual powertrain components. As the standards document notes, “The accuracy of this method is the most difficult to verify by a third party because, unlike the other three methods, it relies upon additional data or information outside of what is gathered in this test procedure.” If a manufacturer is using this method honestly and accurately, though, the results should match a spot check performed using one of the other approaches.
Ultimately, the rated system power, or total combined horsepower, is calculated by adding together the output shaft power of the individual components.
Will Automakers Follow SAE J2908?
J2908 is currently classified by SAE as an “information report” intended to provide educational information for the technical community, but if it gains broad acceptance in the industry, it may eventually graduate to an SAE recommended practice or an SAE standard. Those elevated labels would give it even more clout within the industry, but using it would still be entirely voluntary.
So far, Volkswagen is the only automaker to publicly acknowledge that it’s using the procedures outlined in J2908. On web pages for the 2023 Volkswagen ID4, the company adds a footnote to horsepower figures stating, “Maximum HP calculated via peak performance of the electric motor(s) at peak battery power pursuant to SAE J2908.”
Michael Duoba, chair of the SAE J2908 task force and a research engineer at the Argonne National Laboratory, said several more automakers are likely already following the procedure without advertising it. “I think people have been aligned to the methodology for a few years,” he told MotorTrend.
That comes as no surprise. The SAE task forces that write these documents are made up of industry experts, so the standards often reflect common practices already in use at various automakers. Now that there’s an official SAE guideline on the policy, the real value comes from automakers using the procedures and explicitly saying so in order to pressure holdouts into following suit. Because in the end, it’s not a standard until the majority of the automakers are in agreement.
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