Mild Hybrid Electric Drive Train Design
Full HEVs with parallel, series, or series–parallel configurations can significantly reduce fuel consumption by operating the engine optimally and using effective regenerative braking. 1 − 3 However, a high electric power demand requires a bulky and heavy energy storage pack. This causes difficulties for packing the drive train under the hood and reduces the loading capacity, and also increases the energy losses in the rolling tires. Full hybrid drive trains have structures totally different from conventional drive trains. To turn totally from conventional drive trains to full hybrids, a huge investment of time and money is needed. A compromise is to develop an intermediate product that is easier to convert from the current products, and yet is more efficient than those products. One solution is to put a small electric motor behind the engine to constitute the so-called mild or soft hybrid electric drive train. This small electric motor can operate as an engine starter as well as an electrical generator. It can also add additional power to the drive train when high power is demanded and can convert part of the braking energy into electric energy. This small motor can potentially replace the clutch or the torque converter, which is inefficient when operating with a high slip ratio.