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How Fuel System works in a Gas Station:

 

The Gasoline Storage Tanks

The gasoline sold at service stations is stored underground in buried tanks. Each holds several thousand gallons of gas. There are at least two of these tanks per station and each tank usually holds a different grade of gas. Having the gas tanks underground presents an obvious problem: If the gas must get to a dispenser (and your car's gas tank) located above ground, it has to defy gravity in order to get there -- like a waterfall flowing uphill. But moving the gas from its subterranean hideaway up to street level isn't as difficult as you might think.

Most service stations do the job using one of two types of pump -- a submersible pump or a suction pump:

  • A submersible pump, as its name implies, is submerged below the surface of the liquid, where it uses a propellerlike device called an impeller to move the fuel upward. Slanted blades on the rotating impeller push the water the way the blades on an electric fan push air.
  • A suction pump moves the gas using the principle of unequal pressure. A pipe is inserted in the water. A motor above the fluid level removes enough air from the pipe to decrease the air pressure above the gasoline. The motor continues to remove air until the air pressure above the gasoline is lower than the air pressure pushing down on the gas outside the pipe. The weight of the surrounding air forces the gas inside the pipe upward even as gravity tries to pull it back down. When the air pressure inside the pipe is low enough, the gas simply climbs up into the aboveground dispenser.

The major advantage of a submersible pump over a suction pump is that the impeller can push water over longer vertical distances. However, because the gas tanks at most service stations are located only a few feet below the dispenser, a suction pump is usually more than adequate for the task at hand. There's more to this process, though, and we'll explore it further on the next page.

The Check Valve

The route that the gas takes from the tanks to the aboveground dispenser isn't terribly complicated, though it may take a few minor twists and turns. When pumping is complete and the pump motor is turned off, the gas inside the pipe doesn't simply fall back into the tank. Instead, it's held inside the pipe by a check valve. The check valve, which is located above the gas inside the pipe, creates an airtight seal above the fluid. Although the bottom of the pipe remains open, the vacuum pressure created by the check valve holds the gas in place. This is a process known as keeping the prime.

Using a check valve to hold the gas inside the pipe prevents unnecessary wear and tear on the suction pump and assures that a supply of gas will remain in the pipe so that the next customer won't have to wait for it to be drawn all the way up from the tank. It may not seem like a big deal, but the process can take 10 to 15 seconds. That isn't a very long wait by any means, but it can be an eternity when you're waiting for gas to be pumped.

The power that drives the pumps usually comes from the same electric grid that powers the lights and appliances in your home, though a few states require that service stations maintain a backup power supply in case of power failure.

Now that the gas is on the way to the car and it's time for the customer to start pumping, how does the dispenser know just how much gas the customer has pumped? Considering the volatility of gas prices these days, that may be the only thing the customer may care about. Find out the key to this mystery on the next page.

The Flow Meter

As a driver, your primary objective at the pump is to get your tank filled so that you can get your carback on the road. The goal of the service station owner and the company that supplies the gas, however, is to know just how much gas you've pumped so they can properly charge you for it. That's where the flow meter comes in.

As the gasoline travels upward into the dispenser, it passes through aflow control valve that regulates the gasoline's flow speed. It does this via a plastic diaphragm that gets squeezed more and more tightly into the pipe as the flow of gas increases, always leaving just enough room for the proper amount of gasoline to get through. If you've set a predetermined amount of gas to be pumped, the flow of gas will slow down as you approach the limit.

This pipe also contains the flow meter, which is a cast iron or aluminum chamber containing a series of gears or a simple rotor that ticks off units of gas as they pass through. Information about the gas flow is passed on to a computer located in the dispenser, which displays the metered amount of gas in tenths of a gallon. As the temperature of the gas changes -- on particularly hot and cold days, for instance -- the density of the gas may change, causing an error in the amount of fluid measured by the flow meter. The computer compensates this error by taking the gas temperature into account as it records the flow and adjusts the price accordingly.

Wear and tear on the meter may degrade its accuracy over time, which is why periodic inspections are necessary. Typically, inspectors will use a container of a certain volume, pump gas into it and compare the amount in the container with the amount metered on the dispenser. If the amounts don't match, the flow meter will need to be recalibrated and possibly refurbished or replaced. Although regulations for pump calibration come from the National Institute for Standards and Technology (NIST), the actual inspections are performed locally, usually by a state's Department of Weights and Measures.

Now that the gas is flowing and the amount of flow has been measured, there's only one step left: getting the gas into the customer's car. But that's a trickier process that you might think. For instance, what if the customer doesn't know when to stop pumping? Will he or she get soaked in a potentially lethal eruption of runaway fuel? Let's find out on the next page.

The Blend Valve

One of the first things that a customer will notice at the pump is the variety of choices offered. In most cases, a dispenser will offer several grades of gas -- sometimes as many as five -- each with a different octane rating. The desired octane rating is usually chosen simply by pushing a button. Does this mean that there are five different underground tanks feeding into that dispenser, each offering a different grade of gas? That's not usually the case. In fact, the dispenser can produce as many grades as it wants from as few as two underground tanks, as long as one tank contains the highest grade of octane available at that station and the other contains the lowest. The grades are blended together at the pump -- not unlike the way you'd blend gin and vermouth to make a martini -- producing a kind of octane cocktail. The precise proportion in which the grades are blended determines the octane of the gas that enters the customer's tank.

This feat of gas pump bartending is performed by something called a blend valve. This valve has inputs consisting of two grades of gasoline, each from different tanks. A single, moveable barrier called a shoe is connected to both in such a way that it can be moved across the inputs with a single motor-driven ratchet. As the ratchet opens one valve, it closes the other valve in precise but opposite proportion. This means that when one valve is, for example, 90 percent open, the other valve is 10 percent open, creating a mixture that consists of 90 percent of one octane and 10 percent of the other. By shifting the ratchet back and forth, the blend valve can produce any octane of gas, ranging from the highest to the lowest grades stored in the tanks -- and all octanes in between.

Keep reading to find out how the dispenser makes sure that you don't overflow the gasoline capacity of your tank.

The Automatic Shut-off

When the customer removes the pump handle from its place on the side of the dispenser, this action activates a switch that starts the dispenser operation. (In some cases the switch is spring-loaded and activates automatically; in others, the customer must raise a small lever manually to begin the process.) At that point, the customer simply inserts the nozzle into the car's gas tank and pulls the lever. Stopping the flow of gas is just as simple -- the customer need only release the lever to cut off the stream of fuel.

But what if the tank fills unexpectedly to the brim and the gasoline threatens to overflow? As anyone who's ever operated a gas pump knows, the pump will switch off automatically. But how does the pump know when to stop pumping?

As the gas level in the tank rises, the distance between the dispenser nozzle and the fuel grows smaller. A small pipe called a venturi runs alongside the gas nozzle. When the end of the venturi pipe becomes submerged in the rising gas, it chokes off the air pressure that holds the nozzle handle open and shuts down the flow of gas. Unfortunately, this shutdown can sometimes happen before the tank is full as the rapidly flowing gas backs up on its way into the tank. This can cause the gas handle to spring open before pumping is complete, leaving the annoyed customer to squeeze the handle again and risk the possibility of overflow. Pausing briefly will allow the gas to continue into the tank and the pump nozzle to start pouring gas again.

For more information on fuel and fuel efficiency, take a look at the links on the next page.

Some Questions and Answers

How does a gas pump know when my tank is full?

This mechanism has been around for a long time, so it is safe to say there is not a miniature camerainside the nozzle hooked to amicroprocessor. It's purely mechanical -- and ingenious.

Near the tip of the nozzle is a small hole, and a small pipe leads back from the hole into the handle. Suction is applied to this pipe using a venturi. When the tank is not full, air is being drawn through the hole by the vacuum, and the air flows easily. When gasoline in the tank rises high enough to block the hole, a mechanical linkage in the handle senses the change in suction and flips the nozzle off.

Here's a way to think about it -- you've got a small pipe with suction being applied at one end and air flowing through the pipe easily. If you stick the free end of the pipe in a glass of water, much more suction is needed, so a vacuum develops in the middle of the pipe. That vacuum can be used to flip a lever that cuts off the nozzle.

The next time you fill up your tank, look for this hole either on the inside or the outside of the tip.

How Gasoline Works

In the United States and the rest of the industrialized world, gasoline is definitely a vital fluid. It is as vital to the economy as blood is to a person. Without gasoline (anddiesel fuel), the world as we know it would grind to a halt. The U.S. alone consumes something like 130 billion gallons (almost 500 billion liters) of gasoline per year!

What is it in gasoline that makes it so important? In this article, you will learn exactly what gasoline is and where it comes from.

Is the United States addicted to gasoline?

Ah, petroleum -- used in everything from lipstick and lubricants to motor oil and medications, oil is one product the world just can't seem to get enough of. The United States especially, which consumes roughly 21 million barrels of the stuff a day, has quite an attachment to this ubiquitous product [source: EIA]. And while oil can be refined into a variety of products, Americans seem to prefer theirs in the form of gasoline. In fact, the United States consumes more gasoline thanSouth America, Europe, Africa andAsia combined [source: EIA].

So what's with the United States and its gasoholic tendencies? Is the country truly addicted to gasoline, and if so, what factors led it to get hooked?

While the United States obviously has quite a fixation with the amber liquid, its fondness for gasoline probably doesn't fit the official criteria for an addiction. Rather, the affinity is more like a bad habit spurred on by a number of government policies put into place over the years. Combine a relatively wealthy nation with low fuel taxes, low fuel efficiency requirements and a poor public transportation system, and you have the perfect climate for a gasoline obsession.

As opposed to other countries like Denmark, where high purchase taxes on cars can deter driving, the United States has few roadblocks to impede their gas-guzzling ways. Quite the opposite, in fact -- with a vast road system crisscrossing the country and relatively cheap fill-up stations every few miles, what are American citizens to do? Why, drive of course! And drive they do, as there are more than 244 million vehicles roaming U.S. highways -- 755 cars for every 1,000 people [source: DOT, Pentland].

Lots of cars don't automatically equal high gasoline consumption though. Consider Portugal, which has 773 cars for every 1,000 people, yet consumed less than 45,000 barrels of gasoline a day in 2004 [source:Pentland, ≠EIA]. True, the United States is much larger than Portugal, but that's not the only reason its gasoline consumption far outpaces every other nation. Despite the fact that Americans now own fewer vehicles than they used to, the vehicles they do own travel farther and require more gasoline than those of any other industrialized nation [source: Pentland].

Why the discrepancies? Keep reading to find out.

What speed should I drive to get maximum fuel efficiency?

This is actually a pretty complicated question. What you are asking is what constant speed will give the best mileage. We won't talk about stops and starts. We'll assume you are going on a very long highway trip and want to know what speed will give you the best mileage. We'll start by discussing how much power it takes to push the car down the road.

The power to push a car down the road varies with the speed the car is traveling. The power required follows an equation of the following form:

road load power = av + bv≤ + cv≥

The letter v represents the velocity of the car, and the letters a, b and crepresent three different constants:

  • The a component comes mostly from the rolling resistance of the tires, and friction in the car's components, like drag from the brake pads, or friction in the wheel bearings.
  • The b component also comes from friction in components, and from the rolling resistance in the tires. But it also comes from the power used by the various pumps in the car.
  • The c component comes mostly from things that affect aerodynamic drag like the frontal area, drag coefficient and density of the air.

These constants will be different for every car. But the bottom line is, if you double your speed, this equation says that you will increase the power required by much more than double. A hypothetical medium sized SUV that requires 20horsepower at 50 mph might require 100 horsepower at 100 mph.

You can also see from the equation that if the velocity v is 0, the power required is also 0. If the velocity is very small then the power required is also very small. So you might be thinking that you would get the best mileage at a really slow speed like 1 mph.

But there is something going on in the engine that eliminates this theory. If your car is going 0 mph your engine is still running. Just to keep the cylinders moving and the various fans, pumps and generators running consumes a certain amount of fuel. And depending on how many accessories (such as headlights and air conditioning) you have running, your car will use even more fuel.

So even when the car is sitting still it uses quite a lot of fuel. Cars get the very worst mileage at 0 mph; they use gasoline but don't cover any miles. When you put the car in drive and start moving at say 1 mph, the car uses only a tiny bit more fuel, because the road load is very small at 1 mph. At this speed the car uses about the same amount of fuel, but it went 1 mile in an hour. This represents a dramatic increase in mileage. Now if the car goes 2 mph, again it uses only a tiny bit more fuel, but goes twice as far. The mileage almost doubled!

What's the difference between gasoline, kerosene, diesel, etc?

The "crude oil" pumped out≠ of the ground is a black liquid calledpetroleum. This liquid containsaliphatic hydrocarbons, or hydrocarbons composed of nothing≠ but hydrogen and carbon. The carbon atoms link together in chains of different lengths.

It turns out that hydrocarbon molecules of different lengths have different properties and behaviors. For example, a chain with just one carbon atom in it (CH4) is the lightest chain, known as methane. Methane is a gas so light that it floats like helium. As the chains get longer, they get heavier.

 

The first four chains -- CH4(methane), C2H6 (ethane), C3H8(propane) and C4H10 (butane) -- are all gases, and they boil at -161, -88, -≠46 and -1 degrees F, respectively (-107, -67, -43 and -18 degrees C). The chains up through C18H32 or so are all liquids at room temperature, and the chains above C19 are all solids at room temperature.

So what's the real chemical difference between gasoline, kerosene and diesel? It has to do with their boiling points. We'll get into that on the next page.≠

What does octane mean?

If you've read How Car Engines Work, you know that almost all cars use four-stroke gasolineengines. One of the strokes is thecompression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with aspark plug. The amount of compression is called thecompression ratio of the engine. A typical engine might have a compression ratio of 8-to-1.

Car Engine Image Gallery

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

 

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Last modified: Monday June 01, 2015.