Emissions reductions (%) of new NGVs compared to new gasoline and diesel vehicles (2012)
	LD car	LD truck	HD truck (vs. diesel)
	CNG vs gasoline		CNG	LNG	LNG dual fuel
GHG	13	14	13	13	21
NOx	16	16	40	40	40
Source: NGVAmerica, using data from Argonne National Lab.

Depending upon use, the drop in power and torque with CNG might not even be that noticeable—say, for carrying a crew with a moderate load in the back. In week of driving a 4x4 standard box crew cab version of the truck, the 6.0L Vortec delivered smooth, quiet power with CNG under a range of different load conditions. Gasoline power gives an extra kick up steep hills, but under many conditions, it’s just not necessary. (Hauling a horse trailer would likely be a different story.) Buyers with a need for hefty torque will likely go for the 6.6-liter Duramax B20-capable diesel engine with twice the torque of either the gasoline or the bi-fuel models.

Silverado 2500HD engine options
	Vortec 6.0L (gasoline only)	Vortec 6.0L (bi-fuel)	Duramax 6.6L Turbo-Diesel
Type	6.0L V-8		6.6L V-8
Valvetrain	overhead valve w/ variable valve timing		overhead valves, four valves per cylinder
Fuel delivery	sequential fuel injection		direct injection diesel with high-pressure common rail
Compression ratio:	9.6:1		16.0:1
Power (hp/ kW)	360 / 268	360 / 268 (gasoline) 301 / 225 (CNG)	397 / 296
Torque (lb-ft / N·m)	380 / 515	380 / 515 (gasoline) 333 / 452 (CNG)	765 / 1037
Fuel	regular unleaded	regular unleaded or CNG	diesel or B20

Although Chevy emphasizes the benefits of the 2500HD with the bi-fuel option as a full dual fuel vehicle with a combined range of up to some 650 miles, in more than 500 miles of driving over the course of a week, we used mainly only natural gas. The only reason we used gasoline was for comparison; it wasn’t necessary. We refueled with CNG twice; once locally (with a partial fill, more on this below), and once off the I-5 on the way up to Los Angeles.

Refueling. As with any alternative fuel, be it CNG, LPG, electricity, hydrogen or E85, one of the challenges facing its adoption is both the availability of the fuel as well as the mechanics of refueling. (We’ll set aside the business challenges facing alternative fuel providers.)

There are some 726 public-access CNG stations in the US (i.e., excluding the numerous private stations), according to the US Department of Energy’s (DOE) Alternative Fuels Data Center. As comparison, again excluding private stations, there are 298 biodiesel stations; 8,485 electric plug-in stations; 2,391 E85 stations; 12 hydrogen stations, 56 LNG stations, and 2,718 LPG stations.

(There were 152,995 total retail fueling (gasoline and diesel) sites in the United States in 2013, according to the National Petroleum News’ MarketFacts 2013. That number has been dropping for the last 20 years (there were more than 200,000 in 1994) due, in part, to increased competition, stricter environmental regulations, and shrinking gasoline profit margins.)

There are a wide range of CNG dispensers (“pumps”) and configurations, from slow home-fill systems such as Phill by BRC FuelMaker to ultra-fast-fill private systems.

(Because of moisture and other contaminants inherent in some natural gas supplies, and the inability of some home refueling systems to adequately dry the gas and remove contaminants, Honda, which several years ago was promoting home refueling, currently does not recommend it.)

Despite the difference in size, shape and variety, all CNG dispensers conform to either a fast-fill or a time-fill configuration (or a combination) and are available in different hose configurations and with different flow rates and methods of metering. Public CNG stations, the type with which we are concerned here, are fast-fill.

Unlike a liquid fuel pump, which can just keep pumping until a tank is filled, a gaseous fuel pump is limited by the pressure it supports. In the case of CNG pubic stations, this is currently generally 3,000 psi (20.7 MPa) or 3,600 psi (24.8 MPa).

If you have a 3,600 psi rated tank—as does the Silverado—and you refuel at a 3,000 psi pump, you only get a partial fill. In other words, you might want a full tank, but you’re not going to get one.

There are several types of CNG fast-fill dispensing nozzles (ANSI NGV1-2006), including a pistol grip dispenser reminiscent of a gasoline dispenser, and a barrel-type nozzle with a jaw lock engaged with a 180° rotation of its handle. This secures nozzle jaws onto the receptacle, activating a system of three internal valves that regulate fueling.

(Left) OPW 1000 Series self-service nozzles (NGV1 Type 1), 3600 psi. (Right) Pistol-grip fueling nozzle from WEH (with swivel joint), 3000 psi. Click to enlarge.

We refueled with each of the nozzle types shown in the pictures above. The gas intake to the Silverado’s tank is accessible on the side of the tank—effectively only several inches above the top of the side of the truck bed.

Using the pistol-grip dispenser with a swivel joint was no problem. However, when refilling with the barrel nozzle with the 180˚ rotating locking handle, we hit a slight snag. As you can see in the picture below right, the nozzle has two hoses attached, and its play is rather stiff. Given the orientation of the nozzle at this dispenser, swinging the locking handle through its 180˚ arc was a bit difficult due to the proximity of the top rail of the pickup bed, and took a little fiddling. (Kindly provided by a Clean Energy tech who was servicing another of the CNG pumps.)

The refueling inlet is covered in this picture with its dust cap. The manual shut-off valve is in red. Click to enlarge.		Refueling the Silverado at a 3,600 psi pump, indicated by the yellow handle. A blue handle would indicate 3,000 psi. Click to enlarge. Click to enlarge.

Currently, as required by standards adopted by the National Conference of Weights and Measures (NCWM) in 1994, CNG is priced and sold in gasoline gallon equivalent (gge) units—e.g., we dispensed 8.1858 gge at one pump, at a price of $2.438/gge, yielding a cost of $19.70.

Currently, there is a movement supported by the natural gas fueling and vehicle industry for NCWM to adopt a national standard for dispensing CNG (and LNG) to truckers and other users who typically use diesel fuel in diesel gallon equivalent (DGE) units. From the purchaser’s perspective, the pump display and readings would show the number of DGE units dispensed and information would be provided on the pump to show how natural gas is converted to DGE units.

Under the proposal, which is being considered at NCWM’s meeting this week, LNG would only be sold in DGE units since LNG is expected to be used exclusively as a heavy duty vehicle fuel. CNG would be dispensed in GGE units at retail outlets serving the general public and would be sold in DGE units on pumps selling to trucks and heavy duty vehicles.

The LC8 engine. Click to enlarge.

The engine. The 2015 Silverado 2500 HD Crew Cab 4x4 bi-fuel version is equipped with a Vortec 6.0L V8 engine, engine code LC8. The LC8 is a bi-fuel CNG version of the L96 gasoline engine, also offered in the Silverado (the LC8 also appears in vans with an LPG fueling option). The engine features special hardened valves and valve seats that enable it to run on CNG or gasoline.

The bi-fuel system separately has two fuses that are part of the engine compartment wiring harness. Both are in a plastic holder than is mounted inside the engine compartment, neat the accessory battery tray.

The LC8 is rated at 360 horsepower (268 kW) and 380 lb-ft of torque (515 N·m) on gasoline, and 301 horsepower (225 kW) and 333 lb-ft of torque (452 N·m) on CNG.

Intake flow was improved over previous engines by straightening out and optimizing the flow path from the intake manifold into the cylinder heads. The 6.0L’s cast aluminum cylinder heads feature “cathedral”-shaped intake ports derived from the high-performance cylinder heads that were used on the “C5” Chevrolet Corvette Z06 and support great airflow at higher rpm for a broader horsepower band, along with strong, low-rpm torque. The intake ports that feed the combustion chambers, as well as the D-shaped exhaust ports, are designed for excellent high-rpm airflow. The exhaust ports are also designed for greater flow.

Power and torque chart for CNG. Click to enlarge.		Power and torque chart for gasoline (L96). Click to enlarge.

The Silverado HD is surprisingly quiet. This is partly due to the quiet features built into the engine, which are complemented by an improved engine cradle and mounting system. These help reduce vibrations transmitted through the chassis and into the passenger compartment.

The engine block was developed with math-based tools and data acquired in GM’s racing programs, and provides a light, rigid foundation. A deep-skirt design helps maximize strength and minimize vibration. The bulkheads accommodate six-bolt, cross-bolted main-bearing caps that limit crank flex and stiffen the engine’s structure. A structural oil pan further stiffens the powertrain. Along with the rigid block, the engine’s rotating assembly was designed for optimal strength and duration complemented by features designed to make the engine quiet and smooth.

The 6.0L also features a heavy-duty timing chain developed expressly for quiet operation. The chain, which connects the hollow steel camshaft and crankshaft, is validated for 200,000 miles of operation and fitted with a leaf-spring-type dampener.

The 6.0L features variable valve timing, maximizing engine performance for given demands and conditions. At idle, for example, the cam is at the full advanced position, allowing very smooth idling. Under other conditions, the phaser adjusts to deliver optimal valve timing for performance, drivability and fuel economy. At high rpms, it may retard timing to maximize airflow through the engine and increase horsepower. At low rpms it can advance timing to increase torque. Under light loads, it can retard timing at all engine speeds to improve fuel economy.

The engine has an advanced 58X crankshaft position encoder to ensure that ignition timing is accurate throughout its operating range. The new 58X crankshaft ring and sensor provide more immediate, accurate information on the crankshaft's position during rotation. This allows the engine control module to adjust ignition timing with greater precision, which optimizes performance and economy. Engine starting is also more consistent in all operating conditions.

The exhaust manifolds were developed to improve durability and sealing and reduce operational noise. Cast nodular iron was the material of choice for its basic durability and excellent heat-management properties. The manifolds are fitted with new triple-layer heat shields fabricated from stainless steel and insulating material.

The shields limit heat transfer from the engine to the engine bay, allowing the 6.0L to reach optimal operating temperature more quickly, yet reducing heat in the engine compartment once that temperature is achieved.

The CNG switch (upper far right), next to the Parking sensor switch included in this version of the truck. The flaps are lifted to show three USB ports, two 12V ports, and 110AC outlet. Additional USB and 12V ports are available in the center console. Click to enlarge.

The separate CNG fuel tank—66.6L, 17.6 GGE capacity at 24.8 MPa and 21 ˚C (70 ˚F)—enables the driver to switch between fuels at the flip of a switch, which is integrated into a row of switches beneath the HVAC controls. The vehicle always starts with gasoline; to switch to CNG, the driver presses the switch. If CNG is selected at the time of engine shut-off, the vehicle will restart in gasoline mode, but then switch to CNG as soon as right conditions are reached—in our experience, a brief interlude of about 10 seconds, depending upon ambient conditions.

When the CNG tank is depleted, the system automatically switches to gasoline after giving appropriate low-duel warnings. GM cautions agains running out of gasoline; if the gasoline tank is empty, the vehicle won’t switch automatically to CNG, nor will the vehicle restart until there is gasoline available.

GM’s tier-one supplier installs the CNG fuel tank and complementing fuel system upgrades at a secondary location. Chevrolet dealers deliver the trucks as they would with any optional content.

A single fuel gauge displays fuel levels for gasoline and CNG, depending upon which fuel is currently active. The fuel level for the unused fuel can be shown in the Driver Information Center (DIC). (Tachometer shows the idle level at slightly above 500 rpm.) Click to enlarge.

GM warrants and validates the fuel systems of the 2015 Silverado HD bi-fuel trucks. GM’s five-year/100,000-mile powertrain limited warranty covers CNG fuel systems. All major components associated with the CNG system also carry GM service part numbers.

The Silverado features Electronic Throttle Control (ETC)—i.e., there is no mechanical link between the accelerator pedal and the throttle body. A sensor at the pedal measures pedal angle and sends a signal to the engine control module (ECM), which in turn directs an electric motor to open the throttle at the appropriate rate and angle. The ETC system can deliver outstanding throttle response and greater reliability than a mechanical connection.

Transmission. The Hydra-Matic 6L90 (MYA) six-speed automatic transmission for rear- and all-wheel-drive vehicles is part of GM’s family of electronically controlled six-speed automatics. The modular design of the 6L90 is shared with variants rated for different torque levels to accommodate various engines and vehicle applications.

The modular concept of the Hydra-Matic 6L90 centers on the desire for common components and manufacturing tooling for four different size variants of the six-speed family. All versions use three gearsets: a simple input planetary gearset and two output gearsets, one of which is a compound gearset with three pairs of two pinion gears on the output carrier, one pair meshing with the sun gear and the other with the ring gear.

All versions share a unique output gearset configuration that enables it a wider range of ratios than a conventional planetary gearset automatic transmission. This allows the transmission to be tailored for optimal fuel economy while delivering excellent acceleration traits.

The wide overall ratio spread of 6.04:1 allows a “steep” first gear, as well as a very “tall” overdrive top gear for low-rpm highway cruising.

Performance Algorithm Shifting is available on some applications. Performance Algorithm Shifting (PAS) monitors how assertively the driver is using engine output to determine at what engine speed to upshift or downshift.

Equipped by default with a 4.10 axle ratio with the bi-fuel option, the maximum conventional trailering rating for the Silverado is 13,000 pounds (5,897 kg) on gasoline or CNG. The 4.10 ratio also enhances acceleration and can enable greater efficiency in stop-and-go driving and at speeds below 50 mph (80 km/h).

Chassis and suspension. All Silverado HD models feature fully boxed frame assemblies; 18 different assemblies that accommodate all of the range’s configurations. The front sections are hydroformed. Additional chassis and suspension features include:

• Short-long arm/torsion bar front suspension features forged steel upper control arms and precision-machined cast iron lower control arms.

• Four different torsion bar rates support four different front gross axle weight ratings to help stabilize the range of trim heights of various models under load, while enhancing appearance, handling, durability, tire wear and alignment. The trim height is adjusted on each bar via a single bolt, easily allowing the height to be changed to account for the weight of a snow plow or other accessories.

• An upper shock mount attachment design that’s positively connected to the frame with two fasteners, eliminating squeaks and clunks, while supporting higher load capability and increased durability.

• Asymmetrical leaf-spring design that minimizes axle hop and enhances traction control efficiency.

• All models feature 3-inch-wide (76 mm) leaf springs Z71 Off Road suspension is available on LT and LTZ models and includes 35mm deflective disk twin-tube shock absorbers, off-road jounce bumpers, 33mm front stabilizer bar and a skid plate package.

• Four-wheel disc brakes with hydroboost are standard on all models. Four-wheel, four-channel ABS is standard on all single-rear-wheel models and a three-channel system is standard on dual-rear-wheel models.

Silverado HD safety and crash-avoidance features. Silverado has a strong body structure to absorb crash energy and provide a protective “safety cage” around occupants. Head curtain side air bags with rollover protection and seat-mounted side-impact air bags for driver and right-front passenger are available. The roof-mounted head curtain air bags include design features that provide increased rollover protection and occupant containment, while the seat-mounted air bags provide thorax and pelvic protection.

Additional safety features include:

• StabiliTrak electronic stability control system is standard on all single-rear-wheel models;

• Large, four-wheel disc brakes with four-wheel ABS are standard on all models;

• High-strength steel tubular frame cross member that enhances safety and improves crashworthiness;

• Available rear vision camera;

• Trailer sway control system on all single-rear-wheel models; and

• Hill start assist is standard on single-rear-wheel models.

The trailer sway control system provides an added measure of confidence when towing a trailer. The system senses conditions of trailer sway and intervenes with braking and/or reduced engine power to bring the trailer under control and keep it on its intended path. The system will use electric trailer brakes when a trailer is plugged into the standard wiring harness of the truck and its performance requires no input from the driver.

The hill start assist system is automatically engaged when sensors detect the vehicle is on grade of about 5% or greater. It holds the brakes for about 1.5 seconds or until the gas pedal is pressed, preventing rollback—it is particularly effective when towing, giving the driver time to switch from the brake pedal to the gas pedal without rolling. Based on our experience, it works quite well.

Overall impressions. The integration of the CNG system to the Silverado seems flawless; a buyer ends up with all the very attractive features of the Silverado 2500HD line-up—which are considerable—along with the benefits of CNG, and the additional flexibility that a bi-fuel system provides in terms of range when a refueling infrastructure is still relatively undeveloped. The question, assuming the buyer has an operational need that would be met by the specs of the truck, is the cost.

With respect to our test vehicle, the 2015 Silverado 2500 4WD LT Crew Cab is priced at $42,655; the bi-fuel CNG system adds $9,500 to the sticker. Combined with other content, the truck we drove came in sticker-priced at $58,800. As a point of comparison, the Duramax diesel option adds $7,195 to the base price of the vehicle. The lower cost diesel will offer more power and torque, and lower fuel consumption—but of fuel that is priced significantly higher than CNG.

According to the DOE, the national average dispenser price from 1-15 April 2015 for CNG was $2.15/gge, compared to a national average of $3.65/gallon gasoline and $3.97/gallon diesel, making CNG 41% less expensive than gasoline and 46% less expensive than diesel. (B20’s average per-gallon price was $4.01.)

In a gasoline-use scenario, a fleet customer who puts 26,000 miles per year on the truck would currently save $1.50 per gallon on an estimated 1,650 gallons per year (at about 12 mpgg and 75% CNG use, according to Chevrolet), for an overall annual fuel savings of $2,475. Over four years of service, this would equate to $9,900 in savings, thereby offsetting the bi-fuel option cost. Actual values will of course vary with the truck’s tasks, and how it is driven.

So for buyers who put at least 26,000 miles on their trucks per year, the bi-fuel option pencils out as a good option (with lower emissions), with a shorter time to payback the more the truck is driven—assuming the price spread between CNG and gasoline stays the same.

On top of that, a variety of available federal and state incentives and grants may be available to assist the purchase of CNG-fueled trucks, depending upon the buyer’s location.

Resources

• Compressed Natural Gas Infrastructure, TIAX