Moving on Natural Gas
The fuel that everyone is talking about
By Paul Abelson
We’ve been talking about fuel economy for years, and part of the issue is the country’s dependence on foreign oil imports. That wasn’t always the case. In 1970, the United States spent $3 billion on foreign oil. Most of our needs were sourced domestically. In 1975, expenditures for imported oil had risen to $25 billion. Our payments for foreign oil were $450.8 billion in 2011.
We obviously need to cut imports and start exploiting domestic energy, and that is a large part of the reason that the transportation industry is turning to natural gas. The United States has an estimated 100 years to as much as 420 years of reserves in natural gas. Using this abundant domestic fuel could greatly reduce our dependence on foreign oil.
Back in the 1960s, diesel fuel surpassed gasoline as the predominant fuel for trucks. By 1980, fewer than 2 percent of new Class 8 trucks used gasoline, and most of those were for local use. By the mid-1980s, the superior fuel economy and durability of diesel made it the universal big rig fuel.
Diesel’s strengths didn’t keep visionary engineers from seeking the next great propulsion system. A number of prototype trucks featured turbine engines back in the 1980s. They could burn virtually any liquid fuel, but several undesirable characteristics prevented their acceptance. Fuel economy was poor, even compared to the inefficient diesels of the day, and acceleration suffered because turbine’s had to “spool up” before power could be delivered.
Meanwhile, diesel engine development continued to progress. Horsepower climbed from 268, 290 and 318 in the ’70s to the highly desirable 425 and 444 of the ’80s. In the 1990s, 500 hp was introduced, and today we have 600 hp available, even as mpg doubled.
Engine design began to change in response to exhaust emissions regulations by 1998, but the quest for fuel economy did not diminish. When the dust settled on EPA 2010 engines, many drivers were achieving 7 mpg and more in daily operations. The next round from EPA no longer targets particulate matter (PM) or oxides of nitrogen (NOx). Those regulations are here to stay. The new targets are greenhouse gases, mostly CO2. They must be reduced in 2014 and again in 2017. With hydrocarbon (hydrogen and carbon atoms) fuels, carbon dioxide (CO2) is created in direct proportion to the amount of fuel burned. CO2 emissions can be cut by reducing fuel used, or by reducing the carbon in the fuel.
Burning carbon atoms creates carbon dioxide. When hydrogen burns it creates water vapor. The heavier or more viscous the fuel, the more carbon there is relative to hydrogen. Diesel has as much as 52 percent carbon atoms and 48 percent hydrogen. Natural gas is about 80 percent hydrogen atoms and only 20 percent carbon. That’s why natural gas produces far less carbon dioxide.
Costs and savings
But the lighter the fuel, the less its energy content per unit. Natural gas is sold and transported in units of one thousand cubic feet. To allow us to compare gaseous and liquid fuels, engineers have developed measurements based on each fuel’s energy content. The measure is “diesel gallon equivalent” or DGE.
Traditionally, natural gas has sold from $1.60 to $2.00 less per DGE than diesel. That is one reason that operators like trash collectors and transit bus lines have willingly paid premiums of $50,000 per vehicle and more to use natural gas. Their payback has been as short as two years with a $1.50 DGE differential.
Are those savings achievable for long-haul operations? What are the problems when operating cross-country? A long-haul trucker gets far better mileage than local stop-and-go operations do, which reduces the per-mile payback. But most long-haul operators drive three to five times the distances that those local operators do resulting in payback as good as or better than local operators get.
Assuming 7 mpg for 100,000 miles a year, diesel at $3.75 /gallon will cost about $53,600 per year. With a $1.50 differential or $2.25/DGE, you’ll spend $32,200 per year using natural gas, a savings of $21,400. If a gas-fueled truck costs $50,000 more than a comparable diesel, it will take two and a half years to reach payback through fuel cost savings. Keep your truck longer and your bottom line will increase at least $21,400 per year. If the price difference is more, payback will be quicker. And you’ll get more for your truck at trade-in.
The savings are real, but there are also disadvantages. The first is weight. For those grossing out close to 80,000 pounds, the added weight of fuel tanks could be a major problem.
CNG vs. LNG
America is crisscrossed with pipelines transporting natural gas for use in factories, institutions and homes. Rarely is natural gas unavailable. But we can’t just hook up our trucks to a pipeline and drive off. We need containers to hold the gas, both on the truck and where we refuel.
Natural gas is stored two ways. It is compressed (CNG) or liquefied (LNG).
CNG stations take large volumes of gas, usually from large dedicated pipelines, then compress it and store it. When a vehicle needs refueling, the stored gas is pumped into the vehicle’s on-board tank. For domiciled fleets, vehicles that return home after each shift, refueling a CNG vehicle can take hours. Even with quick-fill facilities, the fill rate is 16 DGE/minute or less. Add in the fact that bulky, heavy tanks wrapped in multiple layers of fiberglass reinforcing are needed to contain 3,500 pounds per square inch (psi) pressure needed to store CNG and it’s clear that while local fleets can transition to CNG, it is totally inadequate for long-haul operations.
The LNG Process
Liquid natural gas does not have those limitations. It can be pumped and stored almost like diesel fuel. LNG takes one-third the volume of CNG, so 225 DGEs of LNG can be stored on board in about the same space as 75 gallons of CNG. LNG does not rely on pressure to stay liquid. Liquefying the gas requires refrigeration down to 260 degrees below zero F. Just as water is a liquid below 212 degrees F and a gas above that, natural gas liquefies when greatly chilled. LNG tanks are constructed like huge stainless steel vacuum bottles. They are heavier than diesel tanks, but much lighter than CNG tanks. A small (75 DGE) LNG tank weighs about 250 pounds, while a CNG tanks comes in at 1,250 pounds or more.
Fuel systems account for much of the cost differential between diesel and gas-fueled trucks, with the rest coming from the engine and related systems. The market leader today is a joint venture between Cummins and Westport Innovations. They convert to spark ignition with new fuel management systems. Cummins Westport engines range from 8 to 12 liters, with power up to 400 hp and 1,450 lb-ft of torque.
Cummins is working on its own proprietary 15-liter spark-ignited engine. Their ISX-15G will produce 475 hp and 1,750 lb-ft. Meanwhile, Volvo is working with Westport to produce its own 13-liter LNG engine featuring Westport’s advanced high-pressure direct injection technology. Navistar, too, is developing a proprietary gas engine. Instead of spark ignition, the MaxxForce 13 liter engine will use diesel to begin the compression ignition process. Once a flame front is established, gas fueling will continue the burn. The ratio is about 15 percent diesel and 85 percent natural gas.
Many contenders have been touted as replacements for diesel as the primary fuel for long-haul commercial trucking. In time, it will be clear if natural gas is the one true challenger.