Only a smart mix of materials ensures a high level of ride comfort and a long service life
Continental is developing customized air springs and vibration-optimized mounts for the electric city bus of a leading manufacturer.
Not all springs are created equal
The big driver of innovation in buses has always been the truck sector. Even when it comes to the switch to electrified drives, buses can learn a lot from trucks, because the basic design is similar and the engines used are usually the same. So it's only natural to think that other components, such as mounts and air springs, could also be adopted one-to-one. However, it is not quite as simple as that, because the requirements placed on the springs are too different.
Basically, the air springs in buses and trucks ensure that the vehicle, load and passengers travel safely and comfortably – especially in poor road conditions. The compressed air filled in the rubber sleeve of the air spring cushions the vehicle in traffic and raises the vehicle to the correct ride height. This is one of the biggest differences between trucks and buses: the springs in trucks are usually designed for large spring deflections to compensate for height differences, for example at loading ramps or when loading containers. In contrast, shorter springs are used in city buses. A positive side effect is that the passenger compartment above can be designed with more space and freedom, and the technical development challenge lies more in the higher comfort requirements.
In the case of the all-electric bus, there are additional factors: because of the batteries, it is heavier than a diesel bus. And because they are located on the roof, the center of gravity is much higher.
Made to measure from a modular system
Defying the discords
For this, e-motors are important trailblazers and their mounts therefore have an important role to play. However, buses and e-motors are a challenging combination. This is because, as with air springs, the requirements for engine mounts in e-buses are quite different from those for combustion engine buses or electrified trucks. In an internal combustion engine, for example, the main task of the engine mount is to secure the unit in the vehicle in such a way that its vibrations are not passed on to the body – otherwise a ride would quickly become very uncomfortable.
Vibrations also occur with the electric motor, but they are hardly noticeable. Instead, there is a different phenomenon here: the high-frequency vibrations of the e-motor can excite surrounding body parts and cause them to vibrate. If the vibration is strong enough, it results in unpleasant whistling and booming noises. In contrast to the loud combustion engine, these noises are much more noticeable in the quiet e-drive.
Continuous strain in city traffic
In addition to the different acoustic conditions, the electric motor is subjected to very different demands in buses or trucks. In the city bus in particular, the mounts are subjected to particularly high loads due to the start-stop traffic. In the case of the e-bus, this is compounded by recuperation, i.e. the recovery of energy while driving. Coupled with the very high torque of the e-motors, the demands on the engine mounts are enormous. Every rubber compound reacts differently under such a high continuous load. If the wrong mix is used here, the service life of the mounts suffers enormously. But here, too, manufacturers benefit from the very broad Continental product portfolio, from which the right engine mounts can be quickly provided – from the right geometry to the right material mix.
The future will be lighter and reach further
In the e-bus of the future, mounts are also expanding their literally supporting role: in the past, auxiliary units such as the air or air-conditioning compressor were usually attached directly to the heavy internal combustion engine. With the lightweight e-motor, this is hardly possible. In the future, the smaller units will have to be mounted independently. It would also make sense to mount the batteries so that the vibrations of the battery packs, which weigh several hundred kilograms, can be better controlled. This not only offers a clear plus in terms of safety, but also protects the bodywork – and thus extends the service life of the bus. And after the trip to the moon and back, there might even be a trip or two around the world left in it – if you've bought the right ticket.
