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5 Things you Need to Know About Honda's Integrated Motor Assist Hybrid
By Bill Siuru
Honda Was the First to Bring a Hybrid to the U.S.
Honda, a recognized leader in offering environmentally-positive technologies, was the first to sell a hybrid electric vehicle in the U.S. when it introduced the 2000 model, two-passenger Honda Insight in late 1999. This was the first introduction of Honda's Integrated Motor Assist – or IMA – hybrid system. The Insight was followed by other Honda hybrids including the Civic Hybrid and the Accord Hybrid sedan that also used variations of the IMA system. Today, an improved version of IMA is used in the 2008 Honda Civic Hybrid. Both the Insight and hybrid version of the Accord are no longer sold, although Honda will be adding a smaller hybrid to its line-up shortly.
Integrated Motor Assist is a Full Hybrid System
Being a parallel hybrid, Honda's IMA provides power with both an electric motor and a gasoline engine. The hybrid is normally powered by the engine alone. When more power is needed, for example for passing or hill climbing, additional power is supplied by the electric motor. IMA is considered a "full" hybrid system rather than a "mild" one. The latter typically features only modest fuel economy improvement via regenerative braking and engine start-stop operation, which saves fuel by shutting an engine off at a stop or while coasting. Unlike other full hybrids, however, the IMA system cannot power Honda's hybrids in pure electric, or battery only, mode. This has led some to incorrectly refer to Honda's IMA as a mild hybrid.
Heart and Brains
A key component of the IMA system is an ultra thin, pancake-type electric motor/generator sandwiched between the engine and transmission. The brains of the IMA system is the Power Control Unit (PCU), which precisely controls the motor assist, regenerative braking, and battery charging functions for both the car's nickel-metal-hydride (NiMH) battery pack and the conventional 12-volt lead-acid battery used for lighting and power accessories. Like all hybrids, a regenerative braking system is used in which the motor/generator functions as a generator to keep the batteries charged. Regenerative braking recoups energy when a car is decelerating and braking, an elegant use of potential energy that would otherwise be wasted or simply dissipated as heat.
How it Saves Fuel
The IMA system saves fuel through a combination of techniques. A big fuel saver is the stop-start feature that shuts down the gasoline engine when stopped at, for example, traffic lights and McDonald's drive through window. Step on the accelerator, put the transmission in gear, and release the brake pedal … and the motor/generator functions as a high-speed starter to instantly and seamlessly restart the engine. The Civic and Insight hybrids also use smaller and less thirsty engines. A 1.0-liter, three-cylinder engine is used in the Insight while a 1.3-liter four-cylinder engine is used in the early Civic Hybrids. Even though these engines produce less horsepower, performance is not compromised since more power can be supplied by the electric motor. Oddly, the strategy of using a smaller and more efficient engine was not followed with the Honda Accord Hybrid, which debuted with a V-6 IMA system and only achieved a few mpg increase over a much cheaper, standard four-cylinder Accord. Taking the high-performance hybrid approach likely led to the demise of this hybrid model.
Long Life Batteries
For a completely new technology, IMA has been relatively trouble-free since its introduction, even the NiMH batteries. Designed to last 10 years under normal driving conditions, there are still a few years left until batteries may need to be replaced in the earliest Insight hybrid models. While NiMH batteries are not cheap, these batteries are now manufactured in mass production quantities because of the many hybrid models using them and costs continue to come down. It is expected that by the time NiMH batteries do need to be replaced in hybrid models, the cost will have diminished substantially to make this an affordable process.
Bill Siuru is a retired USAF colonel who has been writing about automotive technology for nearly 40 years. He has a bachelor's degree in automotive engineering, a PhD in mechanical engineering and has taught engineering at West Point and the U.S. Air Force Academy.
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