Modern Gas Pumps

[PapaSmurf2k3]

So I was wondering the other day where the blend valve is located on a modern single-handle gas pump. In the past, you could go to the gas station and there would be one line/handle for each octane, but many modern ones just have a single line, and you push a button to select your octane.
A blend valve adjusts to mix a higher octane fuel with a low octane fuel to achieve the desired resultant octane... but where is this blend valve located?

If its in the pump unit itself, you get a whole line's worth of low octane gas (assuming the person before you was pumping regular)... not really a big problem for people with big fuel tanks... but what about for bikers and stuff?

[Amays U G37S]

I think its all at a central point within the gas pump itself before the one line exceeds out of the gas pump.

For instance, the main tank is in the ground near the pumps. This feeds a line to a sub running line that feeds into the gas pump. I think only one seperate line goes to the actual gas pump.

so if theres 3 octanes, and 3 tanks in the ground, and you choose which octane, the pump then releases the valve for this tank, that gas then flies towards your gas pump via a line, and into the single hose assembly.

This is why the pressure at some gas pumps differ, because the lines may have to go a certain way through the ground and up to your pump.

However, all 3 lines may run to your gas pump, and when u choose an octane, the gas is blasting towards the valve until it is opened, but this would cause failure and over usage in most gas pump assemblies. But i don't think you get s*** gas before the real stuff comes out. I just run a can of seaform every 3-4 gas tanks. I fill up for $45 and get 350 miles to a tank. Sucks a**, Lol!

I am not for sure, this is my .02 cents

[Dattebayo]

Not all gas stations are the same, not all pumps are the same.

[PapaSmurf2k3]

so if theres 3 octanes, and 3 tanks in the ground,

Gas stations typically only have 2 different types of fuel (and therefore 2 different tanks) in the ground. The octane you select is just a varying mixture of those 2 fuels.

[PapaSmurf2k3]

Yes, I'm saying that even stations with like, 5 different octane levels (say 87, 89, 91, 93, 94) only have 2 tanks underground.
Say one tank is 94 octane, and the other is 87 octane. The blend valve would be completely one-sided for 87 octane (only letting fuel in from the 87 tank), then would blend the 2 tanks to get anywhere between 87 and 94, and would then be completely one-sided for 94 octane (only letting in fuel from the 94 tank).

I just want to know where this blend valve is... I'm thinking its in the pump unit itself, but then you get hosed every time you fill up on premium and the person before you got regular... because everything between the blend valve and the nozzle is going to be whatever the last guy bought.

[Amays U G37S]

The only way you don't get the other persons gas is when you release the handle, and it shuts off the main valve going to that pump.

But if 8 people are using pumps, or 16 people are, then theres no way that the tank can open from under ground. There has be a main feed line into a seperate line which goes to each pump. So when 5 people pump 87, and 5 people pump 93, the gas will feed into a larger line, and then into smaller lines which run into the pumps.

I believe when u release the handle, the pressure from the open valve cuts off immediately and has a draw back. Ya there may be reminants of 87 in ur 91, but you're still pumping 99.9% of 91 octane.

[PapaSmurf2k3]

The only way you don't get the other persons gas is when you release the handle, and it shuts off the main valve going to that pump.

But if 8 people are using pumps, or 16 people are, then theres no way that the tank can open from under ground. There has be a main feed line into a seperate line which goes to each pump. So when 5 people pump 87, and 5 people pump 93, the gas will feed into a larger line, and then into smaller lines which run into the pumps.

uh... what?

So anyway... I guess we are all in agreement that the blend valve isn't in the nozzle, so you're definitely getting a line full of whatever the dude before you bought.

Let's assume that its a .5" feed line going from the pump to the nozzle, and that it's 8' long (96" long). The formula for volume of a cylinder is pi*r^2*h.
SO you get: 3.14*(.5^2)*96 = 75.36 cubic inches of volume in the line, which would mean you get 75.36 cubic inches of whatever fuel the dude before you bought before getting to what you are paying for. That's .32 US gallons.
If you have a 3.5 gallon tank, that's 9.1% of your total tank volume. If you require 93 octane, but the guy before you bought 87, then you have effectively lowered the octane of all the fuel in your tank from an ideal 93, to 92.454.

Not bad I guess, but it only gets worse the smaller your tank gets.

[Dattebayo]

Typical gas hoses are 10-12" long, remember they have a bend up top and a support...

But the machine allows air to fill the hose after the cut-off happens and you drain it by holding the handle down... It's only if the previous guy didn't drain the hose after the initial cutoff that you will get "hosed". lol But that gas isn't metered, so it's free... And you can drain it before you put it into your car if you please. I always have an emergency can...

[PapaSmurf2k3]

But the machine allows air to fill the hose after the cut-off happens and you drain it by holding the handle down...

Are you sure about that? If air goes in, then gas has to come out. Its got a sensor on it to determine when the tank is full, and then shuts off... if it drained the rest of the hose, it could possibly overfill the container.

[ADDirishboy]

Are you sure about that? If air goes in, then gas has to come out. Its got a sensor on it to determine when the tank is full, and then shuts off... if it drained the rest of the hose, it could possibly overfill the container.

A small handheld container, maybe. But I've never had a pump cut me off high enough to overflow with the remaining gas in the line. Hell, most pumps cut you off a good gallon before you're actually full.

[PapaSmurf2k3]

Next time I'm filling up, I'm going to test this "drain" theory. Pump will shut off, I'll yank the hose out, and see if approximately 1/3 of a gallon continues to come out.

[Rev_D21]

Don't forget, people like myself will squeeze the handle after the pump is shut off to try and get every last drop out of the hose.

[PapaSmurf2k3]

^yeah but that re-activates the pump.

[Rev_D21]

Not if you have lowered the activation lever that the nozzle rests on. Once you shut off the pump it's just about letting the residual fuel dribble out of the hose, no pressure used.

[PapaSmurf2k3]

Not really... there is a valve right in the nozzle that shuts off flow from the tip.

http://www.howstuffworks.com/engineerin ... -pump2.htm
http://www.howstuffworks.com/engineerin ... -pump5.htm
http://www.howstuffworks.com/fuel-effic ... l-tank.htm

[Mind-Numb robot]

Modern fuel dispensers use electronic dc valves to (based on flow) adjust for the middle grades. Most midgrade fuels are blended at 30-40% premium, sites that use ethanol blended fuels have a less premium ratio usually 5% and arnt worth the midgrage price.
If the nozzle clicks at the end of your prepay sale your wasting your time draining the hose. If it doesn't click your only getting a few drops hardly worth the effort.
DO NOT fuel up while tankers are delivering fuel. The turbulence caused by new fuel interning the tanks
causes FOD (funky old debris) or water to be pumped to the dispensers. The dispensers fuel filters help to prevent FODs but IMO its not worth the risk. Name branded sites and sites that guarantee their fuel should be okay.

-Rob the gas guy

[Amays U G37S]

Wow good info, i used to try to wait during, or the same day, as the tanker. Many times I have fueled up while they were delievering fuel.

[PapaSmurf2k3]

Thanks for the insight Rob!

[Mind-Numb robot]

Modern gas stations have a pumps in the tank that delivers fuel to the dispensers.
Electronic blending valves are located between the dispenser meter and hose outlet.
An empty hose and nozzle will hold about 0.2 gallons of fuel so I'd imagine you would get that much from the previous grade selected.
Most nozzles have a mechanical lever valve that do not open at zero pressure (valve closed).

[Amays U G37S]

Hahahah yes!

i knew it was in the pump before the single hose to dispense.

[PapaSmurf2k3]

Modern gas stations have a pumps in the tank that delivers fuel to the dispensers.
Electronic blending valves are located between the dispenser meter and hose outlet.
An empty hose and nozzle will hold about 0.2 gallons of fuel so I'd imagine you would get that much from the previous grade selected.
Most nozzles have a mechanical lever valve that do not open at zero pressure (valve closed).

Cool.
On the bright side, I was right. On the down side... we're getting porked with ~.2 gallons of low grade junk.

[ADDirishboy]

Cool.
On the bright side, I was right. On the down side... we're getting porked with ~.2 gallons of low grade junk.

Is that small amount of lower quality fuel really going to mess with anything though? I can't see .2 gallons of regular mixed in with the rest of your premium fuel making a difference.

[PapaSmurf2k3]

If you have a 3.5 gallon tank, that's 9.1% of your total tank volume. If you require 93 octane, but the guy before you bought 87, then you have effectively lowered the octane of all the fuel in your tank from an ideal 93, to 92.454.

Not bad I guess, but it only gets worse the smaller your tank gets.

[Alfador]

If you fill up in $5 increments, it could be a large-ish but still ultimately insignificant portion. If you fill up in larger quantities like you should, it's completely insignificant. It's like when you go to a restaurant and order a 13oz steak. You know it's going to be roughly 13oz, but that they're not going to shave little bits of meat off of one steak and put them on another to make them all match perfectly. It's only something to worry about at all if you have an engine that requires higher grade gas anyway, and let's face it, 90% of people who buy premium grades of gas don't.

Even for a biker with a small tank I'd say it's still a relatively insignificant amount from a fuel chemistry standpoint. It's less than a single digit even in a small tank, and if cars rated for higher octanes were that sensitive to a 1 octane rating drop in their fuel there would be a lot more problems with them. I wouldn't be altogether shocked if the engineers who design the systems had figured this out over the last few decades and designed around it.

[PapaSmurf2k3]

Yeah it's really not a big deal... its just the thought of getting screwed that bothers me haha.

BTW I filled up yesterday and immediately yanked the nozzle out of the tank when it stopped. Maybe half a cup came out... about what would be left in the nozzle from the check valve (in the handle) to the tip.