The vast majority of electric vehicles (EVs) today do not incorporate traditional gearboxes. There are three key reasons for this design choice. Before delving into those, it’s essential to understand the function of a gearbox in an internal combustion engine (ICE) vehicle, as this will clarify why gearboxes are not critical in electric cars. We will also explore scenarios where gearboxes can be advantageous and what benefits they provide.
What is a Gearbox and Its Role in ICE Cars?
Unlike electric motors, ICEs generate insufficient torque at idle to move the vehicle; higher revolutions per minute (RPM) are necessary to produce adequate torque. However, the engine's rev range is limited, which in turn restricts the car's speed. This is where gearboxes come into play. Essentially, a gearbox, in conjunction with the final drive ratio, acts as a torque multiplier. While you might be familiar with the concepts of gear and final drive ratios, let’s clarify their meanings.
Consider an ICE vehicle in 1st gear with a gear ratio of 3:1. This indicates that the torque produced by the engine is multiplied thrice. To calculate the torque at the wheels, we must also consider the final drive ratio. If the final drive ratio is also 3:1, then the total wheel torque becomes nine times that of the engine torque. For instance, if the engine generates 200 Nm of torque in 1st gear with these ratios, the wheel torque would be 200 Nm x 9 = 1800 Nm.
Without a gearbox, this torque multiplication would not occur, leaving the vehicle immobilized, even under full acceleration. The design of an ICE gearbox must effectively harmonize the engine's power and torque characteristics, its efficiency at varying loads, and the maximum speed intended by the engineers for that vehicle.
Despite significant advancements in gearbox technology over the past few decades, these systems still involve compromises, introducing inefficiencies within the drivetrain and contributing to increased fuel consumption and energy loss.
Modern automatic gearbox with 9 gears. The ZF 9HP.The Electric Car Perspective
Electric motors possess three inherent properties that address gearbox engineering challenges and often render them unnecessary: high torque at zero RPM, high RPM capabilities, and electronically limited top speeds. Let's explore these properties in detail.
The availability of high torque from zero RPM negates the need for gearboxes, which primarily serve to amplify torque. Since electric motors offer substantial torque immediately, there is no requirement for multiple gear ratios.
Second, the high RPM capabilities of electric motors — often capable of reaching up to 20,000 RPM — provide the ability to achieve top speeds with a single gear ratio. Lastly, electric cars generally have an electronically limited top speed, influenced not only by the absence of a gearbox but also by the relatively low energy content of today’s high-voltage batteries compared to liquid fuels.
Take, for example, the BMW i4 eDrive 40: it generates a maximum motor torque of 430 Nm from 0 to 5,000 RPM and has a combined gear and final drive ratio of 11.115:1, resulting in an impressive wheel torque of 4780 Nm. This demonstrates why electric vehicles excel in acceleration from a stoplight.
At a top speed of 190 km/h, the motor spins at approximately 16,500 RPM, while at a typical highway speed of 130 km/h, it operates around 11,300 RPM. This example illustrates that the electric motor's RPM range is sufficient to meet all performance requirements without necessitating a gearbox.
The single-speed gearbox of the BMW i4.The Porsche and Audi Approach
The Porsche Taycan and Audi E-Tron GT are notable exceptions as they employ a two-speed gearbox in their rear motors. It’s important to clarify that while gearboxes are not typically necessary in electric cars, utilizing a two-speed system can provide advantages. A short gear ratio can enhance acceleration at lower speeds, while a longer ratio can reduce motor RPM at higher speeds.
Reducing RPM at high speeds improves motor efficiency by minimizing back electromotive force (EMF) and eddy currents, resulting in greater range. Additionally, this setup allows for higher top speeds without excessive motor rotation.
However, implementing two-speed gearboxes presents significant engineering challenges, primarily due to the immense and instantaneous torque generated by electric motors. Nevertheless, manufacturers are exploring the development of reliable two-speed gearboxes for electric vehicles.
If they can produce these systems at scale with high reliability and affordable pricing, we may see broader adoption beyond just high-performance models like the Porsche Taycan and Audi e-Tron GT. For now, the added complexity and expense outweigh potential benefits.
Porsche Taycan’s two-speed gearbox
