Assist hybrids use the engine for primary power, with a torque-boosting electric motor also connected to a largely conventional powertrain. The electric motor is essentially a very large starter motor, which operates not only when the engine needs to be turned over, but also when the driver "steps on the gas" and requires extra power. Honda's hybrids including the Insight use this design, leveraging their reputation for design of small, efficient gasoline engines; their system is dubbed Integrated Motor Assist (IMA). Assist hybrids differ fundamentally from full hybrids in that they cannot run on electric power alone. However, since the amount of electrical power needed is much smaller, the size of the battery systems is reduced.
A variation on this type is Mazda's e-4WD system, offered on the Mazda Demio sold in Japan. This front-wheel drive vehicle has an electric motor which can drive the rear wheels when extra traction is needed. The system is entirely disengaged in all other driving conditions, so it does not enhance performance or economy.
Ford has dubbed Honda's hybrids "mild" in their advertising for the Escape Hybrid, arguing that the Escape's full hybrid design is more efficient. However, assist hybrids should not be confused with actual mild hybrids like the Chevrolet Silverado Hybrid.
