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The New Honda Accord Hybrid: Using Electric Vehicle Technology to Catch Up to the Prius

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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.

Picture of the new Honda engine and 2-motor powertrain

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.

Posted in: Vehicles Tags: , , ,

About the author: David Reichmuth is a senior engineer in the Clean Vehicles Program, focusing on oil savings and vehicle electrification. See Dave's full bio.

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  • http://www.generator-hph.com/ Generator HPH

    Great job Honda!

  • Venus

    Good stuff.

    To be clear, there is nothing “plug in” about this car, correct?

    I only ask because the last sentence of the fifth paragraph reads … “the Accord runs as a plug-in electric car with a gasoline generator at city speeds.”

    That would seem to confuse things, as really the car simply “runs as an electric car with a gasoline generator at city speeds.”

    • http://www.ucsusa.org/about/staff/staff/David-Reichmuth.html David Reichmuth

      Thanks! You are correct that this car is gasoline only, though it shares much of its powertrain with the PHEV Accord. I’ve changed the text as you helpfully suggested to clarify that this car is not a plug-in vehicle.

  • Carcus

    Lots of words, no answers.

    I’ll ask again. How is Honda utilizing variable input power and still retaining generator output efficiency? Where is the “14% to a better motor/generator ” coming from, exactly?

    • http://www.ucsusa.org/about/staff/staff/David-Reichmuth.html David Reichmuth

      The 14% for motor/generator improvements comes from Honda’s R&D site in the paper “Development of SPORT HYBRID i-MMD Control System for 2014 Model Year Accord”. The paper gives details on how the Accord uses intermittent engine operation and battery storage to let the engine run at maximum efficiency. The specifics on motor/generator improvements can be found in “Development of Motor and PCU for a SPORT HYBRID i-MMD System.” The series operation mode allows less variability in engine output (assuming sufficient battery charge) which should enable both the engine and generator to operate under conditions that maximize the efficiency.

      • Carcus

        Thank you

  • Carcus

    Series hybrids have tried and come up short in economy before. The problem? Generator efficiency losses. Most generators are only efficient in a narrow “sweet spot”.

    Honda has somehow solved the problem (and their explanations of immd seem to be intentionally vague.

    Any scientists here know the real story?

    • http://www.ucsusa.org/about/staff/staff/David-Reichmuth.html David Reichmuth

      Thanks for your input. As you point out, the potential drawback of a series hybrid is that instead of burning gasoline to spin a drive shaft and wheels, the series hybrid uses gasoline to spin a generator, makes electricity, and then uses that electricity via a motor to spin the wheels. So there are more steps where you could potentially have efficiency losses in the series hybrid.
      However, the series hybrid gains significant efficiencies that more than make up for the loss in electricity generation and storage. First, separating the gasoline engine from the wheels means that the engine can often run at its most efficient speed and torque. Gasoline engines are less efficient at low power, and the series hybrid lets the engine spend more time at peak fuel economy. Second, the series hybrid eliminates much of the transmission and the associated efficiency losses. Third, the series hybrid can use more efficient Atkinson cycle engine with lower peak power and lower torque, since the high torque electric motor is actually propelling the car at low speeds. For example, the 4 cylinder engine in standard Accord peaks at 185 hp and 181 lb-ft of torque, while the Accord Hybrid I-4 engine only generates 141 hp and 122 lb-ft of torque. However, the 0-60 mph times for the hybrid are actually quicker than the non-hybrid 4 cylinder model. Finally, the series/parallel arrangement of the Honda Accord Hybrid means that the gasoline engine does directly drive the wheels at the high speeds where the gasoline engine is more efficient than the generator/motor combination of the series hybrid.
      All told, Honda’s technical documents attribute 55% of the efficiency improvements to the hybridization of the car, 14% to a more efficient engine, 14% to a better motor/generator, and the remaining 17% to new regenerative brakes and aerodynamics.

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