2005 Honda Accord Hybrid Sedan Powertrain

9/17/2004 2:09:14 PM


Hybrid powertrains were developed originally to enhance fuel efficiency, and the IMA system in the Accord Hybrid is no different in that regard, helping the V-6 sedan achieve an estimated 30 mpg city / 37 hwy. What sets this iteration of Honda's Integrated Motor Assist apart from previous systems is the fact that along with the increase in fuel efficiency comes a marked gain in horsepower, torque and performance - making the Accord Hybrid Sedan the industry's first high performance hybrid car, delivering the power of a high-performance V-6 engine with the fuel efficiency of a 4-cylinder car.

Along with excellent fuel efficiency, the combination of this Accord's 3.0L i-VTEC V-6 engine with Variable Cylinder Management and Honda's 3rd generation IMA system produces 255 horsepower (+6.3% over an Accord EX V-6) and 232 lb.-ft. of torque (+9.4%). Internal testing shows the IMA's motor assist helps the Accord Hybrid improve its 0-60 mph time to 7.5 seconds versus 8.0 seconds for Accord EX V-6 Sedan, with the powertrain producing torque in a constant, flat manner - 90 percent of peak torque (232 lb.-ft) available below 4000 rpm.

Accord Hybrid Powertrain Highlights

  • Peak power: 255 hp @ 6000 rpm, 232 lb.-ft. @ 5000

3.0L V-6 engine

  • Lightweight, durable aluminum alloy block
  • Single Overhead Cam (SOHC) head with i-VTEC
  • Magnesium head cover
  • Variable Cylinder Management (VCM)
  • 4-valves per cylinder
  • Programmed Fuel-Injection (PGM-FI)
  • 6500 rpm redline
  • Active Control Engine Mount System (ACM)
  • Idle-stop feature

Integrated Motor Assist (IMA) Electric Motor

  • 3rd-generation of Honda's patented gasoline-electric hybrid system
  • Thin (68mm), brushless DC motor (vs. 65 mm in Civic Hybrid)
  • Peak assist: 16.1 hp / 100 lb.-ft of torque (vs. 13hp / 46 lb.-ft. in Civic Hybrid)
  • Peak generation: 14 kW (vs. 12.6 kW in Civic Hybrid)
  • Maximum electrical current: 120 Volts (vs. 75V in Civic Hybrid)
  • Intelligent Power Unit - Power Control Unit, Nickel-Metal Hydrid (Ni-MH) battery pack and Integrated cooling system
  • Regenerative braking
  • Improved 5-Speed Automatic Transmission

3.0-Liter SOHC i-VTEC V-6 engine

At the heart of the Accord's power plant is the same 3.0L V-6 engine as that in the Accord EX V-6, but with a few modifications to reduce space and weight and to allow for the installation of the IMA motor and new VCM system. This 60o V-6 engine is designed to require only minimal maintenance, including oil changes every 10,000 miles and its first scheduled tune-up at 105,000 miles.

Aluminum alloy block

Like all Honda engines, the 3.0L i-VTEC engine used in the Accord Hybrid is built around a lightweight and durable aluminum alloy block that provides significant weight and long-term corrosion benefits over traditional iron blocks. The 60¹ V-6 block incorporates extra brackets to allow the mounting of the VCM's magnesium composite head cover and the IMA motor.

To further enhance durability, each cylinder contains cast-iron liners, while the crankshaft and each of the two camshafts are made of forged steel designed to minimize friction.

i-VTEC Engine with Variable Cylinder Management

One major improvement new to this engine is the implementation of the advanced version of Honda's variable valve-timing and lift electronic control (VTEC) system or i-VTEC. i-VTEC adds a level of "intelligence" to the legendary VTEC system by adding a Variable Cylinder Management (VCM) system that switches between six- and three-cylinder combustion. When high output is required, such as during startup and acceleration or when climbing hills, the engine operates on all six cylinders to deliver superior performance. During cruising and at low engine loads, the system idles the rear bank of three cylinders for improved fuel efficiency and reduced emissions. Further, the idled cylinders remain sealed, alleviating the pumping losses associated with intake and exhaust and giving fuel economy a further boost.

Variable Cylinder Management helps maximize fuel efficiency by making the Accord Hybrid's 3.0L V-6 engine effectively run on only three cylinders when the power needs are minimized, yet it can still seamlessly provide the full power of all six cylinders during hard acceleration or other high load situations. When full power is needed, i-VTEC instantly opens the valves and sends fuel to the three previously idled cylinders. When combined with the IMA motor, output peaks at 255 horsepower.

The Accord Hybrid's VCM system is similar to the VTEC-controlled Cylinder Idling System used on Civic Hybrid, but is more sophisticated. Where the Civic Hybrid's cylinder idling deactivates three of the engine's four cylinders during deceleration only to help improve regenerative braking efficiency, the Accord's VCM system, is able to deactivate the engine's rear bank of three cylinders during a variety of driving situations, including steady-speed cruising, light acceleration and braking. Thus, improving fuel efficiency, reducing emissions and making regenerative braking more efficient.

VCM Operation

The Accord Hybrid's i-VTEC V-6 engine features a Variable Cylinder Management (VCM) system that improves fuel economy by idling three of the Odyssey's six cylinders during cruising and deceleration. VCM deactivates the rear bank of cylinders by using the VTEC (Variable Valve-Timing and Lift Electronic Control) system to close the intake and exhaust valves while cutting fuel at the same time.

The rear bank of cylinders was chosen for deactivation because the front bank is best positioned for cooling performance. Idling the rear bank also helps to maintain the temperature of the catalytic converter beneath the cylinder heads, helping to maintain optimum emissions performance. The Accord Hybrid's spark plugs continue to fire even when the cylinders are deactivated. This minimizes plug temperature loss and prevents fouling induced from incomplete combustion during cylinder re-activation. Further, sparking in air alone results in higher voltages than in a fuel-air mixture because of the higher insulation resistance. Therefore, iridium-tipped spark plugs are used for their high resistance to wear at high voltages.

i-VTEC VCM System Designed for Rapid Switching at Low Engine Speeds

In a conventional VTEC system, hydraulic pressure pushes a single synchronizing piston, which is in turn pressed back by a return spring. The VTEC system locks the valvetrain's rocker arms to allow additional valves to open and increase engine breathing. Such systems require relatively high engine speeds since hydraulic pressure is used to push the spring. VCM, however, demands switching at low rpm. This is achieved by creating a hydraulic circuit with two systems, each capable of providing the hydraulic pressure required to push the synchronizing piston in the required direction. This design thereby allows switching from six cylinders to three cylinders in low-speed ranges. To achieve responsive switching to six cylinders when the driver quickly opens the throttle under hard acceleration or driving up hill, the system employs a new three-way solenoid spool valve to control the oil pressure for switching. These enhancements result in highly responsive VCM switching regardless of engine speed.

VCM monitors throttle position, vehicle speed, engine speed, automatic-transmission gear selection, and other factors to determine if the vehicle is cruising or decelerating. During cruising and deceleration, VCM deactivates the three cylinders on the rear bank of the engine. To further enhance control, the system also simultaneously determines whether engine oil temperature is suitable for VCM switching and whether catalytic converter temperature will remain in the proper range. While idling the cylinders, the system controls ignition timing, and turns the torque converter lock-up on and off, thereby suppressing torque-induced jolting during the switch from six to three cylinder operation. The result is a smooth, seamless switch between three and six cylinder modes that is nearly unnoticeable to the driver.

For cylinder deactivation operation, the synchronizing piston moves to isolate the valve-lift rocker arms from the deactivated cylinders' rocker arms. The deactivated cylinder rocker arms are operated by zero-lift cams that deactivate the cylinders by closing the intake and exhaust valves.

Active Control Engine Mount System (ACM)

Due to the inherent increase in engine vibration when VCM has deactivated the rear bank of cylinders, an Active Control Engine Mount system (ACM) is used to minimize the effects of engine vibration. The system uses sensors that alert the Electronic Control Unit (ECU) to direct ACM actuators to move in synch with the vibration, thereby eliminating the vibration being transmitted to the passenger compartment. Additionally, an Active Noise Control system (ANC) works in cooperation with the ACM. An ANC controller with front and rear microphones in the passenger cabin detects the booming noise associated with cylinder deactivation and instructs the audio system's speakers to emit an opposite sound wave. (See the Interior section for more information on ANC.)

Idle stop

As with previous versions of Honda's hybrid powered cars, the IMA system incorporates an idle-stop feature that temporarily turns off the gasoline engine when the vehicle comes to a stop from speeds over 10 mph. In this situation, the IMA's ECU monitors the closed throttle, speed and brake pressure to determine the driver is bringing the car to a stop. The engine is turned off when the car's speed drops below 10 mph (16 km/hr). When the brake pedal is released, the IMA's motor restarts the engine and power is available immediately.

Unlike the idle-stop feature in the Insight and Civic Hybrid, the Accord Hybrid system continues to operate even while the car's automatic climate control system is in use. This is made possible by the introduction of a new hybrid air conditioning system, which uses air compressors powered by both the gasoline engine's drive belt and IMA electric motor (see Interior). Idle-stop is unavailable only when the car is first started on an extremely hot day when maximum cooling is required.

IMA System

The 3rd-generation IMA system is the largest and most efficient to come out of Honda's hybrid development program. As with previous versions, the IMA system consists of an ultra-thin DC brushless electric motor mounted between the gasoline engine and the automatic transmission, and an intelligent power unit (IPU) that stores electric power in a compact battery box and controls the flow of electricity to and from the electric motor. Also new with this generation of the IMA is the mounting of a 15cc air compressor to the electric motor which provides increased cooling capacity to the Accord Hybrid's dual-zone automatic climate control system (see Interior).

IMA Electric Motor

Providing supplemental power boost to the V-6 engine, the IMA's electric motor is designed to provide up to 12 kW (16 hp) and 136 Newton-Meters (100 lb.-ft) of additional power and torque to the Accord Hybrid's engine. Mounted between the engine and the Accord Hybrid's 5-speed automatic transmission, the IMA motor is an ultra-thin (68 mm) DC brushless design and provides a substantial amount of low-end torque to aid in hard acceleration and hill climbing.

In addition to providing supplemental motive power, the IMA motor acts as a generator during deceleration and braking to recapture kinetic energy and recharge the IMA's battery pack during regenerative braking, and provides power to the dual scroll hybrid A/C compressor used in the Accord's dual zone hybrid automatic air conditioning system (see Interior).

For this third generation of IMA motors, a new internal permanent magnet was designed to increase output density and make the motor more efficient than previous motors. At startup, the Accord hybrid provides 26 percent more starting torque than the Civic Hybrid. During driving, the motor provides a 20 percent increase in horsepower assist and more than doubles peak torque (100 lb.-ft. from 46 lb.-ft) compared with the Civic Hybrid's IMA motor; and it is more efficient, now converting 97.5 percent (versus 94.6 percent efficiency of Civic Hybrid IMA) of the available electricity into motive energy in assist mode.

IMA Intelligent Power Unit (IPU)

Power for the IMA system is controlled through the Accord Hybrid's Intelligent Power Unit. Located directly behind the rear seatback, the IPU consists of the Power Control Unit (PCU) - or the IMA's brain, a rechargeable Nickel Metal-Hydride battery module, and an integrated cooling unit.

  • Power Control Unit (PCU) - Electronically controls the flow of energy to and from the IMA's electric motor. Using the latest in computer chip technology, the PCU's response time is quickened over previous versions, and a new inverter and DC/DC Converter help contribute to the IMA's overall increase in peak power.
  • Battery Pack - Energy for the IMA motor is stored in a bank of Nickel Metal-Hydride cells. This bank of 120 1.2V units stores up to 144 Volts of electrical energy for the IMA motor, as in previous versions. But a new SANYO dual module casing reduces weight from previous hybrid battery packs and also allows it to increase efficiency of the electrical flow.
  • Integrated Cooling Unit - To offset the heat generated by the constant flow of electricity to and from the battery pack, an integrated cooling system is mounted directly on the battery pack's outer box. Interior cabin air is continually flowed over the battery pack, and recirculated via a small vent placed on the rear parcel shelf.

Regenerative braking

A key to the long-term viability of hybrid-powered vehicles is their ability to recapture kinetic energy via regenerative braking and store this energy as electricity in rechargeable battery packs. Accord Hybrid is no different, as its IMA electric motor also acts as a generator to recharge its battery pack during steady cruising, deceleration/coasting and braking.

Accord Hybrid's efficiency in generating electricity from braking energy is enhanced by a more efficient IMA motor/generator, the ability of the engine and transmission to minimize friction through the VCM system and a modified 5-speed automatic transmission. Exploiting these friction-saving features during electricity generation, the IMA motor is able to convert 95.2 percent of the available kinetic energy into electricity to store in the IMA's rechargeable battery pack. This is an improvement of almost two percentage points over the previous Civic Hybrid's generator efficiency (93.5%).
This greater efficiency is achieved through more efficient cooperation between the transmission and the IMA motor, and a more sophisticated the idle stop feature that cuts fuel to the engine and reduces excess friction losses due to engine idling.

5-Speed Automatic Transmission

To increase performance through quicker shifts and a wider range of gear ratios and to aid the IMA system's ablity to recapture kinetic energy through regenerative braking, Accord Hybrid employs a new 5-speed automatic transmission.

Sixty millimeters thinner than the version of 5-speed transmission used on the current Accord EX V-6, this transmission contributes to improved fuel efficiency through a new lockup clutch torque converter that shortens shift response during acceleration, more immediate startup after idle-stops by using of an integrated electric oil pump, and smoother ride during hard acceleration. While a new long torsion damper works with the VCM system to increase the rpm range during which VCM is activated to further contribute to improved fuel economy.

5-Speed Automatic Transmission, Gear Ratios
. Accord
EX V-6
First 2.563 2.685
2nd 1.552 1.565
3rd 1.021 1.023
4th 0.727 0.729
5th 0.520 0.530
Reverse 1.846 1.888
Final Drive Ratio 4.428 4.375

Integrated Electric Oil Pump

In order to generate the hydraulic pressure necessary to shift gears through its torque converter, most automatic transmissions experience a lag between shifts while the engine spins the drive shaft and the counter-spinning torque converter spins to create oil pressure. The Accord Hybrid eliminates this lag, which subsequently improves fuel efficiency, by using an integrated electric oil pump to constantly pump oil through the transmission's torque converter. This is particularly important for the Accord Hybrid when restarting after an idle stop when the gasoline engine is providing no power to the drive shaft.

High-response lockup clutch torque converter

In addition to the electric oil pump which maintains consistent pressure within the transmission, the torque converter itself employs an advanced, compact design that improves shift response time to one-third of that in the Accord EX V-6's automatic transmission. Regardless of gear, or vehicle speed, shift engagement takes place in less than half a second, which results in smoother acceleration and improved fuel economy.

In combination with the integrated oil pump, this new high-response lockup clutch torque converter helps improve city fuel economy by more than 1.5 mpg

Fuel Efficiency Comparison*
. Civic EX
(w/ AT)
Accord EX
(L4 w/ AT)
EX V-6
City 31 24 30 21
Highway 38 34 37 30
*Based on 2004 EPA mileage estimates

Internal testing shows Accord Hybrid achieving an estimated fuel economy rating of 30 miles per gallon in the city and 37 mpg highway, up from an EPA-rated 21 city / 30 highway for the 2004 Accord EX V-6 Sedan - a fuel economy gain of 43 percent and 23 percent, respectively. The Accord's Integrated Motor Assist System (60%) accounts for over half the gains in city fuel efficiency, while Variable Cylinder Management (15%) and idle stop (25%) is responsible for the rest. During highway cruising, the VCM system (57%) provides the majority of the increase, while IMA (38%) and a lower drag coefficient and greater aerodynamics (5%) also contribute.