The survey also examined patent activity for other automotive technologies such as navigation systems, transmissions, seats, brakes and entertainment systems.
Overall, Toyota was the top performer in 2015, with 4,214 patents. Hyundai was a distant second with 2,469, while Bosch, Denso and Honda rounded out the top five.
Those same companies also topped last year's list, says Reuters analyst Bob Stembridge.
"There are some fast-moving technologies like mobile phones, where you see rapid shifts" among technology leaders, Stembridge says. "But the auto industry has a slightly longer cycle [of innovation]. There's more stability."
Still, the 2015 list of overall patent leaders reveals a couple of surprises. Chinese truckmaker Beiqi Foton made the Top 10 list, at No. 9. But Volkswagen did not -- despite outspending all rivals on r&d.
According to a Pricewaterhouse study released in February, VW spent $15.3 billion on r&d last year, while Toyota spent $9.2 billion. Stembridge said he could not explain the apparent discrepancy.
While the list of top companies has been stable over time, there has been some fluctuation among which technologies have the most activity.
"Green" powertrains have been generating more patents than any other category, and it has spurred activity among automakers and suppliers. Over the past five years, top innovators in this segment include Bosch, Denso, Samsung, LG Chem and Panasonic.
The green powertrain field includes traditional EV technologies such as lithium ion batteries, inverters and regenerative brakes. It also includes innovations such as stop-start and 48-volt systems, part of the industry's ongoing effort to "electrify" conventional vehicles.
That is where the new action is in innovations, according to Stembridge, and it will likely continue in that area for the next five years.