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Comment by kackerd | Hacker News Reader

The other difference between trucks and cars is that power/weight isn't really important for the latter, and hugely important for the former. If there's an improvement that allows you to carry 5% more weight in your car, that doesn't matter much. Most of the engine's energy is being used transporting the engine around, and the weight of passengers and luggage is not an operative constraint. For trucks it matters a lot, because they just became 5% more efficient.

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The trailer tail aerodynamic devices are supposed to save 5% on fuel. They remain rare and 99% of those are left undeployed. The trucking companies don't seem to care much about easy cost savings.

From looking at it on Google images, could it be that this device has other downsides? Making doors more awkward to open, increasing turning radius etc?

A significant percentage of semis you see are nowhere near fully loaded.

Especially for local deliveries- the truck is often half-empty.

Don’t you mean half-full?

When you're doing deliveries half-empty is the optimistic take.

Can’t leave me hanging, brofus

Don't you mean twice as big as it needs to be?

The marginal cost of dragging 40ft of air is so small compared to the hassle of trying to determine which truck is the correct one for today’s deliveries and pickups, so the companies tend toward having all trucks the same size.

Or... semi full. Ba dum dum tsss

That's odd. Airlines would kill to install a cheap device on their fleet and save 5% on fuel. If diesel is $3/gal and the semi holds 200gal, that's about $30 savings per fillup and would pay for itself quickly.

Wind resistance goes up by the cube of the velocity, it would be a much larger savings on an airplane.

Still even at interstate speeds a 5% savings pays for itself pretty quickly.

The fuel tanks on an aircraft are a major portion of the weight of the whole craft. So make engines more efficient, and you can take less fuel, which means the plane is lighter, so you use even less fuel and can take even less, etc. As you reduce the amount of fuel needed to go a certain distance you can go further and convert two-leg flights to single hops, which saves all sorts of other costs.

The fuel tank of a semi truck is not an important proportion of the weight of the whole vehicle, so it does matter, but not so much.

They seem to be reasonably popular around here. I see them used on trucks on the Interstate probably more than 50% of the time.

They may not be worth it for short haul trucks, but long haul trucks I bet the savings adds up quick.

I was a semi-truck driver for a while. A lot of "fuel savings" gimmicks amount to sticking a card on your bicycle frame so the spokes hit it a lot and make it sound like you're going fast. It's bullshit. Why do some companies do it? Tax breaks and grants. Do they work. No. Why don't all the companies do it? There's actually a limited amount of companies who can apply for the breaks because the laws are incredibly fucking stupid when it comes to commercial trucks (and to drivers). Similar how there are limited amount of carbon tax credits you can apply for. Some companies just don't waste their time. I have nothing against protecting the environment. But bullshit gimmicks won't do a thing.

Crazy idea: don't store hydrogen as a liquid but as a gas. Hydrogen is lighter than air, so you've just reduced the fuel requirements by making the semi lighter than it otherwise would be.

Or just go to the logical conclusion: make semis blimps that use hydrogen to both float and as a fuel source.

... you're aware a variation of this was tried a century ago, yes?

Yes, but tech has improved a tad since then.

In every flight you take, the wings are filled with highly combustable materials, and yet we find ways to keep them from exploding. The same is true for all those batteries in electric cars, and of course gasoline.

Jet A is really not that combustible; it's very close to diesel/kerosene. If you take a lit match and drop it in a bucket of Jet A, nothing would happen, it'd just burn out. It's way safer than gasoline. It's combustible only when turned into a fine mist.

On the other hand, H2 would have exploded just from the spark of lighting the match.

You'd likely get by with dropping the match into gasoline most of the time too. Its not definite that there will be vapor from it in the right mix for it to catch fire before hitting the surface.

Gasoline also isn't quite as combustible as people imagine.

It's worth noting that hydrogen fires burn bright and quickly, but tend to not be very dangerous to people.

Gasoline and batteries are heavy and tend to stick to surfaces (including people) when they burn, almost like napalm. Hydrogen rushes upwards, away from people and surfaces, as it burns, and is generally quickly exhausted.

I strongly disagree with this statement.

I'm not sure what there is to disagree with. Fire around gasoline is dangerous for a number of reasons, but liquid gasoline pooled just isn't as volatile as people imagine.

The vapors are different. See both sides of this coin here: https://www.youtube.com/watch?v=DsZOE1nvlhI

Also, this: https://www.youtube.com/watch?v=vln5-HZpgcA

Don't try this at home. Vapors can change things dramatically and horribly. The videos lightly demonstrate this too, but it can be worse.

Liquid gasoline without the present of vapor is pretty much nonexistent. Dropping a match into some is going to be a problem.

In my experience the combustibility of det fuel and similar petroleum products depends primarily on whether the context of the conversation is praising modern safety engineering or heckling someone for using diesel as a cleaning solvent.

It would be more dangerous when spread over a large surface and allowed to vaporize.

You haven't made them lighter, you've made them more bouyant. A ton of feathers weighs the same as a ton of rocks, but a ton of rocks is far more aerodynamic.

I mean, this is sort of off-topic.... but I just love this video:

https://www.youtube.com/watch?v=-fC2oke5MFg

Sure, but the point is a more buoyant semi would lose less energy to friction between the tires and the road.

You're missing the point. Liquid hidrogen takes much less space, that's all. It is neither lighter nor heavier than the same amount of hidrogen gas.

And for buoyancy to matter, the trucks would need to be huge (zepelin style).

That works until there's a light breeze.

And if you have 20% less weight on the wheels, you now have 20% less traction and thus 20% (or more?) longer braking distance.

You still need the same engine output to accelerate 100 tons, but you only have the traction you would get from an 80 ton load. That's a bad thing. Meanwhile, you save a miniscule amount of energy by having less-loaded tires.

Anyway, you need to compress hydrogen to store any meaningful amount, which increases the density and makes it like a normal gas/liquid instead of making you more buoyant. So it's not even possible.

No, under ideal friction the reduced traction is cancelled out by the reduced mass that needs decelerating, so that braking distance remains constant.

But actually, braking distance vs mass is more like a bathtub curve. With very little weight (talking toy cars here) it's hard to grip a surface, meanwhile very large masses like trucks have huge amounts of kinetic energy to disperse through their contact patches, leading rubber to melt, which reduces its stickiness beyond a certain point. Trucks end up with longer braking distances in practice.

The point is that there is not reduced mass.

You have the same weight, but less traction. That is not the same as reducing weight, in which case the two cancel out as you said.

unless you store them in a black wing shaped bag and let the sun lift the fuel for you

You've confused weight with mass.

It becomes a bit of a transport density problem. A semi does not take up much more room than the cargo it's transporting (if fully packed) but a blimp needs to be significantly larger to carry the same amount. They're big, slow, and require a large amount of space to land on the ground. It just seems like more issues to overcome than hydrogen fuel cell trucks.

Could trains tow blimps?

It gets really exciting when they go through a tunnel or under high voltage lines.

Easy, we just nuke the mountains. No steep gradients and no more tunnels. I’m available for consulting work if anyone at CP or CN need me.

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