The Autopian wants me to write about how a wind tunnel works, and I like the idea. We always see nice wind-tunnel shots of cars, but how do you actually get data from that? I’d love to tell you, as such a story would allow me to touch on many aspects of aerodynamic design, but here’s the hiccup: If I cram all that info in a single article it will turn into a textbook! So in order to protect our collective sanity, I’ll take you for a ride in theory land for a few articles. Don’t leave! I’ll keep this digestible and entertaining! And by the end of today’s post you’ll even understand how a carburetor works.
[Editor’s Note: It’s Manuel, the aerodynamicist from France here to tell you about how aerodynamics works! Last time, he told us about drag coefficients, and now here’s a discussion about how fluids flow, particularly in the context of pressure, velocity, density, and more! -DT]
Lifting The Universe’s Hood Reveals: Thermodynamics
When we talked about drag coefficients, I threw a lot of terms around without giving much explanation. Today we’ll take things from the very start and talk about energy. Yes guys and gals, we’re about to do thermodynamics. Breathe, breathe — it’s going to be fine.
Thermodynamics (or energetics as my teacher used to say) is the science of movement and what creates it. It basically covers the entire realm of energy except for subatomic dark magic. But what is energy exactly? Energy is a quantity of stuff that allows movement, and is measured in Joules. That’s it. It can take a few extra steps, but if your “energy” can’t move something eventually, it’s not energy. Poltergeists? Energy. BDE? Not energy.
Let’s see the different types we’re likely to encounter on a car website:
- Kinetic energy: if something is moving, it’s got energy. It’s written E=½.m.v² (m=mass; v=velocity)
- Potential energy: if you lift something, it can gain speed while falling. Height is energy. The equation is E=m.g.z (m=mass, g=gravity, z=height)
- Electrical energy: it can create magnetic fields that move crap. Electricity is energy. I know nothing about that shit so let’s skip the math and just trust me on this
- Thermal energy: heat can make water boil, shaking a cooking pot’s lid. Heat is energy. E=C. ΔT (C=heat capacity, ΔT=temperature difference)
- Chemical energy: when you burn gasoline it creates heat, which is energy. Gasoline is energy. So is Start Ya Bastard! E=m.LHV (m=mass, LHV=Lower Heating Value)
Now, what’s really amazing is that you can turn one type of energy into another because of THE FIRST LAW OF THERMODYNAMICS: No energy shalt be created, nor shalt energy be destroyed. The number of Joules shalt remain constant for all of eternity or the universe shalt explode like a Holy Hand Grenade.
I know it seems oh so far away from designing race car spoilers, but trust me, thermodynamics can be used for everything and can change your entire view on the world, including driving. You don’t trust me? Here, let me demonstrate via something we all know and love: a road trip.
A Trip To Thermodynamicstan
It is 6:00AM; you sit in your ultimate driving machine, half asleep. You turn the key, allowing the flow of current from your battery to power an electric pump, which makes fuel flow from the tank to the engine. You just converted electricity into kinetic energy in your pump’s motor, which transferred its own kinetic energy to a fluid.
You turn the key one more click, allowing the battery to power a starter motor that gives your inline six its first revs of the day. You’ve converted electrical energy into kinetic energy again.
As the pistons move down, they draw fresh air mixed with a fine mist of gasoline and compress it. Your battery then fuels a spark plug, creating the hot spot that will ignite the mixture, increasing pressure in the cylinder. That pressure pushes against the piston which in turn makes your crankshaft spin. You’ve converted mechanical kinetic energy into fluid kinetic energy and then fluid potential energy (pressure). You then used electricity again to create heat that allowed the conversion of chemical energy into more thermal energy. That heat then expanded a fluid, which pushed against a surface, creating more mechanical kinetic energy from the chemical one. And you’re now ready to go!
9:00 A.M.: After the first part of your trip spent in plains with the gas pedal at the exact same position for hours, the terrain starts showing small elevation changes. You don’t feel like moving your foot, so you lose speed while gaining height, and regain speed as you descend. You’ve traded kinetic energy for gravitational potential energy while climbing hills, and done the contrary when descending.
11:15 A.M.: You’ve now reached the mountains, requiring you to give it the beans. More throttle input on the way up means some brake action on the way down. You’ve increased the car’s gravitational potential energy while maintaining its kinetic energy, meaning you had a surplus of total mechanical energy in the slopes, so you used your brake pads which converted kinetic energy into thermal energy (your discs heated up) that got flung away from your vehicle, decreasing its overall energy level.
1:00PM: Time for lunch! The burger you’re about to swallow whole is also chemical energy!
See? It works with everything.
Thermodynamics covers all the aforementioned fields. It’s a powerful tool that can give you a holistic understanding of any system, no matter the physics involved. (Except for electromagnetism, which is the work of the devil as far as I know. [Editor’s Note: I got a 62.5% on my electricity and magnetism exam in physics in college. The curve brought that test up to a B. I aced the course despite knowing very little about E&M. -DT]).
1/2*m*v²+m*g*z =constant
Indeed, mechanical energy for a solid is nothing but the sum of its kinetic energy (1/2*m*v²) and its gravitational energy (m*g*z) and it remains constant because the first law says so!
Putting The pressure On
Remember how we said pressure is equivalent to energy per unit of volume? I’d like to expand on that as pressure is the clay us aero people build cars, planes and hand dryers with. First, what is pressure? I mean, it’s weird right? There’s air pushing on my skin the whole time, yet I don’t really feel it? My ears pop when driving up mountains? C’est quoi ce bordel ?
Unlike in solids, molecules are not neatly arranged in crystals and immobile relative to each other when they are in their fluid form. If you were to look at the air around you, using the best microscope ever, you’d be able to see the NO2, O2, CO2 and their buddies moving around and hitting each other like drunken sailors on cocaine dancing to Dragon Force. They move around like crazy, and as a consequence, bounce on each other as in the messiest game of snooker ever. That’s called the Brownian motion.
There are two ways to control the intensity of those shocks happening in a given volume of air. You either compress it, pushing the molecules closer to one another (sort of like packing more sailors on the dance floor), or you heat them up (turn the music up), increasing the sailors’ (or molecules, whatever) energy and therefore the number of impacts.
If these moving particles encounter a solid, they will impact it too and push it away, creating a force. That’s what we called static pressure (noted “p”) last time, and it is what’s keeping your tires in a nice donut shape instead of a deflated mess. I said pressure is energy per volume as it’ll take you more sweat (i.e. energy) to inflate you car’s tire to 2 bars (29 psi) than it will to do the same thing to your bike’s because the first one has a more important volume than the second.
That gives us this equation:
E=p*V (p=pressure, V=volume)
The Bernoulli Principle
At this point you are probably wondering where I’m going with all of this physics talk. Rest assured, now is the time where it’s all coming together!
So, we know what pressure is, and that it is energy if we multiply it by a volume. We also learned about the first law of thermodynamics and how it applies to solids (the sum of kinetic energy and potential energy remain constant). Some Swiss dude named Bernoulli put all this together and defined the conservation of mechanical energy for fluids in 1738. It goes like this:
If we divide everything by the volume of fluid we’re analyzing, as aerodynamicists love to do, we get this:
1/2*m/V*v²+m/V*g*z+p=constant
“m/V”, that rings a bell…
1/2*rho*v²+rho*g*z + p=constant
Hey look! It’s our pal density (that’s “rho”)! And his buddy dynamic pressure! Man, I’m so happy to see them again that I’m gonna make this equation even simpler, neglecting the gravity stuff because our systems are not big enough for it to matter in aerodynamics:
1/2*rho*v²+p=constant
Hey that’s rather easy to understand now! This is the fluid version of the conservation of mechanical energy, but expressed in terms of pressure. Indeed, the first term is what we called “dynamic pressure” and the second one “static pressure.” The sum of these is called “total pressure” and just like before, it implies that if we gain speed (½*rho*v²), we lose pressure (p), and vice versa because the total pressure remains constant. And that is the principle behind…
The Venturi Effect
This is probably something you already have heard of, but can you explain precisely how the venturi effect works? And were you aware it’s the foundation on which carburetors are built? Please answer “no,” otherwise there’s no point reading this article!
Now that you know about the first law of thermodynamics, Bernoulli and all that jazz, I’ll throw you one last life-altering information: the conservation of mass. It’s simple; you can’t create nor destroy the amount of merde the big bang threw around. You can rearrange atoms or whatever, but you cannot pull mass out of your derrière.
That is relevant to aerodynamics as it applies to pipes. If a quantity of stuff gets in a pipe, the same quantity goes out. Let’s consider Italian plumbers! If four a-plumbers! enter a pipe every minute, four of them have to come out in the same interval. This is our Mario flow rate (MFR), measured in Mario per minute (mpm) and has to remain constant throughout the pipe, because we can’t create nor destroy a Mario in the ducting. There are several ways to achieve this MFR; you can either push a group of 2 Marios in every 30 seconds, or push them one by one every 15s. Now, a couple of video game heroes needs a bigger pipe to pass together compared to a single mustachioed mushroom enthusiast, but the latter will have to run faster than the Peach pursuing pair to achieve the same MFR of four mpm. So if you want your red sporting character to enter the duct walking with his doppelganger, but leaving running, the pipe needs a bigger cross section at the intake than the exhaust. This is probably intuitive for those of you who have put your fingers over the end of a hose to restrict the outlet area to increase the outlet velocity.
Managing sections in ducting is how engineers manipulate air speed, and therefore air pressure, and that is precisely how a carburetor works. The amount of air going through the carb at any second is dictated by the engine’s displacement and rotational speed, and remains constant throughout the engine’s induction system. At the intake of the carb (1), the pressure is the same as the atmospheric one, but there’s a restriction in the middle of the carburetor (2), accelerating the air speed and conversely decreasing the air pressure. Since the fuel in the carb tank (3) is at 1 bar thanks to a vent, that pressure difference will suck the fuel through the nozzle (6), allowing your Atlantique-300 to fire right up!
Using a restriction in a duct to accelerate a driving fluid in order to pump a driven fluid thanks to the pressure difference is the Venturi effect. A simple application of the Bernoulli principle and the conservation of mass allowing the existence of amazing things such as paint guns, F1 under body down force or, more importantly to a Frenchman, wine aerators.
Conclusion (It’s French for “Conclusion”)
Pfew! Done! No more science today! Light up a Gauloise or a dumpster to celebrate, because we have just covered a majority of the basic concepts of aerodynamics: pressure, Bernoulli and the conservation of mass. All that theory will be put to good use later when we try and design wind tunnels for the next JAM 808.
But before that, our next episode will have to deal with the evil twin of the first law of thermodynamics: the second law of thermodynamics. Stay tuned for your upcoming hit of weird equations, approximate analogies and half-assed schematics!
The notion that Electromagnetics is some kind of Dark Art is self-serving for its practitioners, myself included. It’s better to think of Electromagnetics as a more generalized version of Electricity that applies when wavelength approaches the size of objects in the circuit. For example, 6 GHz WiFi signal has a wavelength of 50mm so a cable will have voltage and current traveling in waves over its length. A 60 Hz AC power signal has a wavelength of 5000 km so the while the same waves are present, the variation is negligible over practical lengths. The same rules govern both cases, but one allows some simplifications.
I still have no idea how the underlying mechanisms work or what I could do with that info. I’ll keep saying it’s dark magic until I can write an article about it!
I bet you can’t!
Great article! There’s another effect of decreasing the pressure in the Venturi chamber of a carb. Because of more math done by guys named Boyle, Charles and Avogadro, we know that the temperature of the gas will decrease as the pressure decreases.
This concept does neat things like make air conditioning and refrigeration possible.
It also means that if your carb pre-heating system fails in your carbureted 1988 Honda Accord DX hatchback, certain atmospheric and driving conditions (like high humidity and temps in the range of 35-50 degrees F) will cause ice to form in the Venturi chamber. This will then clog the fuel nozzle and result in the eventual loss of all engine power, which will then force you to pull over to the side of the road and “wait for the damn carb to melt.”
This joyous circumstance will prompt you, the proud owner of one of the last carbureted Hondas ever, to spend hours digging through a nightmare of vacuum lines and connectors to locate a single failed $1.50 vacuum check valve.
Sounds like fun! /s
That is an interesting phenomenon, akin to the condensation atop aircraft wings under certain conditions.
I’ll make a note in my to do list to write about AC someday, which is a hairy topic! Phase changes and latent vaporization would deserve their own article.
Periodically I go re-read how ammonia refrigeration works, because cooling things by burning propane is a great brain teaser.
Whaaaaat? Care to share a link?
They bend my brain, for sure! These are commonly used for RV refrigerators (at least in North America; presumably everywhere but I don’t know for sure) because they can run off the camper’s propane bottle and a 12V battery when commercial power or generator power isn’t available.
https://en.m.wikipedia.org/wiki/Absorption_refrigerator
The whole thing almost feels like some kind of weird thermodynamic scam, but it clearly works.
Good old carb icing, been there done that more than once. Was difficult to figure out the first time it happened to me as by the time I got to pulling off the (aftermarket) air cleaner the ice had melted and the car restarted and drove off.
PV=nRT!!!! I must have used that equation a million times as an undergraduate in chemistry. It actually sounds like a great exam question. Calculate the amount of fuel that must go through a venturi to condense 2 grams of water vapour on a carb.
I’d say it’s not about fuel but air speed and temperature. The perfect gas law has no impact there as it only allows you to compute density.
You’d need to know the air speed based on the mass flow. If you want to be perfectly accurate you should use this abomination which I’ve now learned by heart because aerospace:
W.Ti^0.5/(pi.A)=M.(gamma/r)^0.5.(1+(1-gamma)/2)^(-(gamma+1)/(2.(gamma-1)))
And you need another equation to compute the air’s static temperature: Ti/T=1+(gamma-1)/2*M^2.
Maybe I could teach you guys about isentropic flow equations but not right now, it’s stuff I can use but hardly explain. Some digging will be necessary!
You’d also need to know how humid the air is, and compute it’s dew point temperature (not sure about the English terms). That’s a whole lotta physics we may talk about again some day ! I actually messed with it a little for aircraft icing.
Your exam sounds hard as hell man!
I love these articles in the days of dumbed-down clickbait and outright pap.
Translate fail, English sucks sometimes 😉
I think you mean greater volume, but honestly, anyone clever enough to make it that far would figure it out for themselves.
Many thanks for sharing your knowledge! I look forward to more!
Oops, my Frenchiness is showing! Sorry about the mistake.
You’re most welcome, I strive to Educate and Entertain 😉
This is peak Autopian. Technical & geeky, but still fun & irreverent! Damn fine job Manuel!
Thank you man! That’s what I was going for!
Oh shit, that was a LOT of info to take in and digest.
I plan to be absent on the day of the test. Already got a note from my Mom.
Yeah I know that’s a lot of stuff crammed in a single article. Imagine if I had put everything in a single piece, adding this with pressure loss and how wind tunnels work.
Rest assured, there are no tests at the Autopian!
Hey Manuel, thanks for the article. What an unexpected turn of events, learning about Mario, 18th century damsels and carburetors, just after reading about cooling challenges, snakes and Jurassic Park… and I haven’t even gotten out of the bed yet!
Oh wait, this is the Autopian, so it is exactly what I should expect from this place 🙂
Seriously, though, I loved the absurd examples you used – that is the best way to learn in my opinion. I assure you I will NEVER not think about Marios per second when I think about the flow of any fluid! In fact, could I request MORE Mario science next time? I was kinda hoping you would explain carburetors using Mariology, but I reckon that it could get a bit silly.
Keep this up, if my teachers were like you, I wouldn’t have dropped out college the first two times (the third one was much more reading than anything else, so I could just pretend to read and earn the degree).
P. S. Nice BMW you got there. I hope the next article needs more pictures of it, maybe with a schematic or two 🙂
Muito obrigado cara! Finally someone who gives the Bangle bimmer the praise it deserves!
I’m not going to Marioify every subject but I like using fun analogies. Next time we’ll learn about pressure loss with our bodies!
Pull my finger? I’ll give you some pressure loss.
I thought about this but went in another direction. You’ll see!
Hey,i DO remember some french words.How about that?
And i thought i took french class purely because of the hot teacher
Well, most French words here are curses so I don’t know how well that reflects on you!
Nice work! As a PhD chemist and a teacher, I could pick a few nits, but you conveyed the essentials well enough, I reckon. And I do applaud you putting all this in terms of energy, which is a very under-appreciated concept IMHO. For any other thermodynamophiles out there, I can highly recommend a short book by the inimitable Peter Atkins (physical chemist extraordinaire) called Four Laws That Drive the Universe (https://archive.org/details/four-laws-that-drive-the-universe). It’s a very accessible and enjoyable book that explains the laws of thermodynamics (yes there are four).
Pick away friend! I’m a little shaky in a few concepts. I’m willing to bet I’m not 100% right regarding the Brownian motion, I’ve never spent time thinking about thermodynamics in that way while working so my understanding may be limited.
And I’ll try and check that book 🙂
It took me two days to read this (it was Friday night), but kudos for writing something even a non-engineer could understand, albeit it took a couple of reads to grasp it (for me). Great style and just enough humor to make it fun.
Thanks man, I’m glad I managed to make it clear enough and enjoyable! I don’t want to be writing only for engineers!
I have a PhD in physics, and some days I think one of the most generally useful things I learned was to think in terms of energy and energy transfer.
So many situations can actually be a lot simpler by thinking in terms of thermodynamics, and it will often cut out a lot of the unnecessary details.
It seems that most of our lives are spent just in the battle against entropy. We fight to maintain and perhaps increase order in our small corner of a universe which is ever marching towards ultimate disorder.
Well written.
Hey I found someone like me! Seeing it all in terms of energy is great at cutting the clutter and getting a quick understanding of physical systems.
Glad you enjoyed the article!
My brain hurts a little bit after reading this article (and the one on drag) but oh boy do I enjoy in-depth stuff like this! (and the suspension stuff)
Complex concepts explained in a way that that dummies like me can grasp, that’s not easy to do and I commend the writers and The Autopian for publishing this kind of article. Fantastic stuff as always
Thanks a lot! I try and minimize the brain pain but it’s still thermodynamics so some is to be expected!
Smarter Every Day does a nice video on this.
https://youtu.be/toVfvRhWbj8
I knew about that vid! The slo-mo is awesome!
You have piqued my curiosity.
I suddenly find myself reading about something I would generally take no interest in. And enjoying it.
That my friend, is the greatest compliment you can make!
Thanks. Good morning and good night.
Well written! So says someone with Aero and Mechanical Eng degrees and is currently a Technical Writer. Simplified enough for everyone, techincal enough for others to follow along.
Thank you! It’s hard to strike that balance when you talk about thermodynamics, I’m glad you liked it.
To improve upon this article, regarding its accessibility to a lay person, I’d recommend explicitly stating that rho means air density. Someone could easily miss that without making sense of the equations presented, and in the US, most people are not mathematically inclined(albeit, I’m certain the readership here is more competent in math than the average, as low of a bar as that is).
Other than that, this was a good read, even to someone like myself who went through textbooks during high school and college while I studied to become an engineer, I found it to be entertaining.
I’m extremely interested in learning how a wind tunnel’s measurements are taken and how a drag coefficient value is derived. That is something that I have not studied and remains a mystery to me. And because I’m obsessed with vehicle efficiency, I’m also obsessed with aerodynamics by default.
Thank you. I’ll keep your comments in mind regarding equations.
And don’t worry about wind tunnels, I actually started writing about them before that article but I realized I wanted to cement some concepts first, so the article is already half written. It’s coming!
I thought it was a good level of detail. Very interesting, easy to understand. More please!
Yes sir!
Another good read, thanx.
Can Mario go supersonic? (-;
Given the mount of rounded surfaces on Mario, I don’t see an angle at which he won’t create very strong shock waves. Therefore, I think he’s destined to remain in the subsonic realm!
Can Mario go supersonic? (-;
Yes. In a vacuum.
In a perfect vacuum there’s no gas, therefore no sound and no speed of sound!
Thank you, I love this! But sometimes I just want to know does it suck or does it blow? 🙂
It’s a distinction without a difference. It’s all about the pressure delta!
Enjoyed it very much, thought the geek level was perfect for this place!
Really enjoyed your common-sense description of carburetors. I have a motorcycle with a manual choke, and I always envision something like that diagram when I’m operating it.
If you would, how about some info on the various jets (pilot, main, etc.) – how do they operate in terms of which one supplies the fuel? I know they each handle different throttle ranges, but how does the carburetor switch from jet to jet?
To be perfectly honest, I have little experience with carburetors. Maybe I’ll dig around some more into this later but I see carbs as a mere excuse to talk about Bernoulli at the moment 🙂
I don’t any questions but loved the article! Great Stuff!
Thank you!
Hey guys, Manuel here! Just like last time, I’ll try and stick around to answer your questions in the comments despite the time difference!
It’s 10:30pm here so I may not stay long (I’m getting old) but come back tomorrow to check if I answered you 🙂
Also feedback is welcome as that’s VERY geeky content, I wonder if you guys are into it or if I should ease up on the physics.
There are enough engineers here that lots of us appreciate the physics, but your writing is accessible enough for everyone. Very Autopian of you!
As an engineer and, first and foremost, a pedant, I do have an objection to your definition of energy as allowing movement. That’s essentially defining energy as the ability to do work, but that’s exergy. Not all energy has the ability to do work, and the loss of that useful, work-doing energy, ie exergy destruction, is a basis of the 2nd law of thermodynamics.
Thanks, making it accurate but accessible is my ultimate goal.
You are right about exergy, but it’s not a term most people are familiar with (even I had never used it before!) AND there’s a whole article dedicated to entropy in David’s inbox right now. That’s why I didn’t want to go in too much detail today!
Enjoyed this. I’m familiar with the basics, having grown up with carburetors and currently working with refrigeration, but I enjoy these kind of articles because being exposed to different explanatory frameworks can help me analyze & troubleshoot. I liked the road trip lead-in there in particular, and will share it with coworkers.
Looking forward to more!
Awesome, glad you liked it!