While we don’t want to get into deep discussions of the nature of vapor pressure and complex subjects like Boyle’s law, there are a couple of basic principles in fuel tank venting that must be understood. To help us with a basic understanding, we reached out to Tanks Inc. to get some insight into the importance of proper fuel tank venting.
First, all liquids have a specific vapor pressure. This is nothing more than the pressure exerted by vapor at a given temperature in a closed system – like a car’s fuel tank for example. Second, a liquid with a high vapor pressure at normal temperature is referred to as volatile. That’s just a fancy way of saying when the temperature of liquid increases, the energy of the molecules in the liquid increases, and some of those molecules transition into vapor.
This is handy information, especially if you are building a still to make bootleg whiskey. The liquid heats up and transitions into a vapor, which is forced out through tubing into a worm tank where the vapor is cooled and returns to a liquid state. For automotive enthusiasts working on a fuel system, we are only concerned about the liquid transitioning into vapor pressure.
In automotive terms, thermal expansion works a little differently. According to Justin Somerville, the general manager at Tanks Inc., thermal expansion in a fuel cell is a big deal. “Gasoline expands and contracts with temperature changes. Additionally, the air inside of a gas tank also changes in volume with temperature changes,” he says.
The change in air and gas volume in a tank will greatly change with temperature change, and probably more so than what most people realize.
“The change in air and gas volume in a tank will greatly change with temperature change and probably more so than what most people realize. It is these changes in volume due to temperature changes that make gas tank venting necessary.”
Basically, an automobile’s gas tank or fuel cell will build pressure naturally. Higher pressure can lead to dangerous conditions like tank bulging, fuel line leaks, fuel pump problems, insufficient fuel to the engine, or potential fire hazards when opening the filler cap.
What To Look For
As stated above, bulging gas tanks or fuel line leaks can be an indicator of high vapor pressure in the gas tank or fuel system. Trouble filling your gas tank can also be an indicator.
“A slow filling tank is typically a venting issue, a filler neck issue or both,” Justin stated. “For a tank to properly fill the air must be able to vent out of the tank to allow room for the gas that displaces it.”
According to Justin, the standard 5/16-inch line commonly used as fuel tank vents, “is not enough to properly vent a tank during filling.” However, most enthusiasts don’t realize this because the tank’s filler neck is larger than the fuel pump nozzle. “This oversized filler neck allows for air to escape during filling,” he pointed out.
“The other issue that we do sometimes run across is simply the filler neck does not have enough drop in it to allow for fuel to gravity feed to the tank,” he added, “Or there are too many tight bends causing the fuel to flow slowly.”
What About Vented Gas Caps?
As anyone that has had to shop for a replacement gas cap can tell you, there is more than one kind of gas cap on the shelves. There are the standard OE-type caps, locking-type caps, or vented-type caps. You can even get an OE-vented, locking-style gas cap! If you pony up the cash and buy a vented cap, most enthusiasts would assume that would eliminate the need to run a vent line from the gas tank.
With today’s modern fuels, the alcohol and additives create a fuel that expands much more than what it used to.J
Justin cautions against that assumption. “With today’s modern fuels, the alcohol and additives create a fuel that expands much more than what it used to years ago when vented caps were common, he said. “Additionally, if an internal pump is added to the system there will be more venting required than what a stand-alone vented cap can provide.”
Internal pumps use the fuel to cool themselves. As a result, this heat transfer raises the temperature of the fuel a few degrees. As we have already discussed, vapor pressure is directly proportional to temperature. Temperature goes up and vapor pressure rises as well. Running a vent line is a safe way to combat the higher vapor pressure.
Running A Vent Line
So you’ve decided that a fuel tank vent line is the best answer, but you want to know how to install one properly. Doing it right the first time is important because there is no one that likes doing the same job twice. We asked Justin what considerations there are when installing a fuel tank vent line.
The first consideration he recommended was to ensure the vent line is run so the line is higher than the highest point on the tank – including the fuel filler neck. On some vintage muscle cars, this will be easier due to the OE placement of the filler in the center rear of the chassis. Others may pose a tougher challenge, but running your vent line higher is critical.
“The vent line cannot have a dip in it where fuel or condensation can get trapped in the line, Justin warns. “If fuel becomes trapped in the line your tank will then build pressure or vacuum until there is enough pressure to purge the vent which will cause gas to come from the vent line.”
There have been times when we’ve witnessed fabricators routing fuel lines and fuel tank vent lines through the passenger compartment. Justin warns against that practice. “Do not vent into the passenger compartment,” he said purposefully.
Do not vent into the passenger compartment!
“If the only way to route the vent line high enough is to go into the passenger compartment, then using something like our in-line vent valve may be necessary. With this, you can vent higher than the tank and neck and then run the line down and out of the passenger compartment.”
Other Considerations For Performance Applications
It is no surprise to anyone in the industry that vintage street muscle is showing up and dominating autocross and street racing events everywhere. With modern aftermarket parts and suspension upgrades, muscle cars are performing like never before. However, the one thing that remains true about upgrading older cars remains. When you make a change and improve performance, it will highlight another issue somewhere else.
In many cases, after an upgrade in power or handling, systems like fuel supply are affected. Sloshing fuel in the tank causes more vapor pressure. As anyone that has watched an autocross event, there is plenty of fuel sloshing going on. Justin has some pointers for these cars experiencing sustained lateral G-forces.
“Depending on the vent location, there could be a possibility that fuel could get pushed up the vent line,” he said. “For Example; if the vent is located on the passenger side of the tank and you had a full tank of gas. There is a real possibility that fuel could come up in the vent line if you were making a long sustained left-hand turn.”
“One way to help combat this is to make your vent line corkscrew as it comes up,” he advised. “Or the addition of a vent canister can be used to prevent this from happening.” The corkscrew or coiled vent line is a popular solution by many fabricators. For aesthetic purposes, fabricators take great effort in making the coils look as even and uniform as possible.
When it comes to performance applications that are more than your typical weekend warrior – something more along the lines of a purpose-built race car – there are a couple of recommendations for safety in fuel venting. “Rollover valves are always a good idea,” Justin stated firmly. “Even if you are not racing, this safety valve will help prevent fuel from coming out of a vent line if there was a crash that caused the vehicle to be on its side or upside down.”
Most race cars have the fuel vent line corkscrewed to help combat the G-forces as discussed in the last section. The same is true of a vent canister. Performance race cars typically invest time and effort in the fuel vent systems for safety and improved performance.
We’ve covered the reasoning and general-purpose behind using a fuel cell vent line. We’ve also discussed how to mount/install as well as route a vent line in the paragraphs above. We also covered many of the concerns associated with fuel cell venting. However, there is still a key element to pay attention to.
“The number one thing to keep in mind with the vent line and vent valve is mounting,” according to Justin, “The biggest issue is ensuring that the valve itself is mounted vertically. If the valve is mounted sideways or at an angle, this could cause the valve to shut and prevent your tank from venting.” This would be creating the exact problem you were trying to avoid.
For more information on fuel tanks, fuel systems, and products associated with fuel systems, visit Tanks Inc. online at www.tanksinc.com.