September 19, 1997

Honda develops new Integrated Motor Assist (IMA) system

Honda Motor Co., Ltd. announced today the development of a new power unit, the "Honda Integrated Motor Assist (IMA) System." This ultra-low fuel consumption power unit achieves consumption figures in excess of 30 km per liter of fuel (10.15 mode).

Fuel consumption reduction (and the consequent reduction in CO2 emissions) is, together with cleaner exhaust emissions, one of the main concerns of the automobile industry today. Honda has contributed to this end with the development and sale of ever more efficient engines combining high power through the use of VTEC technology and reduced fuel consumption in daily driving conditions with the adoption of advanced lean-burn technology.

At the heart of this system is a totally new, compact and lightweight three-cylinder direct fuel-injection engine of 1.0-liter capacity. Friction reduction, lightweight technology, and a widening of the lean-burn range are some of the aspects characterizing this engine. A newly developed direct injection system, allowing fuel to be injected directly into the combustion chamber from a centrally located injector, is combined with VTEC technology to promote ultra-lean-burn combustion and further reduce fuel consumption.

IMA System Improvement Process<
IMA System Improvement Process

IMA System's Engine Efficiency

An ultra-thin motor/generator is attached to this engine to serve as an additional source of power when accelerating, and achieve performances exceeding that of a 1.5-liter engine. For further reduced fuel consumption, this same motor/generator transforms kinetic energy into electric power when braking.

A continuously variable Honda MultiMatic transmission was chosen here for its low transmission loss and the ability to always maintain the engine and motor/generator within their most efficient rotational speed ranges. Ultra capacitor-type accumulators were adopted for their light weight and long life, providing high storage capacity within only 30 kg of weight.

Adding this motor assist function to an already very efficient engine-transmission combination has allowed great strides in fuel consumption reduction. Weight increase over a 1.5-liter car is held down to some 50 kg.

The Honda IMA System

  • Description of the system
The ultra-thin motor/generator is installed between the engine and the transmission of a transversely-mounted front engine/front wheel drive FF powertrain, providing or recuperating energy according to the situation. The Power Drive Unit (PDU) controlling the motor/generator's operation and the ultra-capacitor form a safe, integrated assembly located in front of the rear wheels.
When the brakes are applied, the energy previously wasted in the form of heat is now recuperated and stored in the form of electric power within the ultra-capacitor. When accelerating, this stored energy is called upon to assist the engine, thus providing for both reduced fuel consumption and increased performance.

Honda IMA System Layout
Honda IMA System Layout

  • 3 cylinder 1.0-liter gasoline engine
The heart of this new power unit is a totally new, compact and lightweight 3 cylinder engine of 1.0-liter capacity. Fuel consumption and exhaust emission have both been drastically reduced through detailed friction reduction and improved combustion efficiency.
A newly developed direct injection system using a centrally located injector, is combined with VTEC technology and a compact piston crown combustion chamber for significantly reduced consumption.

3 cylinder 1.0-liter gasoline engine
3 cylinder 1.0-liter gasoline engine

  • Ultra-thin motor
The motor/generator is of an ultra-thin DC brushless type and is directly connected to the engine. Technology developed for the Honda EV Plus served as the stepping stone in the development of this motor boasting world-leading output density and efficiency.

Ultra-thin Motor
Ultra-thin Motor

  • Ultra-capacitor
An ultra-capacitor was selected to store the electric energy required to operate the motor assist system. A capacitor is basically a twin-layer condensor. It allows the easy drawing and returning of very high electric current associated with the power assist/power regeneration process with the added advantage of a long life expectency.



  • Motor-assisted torque variation reduction
4-stroke internal combustion engines are characterized by large torque variations as only one of the four strokes actually provides positive torque, the remaining three strokes creating only negative torque as friction and other losses occur. To reduce these large torque variations, the motor/generator is called upon to provide torque in opposite phase to that of the engine. As the motor/generator alternates between power and regeneration modes, large torque variations can be canceled out at the cost of only a limited amount of electric power.

Torque Variation Control

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