The vast majority of electric vehicles (EVs) do not incorporate a traditional gearbox. This article explores the primary reasons behind this trend, while also defining what a gearbox is and its function in internal combustion engine (ICE) vehicles. By understanding these aspects, we can better appreciate why gearboxes are generally unnecessary in electric cars, and under what circumstances they might be beneficial.
Understanding Gearboxes in Internal Combustion Engine (ICE) Cars
ICE engines produce insufficient torque at idle to initiate movement, requiring higher revs to generate the necessary power. However, the engine's rev range is limited, which restricts vehicle speed. Consequently, gearboxes are employed to optimize this process. In essence, a gearbox acts as a torque multiplier in conjunction with the final drive ratio.
For example, let's consider a vehicle in first gear with a gear ratio of 3:1. This multiplies the engine's torque by three. To find the ultimate torque at the wheels, we also factor in the final drive ratio, which, if it is also 3:1, results in a wheel torque of nine times the engine torque. Hence, if your ICE vehicle generates 200 Nm of torque in first gear with these ratios, the torque at the wheels would be calculated as 200 Nm x 9 = 1800 Nm.
Without a gearbox, this torque multiplication would not occur, leaving the vehicle incapable of movement even with full throttle applied. Gearboxes must be designed to perfectly harmonize the engine’s power and torque characteristics, its efficiency at varying loads, and the vehicle's target top speed.
Despite significant advancements in gearbox technology, they remain a compromise, as they introduce inefficiencies that can increase fuel consumption and energy loss.
Modern automatic gearbox with 9 gears. The ZF 9HP.Electric Cars: A Different Approach
Electric motors possess three inherent characteristics that simplify engineering challenges associated with gearboxes, making them largely redundant. These properties are: high torque from zero rpm, high rotational speed capability, and electronically limited top speeds. Let's examine each of these factors.
High torque at zero rpm is critical because it negates the need for torque multiplication via multiple gear ratios. Furthermore, electric motors can reach impressive rpm levels, often up to 20,000 rpm. This high torque availability across a broad range allows for the attainment of top speeds without multiple gears. Finally, electric cars often feature an electronically limited top speed. This limitation is not solely due to the absence of a gearbox, but also attributable to the relatively low energy density of current high-voltage batteries compared to liquid fuels.
For instance, consider the BMW i4 eDrive 40, which generates a peak motor torque of 430 Nm between 0 and 5,000 rpm, paired with a combined gear and final drive ratio of 11.115:1. This translates to a remarkable wheel torque of 4,780 Nm, suggesting why electric vehicles excel in acceleration from a standstill.
Suppose the maximum speed of the car is 190 km/h. At this speed, the motor operates at approximately 16,500 rpm, and at a typical highway speed of 130 km/h, it runs at about 11,300 rpm. This example illustrates that electric motors have the necessary rpm range to fulfill performance requirements without a gearbox.
The single-speed gearbox of the BMW i4.The Two-Speed Gearbox in Performance Models
The Porsche Taycan and Audi E-Tron GT are notable exceptions, utilizing a two-speed gearbox in their rear motor configurations. This might seem contradictory since we previously discussed the redundancy of gearboxes in electric vehicles. However, these two-speed systems enable quick acceleration at lower speeds through a short gear ratio while facilitating a longer gear ratio at higher speeds to reduce motor rpm.
Reducing the motor's rpm at higher speeds is crucial for two reasons: it enhances the motor's efficiency by minimizing back electromotive force (emf) and eddy currents, which subsequently improves the car's range. Additionally, it supports achieving higher top speeds without over-revving the motor.
Despite the advantages, the implementation of two-speed gearboxes is challenging due to the immense and instant torque produced by electric motors. Nonetheless, some manufacturers are actively developing reliable two-speed gearbox solutions for electric vehicles.
If they can successfully produce these systems at scale while ensuring reliability and affordability, we may see wider adoption beyond just high-performance models like the Porsche Taycan and Audi E-Tron GT. For now, however, the added complexity and costs of two-speed gearboxes outweigh the potential benefits.
Porsche Taycan’s two-speed gearbox
