Regeneration process: Standard in electric cars, not possible in combustion engines

Another factor contributing to the need for the classic combustion engine driveshaft to adapt is the regenerative braking process in electric vehicles. A comparison with the combustion engine world is helpful again here. When a combustion engine vehicle driver releases the gas, the load demand on the driveshaft decreases, allowing it a moment to recover, and the vehicle rolls out or goes into coasting mode. The situation is quite different with the driveshaft in an electric vehicle: Regeneration triggers a negative torque on the driveshaft, similar to the braking process, where positive torque is converted into negative torque. As a result, the driveshaft is almost constantly under load because in an electric vehicle, braking is very often replaced by a high level of regeneration. Controlling acceleration and braking exclusively with the accelerator pedal is also known as One-Pedal Driving. Another important point in the comparison between combustion engine vehicles and electric vehicles is the much higher torque in a BEV. For example, the current Fiat 500 electric offers torque of 220 Nm with a maximum of 118 HP. A Golf II GTI G60 from the 90s achieves similar values and was at the time one of the most powerful vehicles in its class. Driveshafts in electric vehicles must be adapted to these new performance values, which are standard even in small cars.

Electric cars are heavier than combustion engine vehicles

But what does this adaptation process look like, and how does the driveshaft or driveshaft manage to keep up with the ever-increasing performance values of modern electric vehicles? The enormous torques in electric drives could be managed with more material use. However, this would mean accepting a constantly increasing weight, which in turn would reduce the range. What this creates is a classic conflict of objectives between efficiency, weight, and durability. Christian Carlando, Director of Customer Engineering Europe at GKN Automotive explains: "Every component must literally be put to the test on a BEV platform, and we must pay more and more attention to finding the perfect balance between durability, efficiency, and material use. Often, it's a very fine line, but through our test rigs, we can simulate the complete life of an electric vehicle in just a few days. All our customers demand an efficient and stable shaft design. This can minimize heat losses and thus increase the range, even a fraction of a percent increase in range counts here." For this purpose, so-called load spectra are provided by the vehicle manufacturer, which are "replicated" on the test rigs – but in fast-forward. The test criteria here are significantly higher than for vehicles with combustion engines.

Automated vehicles

Even though the fully autonomous Level 5 vehicle may still be a few years away, one thing is certain: It will not be a vehicle with an internal combustion engine, but an electric one. Today, model solutions in public transport with autonomous vehicles for testing purposes already exist. A look at the driveshafts in automated vehicles reveals a new dimension of continuous stress, as the components must cope with multiples of the usual operating time. In extreme cases, this means: continuous operation of the driveshaft in a demanding BEV application scenario with running times of up to 100,000 kilometers per year. And that for ten years. But does the driving behavior of an autonomously driving vehicle match that of a human behind the wheel? "Fortunately not, one might argue," says Christian Carlando. "A self-driving vehicle can of course be optimized for the lowest possible wear level. This means that in normal operation, the vehicle will decelerate and accelerate much more gently than a human driver could. This creates very effective life-extending measures for all vehicle components, whether brakes, tires, or the driveshafts. And of course, the braking and acceleration processes can be exactly reproduced infinitely often - virtually impossible for a human driver."

Human versus machine

The wear limits and life expectancy of vehicle components can be precisely predicted for electric vehicles in autonomous continuous operation. Using test rigs, the life cycles are completely run through and can thus be reproduced again and again. Wear limits can therefore be determined extremely accurately, as the sensor technology of the electric drivetrain constantly compares all vehicle data. The situation becomes more difficult, however, with an electric vehicle that is driven by a human. "A human behind the wheel is naturally unpredictable, personal driving style, and thus also the wear of the components, depends on many factors," emphasizes Carlando. "With combustion engine vehicles, we have been able to gather experience over the decades and with each new vehicle generation, we were able to draw on this knowledge when designing the driveshafts. With the many new vehicle types, automotive manufacturers, and platforms in the BEV sector, we are very often entering uncharted territory, there often are no empirical values. But that's exactly what makes it so exciting, and meanwhile, we have to test much more on real roads again, even though we have expanded our test rig capacities." A continuing trend in BEVs is the still enormous spread of performance values compared to top speed, as even relatively light cars have very high torque. However, the top speed is often much lower than that of comparable models with a combustion engine. "With an electric car, I have an incredible punch in acceleration behavior, and of course, you use that. At the same time, I don't have such a high top speed because my battery capacity is limited. Especially for the driveshaft, this is a huge re-adjustment from the combustion engine vehicle and we have to align our product design accordingly."

Increasing torque?

The future of autonomous driving and whether the trend towards ever-increasing torque and vehicle weight will continue over the next few years is uncertain. Probably not, as it can already be observed on German highways that drivers of electric cars mostly travel at speeds of up to 140 kilometers per hour. On one hand, this is due to the limited battery range, which significantly decreases at higher speeds. On the other hand, it may already be a part of the rethinking process towards electromobility.