Compressed Natural Gas
Compressed natural gas works very well on turbodiesels. It has a very high octane rating which allows the use of a high percentage of gas to diesel. CNG is less expensive than propane, often substantially less, which will reduce operating cost significantly. It offers the same power increase as propane, around 100 horsepower on the current diesel engines from Detroit. Since it is stored as a gas and not a liquid, it doesn't have the range of propane. More frequent refueling is required, but with a refueling appliance attached to a residential gas supply, you can refuel frequently and conveniently at home. Tanks are available from 2.5 gal to 20 gal GGE (gasoline gallon equivalent).
Since cng isn't liquified, it is sold by the GGE. This means that you are buying the equivalent amount of energy as in a gallon of gasoline. Gasoline is approximately 125,000 BTUs per gallon. Natural gas varies in energy content depending on its composition. Natural gas is mostly methane, but contains propane, butane, and a number of other hydrocarbons. Because of this, a cubic foot of natural gas can range from 1000 to 1500 BTUs. The weight of the fuel is proportional to its energy content, so rather than sell it by the cubic foot, it is sold at refueling stations by the pound. For every 5.66 lbs of fuel, you are charged for a GGE. This way, no matter the energy content of the fuel, you are paying by the BTU rather than the volume. CNG tanks are often sized by the GGE rather than the liquid volume the way propane tanks are sized. A tank that would hold approximately 40 liquid gallons will hold around 10 GGE at 3600 PSI.
Prices vary a lot for CNG at refueling stations, ranging from as low as 65¢/GGE in Utah to over $2.00 elsewhere. The real savings comes by refueling from home. Small compressors, often called home refueling appliances, are available to refuel at home. It attaches to the gas supply in the house, and is located in the garage or area where the vehicle is parked. A hose attaches to the vehicle, allowing refueling anytime the vehicle is home. We now have compressors available starting at 3 GGE per hour pumping rate. This would fill a 12 gallon tank in about 4 hours. Storage tanks are available to quick fill vehicles.
The supplemental CNG system used on a Chevrolet Silverado with Duramax engine has increased the diesel mileage from 18 mpg to 50 mpg. Bear in mind that there is no magic occurring here, the diesel is being replaced with cng. The approximate cng mileage achieved was 25 mpg. The following shows the math:
#2 diesel price $4.75/gal
CNG price $1.70/gal
Price to drive truck at 18 mpg running on #2 only $4.75/18 = 26.4¢/mile
Price of #2 to drive truck at 50 mpg running on #2 and cng $4.75/50 = 9.5¢/mile
Price of cng to drive truck at 25 mpg running on #2 and cng $1.70/25 = 6.8¢/mile
Combined price of the #2 and cng per mile 9.5¢ + 6.8¢ = 16.3¢/mile
Savings per mile is 26.4¢ - 16.3¢ = 10.1¢/mile
The savings will be greater using fuel compressed at home. A typical price for gas is 72¢ per therm. A therm is 100,000 BTUs, so when adjusted to a GGE it costs 72¢ x 125,000 btu/100,000 btu = 90¢/GGE. The compressor will add a cost of somewhere between 10¢ and 20¢/gal in electricity to operate.
Uncompressed natural gas 90¢
Ongoing electricity cost 10 - 20¢
Total cost of gas not including compressor cost $1.20 - $1.30/gal
CNG is also excellent to use as a supplemental fuel on stationary engines. If natural gas is available, there are no pumping costs to compress the gas. It works well on diesel gensets and other uses, such as tractors that are used to grind feed and run long periods without being moved. Hybrid units are available that allow low pressure gas and high pressure gas to be used. When using low pressure gas, the line is attached to the unit with quick couplings.
Keep in mind that besides the savings, you will have more available horsepower. If you would like to find out more, contact us. Below is a picture of a typical CNG injection assembly.
This unit is installed on a HPCR Cummins