7.2 Hybrid and plug-in hybrid vehicles
As mentioned earlier in Section 5.3, combining an internal combustion engine (ICE) with a small storage battery and an electric drive motor is one way to improve the overall engine fuel efficiency. Petrol–electric hybrids were initially introduced in the late 1990s by Toyota and Honda, and are now offered by a large number of car manufacturers.
Overall, hybrids reduce fuel consumption through a combination of the following:
- reducing wasted energy during idling and low output, generally by turning off the ICE off and using the electric drive motor
- recapturing waste energy through regenerative braking
- using the internal combustion engine only when it can be run close to its maximum efficiency, substituting an electric motor at lower power and using it to boost the ICE for acceleration.
The battery is charged by the ICE when the engine loading is low.
The first generation of family hatchback hybrids, such as the Prius, introduced in 1997 (Figure 11), incorporated a 30 kW (40 hp) electric motor and a 1.7 kWh nickel metal hydride (NiMH) battery (i.e. only a slightly larger storage capacity than a normal car lead-acid starting battery).
They had CO2 emissions of about 90 grams per km compared to 130 grams per km for a comparable standard petrol car.
Plug-in hybrid electric vehicles (PHEVs)
In the earliest hybrid designs, the vehicle would only be driven in full battery mode when starting, or at low speed, such as in slow traffic or when parking. However, increasing the storage capacity of the battery, to 4.4 kWh in the later 2017 Prius design, together with a larger, 60 kW (80 hp) electric motor, has allowed more flexible operation. The battery technology also moved from Ni-MH to lithium-ion technology. This, as shown in Figure 10, has a higher energy density per kilogram, so the larger storage capacity did not incur a serious weight penalty.
A PHEV can be operated in a full battery electric mode for short journeys, as well as in a hybrid mode with the ICE. The battery can either be recharged from the ICE or from the mains, thus saving on fossil fuel. Depending on the vehicle model, the battery capacity and the care taken in driving gently the electric range can be 20–60 kilometres.
Although the PHEV can be seen as a modification of a basic hybrid design, it may also be described as an extended range electric vehicle. It is essentially a battery electric vehicle whose internal combustion engine is only used on long journeys.
Key advantages of this configuration are that:
- most car trips are short and so for these the car will be running in pure electric mode
- the larger battery allows even higher efficiency improvements to the ICE performance, improving its energy efficiency and reducing its emissions
- it overcomes the range limitation of a pure battery electric vehicle.