Inside Honda's new two-motor 50-mpg Accord Hybrid
The all-new 2014 Accord Hybrid employs Honda’s new i-MMD (intelligent Multi-Mode Drive) two-motor hybrid system for midsize vehicles that debuted earlier this year on the Accord Plug-in Hybrid sedan.
The Accord Hybrid has EPA fuel-economy (FE) ratings of 50/45/47 mpg city/highway/combined, which Honda claims is a best-in-class city rating for a four-door sedan. The car also boasts a driving range of 673 mi (1083 km)
The key difference between the plug-in (PHEV) and hybrid versions—besides the presence of an onboard charging system and cord for the PHEV—is the size of the onboard battery. The Accord Plug-in Hybrid uses a 6.7-kW·h lithium-ion pack, whereas the new hybrid model employs a more compact 1.3-kW·h pack that is charged via the 2.0-L "Earth Dreams" Atkinson-cycle gasoline engine, regenerative braking, or a combination of the two.
Blue Energy, a subsidiary of GS Yuasa Corp., supplies its EH5 Li-ion battery for the Accord Hybrid and its EH19 model for the PHEV.
“The engine in the two-motor system is the same in the hybrid and the PHEV,” Hiroo Shimada, Chief Engineer, Assistant Large Project Leader, Powertrain Design, explained to SAE Magazines through an interpreter. “In the rear, the battery unit is different between the two. But there’s very high commonality in terms of the parts between the hybrid and the plug-in hybrid.”
Honda’s two-motor hybrid approach continuously cycles between three different modes: EV drive, hybrid drive, and engine drive. (Go to http://video.sae.org/11763/ to see a Honda video illustrating how the system operates.)
The new i-MMD system
Shimada has spent 15 years on hybrid powertrain engineering at Honda, including work as Project Leader on the IMA (Integrated Motor Assist) system for the first-generation (MY2000) Insight. He also is responsible for research that led to development of the i-MMD hybrid system.
Shimada said he couldn’t discuss how long it took to develop the two-motor hybrid system, but he did share some challenges encountered during its development.
“Honda originally had an IMA hybrid system, so in contrast to that, how did we want to make our new hybrid system? There’s lots of things that we did in each of the different systems. We looked at where efficiency was poor, looked at things that were larger and heavier, and that led us to this new i-MMD hybrid system,” he said.
One of the focus areas was the motor. “We tried different positioning of the magnets within the motor to optimize it,” Shimada said. “The magnets are kind of at an angle; normally they’re [positioned] concentrically. By angling the magnets the way we did, the reductor stroke creates a pulling force on the metal, and this motor uses that. So rather than the standard design, this [new design provides] about 1.8 times more torque. And it’s smaller.”
Output for the ac synchronous permanent-magnet propulsion motor is 124 kW, and electric-drive motor torque is 226 lb·ft (306 N·m). A separate generator motor is powered by the gasoline engine to generate electric energy to drive the propulsion motor when the vehicle is operating in hybrid drive mode. This motor can also restart the gasoline engine when the vehicle is in idle-stop mode.
The new motor design requires fewer magnets. “And one other thing,” Shimada continued. “Normally in a magnet you have rare-earth metals spread evenly throughout the whole thing; it’s very homogenous. This magnet that we’re using, the rare earths are only on the surface, not in the rest of it. So that also reduced the amount of [expensive] rare-earth metals used.”
Located behind the rear seat, the Accord Hybrid’s Integrated Power Unit (IPU) packages the 1.3-kW·h battery, which contains 72 Li-ion cells, and a dc-dc converter. Battery temperature is controlled by a fan system that pulls air from the vehicle’s interior via vents on the driver’s side of the rear seatback.
Taking the place of a conventional transmission is the electric continuously variable transmission (E-CVT), whose operation is coordinated by the IPU. When cruising at medium to high speeds, the Atkinson-cycle i-VTEC gasoline engine provides propulsion via the E-CVT with lock-up clutch, which connects the drive motor to the generator motor to transmit engine torque directly to the drive wheels. In EV drive mode, a clutch disengages the stopped gasoline engine from the drivetrain to eliminate efficiency loss from mechanical friction in the engine.
“The ‘secret sauce’ in all of this is that the entire process is accomplished without the use of a conventional transmission or even a torque converter—that is what the models out there right now are using,” said Art St. Cyr, Vice President of Product Planning and Logistics. “So there’s no pulleys and only one fixed gear. This is a unique aspect of our technology; no one else is doing hybrid propulsion quite like this.”
Honda claims that unlike a conventional CVT, its E-CVT is able to control both engine and electric motor rotation to deliver higher fuel efficiency and quicker engine response in each driving mode.
“The Accord Hybrid will not be the last application of this two-motor system in the Honda lineup,” promised St. Cyr.
Achieving 50 mpg
While the two-motor hybrid system was “a big factor” in achieving the car's 50-mpg highway/47-mpg combined fuel efficiency, several other factors contribute as well, including the new Atkinson cycle I4 that Shimada calls “the world’s most efficient gasoline engine.”
The Accord Hybrid is about 99 kg (218 lb) heavier than the conventional non-hybrid version, mainly due to the battery pack. “If we just limit talk to the engine,” Shimada added, “the regular gasoline vehicle has a 2.4-L engine—a bigger engine. So total weight between the two engines isn’t so different.”
The hybrid model features the same fundamental body and chassis architecture as the standard Accord sedan, but it (along with the Accord Plug-In) does include some exclusive features to aid fuel efficiency. These include low-rolling-resistance tires and an all-aluminum front subframe that replaces the steel-and-aluminum subframe used on the conventional Accord sedan and coupe, resulting in an 8.8-lb (4.0-kg) savings. An aluminum rear bumper beam helps offset a little of the additional weight of the rear-mounted battery pack.
The Accord Hybrid also features wider-diameter stabilizer bars—19 mm (0.75-in) front and 16 mm (0.63 in) rear—that help trim body roll during cornering and handling maneuvers.
Shimada singled out the electro servo-brake system as another key component in achieving greater fuel efficiency. The braking system is similar to a traditional system in that it’s fully hydraulic from the master cylinder to the 4-wheel disc brakes. According to Honda, the key difference is that the braking function is electronically controlled rather than a purely mechanical activation, allowing regenerative braking from the electric drive motor to slow the vehicle, rather than the hydraulic friction brakes under most circumstances.
Friction braking often is not needed until the vehicle speed drops below 5 mph (8 km/h).
Honda claims that its new electric servo-brake system yields an 8% improvement in regenerative braking effect compared to its IMA hybrid system, along with improved brake control. Regenerative braking begins when the driver releases the throttle pedal, with a “strong” regenerative braking effect beginning when the brake pedal is depressed and continuing until the vehicle speed drops below 1 mph (1.6 km/h), when the friction brakes fully engage.
Electric power-assisted rack-and-pinion steering (EPS) also contributes to the Accord Hybrid’s increased fuel efficiency, Shimada said. EPS eliminates the traditional hydraulic power steering pump and reduces steering effort.