The Chevrolet Volt is nothing if not a very complex engineering project. Over the years, we've heard a lot of information from General Motors about the new powertrain developed for this car (and others, like a plug-in Cadillac SRX, maybe), but now we've got the scoop on the gas tank used to keep liquid fuel in the Volt from evaporating or going stale for up to a year.
Back in 2009, we learned that the Volt can move without gas in the tank, but that's only part of the problem that GM engineers had to solve. They also needed to figure out how to keep fuel from simply evaporating away. Standard vehicles use charcoal canisters to trap the gas, and then return it to the tank once the engine starts running. In the Volt, though, the engine could potentially remain off for a very long time even under daily use – up to six weeks, before the Volt's "maintenance mode" automatically kicks in. That's why a special sealed steel tank was developed. Read on past the jump for the tasty details.
Recently, we spoke with Jon Stec, fuel system integration engineer for the Volt, about the sealed tanks came to be. Stec said that the Volt program, at just 29 months, was much faster-paced than most new vehicle programs at GM, which typically take 36 months or more. "We knew early on that it was a fast program," he said, and so GM began talking with other OEMs and CARB to figure out how to design a fuel system that would work with a plug-in vehicle, because a conventional system wouldn't work. The most important thing to figure out was how to handle evaporative emissions and prevent hydrocarbons from getting into the air.
Typically, in a standard vehicle, the fuel system captures errant hydrocarbons in a canister and, when the engine runs, it purges that canister. "This time," Stec said, "we don't have the engine running all the time, so what do you do? Do you put a gargantuan canister down there and hope that whatever hydrocarbons you're going to develop – typically because of temperature changes and fuel fill – will be caught? That's not very feasible."
GM knew other technologies were possible. On earlier Prius models, for example, Toyota tried a bladder tank, where there was an outer shell with a flexible shell on the inside that would shrink to try an reduce the space where vapors could form and deal with emissions this way. "That didn't work so well for them," Stec said.
This time around, the OEMs involved collectively decided to make a pressurized vessel, to seal it and then to give the the powertrain the controllers to open and close the system. The reason that GM worked with others to develop this system was because having each manufacturer come forward with its own proposal and hoping that the regulations concerning the tank emissions were flexible enough to allow for the different solutions seemed unlikely. "It had to be an agreement between all the OEMs, and it was resolved three or four years ago that we do this process (use a sealed system)," Stec said. "All the OEMs, as far as I know, are going to pursue this technology."
Of course, if your fuel tank can't be sealed all the time. Since the Volt uses gas, drivers were going to have to open it up at some point to refuel it, so "we had to develop a whole new control process to unseal the tank without getting any hydrocarbons in the atmosphere and allow the customer to refuel the tank," Stec said.
GM has proprietary leak check diagnostics that are used on The General's more conventional vehicles. While some manufacturers use a vacuum pump system to check for leaks, GM uses the temperature decay profile to determine if there are any holes in the system. The problem here was that, "we couldn't do that without the engine running," Stec said.
So, with all of this close attention paid to the vapors, what happens if you put in regular gas? Not much, Stec said. The pressure system can handle different octane levels, and the premium fuel certification was mostly done to get an improved fuel economy rating. "Will the world end if you put in regular? No, but we recommend premium fuel," Stec said.
Technical details are available in GM's press release. Just click below.
Show full PR text
Special Gas Tank Caters to All-Electric Chevy Volt Drivers
Pressure-Sealed steel fuel tank contains gasoline vapors from sporadic engine use
WARREN, Mich. – Ben Short isn't complaining about the rising price of gasoline. In fact, the Pittsford, N.Y., Chevrolet Volt owner doesn't pay attention to the rising prices since he's only used about a half a tank of gas in the last three weeks.
Short is an early Volt purchaser whose idea of an electric car with extended range is that it is an electric car first and foremost. Like the typical Volt driver, Short counts on the Volt's 16 kWh lithium-ion battery to take him about 35 miles between charges. Interviewed recently by WHAM-TV in Rochester, N.Y., Short said he remembered gas was about $3 a gallon when he last bought it.
If he gets really good at going without gas, Short will find his Volt telling him the engine generator needs to start every so often, while the Volt's specially engineered fuel tank makes sure the gas inside stays fresh.
"Gasoline readily evaporates at normal ambient temperatures and it also degrades over time from oxygenation and condensation, "said Jon Stec, fuel system integration engineer for the Volt. In order to ensure that the gasoline in the tank doesn't hurt the Volt's performance or emissions when it is needed, engineers pressure sealed the 9.3 gallon steel fuel tank to contain the gasoline vapors.
"Using a sealed tank limits this evaporation when the engine is off," Stec said.
For the last several decades, new vehicles have been equipped with charcoal canisters to trap evaporating gasoline and then feed it back to the engine. Since the Volt's engine may not fire up for extended periods, hydrocarbons potentially could build up in the canister, so the sealed tank contains the vapor. The extra pressure meant that a lightweight steel tank had to replace the lightweight plastic tanks used in most modern vehicles.
Volt engineers and supplier Spectra Premium Inc. developed the tank from 1.4 millimeter thick hot-dip tin-zinc coated steel to resist corrosion from both inside and outside. Despite the strength of the tank, it has a mechanical pressure relief valve that begins opening at 3.5 psi and a vacuum relief that opens at -2.3 psi, levels that are rarely exceeded.
Even with a tank that resists fuel vapors escaping or humidity getting in, the gas inside still needs to be used up and replenished periodically. That's where the Volt's "maintenance mode" comes in. If the engine hasn't started after six weeks, the powertrain controller sends a message to the driver telling him the engine needs to run for maintenance.
Volt drivers can defer the engine maintenance mode for up to 24 hours, after which the engine will run for a while on its own to use up some of the gas and keep the internals lubricated and ready for use. If a driver manages to go a full year between fill-ups, the fuel maintenance mode will run the engine until the old gas is used up or the driver adds fresh fuel.
"For the driver who starts the year with a full tank of 9.3 gallons and runs 15,000 miles on electricity, the maintenance mode will use just enough gas to average a very respectable 1,613 miles per gallon," Stec said.
Founded in Detroit in 1911, Chevrolet celebrates its centennial as a global automotive brand with annual sales of about 4.25 million vehicles in more than 120 countries. Chevrolet provides consumers with fuel-efficient, safe and reliable vehicles that deliver high quality, expressive design, spirited performance and value. The Chevrolet portfolio includes iconic performance cars such as Corvette and Camaro; dependable, long-lasting pickups and SUVs such as Silverado and Suburban; and award-winning passenger cars and crossovers such as Spark, Cruze, Malibu, Equinox and Traverse. Chevrolet also offers "gas-friendly to gas-free" solutions including Cruze Eco and Volt. Cruze Eco offers 42 mpg highway while Volt offers 35 miles of electric, gasoline-free driving and an additional 344 miles of extended range. Most new Chevrolet models offer OnStar safety, security and convenience technologies including OnStar Hands-Free Calling, Automatic Crash Response and Stolen Vehicle Slowdown. More information regarding Chevrolet models can be found at www.chevrolet.com