A NASA Engineer Discovered a World of Semi Truck Aerodynamics by Accident | Hacker News Reader

A NASA Engineer Discovered a World of Semi Truck Aerodynamics by Accident

thedrive.com

61 pointsPaulHoule58 comments

As someone who works in commercial fleet fuel economy (powertrain, not aero, but you keep an eye on the whole stack since technologies compound), it's worth noting how far the industry has moved since the 1970s NASA Dryden work.

The sheer amount of "snake oil" bolt-on products sold to fleets in the 90s and 00s means the industry now demands rigorous third-party validation. The best open source testing I've seen comes from Mesilla Valley Transportation Services (https://www.m-v-t-s.com/certified-technologies/). They don't rely on ECM dashboard readouts or pump receipts. They run a control truck and test truck simultaneously on a track or controlled highway loop so weather, air density, and wind are identical. Anemometers on the trucks algorithmically factor out wind speed/direction, tyre temp, and weight. Tests are designed to answer the actual question rather than tick a standard's boxes. Their chief test engineer was ex NASCAR, IndyCar and F1.

My team actually tested a dual-fuel system with MVTS. The engineering was sound, testing proved a slight thermal efficiency improvement, but cheap US diesel versus the cost of the alternative fuel meant the ROI was non-existent. The physics worked, the math killed the project. (The economics work in Europe, thankfully.)

A lot of what's being discussed in this thread is already standard too. Mirrorless cabs are more the rule than the exception now (as pjc50 linked above), close-coupled trailers with gap fairings, wheel covers, under-trailer aero, all commonplace. A couple of current ones worth knowing about: the Scanias with the "Super" powertrain run a really aggressive overdrive at around 900rpm at the 90km/h (56mph)EU limit, where a conventional truck sits around 1200rpm. Requires huge low-end torque but claims 5-8%. The Mercedes Actros L claims 3% from its slippery front end alone. It is absolutely hideous though, so make of that trade-off what you will.

The biggest factor though is driver variability. All the aero and powertrain hardware in the world doesn't matter if the driver thinks they are the stig. The biggest shift in the last decade has been removing human inconsistency from fuel economy entirely.

Automated Manual Transmissions are now completely dominant in modern fleets. They shift perfectly for fuel efficiency every single time, totally capping the penalty of a bad driver. Layer on top of that Predictive Cruise Control. Modern trucks use GPS and 3D topographical maps to "see" miles ahead. The truck's computer knows exactly when to back off the throttle before cresting a hill to coast over the top, and exactly how much momentum to carry into the next dip. A human driver relying on their eyes simply cannot compete with a powertrain that knows the exact gradient of the road three miles ahead.

Thanks for the informative post. In America, I frequently see/hear the diesel engines idling all night at rest/truck stops. Smart cruise control that coasts perfectly over the top of a hill sounds great, but if the driver leaves his engine idling for 8 hours so he has ac… feels like that matters more, no? Has there been any sort of push for batteries or solar to power the sleeper cab amenities instead of running that engine while not driving?

A lot of the time, the idling you may hear next to a truck that's parked up on break may actually be a refrigeration unit. Most fleets have telematics that would give a driver a red flag if they idled excessively, a lot of trucks even turn themselves off (although it's easy to bypass).

On the market, there's battery electric APU's that drive AC units, like: https://voltaair.com/product/iq-no-idle-ac-unit-for-trucks/#...

But unless they are mandated, an idling truck uses <1 litre per hour, assuming AC over summer months only (as they will have a webasto style diesel heater for the winter), results in about $1,000 per year in idle fuel. Assuming a fleet has their vehicles on a 3 year rotation, the APU needs to be $3k or less, with zero failures for it to be economically worthwhile. (additionally, the charge for those batteries needs to come from somewhere, you can do retarder/brake enabled alternator charging which would help though).

So unless mandated by carbon emissions targets, the lower risk option is just to burn diesel.

However, at around 1k litres per year, that's around 2.5tonnes of CO2, so if cost isn't a lever that can be pulled to change this behaviour, CO2 may be.

Many trucks have auxiliary power units (APUs) -- essentially small generators -- to power the AC and other house loads at rest stops. Some states have anti-idling laws.

Although this has dropped off the FP now, I wanted to finish my response, but wanted to follow up with some data:

https://www.mikeayles.com/torque-rpm-time.png

So, here is approximately a weeks worth of data (around 700l of diesel), looking at the histogram on the right hand side, theres an awful lot of time spent at 0-10% this is around 60% of the trucks time.

https://www.mikeayles.com/torque-rpm-volume.png

However, when you multiply the time by the fuel consumption within that same time period, you get a volumetric chart, where we can see that 60% of the time, accounts for only 2% of the fuel use.

So, for idling, whilst it's noisy, produces CO2, a 5% improvement at highway speed will get you so much more of a fuel and co2 saving than eliminating idle. although, both, do both.

What are the axes on the chart? Rpm?

Correct, RPM and Torque (%)

edit: images may be down for 5mins, migrating my blog to astro.

Roger that - thanks for the OC graphs. In the first chart, the sloping cluster of data points, is that just the max torque output of the engine then? And the vertical clusters at ~950, ~1050, and ~1110 - can we assume those are some sort of cruise control or gearing outcome?

Also, your 28.5M+ miles driven is such a great stat - thanks for sharing this knowledge!

Correct, the engines will have a peak reported torque at one particular RPM, usually quite low (maybe 900RPM off the top of my head), at higher RPM, the peak torque may only be 90% of the peak torque, but at 50% more RPM, you end up with (100 * 0.9 * 1.5)=35% more 'power' despite the torque being lower.

This is detailed in J1939 (canbus protocol) PGN (parameter group number) EC1 (engine configuration 1) if you fancy a rabbit hole.

The vertical clusters will be a combination of gearing and cruise control, exactly. At the vehicle speed limit in the top gear (usually 12) most trucks will cruise around 1200rpm (exception being the scanias with supercruise). It's likely this truck did a lot of urban driving or was in average speed zones.

You also get a lot of drivers (suprisingly more younger than older drivers) that are happy to set it at 53mph, get the green tick for the telemetry to try and get a bonus, but then have a really low stress drive. It creates a bigger speed differential so they can be overtaken by other trucks easier, and realistically results in 10-20mins over a 10hr shift, to some, a 10hr chilled out shift is better than 9hr40mins of pushing it.

Bit of googling suggests this is a whole, fascinating world of little improvements. However it's also both constrained and pushed forward by what's road legal.

https://www.volvotrucks.co.uk/en-gb/news/insights/articles/2... : removing mirrors allows for much smoother cab airflow

https://go2stream.com/blog/aerodynamic-truck-legislation-rea... : UK legalization of fishtail-like devices

https://www.kudauk.ltd.uk/aerodynamics-explained : Kuda on the UK allowing higher loads, and therefore benefiting from extra wedge devices on the top of the cab.

I'm sure there's a lot more out there. The eventual switch to electric will probably come with another round of aerodynamic improvements to maximise range, as with cars.

> removing mirrors allows for much smoother cab airflow

On brand new trucks (designed without mirrors) yes. However on older trucks the mirror design might be an essential part of the aerodynamic design and required for a low drag coefficient.

> Kuda on the UK allowing higher loads, and therefore benefiting from extra wedge devices on the top of the cab.

Cab-top deflectors are extremely common on every truck where the cab is not tall enough to cover a standard trailer (which is common, usually only the highest end sleepers are that tall e.g. Scania's highline cab on the R and S, Volvo's globetrotter xl and xxl, ...)

For instance on this hero image from scania's site every truck but the very shortest and the very tallest have a deflector: https://www.scania.com//group/en/home/products-and-services/...

Obviously if you run higher than standard trailers, you need a custom deflector.

I don't know too much about the USA side of trucking but in the EU the cab over trucks have been able to steadily improve their aerodynamics, and there has been a lot of effort into improving this. Obviously there is more frontal area compared to a passenger car but comparatively modern EU trucks are quite aerodynamic. (even though they may not look it)

There are other things to consider as well when building a truck, as mentioned the maximum length limits, comfort, power price and more.

also check Luigi Colani's quite bold designs:

https://www.colani.org/luigi_colani_Product_design_museum/Tr...

Meanwhile in Hollywood: https://www.fortressofsolitude.co.za/the-highwayman-reboot-t...

In one episode the helicopter actually detached and flew!

One of the comments on the original article plays aerodynamics off as unimportant, saying only up to 0.4 miles per gallon difference. Even if that number was true, this is an industry that gives drivers patches on their shirt or hat for averaging above 7 miles per gallon of fuel for the year. An extra 0.4 miles out of every gallon is insignificant at all.

Another development in trucking efficiency that seems really interesting is the Achates opposed piston two stroke. If their materials are accurate, through fluid modeling they've achieved a massive efficiency boost and emissions exceeding regulatory targets without urea injection. In a piston ported two stroke.

https://www.ccjdigital.com/regulations/article/15291029/acha...

This is the most recent news I can find about them, though, so unclear if these engines will ever be produced for road vehicles:

https://www.govconwire.com/articles/ga-asi-david-alexander-a...

I was hoping we'd see these engines running generators in Edison trucks one day.

> would increase fuel economy by 15-25%

Makes you wonder if/why truck companies don't have aerodynamics experts on their payroll.

Those figures are from the 1970s, when presumably there was a lot more low-hanging fruit. Still, I wouldn't be surprised if they invest a decent chunk into computation aerodynamical work as part of their body design.

You’re also touching on a rather large ignored issue in this article, that this NASA truck aerodynamics project was directly connected to the 1973 oil crisis and a government wide effort to reduce oil consumption, not just some epiphany by some guy on a bike.

Both can be true. Sounds like the classic: necessity is the mother of invention

I don't understand why you put such a negative spin on this. Who loses when trying to become more energy efficient?

They absolutely do design trucks for aerodynamics. Fuel economy is a huge avenue of competition for semi trucks.

The reality is just that with the drive cycles and costs and tradeoffs of implementing all this it's not worth it to go this far.

Second, this sort of "round the whole thing" approach has mostly been replaced with "the simulation says we can do a 90/10 if we just do X, someone print up a block and toss it in the wind tunnel and see how it does" type approach so the result tends to be more surgical modifications and use of dead air and less "smooth out the whole thing with fairings".

> They absolutely do design trucks for aerodynamics

I know next to nothing about trucks, and vehicles in general, but something I've noticed, and probably everyone else, is that trucks in the US looks very different from trucks in Europe, it's very easy to identify which one is a US-like truck vs a Europe-like one, because of the shape of the "cab" or whatever that part is called.

So one design has to be clearly better than the alternative, given that aerodynamics works the same all around the world, but still the designs are uniquely... unique.

But why is it like this? If trucks were designed for aerodynamics, shouldn't one of the designs have "won" by now, or are they truly equal in terms of aerodynamics?

Did a bit of research: https://news.ycombinator.com/item?id=47121031 - different countries have different legal requirements, despite air being the same everywhere.

Judging by the images only, those are all the ones I called "European" trucks though, the American ones are much longer. Is that also regulated so European trucks aren't allowed in the US and American ones aren't allowed in Europe? Because here in Europe it's really uncommon to see the US ones, and I'm guessing it's the opposite in the US?

One of them must be better aerodynamically though, must'n it?

Europe has length limits on the entire thing, so US trucks would require shorter trailers, which nobody really wants. Euro trucks also have significantly smaller turning radii, which makes navigating european cities and country roads… feasible.

Furthermore Europe has relatively strict speed limits on trucks, which makes aero something of a lesser factor since drag grows to the square of speed: european trucks at european speeds have a pretty significantly higher efficiency than US trucks at US speeds.

I could be wrong, but I thought the US had a mostly-national speed limit of 55mph, while the UK truck speed limit is 60mph, and the French truck speed limit appears to be 90kmh (56mph)

US speed limits are highly variables, they’re generally 55 in the north-east but on the western half they’re 65 to 70, and 75 in TX.

And that assumes the speed limits are respected at all, but the EU has required a hard 90kph limiter since 2005, tampering with the limiter is a criminal offense, so is tampering with the (also mandatory) tachograph which would reveal the first.

So while nothing prevents speeding up to that (and it very much happens) going higher becomes extremely dicey.

> US speed limits are highly variables, they’re generally 55 in the north-east but on the western half they’re 65 to 70, and 75 in TX.

I fear even that is misleading. Yes, the speed limits on undivided highways are often 55 in the north-east (or even 50 in a couple), but the speed limits on divided highways are all higher, ranging from 65-75 mph. See the two maps on the top right: https://en.wikipedia.org/wiki/Speed_limits_in_the_United_Sta.... I suspect that the majority of truck miles are on divided highways.

A number of the major freeways immediately around Boston are 55. Almost nobody actually drives slower than +10 mph. If you go 55 in the slow lane you'll get passed by other people in the slow lane!

I've seen truck speed limits of 55 pretty much everywhere I've been except Texas, though.

American designs are aerodynamically better. they are potenitilly safer (crumple zone) though I haven't seen any safety studies.

> American designs are aerodynamically better

But that goes directly against TFA, doesn't it? The final image is closer to the European design than the American one? Or am I misunderstanding the article?

> they are potenitilly safer

Maybe for the occupants, but for everyone else they seem strictly worse, not to mention the visibility much be much much worse, making it a somwhat iffy tradeoff.

You're misunderstanding the context. The US at the time regulated truck length to a patchwork of shorter lengths state by state and so cabovers proliferated and that's what NASA chose as a base. So if you start with that and "add aero" it's going to trend toward what looks like a modern European design.

There are no crumple zones in a truck really.. You have straight frame rails. I guess the cast iron engine block gets a little crumply if you hit it hard enough..

Any space becomes a crumple zone - rails are great at that, but it is still something. Though as I said - I have never seen a study of relative safety.

In a cabover truck you're the crumple zone >_<

Is shipping either design of large truck across the ocean even feasible? I'm thinking they might be too tall for standard car carrier ships, but IDK.

It’s not hard. There are European cabovers in the US (see the Bruce Wilson YouTube channel).

> despite air being the same everywhere.

The air may be the same everywhere, but roads and safety laws aren't.

This is down to regulations. The total length of the vehicle is capped in Europe so to get more useful cargo space, the cab is shortened to a cab over engine design with a completely flat (short) nose. Making the cab longer or more pointy/aerodynamic cuts from the length available to the trailer. There’s usually more money inside the trailer than in the fuel savings so everyone accepted this solution.

An other important regulation is truck speed limits, because drag grows to the square of speed.

The energy of a potential impact is the main driver of that regulation. Also increasing with the square of the speed, and with the (sizable) mass.

That does not change the result: drag is much less of a concern for trucks in Europe than it is in the US.

> important regulation is truck speed limits, because drag grows

No I just meant that drag was a second thought when the regulation was conceived on either side of the ocean. The regulation was not created "because of drag". The main driver was road safety. Efficiency and pollution were secondary. There are other ways to achieve them like improving aerodynamics or power trains, and much of this cost is paid by the freighter alone. This is why speed limits didn't increase as trucks got more efficient and drag went down. Because there's no way to reduce the potential energy of a 40-50t vehicle (or up to almost 90t Finland) travelling at 130km/h.

> No I just meant that drag was a second thought when the regulation was conceived on either side of the ocean.

Again, not relevant.

> The regulation was not created "because of drag".

That is not a claim I made. Please do not involve me in your fights against the ghosts you made up.

All trucks have standard shipping container trailers. Europe has strict length limits for the overall cab/trailer that preclude a hood in front of the cab. There is great resistance to changing the law, even though long-nosed trucks are vastly more aerodynamic, because the law provides protection for european truck makers from American competition.

Even though several Europeon truck makers compete is America. they have the designs and ability to compete

> even though long-nosed trucks are vastly more aerodynamic

Are they? I mean by intuition I'd assume the same, but seems it isn't considered so black and white as far as I can tell, I can find reasonable research pointing both ways, but I'm guessing you're basing that on something more conclusive? Mind sharing the link so I could too read up on it?

TFA literally has NASA doing research and seemingly reaching a different conclusion than "long-nosed trucks are vastly more aerodynamic", seems to be about more things than just the hood, and those can be/are optimized on both variations.

But again, I know little about all of this, but would welcome being educated on it more.

My understanding is that in Europe regulations focused on the length of the whole vehicle, whereas in America the length of the cab and trailer were regulated separately.

Also American truckers do a lot more long-haul work and American roads are noisier, so they prefer being more insulated from the engine.

This is a bit counter intuitive, but Euro style cabovers are much quieter inside than a 'conventional'. Plus cabovers can have air spring suspension, they're often more comfortable. The downside is much less space inside, which is a big deal for the long haul drivers in the states, whereas in europe, they rarely do more than a month at a time.

AFAIK nothing precludes having air bags on conventionals, it’s just optional / uncommon whereas it’s completely expected on euro trucks.

To clarify, the air bags isolate the cab from the chassis.

There is also suspension between the axles and the chassis which is 99% of the time air on the rear, leaf spring front.

I haven't come across a cab that is suspension isolated from the frame of a conventional, even though the axles are on air. Theoretically as the driver is in the sweet spot of a much longer wheelbase, rather than sitting directly over an axle.

> I haven't come across a cab that is suspension isolated from the frame of a conventional, even though the axles are on air.

They are very often on a simple suspension. The cab will have a pivoting mount at the front and sit on air springs in the back.

> If trucks were designed for aerodynamics, shouldn't one of the designs have "won" by now,

Because aerodynamics is not the only concern the manufacturers are thinking about.

The difference is in the regulation. The maximum allowed length for a semi-truck is shorter in the EU than in the USA. And since they both want to transport the same length of container the European truck had to be designed shorter.

My dad used to drive concrete delivery trucks in europe. Most were the european arrangement with the cab over the engine, but one of them he drove had the American style long-nose. It was so unusual that they nicknamed the truck after this feature. They called it "csőrike" which is hungarian for "the one with the beak".

He also said it was a lot easier to maintain the long-nosed vehicle. With the european cab-over design you have to first secure everything in the cab and then tip the cab forward to have access to the engine and transmission. While with the long-nosed concrete truck you could just open the hood and get to work.

Mostly length regulation, and the prevalence of "sleeper" cabs and other trip length related factors. Economies are a little more self contained and less overland-transcontinental in Europe as well - the amount of freight going Rotterdam to Istanbul is a lot smaller than the amount of freight going LA to Boston, and it's a significantly shorter journey regardless.

Front-engine designs are much easier for a mechanic to work on. Cabovers are much more length-efficient.

If we were overly concerned with aero, you would see multiple unit designs like Turnpike Doubles or triples (or in an extreme, Australian road trains) - https://www.fhwa.dot.gov/policy/otps/truck/wusr/chap02.cfm. For areas where we're too operationally locked in to singles, you would see 60 footer container trailers like they have in a few states in the western US.

You would also, in 2026, hopefully see multi unit designs where the trailers are self-propelled vehicles that can separate and dock automatically at half a mile an hour.

Videos can show the difference visually and note some wider patterns where Euro trucks continued to evolve in other ways -

https://www.youtube.com/watch?v=WpenLsHEHaY

https://www.youtube.com/watch?v=GZRHjhoURz0

https://www.youtube.com/watch?v=LVDIGe0y-to

https://www.youtube.com/watch?v=C0iUiyQOn5E

https://www.youtube.com/watch?v=_ILjQpEX_8Y

https://www.youtube.com/watch?v=KxRKFO_OiQM

They do, look for some semi-trucks on long haul routes, they have trailer skirts and/or trailer tails.

Part of the issue is the extreme constraints imposed on designers by road regulation, another is simply that the majority (75%?) of drag reduction is in a fishtail that needs to go on the container, the part of the truck that gets left behind at the loading dock, that is often not "owned" by the person who owns the truck.

Because the individual logistics companies are not developing the trucks. The aerodynamic properties of a truck are mostly determined by the model/brand, and this is a huge consideration at companies like Scania, Volvo or Mann. Not so much at the individual logistics companies.

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