Driving Chevrolet’s 2015 Impala Bi-Fuel sedan

Chevrolet offers the 2015 Impala full-size sedan as an all-new bi-fuel model—the only manufacturer-produced full-size sedan in North America that can run on both compressed natural gas (CNG) and gasoline. Available at the end of 2015 to both fleet and retail customers in either LS or LT trims, the bi-fuel Impala features a dedicated version of the standard 3.6L V–6, with specific valves and hardened valve seats for improved wear resistance and durability with the CNG fuel system. The engine, mated to a six-speed automatic, is rated at an estimated 260 hp (195 kW) on gasoline and 230 horsepower (172 kW) on CNG.

EPA figures put combined fuel economy in gasoline mode at 20 mpg combined (17 mpg city/25 mpg highway), and 19 mpg combined (16 mpg city/24 mpg highway) with natural gas. That works out to 437 grams CO₂ per mile with gasoline, and 343 g CO₂/mile with natural gas.

The specific natural gas fuel components in the Impala include:

• CNG fuel rail assembly and gaseous fuel injectors
• CNG fuel tank. Mounted in the trunk, the CNG tank capacity is US 3,600 psig @ 70 ˚F, 7.8 gasoline gallon equivalent (GGE). The CNG tank has pressure relief devices that are designed to release pressure if the CNG tank is overheated or over-pressurized. If a pressure relief device releases CNG, the tank will empty and the vehicle will switch to gasoline. A loud rushing noise may be heard when the CNG is released.

• Fill receptacle
• High pressure lock-off (HPL)
• High pressure regulator (HPR)
• Hose Assembly fuel rail feed
• Manual CNG shutoff valve. A manual shutoff valve is on the driver side of the vehicle in front of the rear tire. A label is on the outside of the vehicle. One-quarter turn clockwise shuts the valve off.

• High pressure CNG fuel filter.

The CNG fuel system is validated by GM and covered by GM’s three-year/36,000-mile new vehicle limited bumper-to-bumper warranty and five-year/100,000-mile limited powertrain warranty, whichever comes first.

Driving the Impala. The Impala underwent a major redesign in 2014; Chevrolet basically maintained the changes for the MY 2015 line-up, of which the Bi-Fuel model is a member. Overall, the Impala is a very attractive large sedan: quiet, roomy and with a large trunk—albeit the latter attribute to reduced in the Bi-Fuel model.

The 2015 Chevrolet Impala comes standard with antilock brakes; traction and stability control; front-seat side airbags; full-length side curtain airbags; and front knee airbags. The Impala offers a choice of powertrains: the base 2.5-liter with standard stop/start; the gasoline-powered 3.6L V6; and the Bi-Fuel 3.6L V6.

The Impala runs primarily on CNG; once the fuel tank is depleted, the system automatically switches to gasoline. Drivers who wish to change fuels while driving can do so by simply pushing a button. A light on the instrument panel indicates when CNG is being used and there is no interruption in the vehicle’s performance.

Nor is there much discernible difference in most driving circumstances between the performance of the Bi-Fuel engine under gasoline or CNG modes. Equipped with the 3.6L engine, the Impala is more than adequately powered; even the step down in power output under CNG doesn’t affect the feel and performance of the car. Exceptions might be conditions such as a hard kick-down on a steep incline; even in that condition, however, the Impala felt solid and sufficiently powered.

From an driving point of view, then, the Bi-Fuel Impala inherits almost all the attributes (performance, ride and handling, connectivity, infotainment, etc.) of its conventionally-fueled 3.6L sibling—the two notable exceptions being the reduced truck space noted above, and refueling.

Refueling the Impala Bi-Fuel. There are two methods of refueling the CNG tank: fast fill or time fill. Fast fill is normally used in fuel stations for NGVs, and takes about two to five minutes for a full fill. Time fill is done with a refueling appliance, including home appliances, or a time-fill post provided by the fleet operator. Refueling time varies depending upon the refueling system used.

During fueling, CNG needs to be delivered to the vehicle at the appropriate pressure in relationship to the ambient temperature. This can be done automatically by a temperature compensation system on the CNG fuel dispenser or manually by stopping the CNG fill at a recommend pressure. GM recommends that customers use CNG refueling stations that have a temperature compensation system whenever possible.

Depending upon the CNG fill station equipment/performance, the CNG fill volume will vary—affecting overall vehicle range. In other words, even if you “fill up”, you might not actually fill the tank to its full capacity. This has nothing to do with the specifics of the Impala; it is a general attribute for NGVs with the current refilling infrastructure.

What is specific to the Impala, however, is the design of the filling receptacle. Chevrolet combined the natural gas receptacle in the same port with the gasoline fuel filler neck.

Gasoline filler neck (bottom) and CNG receptacle (top). CNG filling requires removal of the dustup (inset). Click to enlarge.

While this design works from an aesthetic and driver convenience point of view in terms of location (no trying to remember the location of two fill ports), the proximity of the two, combined with the downward angle of the CNG receptacle, in practice can make it extremely difficult, if not impossible, to use certain CNG nozzles—which, unlike the nozzles used for liquid fuel, are far from uniform in form factor. A pistol grip nozzle, similar to a standard liquid fuel nozzle, fits with no problem at all—refilling is a snap (see picture below). A barrel-type nozzle may not be able to fit, on the other hand, and if it does, the space available for turning the lever is limited. On top of that, the hoses to the nozzle are stiff, and not all that amenable to creative repositioning to work the connection. The bottom line: depending upon the type of NGV station, you might not be able to fill the Impala—a situation that occurred for us when we stopped at the local waste management company for a fill from their publicly available CNG pumps. (As a side note, refilling from this particular station was no problem with the Silverado CNG we had tested earlier, due to the location of the receptacle. Earlier post.)

Top. Filling the Impala Bi-Fuel with a pistol-grip nozzle. Bottom. Example of a barrel nozzle with lever. Click to enlarge.

Bottom line. The Impala is an excellent full-size sedan, well worth consideration. The Bi-Fuel option provides an avenue for enjoying the benefits of the large sedan while reducing its carbon footprint. On the other hand, the Bi-Fuel Impala is not inexpensive (the LT model we drove tickets out at $42,625), nor is it the most fuel-efficient.

Staying with the 2015 model lineups for comparison, the Toyota Avalon (24 mpg combined, 362 g CO₂/mile); the Hyundai Azera (23 mpg combined, 389 g CO₂/mile); the Chrysler 300 (23 mpg combined, 386 g CO₂/mile) all beat the Bi-Fuel Impala’s 20 mpg combined, 437 grams CO₂ per mile (gasoline). The benefit to the Impala, of course, is the lower CO₂ output with CNG (343 grams CO₂ per mile).

However, Toyota offers a hybrid version of the Avalon, with 40 mpg combined and an EPA rating of 233 grams of tailpipe CO₂ per mile, MSRP $36,470 - $41,700.

Consumers looking for a fuel-efficient or lower carbon full-sized sedan thus have a range of options beside the Impala, despite that vehicle’s solid quality. On the other hand, from a fleet point of view, the Impala Bi-Fuel could be quite attractive, especially if natural gas vehicles are already part of the fleet or planned.