Under the Hood of the Accord Hybrid
17 September 2004
Honda’s new Accord Hybrid, due to be in dealer showrooms 3 Dec 2004, is a mild hybrid. (See earlier post: A Short Field Guide to Hybrids.) The electric motor in the Integrated Motor Assist (IMA) powertrain supports the gasoline engine, but it is never the sole source of motive power as the motor in the Toyota Prius can be.
The result, while increasing fuel economy, delivers gains in horsepower and torque over a standard Accord EX V-6 sedan. The result is the highest-performance hybrid on the market to date—and an acceleration of the inevitable blurring of the popular interpretation of “hybrid”.
Let’s take a look at some specs. The table below compares the new Accord Hybrid to the 2005 Prius and 2005 Civic hybrid.
|2005 Accord Hybrid Sedan||2005 Toyota Prius||2005 Honda Civic Hybrid|
|Type||V-6||Inline 4||Inline 4|
|Acceleration 0-60 (seconds)||7.5||10.4||11.6*|
|Power Output (hp)||16||13.4|
|Type of Battery||Ni-MH||Ni-MH||Ni-MH|
|Output||144V (120 cells @ 1.2V)||201.6V (168 cells @ 1.2V)||144V (120 cells @ 1.2V)|
|EPA Mileage Estimates|
*Results from testing on 2003 Civic Hybrid. One independent review produced a 9.8 sec 0-60 figure for the 2004 Civic Hybrid, but that is an outlier.
Note that the engine in the Accord is literally twice the size of the engine in the Prius, and packs more than 3 times the horsepower. When the IMA electric motor kicks in, the Accord hybrid offers a combined 255 hp, and it accelerates more rapidly than the standard Accord.
The electrical side of the hybrid is smaller and less capable than that of the Prius—and that makes perfect sense given the different roles the two electric motors have.
The gasoline engine in the Accord hybrid is basically the same as that in the Accord EX V-6, except for a few modifications to reduce space and weight and to allow for the installation of the IMA motor and new Variable Cylinder Management (VCM) system.
VCM, Honda’s version of cylinder deactivation, switches the engine 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 enhanced 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
The Accord hybrid is the first to feature such cylinder deactivation on the gasoline engine. The 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 deactivates the engine’s rear bank of three cylinders during a variety of driving situations.
Other features in the hybrid include:
Idle Stop and Start. 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. 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 Hybrids, 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 the electric motor. Idle-stop is unavailable only when the car is first started on an extremely hot day when maximum cooling is required.
Regenerative 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%).
Intelligent Power Unit (IPU). Located directly behind the rear seatback, the IPU controls the power for the IMA system. 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.
With all of that, the Accord hybrid delivers improved fuel efficiency over its non-hybrid cousins, but still falls far below that offered by the Prius—half of the city mpg, in fact. (See table above.)
By Honda’s calculations, the Integrated Motor Assist System accounts for 60% of the gains in city fuel efficiency, while Variable Cylinder Management (15%) and idle stop (25%) are responsible for the rest. During highway cruising, the VCM system provides the majority of the increase (57%), while IMA (38%) and a lower drag coefficient and greater aerodynamics (5%) also contribute.
As more mild hybrids appear on the market, or as larger full hybrids emerge, the size of the aggregate fuel economy benefit due to hybrid technology will begin to decrease. In other words, unless everyone drives a Prius, we’re still looking at fuel economy ranges that, while much better than that provided by pure gasoline platforms, are very similar to those offered by non-hybrid diesels.
There is no magic technology right now that will let drivers roar around in big, powerful cars and trucks while only sipping fuel. At this point, radical adjustment in consumption has to come from consumer behavior and buying patterns. The incremental benefits of hybrid technologies are important, and should be implemented. It’s just not enough on its own.
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