How Things Work: Fuel Tank [VIDEO]

The fuel tank in our cars is something that we don’t often think about, however, it is a surprisingly complex component and a very crucial one. In this episode of How Things Work, we will be taking you through the overall fuel system.

Estimated reading time: 7 minutes, 56 seconds

If you can’t watch the video, you can read the video transcription below: Edited for clarity and readability

Pieter: (00:00)

Welcome to this week’s episode on How Things Work. And in today’s segment, we’re going to be taking you through the humble fuel tank that we have on our cars. Now, this is something that you perhaps, have never even bothered to think about, but it’s a surprisingly complex component and a very critical one in terms of being part of our overall fuel system. So, in addition to just storing fuel, the fuel tank does a few other things, including preventing vapours from escaping out into the atmosphere, and it also prevents contaminants from getting into the fuel system. But most importantly, it contains a whole lot of components that are critical in terms of pumping and measuring the fuel that goes from the tank to the engine itself. So let’s have a look at some of the components that we find inside the fuel tank itself or on the fuel tank.

Pieter: (00:55)

The first is the fuel filler neck. Now most of us sometimes get to the fuel station and we forget which side our fuel cap is on. So an easy way to figure it out while you’re still sitting inside the car…. So, if you’re driving somebody else’s car or you’re driving your partner’s car and you get up to that fuel pump, 99% of the time, the fuel filler cap is on the opposite side to where your indicator stalk is on. So if your indicator stalk is on the left-hand side, the filler cap is going to be on the right-hand side of the vehicle and vice versa. Of course, most manufacturers have actually printed on the cluster itself where the fuel pump icon is, a little arrow that tells you which side the filler cap is on. So between those two, you shouldn’t have any problem figuring out where your fuel filler cap is next time you pull up to the fuel pump and you can avoid those embarrassing moments where you’re trying to turn around the vehicle and figure out which side the cap is on.

Pieter: (01:44)

The second thing that we find, obviously from the fuel tank, are the fuel lines. Now there are quite a few fuel lines that go out of the fuel tank and then return back in and they serve different roles. But in the main, we have an outlet pipe that goes through to the injectors and we have a return pipe that comes back in and vents any excess fuel back into the tank itself. And of course, we need to have a fuel pump to pump the fuel. And these days, most fuel pumps are actually immersed in the tank itself. And in fact, they’re in a splash chamber and that splash chamber just allows for the pump to have a constant supply of fuel, even though we might be going around corners and going over bumps, which would cause the fuel to splash around.

Pieter: (02:40)

Now, it’s very important that we understand as well, that it’s not a good idea to run our tanks on empty because we need to have that fuel to actually cool the pump itself. So you run the risk of damaging your pump if you constantly drive on an empty tank. And I know that in these tough economic times, that’s always easier said than done, but try to avoid running your tanks on empty because you are putting your pump in danger of overheating. 

Now, another critical component that we have inside the fuel tank is of course our fuel level sensor, and this tells us exactly how much fuel we have in the tank. And the way this works is very simple and very similar to the ball system that we have in our toilet systems. So as the ball floats, or there is a float on the fuel itself; as the fuel goes up or down, so does this float move up and down.

Pieter: (03:34)

And that float is connected via an arm to a potentiometer, and that potentiometer measures a change in resistance that occurs due to the change in height of that float. And that then sends a signal to your instrument cluster, which then gives you an exact indication of how much fuel you’ve got left. Now, a common question is when my reserve light goes on, how much fuel have I got left? And therefore, how much range have I got? Now that does vary from car to car, but generally on your small compact vehicles, we have about five litres of reserve fuel. So as that reserve light comes on, you’ve got about five litres of fuel and on the bigger cars, your SUVs and the like, we can have up to 10 litres of fuel. So we have therefore, a range of anywhere between 50 and a hundred kilometres of reserve fuel.

Pieter: (04:26)

A critical part of our fuel tank is, of course, the ability to be able to vent it. You can imagine that if we go from a full tank of fuel, as the fuel levels drop, we’ve now got a sealed closed system and therefore we’re going to create vacuums within the fuel tank, and that can actually cause the fuel tank to collapse in on itself, and certainly, it can cause the fuel pump to really battle to pump under those vacuum conditions. So we need to be able to allow the tank to breathe. Now, this can happen not only when the fuel drops in level, but also if there’s increases in temperature, you can imagine that the pressure will increase in the tank itself. So we want to maintain a consistent pressure in the tank and we use, obviously, valves to assist us and those valves work in conjunction with our evac system, or as it’s better known, our charcoal filter. You may have noticed with modern cars, you can’t smell the modern car after it’s had fuel put in. In the old days, you could always smell a petrol driven vehicle, for example. There was that real raw smell of petrol fumes, and obviously from an environmental point of view, that became unacceptable and it was also relatively unsafe.

Pieter: (05:30)

So these days we have charcoal filters and those charcoal filters then process the evaporated fuel gases and take away that smell and return those gases and fuel back to the tank itself. 

So of course, there are some common questions that we typically get about our fuel tank. And the first one is, is it dangerous to run my vehicle on an empty tank? Now we’ve already alluded to the fact that we need to have fuel surrounding the fuel pump itself in order to cool it. So the real danger exists that we will damage the pump, if we run the tank low. I’m not too concerned about contaminants that the pump will pick up because we’ve got filters for that, in any case, it’s more about protecting the temperature of the pump.

Pieter: (06:23)

The second question that we get asked is in terms of saving fuel, is it better to run your vehicle with a full tank of fuel or with a half a tank of fuel? And it’s quite a cool question. So the thinking behind the full tank of fuel is that there’s less vapours that can escape. Now, as I’ve told you, the fuel tank system and the fuel circuit is a closed circuit. So even if there are fuel vapours that are created by a half a tank of fuel, those still remain in the system. And as I’ve said already, after those have passed through the charcoal filter, they enter back into the fuel tank itself. So unless you open up your fuel filler cap, which will then allow those vapours to escape, it’s probably not going to make any difference whatsoever.

Pieter: (07:13)

However, there is some merit, I suppose, in the fact that if you’re driving around with a half a tank of fuel, you’re driving around with less weight and obviously weight adds to fuel consumption. So it’s quite an interesting debate. And then lastly, of course, every time there’s a fuel price increase people scramble off to make sure that they fill up their tank of fuel before the price increases. And I think that’s almost a bit of a waste of time. You know, every time there’s a fuel price increase, it’s, let’s say between 50 cents and a Rand and on an average size fuel tank, you might have a saving of perhaps 30 to 40 Rand. I don’t know if it’s worthwhile, you probably spend 30 or 40 Rand just getting to the fuel station and then you’ve got to sit in queues, potentially waiting to have your tank filled up. So I don’t generally bother with that. In any case, we get used to that new fuel price increase very quickly. 

So I hope you found that useful, some interesting titbits about the humble fuel tank. And we look forward to seeing you on future episodes of How Things Work.

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