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.
IMA Modes of Operation
- Vehicle stationary - The engine is turned off and fuel consumption is zero.
- Startup and acceleration - The engine operates in low-speed valve timing mode with motor assist.
- Rapid acceleration - The engine operates in high-speed valve timing mode with motor assist.
- Low-speed cruising - The valves of all four of the engine’s cylinders can be closed and combustion halted, the electric motor alone can power the vehicle.
- Gentle acceleration and high-speed cruising - The engine operating in low-speed valve timing mode powers the vehicle.
- Deceleration - The valves of all four of the engine’s cylinders are closed and combustion halted. The motor recovers a significant portion of the energy normally lost during deceleration and stores it in the battery.
Integrated Motor Assist (IMA) Applications
Vehicles Using IMA:
Primary motive power for the Insight's IMA system is provided by the 1.0-liter, 3-cylinder, 12-valve VTEC-E gasoline engine. Although the gas engine provides the main propulsion, a permanent-magnet electric motor mounted between the engine and transmission provides assistance whenever extra power is needed under certain conditions (such as initial acceleration from a stop). In this arrangement, the electric motor cannot operate independently of the gas engine - the gas engine must be running for the car to move. The electric motor can generate electricity for the battery or consume electricity from the battery, but not both at the same time.
The electric motor on the Insight helps in several ways. It can:
- Assist the gasoline engine, providing extra power while the car is accelerating or climbing a hill
- Provide some regenerative braking to capture energy during application of the brakes
- Start the engine, eliminating the need for a starter
As the IMA gasoline engine enters its mid- to high-rpm operating range, the electric motor assist ceases and power is solely supplied by the engine, which is operating in its high-rpm 4-valve mode. Power for the electric motor comes mainly by recapturing energy from the forward momentum and braking of the vehicle, rather than from the gasoline engine. When the Insight is coasting or its brakes are applied, and the vehicle is in gear, its electric-assist motor becomes a generator, converting forward momentum into electrical energy, instead of wasting it as heat during conventional braking. If the charge state of the IMA battery is low, the motor/generator will also recharge while the Insight is cruising.
In its first generation, IMA technology could not power the car on electricity alone, and could only use the motor to assist or start the engine. For the 2006 model year, however, the Civic Hybrid's new, more efficient fourth generation IMA system is capable of powering the car entirely on electricity at low cruising speeds. The electric motor is now able to propel the car from a stop to speeds up to 35 mph; while the Civic Hybrid cannot start from a stoplight under electric power alone, it can activate the electric motor while the vehicle is coasting without turning the ICE on, providing the sole means of propulsion.
The Civic Hybrid uses a 3-Stage i-VTEC valve control system that provides normal valve timing, high output valve timing and cylinder idling functions to the benefit of low fuel consumption, high output and greater electrical regeneration capabilities. A new Variable Cylinder Management (VCM) system is an advanced form of the three-cylinder Cylinder Idling System used in the previous generation. VCM allows the regenerative braking system to reclaim as much energy as possible during deceleration, while also allowing the electric motor to propel the vehicle in certain steady state cruising situations. The previous generation system in the 2005 Civic Hybrid uses 2-stage VTEC that provides normal valve timing and 3-cylinder idling. The new 3-stage system adds high output valve timing and 4-cylinder idling. The high output valve timing contributes to the engines increase in output, while the added cylinder deactivation reduces pumping losses to help improve electrical regeneration capability.
Since the electric motor, which also acts as an electric generator, is attached directly to the crankshaft of the engine, the engine needs to provide as little resistance as possible during deceleration to allow the generator to produce high levels of electricity and charge the batteries. In a traditional engine, the pumping action of the cylinders will actually provide a moderate amount of resistance, or "engine braking," during deceleration. VCM virtually eliminates that effect.
The all-new 2005 Honda Accord Hybrid was the first V6-powered hybrid vehicle to market, adding more power to Honda's hybrid lineup while still retaining gas mileage and emissions levels more in line with the Civic Hybrid. In fact, the Accord Hybrid combines its V6 and electric motor to generate a total of 253 horsepower - 9 more than the standard gasoline-only V6 engine.
The Accord Hybrid utilizies the latest generation of Honda's advanced IMA full hybrid system and, like the Civic Hybrid, also incorporates Variable Cylinder Management (VCM) cylinder deactivation technology. When cruising at steady speeds where less engine power is required, the VCM system deactivates the V-6 engine's rear bank of cylinders, closing both the intake and exhaust valves for reduced fuel consumption. Under light acceleration from a cruising state, the IMA motor can provide power assistance to the engine in three-cylinder mode. When coming to a stop, the IMA system shuts off the vehicle's engine at speeds below 10mph to reduce fuel consumption and exhaust emissions. At the moment the driver releases the brake pedal, the IMA motor instantaneously restarts the gasoline engine.