Chevy Volt Technical Sidebar

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When buyers think of the new group of plug in hybrid cars, they are most aware of technologies such as the battery pack, the motor/generator, as well as the different displays and instrumentation. But looking deeper reveals a host of other items that are quite different than what we’ve become used to with internal combustion engine (ICE) technology. The fuel system required a complete rethink for not just GM, but every other Plug in Hybrid Electric Vehicle (PHEV) manufacturer. With the ability to drive 30, 40 or more miles per charge, some users will find themselves rarely utilizing the ICE as a generator. In theory, weeks or months could pass and the fuel in the tank would remain unused. That presents a problem since gasoline has a finite shelf life. Refineries vary the product by season depending on geographic location to better suit the expected temperature ranges, and gasoline in the tank could be stale or “out of season”.

A Rare Agreement
Interestingly, manufacturers took it upon themselves to collectively agree on a fuel management system that was then adopted. It’s an important issue because the EPA mandates controls on emissivity, and by banding together, they felt that time could be saved by submitting a single collaborative proposal to the government.  The resultant system is quite interesting. In normal use, the tank is sealed and pressurized to control evaporation, and to stall degradation from oxygenation and condensation. When the driver refuels the system, a button is pressed to open the fuel door, but it doesn’t open instantly.  The dashboard displays a “wait to refuel” message. Unseen by the driver, a vacuum pump is purging the tank of vapors and storing them in a canister. Once the purging of vapors and pressure is completed, refueling may take place.

Big Brother is Watching
The control of vapors is only one aspect of the unusual fuel system. The Volt tracks fuel use and replenishment, and sensors are tracking temperature ‘degree days’ to calculate fuel degradation over time. The Volt will trigger an alert if the fuel is approaching a condition that is calculated to be detrimental by triggering a dashboard display encouraging the driver to burn some gas. In this instance, maybe the family decides to use the Volt for a long distance trip as opposed to the other car in the garage. If another warning and further time passes, and the fuel has still not been consumed, the Volt will begin a “fuel and engine maintenance mode” by starting the engine and warming it, circulating the oil and burning off stale gas, and then shutting off after roughly ten minutes. This occurs about every 6 weeks of no fuel use. In the rare event that this isn’t enough, and the Volt determines the same fuel has been in the tank for approximately a year, it will initiate the “fuel maintenance mode” and will burn off enough fuel to prompt a “refueling event”. 

Burning gas to get rid of it? The horrors!
While burning gas just to keep it from going stale seems, on the surface, to be terribly wasteful, keep a few things in mind. First, the tank has a maximum capacity of just over 9 gallons. Second, the engine must be started to ensure its health and longevity. So under no circumstances will the year mark (and the fuel burn mode) occur with a full 9 gallons. Remember, Volt drivers value the extended range capabilities of their choice of car. If not, they likely would have chosen a battery only based EV. So it’s unlikely the yearly fuel maintenance mode will be triggered at all. And if it is, it’s likely that it will be a few gallons at most. The critics among you might ask, “How does that affect mileage?” Let’s run some hypothetical numbers. In the extreme case, starting with 7 gallons in the tank, driving a round trip of 30 miles each day and charging each night so that over the course of a year, the engine has never had to start to generate a charge, what would the mileage be? Well, assuming a two week vacation from work, 10 holidays, a couple sick days and a couple extra trips other than work, the car will have traveled about 7500 miles. Each 6 weeks the engine started and burned off a half gallon, and at the end of the year it burned off the remaining 4 gallons. That’s roughly 1,070 MPG. Assuming a nightly electric charge of $1.35, and gas prices of $4 per gallon, the motive costs would be $352 a year.

Blissfully Unaware
Of course, that’s one extreme. Most will use the Volt for more varied trips, and will likely never even have a clue that such operations and modes even exist. They might think it’s odd that they need to wait a short minute or so before refueling, but even that’s unlikely as most will be finding the credit card, chatting to a friend, or trying to remember which side the filler door is on. No shame in forgetting that in a car that needs so little refueling.

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