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Comment by exDM69 | Hacker News Reader

I took an interest in this and looked up "retarder" on Wikipedia and it mentions engine braking (and exhaust braking), hydraulic retarders and electric retarders which are eddy current brakes more commonly used in the railways. Which one are you referring to?

According to the article, most commonly these are not used for regenerative braking, the energy is wasted as heat instead.

Eddy current brakes can be used for regenerative braking if there's an electric drive train (and high power electronics to handle it) but more typically the energy is just wasted as heat in a cooled resistor grid.

Mechanical/hydraulic braking can be used to spool up a flywheel and this was used in Formula 1 in late 2000's (now they use electrical regenerative braking instead).

Which of these techniques is commonly used in trucks?

https://en.wikipedia.org/wiki/Retarder_(mechanical_engineeri...

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Sorry, for not checking Wikipedia. Because I work in the industry as an electrical engineer defining standards, and owning a historic truck myself just for fun, because I can.

Originally, it is right that retarders were introduced to just having a non-destructive brake by just wasting the energy. That was around the 1980s. This was just to save maintenance cost as brakes were a big part of that costs.

The trucking industry is about costs and every little penny you can safe. So, years ago it started: Why do we waste that energy and do not re-charge the battery with that, because that reduces load from the generator and that will reduces fuel consumption?

Retarders have a huge impact in costs when your are counting miles. Retarders are having the disadvantage you can't brake to full stop with them. That's why they never worked in cars, because they are always additional to conventional brakes. So they add complexity and weight, both of which you want to reduce in cars.

My 2013 Mitsubishi Outlander Sport has regenerative braking. Seems to work ok but I'm not sure how much it actually helps.

Do note though that engine braking is banned in many population areas because it is so damn loud!

What fraction of the energy recovered by braking is actually stored? I would have thought the battery on a non-electric or non-hybrid truck would be far too small. Surely the battery gets topped off and then the extra power is dumped as heat right?

Is the energy from braking really so small that it can't even fully charge the starter battery?

Honest questions. I figured that since regenerative braking in cars seems to double fuel efficiency, that order of half the power used to accelerate was recovered and I would have thought that was a very large amount of power to store.

Or is it because of the nature of long-haul trips? I know hybrids have much less advantage on the highway, maybe for long-haul trips the ratio of energy lost as air friction to energy lost in braking is vastly different.

There will always be situations where you can no longer recover energy, fully electric or hybrid. Your battery could be full, or overheating. At that point, you have to waste the energy.

I believe the normal way to handle this scenario is to just fall back to mechanical braking.

Why do most/all electric and hybrid cars have regenerative braking while ICE cars don't? It's because, as the GP said, the battery would be quickly topped up and extra energy wasted. Only electric vehicles doing frequent stops/downhills can make productive use of regenerative braking.

Potential energy lost by a truck going 10 m (vertically) downhill: 40 000 kg * g * 10 m = 4e6 J.

Capacity of car starter battery: 40 Ah * 12V = 400 Wh = 1e6 J. Let's guess a truck battery has 4 times the capacity of a car battery, so 4e6 J to full charge.

That's just down one small hill and all subsequent braking energy for the entire trip is wasted.

To drive the point home, a Swiss company is working on a mining truck that will be a net generator of electricity - it goes up the mountain empty, but when it goes down it is full of heavy ore, and regenerative breaking charges the batteries more than was used to climb the mountain. The extra 10kWh can then be fed back into the grid.

https://arstechnica.com/cars/2017/09/this-cement-quarry-dump...

... Also because electric regenerative braking requires a battery and electric motor (for propulsion, the starter is useless for this). Once you fit regenerative braking into an ICE vehicle it becomes a hybrid vehicle, leaving no ICE vehicles with regenerative braking. :)

(I don't think you would call a vehicle with mechanical KERS system a "hybrid", although it technically is as there are multiple propulsion systems in the vehicle.)

> What fraction of the energy recovered by braking is actually stored? I would have thought the battery on a non-electric or non-hybrid truck would be far too small. Surely the battery gets topped off and then the extra power is dumped as heat right?

It is always the question of what type of truck you are running. Do you just have storage goods, or do you have goods that need refrigeration? In the later you need energy to power that. As your truck does not know in advance what kind of trailer will be towed (connected, what is the right word?) the truck needs to be prepared.

Yes, extra power is dumped. But that is true for any kind of vehicle.

> Or is it because of the nature of long-haul trips? I know hybrids have much less advantage on the highway, maybe for long-haul trips the ratio of energy lost as air friction to energy lost in braking is vastly different.

Actually, Hybrid is very good concept for long-haul. Because on highways you can do coasting (sailing as we say in German) very efficient when you are in cruise control. Because with all the little hills up and down at the same speed, hybrid takes its full advantage.

Regenerative brakes in an electric car use the engine, and add no weight and no complexity.

*motor. Engines are a type of motor that convert heat into mechanical energy (like, say, an internal combustion engine).

(Just being pedantic)

Thank you. I always wondered what was the difference.

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