Integrated Motor Assist (commonly abbreviated as IMA) is Honda's hybrid car technology, introduced in 1999 on the Insight. It uses both a gasoline engine and a thin, compact permanent magnet electric motor/generator mounted between the engine and transmission to act as a starter motor, engine balancer, and assist traction motor. The internal combustion engine (ICE) provides all the power needed for most driving situations. When additional power is needed, such as under initial acceleration from a stop, during passing or when climbing grades, the integrated electric motor/generator performs in ways similar to a supercharger, seamlessly kicking in to supply additional power. The motor/generator also functions as a high-speed starter and as a generator for battery charging during regenerative braking. A nickel-metal-hydride (NiMH) battery pack is used in all Honda hybrids; it features stable output characteristics regardless of the state-of-charge status and is also extremely durable, designed to last 10 years under normal driving conditions.
The theory behind IMA is to use Regenerative Braking to recapture some of the energy lost through deceleration, and reuse that energy later on to help accelerate the vehicle. This has two effects: it increases the rate of acceleration, and it reduces the work required of the gasoline engine. The acceleration boost is important, as it allows the engine to be scaled down to a smaller but more fuel-efficient variant without rendering the vehicle overly slow or weak. This smaller engine is the primary reason cars equipped with IMA get better highway mileage than their more conventional counterparts.
Additionally, vehicles equipped with IMA incorporate an 'idle-stop' feature. As the vehicle comes to a stop, the engine automatically shuts off to save fuel and minimize emissions. The electric motor rapidly restarts the engine as soon as the brake pedal is released. They also have a conventional starter as a backup, making it the only production hybrid system which can operate with its high voltage electric system disabled, using only its ICE like a traditional vehicle. However, since the IMA also acts as the vehicle's alternator, eventually the 12 V accessory battery would require an external charge.