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Published on August 13th, 2018 | by Michael Barnard


Tesla Semi Trucks Won’t Use Runaway Truck Lanes As Often As Others

August 13th, 2018 by  

The Tesla Semi has a couple of reported advantages out of the box over normal semis. It will go up hills a lot faster and it won’t jackknife. But what about one of the other bugaboos of trucks: going so fast down hills that it needs to turn into a runaway lane with loose gravel instead of crashing?

All else being equal, Tesla Semi’s will have fewer problems slowing down hills.

What causes a runaway truck problem with existing 18 wheelers? Two things. The first is picking up too much speed and not being able to downshift for drivetrain braking without destroying a transmission. The second is reduction or loss of braking from other brakes due to air pressure loss for the air brakes (less likely on modern trucks) or drum fade due to prolonged braking.

Engine braking will be different for the Tesla Semi. It has no gears. It has no gearbox. The four electric motors on the drive axles will use regenerative braking under almost any conditions to provide braking force, something engine-braking in a diesel semi can’t permit as often. As a result, inability to downshift to get engine braking is no longer a problem.

But that’s only providing braking to 8 of the 18 wheels of one type. Engine braking is one of four braking systems, including air brakes (aka service brakes), parking brakes, and emergency brakes. Regenerative braking doesn’t eliminate air pressure loss or brake drum fade to all wheels.

One question that could be asked is whether regenerative braking will be sufficient to brake an 80,000 pound, fully loaded semi truck. Well, let’s look at something else that uses electric traction motors to provide braking: 8 million pound diesel-electric freight trains. They use something called dynamic braking instead of regenerative braking. All that really means is that the electricity generated from braking has nowhere to go on a diesel-electric freight train, as the electricity for the motors is supplied by a massive diesel generator instead of a battery. As a result, they route the electricity into heating coils on the roof of the locomotives.

There is a clear safety proviso that must be taken into account with Teslas. Due to their ability to go much faster uphill as well as accelerate faster, there will be more situations where too much speed is carried into a downhill. There’s no replacement for safe driving. It’s unproven how Tesla Semis that are in Autosteer will deal with upcoming steep downhills. That’s not necessary to account for with their cars.

In net, it’s highly likely that there will be fewer runaway Tesla Semis, but there will still be some.

Related: The EV Safety Advantage (2018) 


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About the Author

is Chief Strategist with TFIE Strategy Inc and co-founder of two current startups. He works with startups, existing businesses and investors to identify opportunities for significant bottom line growth and cost takeout in our rapidly transforming world. He is editor of The Future is Electric and designing for health. He regularly publishes analyses of low-carbon technology and policy in sites including Newsweek, Slate, Forbes, Huffington Post, Quartz, CleanTechnica and RenewEconomy, and his work is regularly included in textbooks. Third-party articles on his analyses and interviews have been published in dozens of news sites globally and have reached #1 on Reddit Science. He's available for consulting engagements, speaking engagements and Board positions.

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