The newly released Honda Accord Hybrid scores a noteworthy 50 mpg around town according to the EPA. That’s nearly the fuel efficiency of the popular Toyota Prius hybrid—but in a larger car with sedan styling. However, what really stands out is the technology that lets this car achieve such a high fuel economy mark.
It uses a series/parallel hybrid powertrain system very similar to the plug-in hybrid version of the Accord. While this new sedan can’t be refueled from an electric outlet, it uses a battery, generator, and electric motor to greatly reduce the amount of gasoline needed. This is a significant change in Honda’s approach to hybrids, which until now had relied on “mild hybrids” that had limited electric drive capability. These previous Honda hybrids had failed to match the fuel economy performance of Toyota’s popular hybrid synergy drive system. It also shows that some of the technical advances in plug-in vehicles can improve the efficiency of gasoline-powered cars.
Honda’s new 2-motor series/parallel hybrid powertrain (credit: American Honda Motor Co., Inc.)
A 1-speed transmission?
One of the key advances of the Accord Hybrid is the elimination of much of the transmission. Every conventional car on the road needs a transmission to match the output from a fast-turning engine to wheels that are often turning slower.
The type of transmission used can have an important impact on the vehicle’s overall fuel economy. Increasing the number of gears or using a continuously variable transmission (CVT) allows the engine to run more efficiently. However, adding complexity to the transmission can increase vehicle weight or friction losses, offsetting some of the engine efficiency derived fuel savings.
The new Accord gets some of its fuel savings by tossing out most of the transmission that every other gasoline car has. Instead, the gasoline engine is only connected to wheels at highway speeds and even then there is a single, fixed gearing. At city speeds, the gasoline engine is disconnected from the wheels via a clutch and instead only spins an electric generator. At speeds of less than about 40 mph, the car is completely reliant on electric motors for acceleration. Essentially, the Accord runs as an electric car with a gasoline generator at city speeds.
This system lets the Accord rely solely on a powerful electric motor at lower speeds and only directly engaging the gasoline engine when it is most efficient: cruising at highway speeds. The Toyota Prius is a “transmission-less” series/parallel hybrid as well, but uses a power split system that keeps the motor and engine always linked.
Getting the most out of a drop of gasoline
Electric motors have the advantage of not consuming power when idling or coasting. Other hybrids and some conventional cars can shut the gasoline engine off when not needed. However, it’s impossible to perfectly time the shutdown of the engine—so a series hybrid wastes less fuel in electric drive mode by only running the motor when needed for acceleration. In addition, Honda is using its latest high efficiency gasoline engine that achieves about 10% fuel economy improvement through various techniques like improved exhaust gas recirculation and variable valve timing systems.
Half the Oil Progress
More efficient gasoline-powered vehicles, as well as ones that use no gasoline at all like plug-in electric and fuel cell vehicles, are key solutions for reducing our projected oil use in half in the next 20 years. Another 50-mpg car that is available in a top-selling model (the Accord is #2 in midsize cars) is welcome news, as are the initial reviews (examples:1,2,3) confirming the impressive EPA label rating. Honda’s new generation of its hybrid drivetrain demonstrates there’s continued room for improvement to hybrid technology to deliver even better fuel economy to consumers. It also means Toyota, the hybrid technology leader for more than a decade, will have to continue to innovate to stay in the lead. It will be exciting to see these electric vehicle-inspired technologies advance and spread to other makes and models.