Dedicated hybrid transmissions explained
March 30, 2023
Dedicated hybrid transmissions (DHT transmissions) are unique because they can operate with at least two power sources. They are commonly found on vehicles with both an internal combustion engine (ICE) and an electric motor.
DHT transmissions are becoming more common. Hybrids currently account for about 30% of new vehicles sold in Australia. They are the preferred option for consumers compared to pure electric vehicles, and a DHT allows them to operate at peak efficiency.
Dedicated hybrid transmissions handle tasks essential to vehicle operation. For example, they control torque and engine speed so that the automobile responds to drive input and conditions. As they do, they combine the available propulsion sources to achieve the best possible balance between energy consumption and performance.
An important distinction between dedicated hybrid transmission cars and standard transmissions on hybrid vehicles is that the standard option treats the hybrid components as add-ons that do not play an active role in vehicle propulsion. This limits their effect on efficiency and performance. Many so-called mild hybrids fall into this category.
Dedicated hybrid transmission cars adjust both power sources to maximise efficiency and performance. They typically achieve this by using the electric component to keep the ICE operating within its peak efficiency zones.
At the same time, it is important to note that dedicated hybrid transmissions do not require two power sources. They can operate in full ICE mode when needed and can drive the car effectively without electric motor assistance (though this would lower efficiency in the long term). In some instances, such as with a plug-in hybrid, the transmission can run vehicles in a full electric mode without involving the fuel-powered engine at all.
Different Types of Hybrid Transmissions
All dedicated transmissions of hybrid electric vehicles have a dual propulsion setup that balances performance and efficiency, but their designs can differ significantly.
The term “hybrid transmission” can have different meanings depending on the vehicle’s powertrain design. The distinctions have to do with the way the battery and internal combustion engine interact.
The different varieties include parallel hybrids, series, power splits, and through-the-roads.
Parallel Hybrid Transmissions
Parallel hybrid transmissions combine two power sources: a small electric motor and an internal combustion engine. These two work together to provide power to the wheels simultaneously. Unlike some other hybrid transmissions, this type does not use the engine to power the electric motor.
Parallel systems can run on either battery or gas power if the other part of the system shuts off. For example, if the battery is drained, the vehicle can rely solely on the ICE for propulsion.
This transmission excels at using the parallel electric motor to maximise the efficiency of the ICE.
Series Hybrid Transmissions
Series hybrids get all their power from an electric motor. Even though hybrids with this type of transmission still have an ICE on board, it does not assist with propulsion. Its sole purpose is to charge the battery that powers the electric motor.
Series hybrids require a much larger battery than parallel hybrid designs. However, they are much simpler and easier to maintain, and they tend to be more efficient than parallel models for certain types of driving. For example, these cars perform better in stop-and-go city driving, where fuel-powered vehicles are usually the least efficient.
Power-Split Transmissions
Vehicles operating in power-split mode can switch between parallel and series hybrid modes. This design can combine the advantages of each. However, it only works on transmissions that do not have gearing. They require an electric variable transmission (EVT) that automatically shifts to an optimal gear ratio using a semi-flexible belt.
In these systems, the engine torque and rotation are decoupled from the vehicle’s drive shaft. The design allows the powertrain to operate at peak efficiency continuously. However, it adds complexity and can be more difficult to maintain.
Through-the-Road Hybrids
Through-the-road hybrids are four-wheel-drive vehicles with one axle powered by an electric motor and one by an ICE. Since the two propulsion sources are decoupled, they function independently and can operate with a standard automatic transmission.
The transmission ensures balanced power between the front and rear axles and manages the response to road conditions and driver input.
Advantages of Dedicated Hybrid Transmissions
Hybrid vehicles bring specific benefits. They do not rely solely on battery power, so their range is never an issue, but they offer some of the lower operating costs associated with EVs.
Dedicated hybrid transmissions allow these dual-propulsion vehicles to balance performance and efficiency. Here is a closer look at the advantages of DHTs:
- DHTs are compact. They can operate with fewer gear steps than traditional transmissions. This makes them lighter and easier to maintain.
- DHTs ensure efficiency. They always keep the internal combustion engine in its ideal efficiency range by relying on support from the electric motor. Though efficiency can vary depending on design type and driving habits, the transmission always manages torque and RPM with electric motor support to limit fuel use.
- DHTs improve performance. Because they manage electric and ICE components simultaneously, these transmissions can improve performance. For example, an electric motor provides instant torque. A DHT may favour output from the electric motor when the driver accelerates quickly from a full stop.
- DHTs balance performance and efficiency. In a traditional ICE car, a rapid acceleration maneuver burns excessive fuel. Fuel-efficient ICEs would take a long time to accelerate to full speed. However, you get the best of both worlds in a hybrid with a dedicated transmission. Cars with hybrid technology can be fun and responsive to drive while also being fuel efficient.
This best-of-both-worlds functionality makes DHTs a popular option for vehicle owners.
The Future of Dedicated Hybrid Transmissions
Though vehicles with dedicated hybrid transmissions previously outsold electric vehicles (EVs) in Australia, all-electric models recently captured the majority of sales.
Due to the demand for increased efficiency, increasingly effective transmissions will be necessary to ensure that ICE vehicles operate with minimal fuel consumption while maximising performance.
If you require a new dedicated hybrid transmission or servicing of your hybrid vehicle’s existing transmission, book a service with the team at Automatic Transmissions R Us.
