Would a purely electric drive-train powered by a gas engine be more efficient than a hybrid vehicle?

With the increasing demand for sustainable and efficient transportation, the concept of a range-extender battery electric vehicle (REx) has gained significant attention. However, the idea of a purely electric drive-train powered by a gas engine poses various challenges and questions regarding its efficiency compared to traditional hybrid vehicles. In this article, we will explore the intricacies of this concept and its potential applications in the automotive industry.

Understanding Range Extender Technologies

Range Extender Battery Electric Vehicle (REx): A REx is a type of vehicle that combines an electric motor for propulsion with a small, dedicated internal combustion engine (ICE) that generates electricity to replenish the battery when its charge is low. This technology is similar to the hybrid electric vehicle (HEV) but with a dedicated ICE that operates at its most efficient point, thereby enhancing overall efficiency. The primary function of the ICE is to generate electricity, which is then used to power the electric motor through a generator.

Efficiency Considerations in Hybrid Vehicles

The efficiency of hybrid vehicles, including the aforementioned REx, is a result of managing load and RPM conditions of the internal combustion engine. An ICE is most efficient at a specific combination of revolutions per minute (RPM) and load. However, real-world driving conditions often deviate from these optimal points. To circumvent this issue, HEVs utilize a battery to store and manage the energy, allowing the ICE to operate closer to its most efficient state.

The concept of regenerative braking is often misinterpreted as the primary reason for HEV efficiency. In reality, the main efficiency gain comes from the careful management of the ICE's load, which is controlled by the battery. The electric motor-assist is a tool used to optimize this control, and regenerative braking is a secondary feature that helps in recharging the battery without additional hardware.

Drivetrain Types and Their Efficiency

Hybrid vehicles can be categorized into three main systems based on their drivetrain configuration. Each configuration has its own set of advantages and trade-offs:

1. Parallel Hybrid Drive

In a parallel hybrid system, the electric motor shares the mechanical drive train with the internal combustion engine. The electric motor supplements the ICE, but the ICE remains the primary power source. This system is simpler and more efficient when cruising at high speeds. The primary disadvantage is that the electric motor is less powerful and cannot operate at its most efficient load for extended periods.

2. Serial Hybrid Drive

Serial hybrids, on the other hand, have the generator and motor separate from the ICE. The ICE runs the generator, which converts mechanical energy into electrical energy. This electricity then drives the motor to power the vehicle. While this system provides greater flexibility in managing the ICE's load, it is less efficient due to the conversion losses between mechanical and electrical energy.

3. Serial-Parallel Hybrid Drive

Serial-parallel hybrids use a power split device to control the ICE's output, creating both serial and parallel drive paths. This system offers a balance between the simplicity of parallel hybrids and the flexibility of serial hybrids. However, the power split device adds complexity, and the electric motor must still be powerful to meet driving demands.

Challenges in Implementing REx Technologies

While the idea of a purely electric drive-train powered by an ICE sounds promising, several challenges must be addressed for such a concept to be viable:

Weight and Complexity: The REx configuration requires multiple components, including the ICE, generator, battery, and motor. Each component adds weight and complexity, increasing the overall cost and reducing efficiency. Size of Components: To ensure the REx can meet most driving demands, the motor and battery must be sufficiently powerful. This can lead to larger and more expensive components, which may compromise the vehicle's overall efficiency. Hybrid Systems vs. REx: Traditional HEVs manage energy more efficiently by using a battery pack and a smaller, more efficient ICE. These systems can switch between ICE and electric drive modes more seamlessly, minimizing efficiency losses. Market Examples: The BMW i3 REx failed due to the inability to balance a large battery with a large ICE-generator system. This highlights the practical limitations of combining high-performance components in a single vehicle.

In conclusion, while the idea of a purely electric drive-train powered by an ICE has theoretical merit, it faces significant challenges in terms of efficiency and practicality. The complexities and weight of the components, coupled with the limitations of current technology, make it difficult to outperform the well-established and proven HEV systems.