The Parallel Hybrid or Hybrid Electric Vehicle abbreviated HEV. Called the “Self-Charging Hybrid” by Toyota as the battery pack cannot be independently charged from a plug.
The mechanical output of the electric motor is connected to the engine (fuel to mechanical). This means that the electric motor can be used to assist the engine and hence the engine can be optimised for fuel efficiency.
The motor can absorb energy during braking events and return that energy during acceleration. This operation means the control system keeps the battery at a lower SoC and “looks” for opportunities to recover energy that would otherwise be lost to heat in the brakes.
Supporting the engine during acceleration means that the engine design can be biased towards fuel economy. Toyota use the Atkinson cycle in their hybrid electric vehicles.
| Engine | Thermal Efficiency |
| Third generation Prius | 38% |
| First and second generation Prius (Atkinson cycle) | 37% |
| Passenger car diesel engine | 34% |
| Passenger car gasoline engine (Otto cycle) | 28% |
The main advantages of the hybrid system are:
- lower emissions
- more fuel efficient
- smoother and quieter operation
- reduced maintenance costs
Toyota Prius
This is a HEV pack and so quite a small total energy of 1.3kWh and using a smaller 40% SoC window. This small window is so that the pack will be able to last the lifetime of the car with high charge and discharge rates from braking and acceleration.
References
- Atkinson Thermal Efficiency, TDiClub Forum
- Toyota hybrid – how does it work?, Toyota
Hybrid System Block Diagrams
A high level definition of the hybrid system type. The pros and cons and a look at the battery.
The mechanical output could be to the wheels of a vehicle, the propeller of a boat or aircraft. The requirements for each system will influence the sizing of the system and the balance between battery storage capacity and onboard fuel storage. Thus changing the pros and cons of each system to some extent.