In both the parallel hybrid topology and the combination hybrid topology, the ICE engine is used as the steady state propulsion and the electric traction is used for dynamic propulsion. This is due to the rather sluggish reponse of the ICE engine as compared to the electric motor. If the electric motor handles the torque dynamics, then the ICE engine would handle the steady torque demand. This would mean that under these conditions, the ICE engine would see a highway like drive cycle even in a city drive. This would improve significantly the kilometre per litre ratio (mileage). As the electric motor is used as an auxillary drive to assist the power demand of the main ICE propulsion, this mode of operation is called the power assist mode. Here the battery size can be small as it needs to store just the amount of energy required for dynamical loading. If the electric drive becomes the primary propulsion system, then the ICE is used as an aid to extend the range of the vehicle and such mode of operation is called the range extender mode. The advantage of this control strategy is that most of the energy used is electrical which would reduce the overall emissions at the wheels. Further, as the electric motor is rated to handle the starting torque conditions of the vehicle, the need for a separate starter motor is avoided as the vehicle is in motion when the ICE is required to be turned on.
Figure 2: Combination hybrid with planetary gear train
In the power assist mode of operation, the ICE is operated in its most efficient region of its torque-speed characteristics thereby reducing full-throttle emissions. The electric motor provides the fluctuating torque requirements like acceleration conditions. In the parallel hybrid topology, the generator and rectifier are absent and therefore the battery cannot be charged directly by the ICE when at standstill. However, under running conditions, the battery can be charged through regeneration from the wheels during down-hill driving and braking conditions. It should be noted that compared to the range extender mode of operation, the battery size requirements for the power assist mode of operation is very small as the battery needs to act as only a buffer to supply peak load demand.
One could also achieve battery charging from the ICE during standstill conditions by introducing a planetary gear set and connecting a generator to it as shown in Figure 2. While the vehicle is in standstill, the ICE will drive the generator through the planetary gear set. The generator will in turn charge up the battery. A topology similar to this has been used in the Toyota Prius hybrid electric vehicle.
One of the significant advantages of either the EVs or the HEVs over the conventional ICE vehicle is the inherent bi-directionality of the power flow. An EV propulsion can convert energy stored in the batteries into vehicle motion just like the conventional (ICE) vehicles, further it can also convert the kinetic energy of the vehicle in motion back into electrical energy that can be stored in the battery during braking. This is referred to as regenerative braking which is not a feature in ICE vehicles. The significance of regeneration becomes apparent when one considers a typical city drive cycle.