Electrified vehicles, particularly those with electrically assisted internal combustion engines (ICE), are experiencing a surge in popularity among American consumers. This trend signifies a shift towards more fuel-efficient and environmentally conscious transportation options. Automakers are responding to this demand by integrating hybrid technology into a wider range of models.
Toyota, a pioneer in hybrid technology, reported selling over one million electrified cars last year, underscoring the strong market acceptance of their hybrid offerings. Honda also saw significant success with its electrified vehicles, accounting for a quarter of its 1.3 million total sales. The increasing popularity of Hyundai’s Santa Fe and Tucson hybrids further illustrates this growing trend. These vehicles provide a compelling blend of improved fuel economy and reduced emissions without the range anxiety associated with fully electric vehicles (EVs).
While all hybrid vehicles share the common goal of improving fuel efficiency and reducing the load on the internal combustion engine, the methods they employ vary significantly. This has led to the development of several distinct types of hybrid powertrains, each with its own unique architecture and operational characteristics.
The hybrid landscape includes five primary categories: mild hybrids, plug-in hybrids (PHEVs), parallel hybrids, series hybrids (also known as range extenders or E-REVs), and power-split hybrids (also called series-parallel hybrids). It’s important to distinguish between these types, as the term “mild hybrid” is often misused to describe any hybrid vehicle without a plug. In reality, a mild hybrid specifically refers to a vehicle equipped with a 48-volt starter generator, which primarily assists the engine by providing torque fill and boosting efficiency. Traditional hybrids, on the other hand, can typically power the wheels using electricity alone for short distances and encompass parallel, series, and power-split configurations.
| Hybrid Type | Description | Examples |
|---|---|---|
| Mild Hybrid | Uses a 48V system to assist the engine, primarily for start-stop and torque fill. | Ram 1500 eTorque |
| Plug-in Hybrid (PHEV) | Can be plugged in to recharge the battery, offering a longer electric-only range. | Toyota Prius Prime, Hyundai Santa Fe PHEV |
| Parallel Hybrid | Engine and electric motor can both directly power the wheels. | Hyundai Santa Fe Hybrid |
| Series Hybrid (Range Extender) | Engine only charges the battery; the electric motor powers the wheels. | BMW i3 with Range Extender (REx) |
| Power-Split Hybrid | Combines features of series and parallel hybrids using a planetary gearset. | Toyota Prius |
The Hyundai Santa Fe Hybrid serves as an excellent example of a parallel hybrid system. In this configuration, both the internal combustion engine and the electric motor can independently or jointly power the wheels. This design offers a balance of performance and efficiency, allowing the vehicle to leverage the strengths of both power sources.
The latest Santa Fe Hybrid features a 1.6-liter 4-cylinder turbo engine paired with a 44.2-kilowatt electric motor and a 1.5-kilowatt-hour lithium-ion battery. This combination delivers a total output of 231 horsepower and 271 pound-feet of torque. The electric motor is strategically positioned between the engine and the six-speed transmission. A clutch mechanism enables the engine and electric motor to work in tandem or independently, depending on driving conditions and power demands. This parallel arrangement allows for seamless transitions between electric-only, engine-only, and combined power modes.
One of the key advantages of the parallel hybrid system is its ability to mitigate turbo lag, providing smooth and responsive acceleration from a standstill. Additionally, regenerative braking captures kinetic energy during deceleration, converting it back into electricity to recharge the battery. This stored energy can then be redeployed to assist the engine, further enhancing fuel efficiency. The compact packaging of the electric motor and clutch, housed within a 40-millimeter space, demonstrates the ingenuity of this design. Unlike some Toyota hybrids, the Santa Fe Hybrid’s battery pack is located at the rear right side of the floor, contributing to its relatively small footprint. The use of a conventional six-speed automatic transmission distinguishes it from power-split hybrid systems, which typically employ an e-CVT.
Interior of a Hyundai Santa Fe Hybrid
Power-split hybrids, also known as series-parallel hybrids, have become increasingly prevalent among automakers. This sophisticated system, exemplified by the Toyota Prius, offers a unique approach to blending the power of the internal combustion engine and electric motor(s).
In a power-split hybrid, the engine’s power is routed through a planetary gearset, which consists of three main components: the sun gear, planet gear, and ring gear. These gears, each with different sizes and operating speeds, work together to seamlessly blend the power from the engine and electric motor(s). While planetary gearsets are also used in conventional automatic transmissions, their role in power-split hybrids is to manage the flow of power from two distinct sources.
Typically, the engine drives the planetary gear, while the electric motor drives the sun gear. The combined output is then channeled through the ring gear towards the driveshaft and ultimately to the wheels. This arrangement allows the engine to directly power the wheels with fixed gear ratios, but it can also continuously adjust those ratios when the electric motor engages. This dynamic adjustment creates a continuously variable transmission, often referred to as an e-CVT. The Toyota RAV4, Honda CR-V and Accord hybrids, and Ford Escape hybrid are other examples of vehicles utilizing this power-split hybrid technology.
Toyota Prius, a classic example of a power-split hybrid.
The key difference between parallel and power-split hybrids lies in their transmission systems. While the Hyundai Santa Fe Hybrid utilizes a traditional six-speed automatic transmission, the Toyota RAV4 employs an e-CVT. This e-CVT serves two primary purposes: enabling the engine to operate within its most efficient power band and leveraging dual electric motors to deliver greater power and fuel savings compared to the single-motor setup found in parallel hybrids.
As Paul Turnbull, lead engineer at Munro & Associates, explains, power-split hybrids can optimize engine speed to deliver power to the wheels in the most efficient manner for every speed and torque demand. This optimization contributes to the RAV4 Hybrid XSE’s slightly higher city fuel efficiency (41 mpg) compared to the Tucson Hybrid Limited (35 mpg). However, overall combined city and highway fuel efficiency figures are generally similar between the two systems. It’s important to note that factors such as tire sizes and driving patterns can also influence fuel efficiency.
Beyond fuel efficiency, hybrid systems also play a crucial role in recharging the low-voltage battery, which powers essential in-vehicle systems such as lighting and climate control. Toyota’s hybrid SUVs often incorporate a more complex three-motor system, with a third electric motor at the rear providing electronic all-wheel-drive capability in low-traction situations. This eliminates the need for a mechanical driveshaft to the rear axle while maintaining exceptional fuel efficiency.
As the transition to fully electric vehicles progresses, hybrids are poised to become even more sophisticated, featuring higher energy density batteries and more efficient motors. These advancements will further enhance their fuel economy and performance, making them an increasingly attractive option for consumers seeking a balance of efficiency, practicality, and environmental responsibility.
| Feature | Parallel Hybrid (e.g., Hyundai Santa Fe Hybrid) | Power-Split Hybrid (e.g., Toyota RAV4 Hybrid) |
|---|---|---|
| Transmission | Traditional Automatic (e.g., 6-speed) | e-CVT (Electronically Controlled Continuously Variable Transmission) |
| Electric Motor Setup | Typically Single Motor | Typically Dual Motors |
| City Fuel Efficiency (Example) | ~35 mpg (Tucson Hybrid Limited) | ~41 mpg (RAV4 Hybrid XSE) |
| Power Blending | Clutch-based system | Planetary Gearset |
