Molly Ayala
Determining the horsepower of hybrid and electric vehicles is a complex task due to the interaction of multiple propulsion units. The total horsepower of a hybrid vehicle is often less than the sum of the power of its individual components, as the engine and motor(s) rarely produce peak power simultaneously. To address this challenge, the SAE J2908 standard has been introduced to provide a consistent method for measuring the horsepower of hybrid, plug-in hybrid, and electric vehicle powertrains, enabling direct comparisons with gas and diesel engines. This standard considers the unique characteristics of electric powertrains, such as the torque curve and hp curves, to ensure accurate representations of their performance. While currently classified as an information report, broad industry acceptance of the SAE J2908 standard could lead to its elevation as a recommended practice or a mandatory standard for measuring and advertising vehicle horsepower.
| Characteristics | Values |
|---|---|
| Method to determine HP | SAE J2908 details methods to measure the horsepower of hybrid, plug-in hybrid, and EV powertrains so they can be compared to gas and diesel engines |
| Power output | It is easy to measure the power output of an electric motor or gas engine in isolation, but when installed in a vehicle, the total system output is affected. |
| Total horsepower of a hybrid | The total horsepower of a hybrid is almost always less than the sum of the power made by the engine and each motor independently. |
| Exceptions | The 2024 Chevrolet Corvette E-Ray, which makes a claimed 655 hp from its 160-hp motor and 495-hp V-8 |
| Serial hybrids | Honda Insight, Clarity, and CRV Hybrid, where the peak HP can only be felt at 90mph or higher due to the engine only providing peak power at a high rpm |
| Parallel hybrids | Prius, which have variable gear ratios so both the electric motor and engine could reach their peak power simultaneously |
| System power | A widely used term to denote the peak actual power of the combined powertrain, which is usually lower than the sum of all engines |
| Example | The RAV4 Prime PHEV in hybrid mode has good acceleration at city speeds because it can use the EV motor torque from a standstill |
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What You'll Learn
Serial vs parallel hybrids
The total horsepower of a hybrid vehicle is usually less than the sum of the power produced by the engine and each motor independently. This is because the engine and motor(s) rarely produce peak power simultaneously. However, there are exceptions, such as the 2024 Chevrolet Corvette E-Ray, which produces a claimed 655 hp from its 160-hp motor and 495-hp V-8 engine.
Hybrid vehicles typically use one of three types of hybrid systems: series, parallel, and series-parallel. In a series hybrid, the electric motor is directly connected to the transmission, resembling a fully electric vehicle. The gas engine serves as a generator that powers the electric motor and recharges the battery, along with regenerative braking. The vehicle's computer determines the power source based on driving conditions, and the gas engine only operates when necessary. Series hybrids are particularly efficient in slow or stop-and-go traffic, where the computer may opt to use battery power exclusively.
In contrast, a parallel hybrid system combines the engine and motor(s) to power the drivetrain directly. The vehicle's computer splits the power demands between the two systems to optimise efficiency. Unlike series hybrids, parallel hybrids do not need to convert the engine's mechanical energy into electrical energy and then back into mechanical energy through the motor. This makes parallel hybrids more efficient for highway driving but less efficient in stop-and-go traffic. Parallel hybrids cannot drive in fully electric mode.
The series-parallel combination, also known as a power-split hybrid, offers the flexibility of using both power sources independently or in conjunction. This provides the benefit of extended range from the refillable gasoline engine and emissions-free electric-only driving when possible, such as at lower speeds or under light-load conditions.
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Torque and hp curves
Torque and horsepower (hp) curves are essential for understanding the performance characteristics of electric vehicles and hybrids. Electric motors deliver maximum torque at 0 rpm, resulting in immediate full torque from a standstill, providing excellent acceleration. This is in contrast to internal combustion engines, which typically only produce maximum torque at specific rpm levels.
The torque curve of an electric motor shows how the motor's torque output varies with rotational speed. In the low-speed range, the motor delivers maximum torque, which is crucial for applications requiring high force, such as accelerating from a stop. As the speed increases beyond the base speed, the motor enters a field weakening zone, where torque decreases to maintain constant power, facilitating higher speeds.
Manufacturers of electric motors typically provide two torque curves in the technical specifications: peak torque and continuous torque. Peak torque represents the maximum torque the motor can sustain for a limited duration, such as 30 seconds, beyond which thermal overloading may damage the motor due to high stator currents. Continuous torque, on the other hand, is the maximum torque the motor can sustain indefinitely without sustaining damage.
The hp curve of an electric motor represents the power output as a function of rotational speed. Horsepower is defined as torque times rpm, divided by a conversion constant. Electric motors deliver nearly 100% of their maximum torque across their rpm range, resulting in a relatively flat torque curve. This characteristic of electric motors provides more push or thrust at equal horsepower ratings compared to internal combustion engines.
The interaction of the torque and hp curves gives a more comprehensive understanding of the performance of electric vehicles and hybrids. The torque curve determines the acceleration capabilities, while the hp curve indicates the overall power output at different speeds. Together, they influence the driving experience, such as the initial acceleration, high-speed performance, and overall responsiveness of the vehicle.
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SAE J2908 methodology
The methodology includes test methods for evaluating the maximum power of electrified vehicle powertrain systems. This is achieved through direct measurement at the drive wheel hubs or axles, using either a chassis or hub dynamometer for all driven wheels. Additional tests are included for plug-in hybrid electric vehicles (PHEVs) to measure electric-only propulsion power and for hybrid electric vehicles (HEVs) to measure electric power assist and regenerative braking.
The SAE J2908 procedure starts with a wheel power test (WPT) to determine the road speed at which the vehicle produces maximum power. However, the power output at the wheels is affected by friction in the drivetrain, which makes it unsuitable for direct comparison with the SAE net power ratings of gas and diesel engines. To address this, the J2908 guidelines outline a process for determining the power of each propulsion device at the output shaft under the same conditions as when the vehicle achieves maximum wheel power.
The rated system power, defined as the maximum sum of the mechanical power outputs of the individual power-producing components used in propulsion, is then calculated. This methodology provides an objective determination of the powertrain power among all types of electrified vehicles, allowing for a more accurate comparison with traditional internal combustion engine power ratings.
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System power in the US and EU
The adoption of plug-in electric vehicles varies worldwide, influenced by consumer demand, market prices, availability of charging infrastructure, and government policies. In the US, the term "system power" is used to denote the peak actual power of the combined powertrain, which is usually lower than the sum of all engines' outputs. This is likely due to the interaction between the propulsion units, which can affect the total system output.
In Europe, the use of electric cars has been steadily increasing. Germany, for instance, had the largest stock of plug-in electric vehicles in Europe in 2022, with 1,184,416 plug-in cars in circulation, representing 2.5% of all passenger cars on German roads. Norway, on the other hand, had the highest number of new battery electric vehicles (BEV) registrations in 2023, accounting for 83% of new car sales. The EU market has also seen a steady increase in the number of electric car registrations, with electric vehicles accounting for 22.7% of new car registrations in 2023, up from 20% in 2022.
To address the environmental concerns associated with the transport sector, the EU has set targets for a 15% reduction in emissions by 2025 from new cars and vans. This has resulted in the introduction and promotion of more fuel-efficient and less polluting vehicles, including electric and hybrid options. In 2023, extra-EU imports of hybrid and electric cars made up 44% of all extra-EU car imports, with China being the largest origin for these imports.
While the US and EU differ in their specific approaches to electric vehicles, both regions recognize the importance of reducing emissions and are working towards integrating more electric and hybrid vehicles into their markets.
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Real-world performance
The real-world performance of a hybrid or electric vehicle is a key consideration when determining its overall horsepower. While laboratory tests provide important insights into a vehicle's potential, how that power is delivered and utilised in everyday driving situations is crucial.
One of the key benefits of hybrid vehicles is their ability to combine power from two or more propulsion units, often an internal combustion engine and an electric motor. This combination allows hybrids to offer increased fuel efficiency and reduced emissions without compromising performance. The electric motor, powered by the battery pack, can provide additional torque and power to complement the gasoline engine, particularly at slower speeds.
For example, the Lexus GS 450h hybrid sedan features a 3.5-litre V6 engine that produces 292 horsepower on its own. However, when the contribution of the electric motor is factored in, the total horsepower output rises to an impressive 339 horsepower. This additional power can result in improved acceleration and overall driving performance.
It's important to note that the interaction between the engine and motor in hybrids can be complex. The total horsepower of a hybrid vehicle is often less than the sum of its individual components due to the engine and motor rarely producing peak power simultaneously. This makes direct comparisons with traditional internal combustion engines challenging and highlights the need for standardised measurement methods, such as the SAE J2908 standard.
In real-world driving, the performance of a hybrid vehicle may be influenced by various factors, including driving conditions, battery charge levels, and the specific hybrid system in use. Some hybrids, like the Porsche Panamera S Hybrid, excel in quickness and handling, offering a dynamic driving experience. Others, like the BMW i8, boast impressive acceleration and fuel economy, showcasing the versatility of hybrid technology.
Overall, when assessing the real-world performance of hybrid and electric vehicles, it's essential to consider not only the horsepower but also how that power is delivered and utilised to enhance the driving experience. Standardised measurement methods and real-world testing help provide a comprehensive understanding of these vehicles' capabilities.
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Frequently asked questions
The SAE J2908 standard details methods to measure the horsepower of hybrid powertrains so they can be compared to gas and diesel engines. The total horsepower of a hybrid is almost always less than the sum of the power made by the engine and each motor independently.
The combined horsepower of a hybrid vehicle is calculated by adding the power of the electric motor at peak battery power to the power of the combustion engine. This is known as the "system power". However, the combined horsepower may not always be reached, as the engine and motor rarely produce peak power at the same time.
The combined horsepower of a hybrid vehicle is calculated based on the peak power of each component, but in real-world situations, the vehicle may not be able to maintain this power output due to various factors such as battery charge, gear ratios, and speed.
