nothing venturi’d, nothing gained

Entropy is a harsh mistress. Three years ago, when I moved to Los Angeles, I bought the first car I'd owned in 20 years: a brand new 2007 shiny red Prius. It was all downhill from there, starting with the first little ding, a scrap on the bumper, a new back fender thanks to some jerk who sideswiped the Prius in a parking lot, knocking loose the bumper, and then didn't bother leaving a note, and so forth. Sure, I bring it in for regularly scheduled tuneups, but hey — things fall apart. Entropy happens, and I'm okay with that. And as a conscientious car owner, I've familiarized myself with the basics about how it works. But every now and then something happens that makes me realize how little I really know about my car.

Take last week, when I went to fill the car up with gas, and for some reason, the pump didn't shut off automatically when the tank was full, as pumps are supposed to do. Fortunately, I do know how many gallons per fill-up I normally get, so I sensed something was wrong and stopped pumping. It wasn't quite soon enough: gas began burbling out of the tank and running down the side of the car. This station had a mechanic on duty who helped me siphon out some of the excess gasoline, and rinse off the spilled gas.

Then he pointed out that I seemed to be missing the little metal doohickey that usually covers the tube leading to the gas tank –  you know, the thin piece of metal that you see whenever you unscrew the gas cap to insert the pump handle. His theory: the gas pump didn't stop automatically when the tank was filled because that piece of metal responds to the rising levels of gas in the tank, and the pump senses that rising back pressure and stops. Without that piece of metal, there was no way for the pump to know when to stop. Neither one of us could think of what the doohickey is actually called (and neither could the mechanic at the Toyota dealership), but apparently it's pretty vital. When it comes to car parts, size doesn't matter. Even a tiny piece can have serious ramifications if something goes amiss. 

I screwed in the gas cap and drove off, and brought the Prius in for repair a few days later, since it was due for another tuneup anyway. No doubt there's a few car buffs out there who've already figured out that the gas station mechanic was wrong about the cause of the trouble. The Prius doesn't have one of those metal doohickey things over the gas valve — never had it, never will. The Prius repairman was a bit vague on the details, but apparently all that pressure sensing equipment is located elsewhere in the tank on the Prius. Which means that it wasn't my car that was at fault; it was the actual pump at the gas station that malfunctioned.

Being of a curious bent, once I got home, I turned to teh Google and started nosing around the Interwebz looking for information on gas pumps, gas tanks, and so forth. The ever-reliable How Stuff Works provided an excellent explanation for how gas station pumps work, as well as a brief summary of how it knows (ideally) when to turn off. It's purely mechanical: there's a small hole at the top of the nozzle, connected to a small pipe that leads back to the storage tank in the pump. A device called a venturi (basically a small pipe) is used to create suction at one end. If your car's tank isn't full — which, at the outset, it probably isn't, otherwise why are you pumping gas? — air gets drawn through that hole in the nozzle because of the vacuum that's been created. But eventually the level of gas in the tank gets high enough that it blocks the hole. Another bit of mechanics in the pump handle senses the change in suction, and the pump automatically turns off.

But what happens when that pressure sensing mechanism fails, as it did in my case? Well, the tank overflows. Duh. This is potentially a bigger problem in a Prius because rather than a solid tank, it has a flexible rubber fuel bladder, purportedly because it does a better job of preventing gas fumes from escaping. So when I over-filled my tank, I basically created a water balloon, and the flexible bladder squirted the excess fluid back out the way it came in. If this happens once, it's not such a big deal (although it did screw up my gas gauge reading for awhile), but routinely over-topping when you fill the tank can eventually lead to destruction of the vapor-capture system — a pricey repair.

(Incidentally, that flexible rubber bladder is also why there's often such a big discrepancy between the "official" fuel tank capacity cited by Toyota for the Prius (11.9 gallons for the 2010 model) and real-world usage. That 11.9 gallons only applies in temperatures above 70 degrees F, apparently. In colder weather, the rubber gets more rigid, just like what happens if you put a rubber band into the freezer. It has less "give" and thus holds less fuel — sometimes as much as 2-3 gallons less — because of the above described mechanism that causes a gas pump to shut off automatically in response to back pressure from the tank.)

So there was nothing wrong with my Prius, after all, and now that's it had a full tune-up, it's purring along almost as good as new — at least until entropy starts taking its toll again. And as a result of my little adventure, I stumbled across a physical concept that doesn't get bandied around much these days at cocktail parties (in the blogosphere or otherwise); the Venturi Effect.

The name derives from Giovanni Battista Venturi, an Italian scientist who started out as an ordained priest, going on to teach logic at a local seminary. Logic leads to geometry and philosophy, and eventually Venturi found himself teaching physics at the University of Modena in 1776 (he was contemporary with Lagrange and Laplace). Venturi championed Leonardo da Vinci's scientific achievements — not just Leonardo's artistic gifts — and also published many of Galileo's surviving manuscripts and letters.

There are scant details readily available about Venturi, but at some point he became intrigued by fluid flow and the work of Daniel Bernoulli, best known for developing the Bernoulli Principle detailing the relationship between a fluid's pressure and its velocity. But what happens when a fluid flows through part of a pipe, for example, that has been constructed? Well, the pressure drops — not because the fluid is traveling more slowly through the constricted pathway, but because it is flowing more rapidly, in accordance with something called the equation of continuity. That's just a differential equation that describes the transport of some kind of conserved quantity, usually things like mass, energy, momentum, or electric charge. If something is conserved, that means that quantity — say, of energy — cannot increase or decrease, it can only move from place to place, or take a different form. Energy conservation also dictates the drop in pressure: per Wikipedia, "the gain in kinetic energy is balanced by a drop in pressure." And as someone who now has a passing familiarity with calculus, I was pleased to read that Venturi mathematically derived an equation for his eponymous effect from a combination of Bernoulli's principle and the equation of continuity.

So now we know why the pipe in gas pumps that keeps track of the rate of fluid flow is called a venturi. There's a similar device known as a de Laval nozzle: a tube pinched in the middle so that it forms a kind of hourglass shape, that can be used to get a hot pressurized gas to speed up as it passes through. You'll find de Laval nozzles in steam turbines, rocket engines, and supersonic jet engines, for example — basically anything that involves the combustion of a hot gas.

As for the Venturi effect, it's well nigh everywhere. It can be found in those perfume atomizers wielded by aggressive shopgirls that spray you with unwanted scent as you maneuver your way through a department store, as well as the nozzles of fire extinguishers and automated pool cleaners that use water pressure flows to collect debris. It's used to regulate the slow of oxygen in scuba diving gear; in recoilless rifles; in the capillaries of the human circulatory system (blood flow); and in the barrel of modern clarinets, where it helps speed up air as it flows down the tube to produce better tone. The Venturi effect also comes into play in your car's carburetor. Diandra's the resident expert on automobiles at the cocktail party, so I'll let her explain:

A venturi is used to create a region of low pressure where the gas is mechanically sucked into air flow and into the intake manifold. The carburetor was invented by Karl Benz (Yes, of Mercedes-Benz) in 1885. The throttle on a carbureted car does not control the amount of fuel going inot the engine. It actually meters how much air is being pulled into the entire and the speed (which determines the pressure), automatically puts the right amount of fuel into the airstream.. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream.

A carburetor basically consists of an open pipe, a "throat" or "barrel" through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi: it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. 

Oh, and there's one more interesting application for the Venturi effect, specifically for hardcore oenophiles: it's exploited in the wine aerators that are used as a speedy shortcut for decanting wine. We received one as a gift last year — along with a traditional decanter — and it fits neatly over the mouth of the decanter, essentially mixing air in the wine as it flows through the device. But the type that uses the Venturti effect is a handheld injection-style acrylic aerator — similar to the devices used in engineering — like the carburetor system Diandra describes above. That's a whole 'nother blog post in its own right, and I know Diandra's got one in the works. So I'll let a Florida wine-maker named Tim have the last word on wine and the Venturi effect:

Tim's Wine – The Venturi effect from Pixel 1080 on Vimeo.