Yutong extreme heat challenge in Saudi Arabia. Source: Yutong
One of the highlights of my recent trip to Yutong’s factory and testing grounds in Zhengzhou, China was hurtling around their high speed test track at 80-90km/h in an electric bus.
There were a few shakes and rattles, plus something beeping as we flew around the steepest part of the track, but I felt completely safe and confident inside that bus.
Even if the bus were to somehow lose control and crash into a barrier, I was reassured by the promise we’d have 2 hours to escape from the bus before any potential battery fires entered the passenger compartment, as I’ll explain below.
Yutong not only leads the world in terms of bus and coach sales, the Yutong Electric Safety System or YESS has been at the heart of all their battery electric buses since 2021, enabling safe and reliable operation under any climate or road conditions around the world.
YESS consists of 5 different levels of protection, developed through extensive research and real world experience operating electric buses in diverse and harsh climates. From -33°C within the Arctic Circle at Kirkenes in Norway, to Jeddah in Saudi Arabia where surface temperatures surpassed 60°C during extreme heat testing, pictured above.
During the factory tour, we got to experience many of the rigorous validation procedures that every single Yutong bus passes through before leaving the factory. Yutong is clearly proud of their testing and validation procedures, as evidenced by a proverb displayed on the hill climb portion of the test track.
“Don’t consider the market as a test ground, consider the test ground as the market.” This is in stark contrast to some electric vehicle makers which seem to revel in using their buyers as beta testers.
The durability of Yutong battery packs and high voltage components is impressive. We witnessed one of their electric motors and motor controllers submerged under 2m of water during a 72 hour test, demonstrating how they exceed IP68 plus IP6K9K standards for dust protection, water submersion and high-pressure, high-temperature water jets.
To help speed up testing under extreme climate conditions, Yutong has an environmental chamber in the factory where they can set the temperature, humidity and wind to whatever parameters they desire. Engineers said this allows them to verify operation in extreme climates in around 1 week, much quicker and cheaper than shipping buses overseas.
As we passed through the environmental chamber in operation, it was set to -20 or -25°C which felt nice for the first minute or two compared to the nearly 40°C outside temperature. However, it soon became uncomfortable so we moved on before developing icicles like the windscreen of the bus being tested.
As mentioned already, Yutong Electric Safety System or YESS is made up of 5 different layers of protection that covers everything from the overall vehicle structure and design, battery pack safety and last but not least, continuous monitoring of bus components.
Firstly, Yutong electric buses are designed with passive safety features to protect battery packs from being damaged during collisions with other vehicles, including heavy trucks and pick up trucks travelling up to 50 km/h.
Individual battery packs are installed at the top, rear and bottom of the bus chassis, located inside cage-like structures made out of 1500MPa ultra-high strength steel. Steel plates and thermal insulation material add further layers of protection between the battery compartments and passenger compartment.
Overall, these passive design features greatly reduce the risk of short circuits caused by battery intrusion and also increase the amount of time passengers have to safely evacuate a bus if there was a fire in one of the battery packs, from 5 minutes to 2 hours.
The most interesting aspect of YESS for me was the nitrogen protection system that Yutong has developed, which helps prevent thermal runaway from occurring in their battery packs. This system works by continuously injecting inert nitrogen gas into the battery pack, effectively cutting off the oxygen supply and removing one of 3 necessary ingredients for fire.
Nitrogen is harvested from the surrounding air using aviation-grade high-purity nitrogen separation technology, and pumped into the battery pack to maintain less than 1% oxygen concentration and greater than 98% nitrogen concentration. Nitrogen also has a cooling effect on the battery cells, further reducing the risk of thermal runaway.
Improved resistance to high humidity environments and water ingress are added benefits of the nitrogen protection system, as slight positive pressure inside the battery pack is maintained, expelling moisture and helping to reduce corrosion from occurring over time.
When asked about the efficiency impact of the nitrogen system continuously running, Yutong said it has less than 1% impact on the range of the bus which is impressive. I also wondered whether it could be scaled down to work with passenger EVs, although it would probably take up valuable cargo space in a car and have a greater impact on range.
During the factory tour Yutong also performed a live demonstration using 2 battery packs, one fitted with the nitrogen protection and one without. Obviously Yutong was not concerned with our safety during the test as we were standing less than 10m away behind a simple seat-belt style barrier you find at airport check-in lines.
As expected the battery pack on the left hand side without nitrogen protection caught fire, with a loud bang initially as the safety valve let go. Real flames can be seen coming out of the pack near the flames on the banner behind. It was soon extinguished, but the lingering smell was still pretty bad so the crowd quickly dispersed to enjoy our dinner and drinks in the factory nearby.
Yutong designs and builds their own battery packs, using LFP battery cells sourced from Contemporary Amperex Technology Co., Limited or CATL as they are widely known. With CATL and Yutong working in partnership, these battery packs are designed with a 1.5 million kilometre, 15 year warranty.
Each individual 88 kWh battery pack is fitted with its own short-circuit protection device, which quickly detects any abnormal overload or short-circuit and disconnects that battery pack from the rest of the high voltage system on the bus.
Battery packs also include a mica layer on top, enclosed within a steel case and then a high temperature ASC cover, which means a battery pack can withstand an external fire burning at 1,300°C for 2 hours without catching fire itself. This is pretty insane and should put anyone at ease who worries about the fire risk of a Yutong battery electric bus.
To ensure Yutong battery packs are kept at the optimal operating temperature, they employ liquid heating or cooling systems depending on the environment. To reduce the risk of high-voltage failures, there are no high-voltage heating components within the battery boxes.
Instead, each battery pack is connected to a single high-efficiency liquid heating system. The efficiency of this centralised system combined with the pack level insulation from the sandwich structure described above also contributes to higher efficiency of the bus overall.
Monitoring is an often overlooked area of system design, either completely missing or haphazardly added as an afterthought. My years at Google as a network and site reliability engineer taught me how important good monitoring systems are, you simply can’t tell if something is working properly without decent monitoring.
Thankfully, Yutong seems to understand the importance of good monitoring, as their intelligent battery management system (BMS) monitors various components of the battery pack 24-hours a day and provides at least 26 kinds of safety early warnings according to one of the plaques next to a battery pack I saw on display.
24/7 monitoring includes nitrogen levels and air tightness of the battery pack, ensuring the nitrogen protection system described above is functioning correctly at all times. All of this monitoring data is streamed remotely, allowing fast detection of problems and timely repair of buses in the field.
I found the level of safety and monitoring designed into Yutong’s electric buses to be both interesting and reassuring, it is no wonder their buses are among the safest and most reliable. Yutong has also published this short Youtube video summarising YESS if you prefer to watch a video describing these technologies in more detail.
Yutong has not stopped innovating since YESS was introduced in 2021 either. In June 2023 they launched Yutong Electric Architecture (YEA) which was the first EV specific platform in the commercial vehicle industry to integrate both hardware and software developed by Yutong, further boosting efficiency and reliability.
YEA combines the Yutong C-architecture or cross-domain integration architecture, with Yutong Operating System or YOS and continuous OTA updates. C-architecture features a centralised computing unit and less than 10 hardware controllers compared to several dozen in traditional designs.
After receiving valuable feedback on my previous Yutong article, I hope to learn and share more about what other bus makers are doing to design and build safe, reliable battery electric buses for the future of public transport.
Note: The author travelled to China as a guest of Yutong.
Tim has 20 years experience in the IT industry including 14 years as a network engineer and site reliability engineer at Google Australia. He is an EV and renewable energy enthusiast who is most passionate about helping people understand and adopt these technologies.
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