Do Electric Vehicles Need More Than One Gear?
Do electric vehicles need a transmission with more than one speed? Well, you might assume not, as most current electric vehicles get by just fine with one gear. However, like almost all engineering questions, the answer is: it depends. In this article, we look at the potential benefits of using a multi-speed transmission in an electric vehicle, what applications stand to benefit most from a multi-speed transmission, and the potential drawbacks of such a solution.
To start, let’s look at an idealized torque curve for a high-performance PMSM producing 500 Nm of torque with a power output of 315 kW and a maximum speed of 16,000 rpm:
Typical Electric Motor Performance Curve
As we can see, such a motor provides excellent torque for acceleration from a standstill and constant power all the way to its maximum speed. However, for most vehicle applications, this motor is going to require additional gear reduction to produce usable speeds and torque at the wheels. This is where we start to encounter tradeoffs between peak torque at the wheels, vehicle top speed, and powertrain efficiency.
The chart below shows this the achievable wheel torque and vehicle speed using this motor as a single drive unit in a car with two different gear ratios: 8:1 and 16:1. Assumptions were made as to tire size and the effects of efficiency loss are ignored.
Wheel Torque vs. Vehicle Speed for Two Different Gear Ratios
As the chart shows, the higher gear reduction gives us higher wheel torque but a lower top speed. Now, this will be obvious to most, as this is simply how gear reductions work. However, this still provides a useful illustration of the benefit of using a multi-speed transmission in an EV. The tire size, gear ratios, and motor power are in line with the rear EDU of a performance EV, such as the Porsche Taycan. Also shown is a hypothetical traction limit based on the curb weight of the Taycan, along with assumptions for tire performance and weight transfer during acceleration. Clearly, the 8:1 ratio would leave significant performance on the table, as the peak torque is significantly lower than the traction limit. At the same time, a higher gear ratio would provide optimal acceleration but with a top speed that falls short of customer expectations.
This demonstrates the first benefit of a multispeed transmission in an EV application: The ability to achieve both high output torque and high vehicle speed. This is beneficial not only in performance car applications, but also for commercial vehicles and off-highway applications. These vehicles typically carry high loads and have demanding requirements for grade and obstacle climb. Multiple gear ratios enable the high torque required for these conditions, while preserving higher speed performance and allowing electric motor downsizing for these applications.
Another reason to use a multispeed transmission is to improve the operating efficiency of a vehicle, particularly for vehicles that require high output torque. For many applications, the lowest useable gear ratio will provide the best efficiency. By reducing speeds through the gear train, gearbox losses are minimized. Additionally, the efficiency of a motor tends to increase as torque increases, up to a certain point. However, applications that require high output torque typically require a higher than ideal gear ratio, thus limiting the achievable efficiency with a single speed.
The chart below shows a simulation of our hypothetical performance car on the UDDS and HWFET EPA dyno cycles overlaid on a representative efficiency map for our idealized PMSM. Operating points are shown for both an 8:1 gear ratio and a 16:1 gear ratio. Higher efficiencies are shown in red and lower efficiencies in blue.
UDDS & HWFET Operating Points at Two Gear Ratios
As we can see, the 8:1 gear ratio reduces the speed of the rotating components while increasing the operating torque of the motor. This increases the average efficiency of the duty cycle. It is important to note that a lower gear ratio does not increase the efficiency of all operating points. To understand the real benefits, detailed analysis is needed to account for the effects of gearbox losses, the duty cycle of interest, and efficiency characteristics of a particular motor & inverter combination.
So far, we have covered the major benefits of using a multispeed transmission but, of course, there are drawbacks. Firstly, many passenger car applications do not require the type of performance that justifies a multispeed transmission and are better served with the simplicity of a single speed. A multispeed transmission is a significant increase in complexity, particularly when it comes to the control logic and integration needed for smooth shifting. A multispeed transmission will come with higher BOM cost and higher development costs than a single speed transmission. In addition to higher costs, a multispeed gearbox will usually be less efficient than a single speed transmission due to the additional losses associated with additional gears, bearings, and other components. This reduced efficiency can eat into the efficiency gains possible from optimizing gear ratios and must be carefully considered when deciding whether to use a single or multispeed transmission.
How do you know if a multispeed transmission is right for your application? This requires careful analysis and a thorough design study of the different tradeoffs. At Coder Research & Engineering, we have developed a set of in-house tools that allow us to rapidly conduct design studies, vehicle performance analysis, and tradeoff studies to determine the optimal transmission ratio – or ratios – for your application. We have over a decade of experience in sizing powertrains in various applications and have helped our customers developed innovative powertrain architectures for the most demanding use cases.
Reach out to our team today for help with your powertrain sizing & architecture development projects!
